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enzootic bovine leukosis

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Datasheet

enzootic bovine leukosis

Summary

  • Last modified
  • 14 July 2018
  • Datasheet Type(s)
  • Animal Disease
  • Preferred Scientific Name
  • enzootic bovine leukosis
  • Overview
  • According to Olson and Miller (1987), the initial descriptions of enzootic bovine leukosis (EBL) were published in German...

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Identity

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Preferred Scientific Name

  • enzootic bovine leukosis

International Common Names

  • English: adult bovine lymphosarcoma; adult lymphosarcoma; adult multicentric lymphosarcoma, sporadic bovine leukosis; bovine leukosis; bovine lymphosarcoma; bovine malignant lymphoma; deltaretrovirus enzootic bovine leukosis; EBL; enzootic adult lymphosarcoma, lymphoma, bovine leukosis, blv; leukemia; leukemia in cattle; lymphosarcoma; malignant lymphoma

English acronym

  • BLV
  • EBL

Overview

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According to Olson and Miller (1987), the initial descriptions of enzootic bovine leukosis (EBL) were published in German medical literature in 1871. An increasing number of reports of EBL in virtually every other Eastern European country were published after World War II. European cattle that probably were infected with bovine leukaemia virus (BLV) were imported from the shores of the Baltic sea to the USA at the end of the 19th century. Reports of EBL in the USA were published first before World War II. During the latter half of the 20th century, EBL was reported in South America, Africa, Australia, Japan, and Israel. Now, BLV infection can truly be considered pandemic, even if the reported incidence of adult bovine lymphosarcoma varies greatly among affected countries (OIE, 1999). BLV, a retrovirus, is a member of the family Retroviridae. BLV’s specific genus, deltaretrovirus, is one of seven newly-created genera of animal and human retroviruses that was established by the International Committee on Taxonomy of Viruses. Apparently the "subfamilies" of oncovirinae, lentivirinae, and spumavirinae are now obsolete terms. BLV causes a fatal disease and a non-fatal disease, adult bovine lymphosarcoma and persistent lymphocytosis, respectively. The natural hosts for BLV infection are dairy and beef cattle. The prevalence of BLV infection is high in comparison to the prevalence of adult bovine lymphosarcoma. The adverse economic impacts of BLV infection are rather inconsistent, however, they include death, culling, impacts on milk production, and barriers to international trade. BLV infection is not transmissible to humans.

Attempts to identify an infectious cause of bovine leukosis failed for many years. A significant breakthrough arose in 1969 when it was reported that the leukocytes of some cattle produced virus particles after being cultured for 2 to 3 days (Miller et al., 1969). The particles were similar to virus particles from cells of other animal species with leukaemia. The virus was isolated only from cows with adult lymphosarcoma, and from cows with persistent lymphocytosis, but not from cattle with the calf, thymic, and skin forms of leukosis. The virus was not isolated from cattle in EBL-negative herds. The transmissibility of the agent was confirmed after calves which were inoculated with cell cultures containing the virus became infected themselves and developed persistent lymphocytosis. Large-scale sero-epidemiological studies of the bovine population were conducted. The results of all of these early studies provided sufficient evidence to meet the virological and sero-epidemiological criteria necessary for the establishment of a cause-effect relationship between BLV and EBL.

This disease is on the list of diseases notifiable to the World Organisation for Animal Health (OIE). The distribution section contains data from OIE's WAHID database on disease occurrence. Please see the AHPC library for further information on this disease from OIE, including the International Animal Health Code and the Manual of Standards for Diagnostic Tests and Vaccines. Also see the website: www.oie.int.

Hosts/Species Affected

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Cattle are the only animals that are thought to be susceptible naturally to bovine leukaemia virus (BLV) infection. Sheep are susceptible to experimental infection with BLV, and seem to be more rapidly susceptible to the pathological effects of BLV infection. Thus, sheep are commonly used in laboratory studies of BLV infection. There is no breed predisposition to BLV infection in cattle or in sheep. Nor is there a breed predisposition to lymphosarcoma in these two species. Although the data are limited, it indicates that breeds within each of these species are equally susceptible to BLV infection and to lymphosarcoma.

Distribution

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The results of epidemiological studies suggest that bovine leukaemia virus (BLV) infection is globally distributed. Regardless of whether a study was reported by a highly developed country or by a poorly developed country, absence of BLV infection is reported rarely, if at all. Some of the more statistically valid national studies were reported by the USA, the European Union, Canada, and Australia. A 1980 national study of BLV infection in Canada showed that 40% of its dairy herds and 11% of its beef herds were infected (Samagh and Kellar, 1982). BLV control programmes have been established in member countries of the European Union (EU) since the 1980s. According to Lorenz and Straub (1987), the seroprevalence in the entire EU cattle population rarely exceeds 0.5 to 1.5%. In the USA, epidemiological studies of BLV infection before 1996 had been restricted to a few states, small regions within those states, or single herds (Johnson and Kaneene, 1991a, b). The seroprevalence varied greatly among those herds from 0% to 95% of the cattle sampled. An assessment of BLV prevalence in USA dairy operations was part of the national dairy and beef studies by the USDA (USDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999). The dairy study showed that 89% of USA dairy operations and 43.5% of USA dairy cattle were seropositive for BLV. Herd prevalence in 4 different geographic regions was 87% to 99%. Virtually all animals tested in some herds were seropositive. The within-herd seroprevalence was at least 25% in 75% of the positive dairy herds. The beef study showed that 38.7% of USA beef operations and 10.3% of USA beef cattle were seropositive for BLV. Herd prevalence in various geographic regions was 13.3% to 51.1%. The within-herd seroprevalence was 0.92% to 17.1% in the positive beef herds. South American countries have begun to report BLV prevalence recently (Carvalho et al., 1996; Samara et al., 1997; Alfonso et al., 1998; Molnár et al., 1999a, b). BLV infection has been reported by many other countries, but valid national estimates of seroprevalence are rare for these countries (Bhattacharjya, 1999; El-Sebaie et al., 1999).

For current information on disease incidence, see OIE's WAHID Interface.

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Asia

AfghanistanNo information availableOIE, 2009
ArmeniaDisease not reportedOIE, 2009
AzerbaijanDisease not reportedOIE, 2009
BahrainDisease not reportedOIE, 2009
BangladeshDisease not reportedOIE, 2009
BhutanNo information availableOIE, 2009
Brunei DarussalamDisease not reportedOIE Handistatus, 2005
CambodiaNo information availableOIE, 2009
ChinaNo information availableNULLOIE, 1999; OIE, 2009
-Hong KongNo information availableOIE, 2009
Georgia (Republic of)Last reported1996OIE Handistatus, 2005
IndiaDisease not reportedNULLOIE, 1999; OIE, 2009
IndonesiaDisease not reportedOIE, 2009
IranPresentNULLOIE, 1999; OIE, 2009
IraqNo information availableOIE, 2009
IsraelPresentNULLOIE, 1999; OIE, 2009
JapanPresentNULLOIE, 1999; OIE, 2009
JordanDisease never reportedOIE, 2009
KazakhstanDisease not reportedOIE, 2009
Korea, DPRDisease not reportedOIE Handistatus, 2005
Korea, Republic ofPresentNULLOIE, 1999; OIE, 2009
KuwaitDisease not reportedOIE, 2009
KyrgyzstanRestricted distributionOIE, 2009
LaosDisease not reportedOIE, 2009
LebanonDisease not reportedOIE, 2009
MalaysiaDisease never reportedOIE, 2009
-Peninsular MalaysiaDisease never reportedOIE Handistatus, 2005
-SabahDisease never reportedOIE Handistatus, 2005
-SarawakDisease never reportedOIE Handistatus, 2005
MongoliaNo information availableOIE, 2009
MyanmarNo information availableOIE, 2009
NepalDisease never reportedOIE, 2009
OmanDisease not reportedOIE, 2009
PakistanDisease not reportedOIE, 2009
PhilippinesDisease never reportedOIE, 2009
QatarNo information availableOIE, 2009
Saudi ArabiaDisease not reportedNULLOIE, 1999; OIE, 2009
SingaporeDisease never reportedOIE, 2009
Sri LankaDisease never reportedOIE, 2009
SyriaDisease not reportedOIE, 2009
TaiwanLast reported2003OIE Handistatus, 2005
TajikistanDisease not reportedOIE, 2009
ThailandDisease not reportedNULLAiumlamai et al., 1992; OIE, 2009
TurkeyNo information availableNULLUysal et al., 1998; Carli et al., 1999; OIE, 2009
TurkmenistanDisease not reportedOIE Handistatus, 2005
United Arab EmiratesNo information availableOIE, 2009
UzbekistanDisease not reportedOIE Handistatus, 2005
VietnamAbsent, reported but not confirmedOIE, 2009
YemenNo information availableOIE, 2009

Africa

AlgeriaDisease not reportedNULLOIE, 1999; OIE, 2009
AngolaDisease never reportedOIE, 2009
BeninDisease not reportedOIE, 2009
BotswanaDisease never reportedOIE, 2009
Burkina FasoNo information availableOIE, 2009
BurundiDisease not reportedOIE Handistatus, 2005
CameroonNo information availableOIE Handistatus, 2005
Cape VerdeDisease never reportedOIE Handistatus, 2005
Central African RepublicDisease not reportedOIE Handistatus, 2005
ChadNo information availableOIE, 2009
CongoNo information availableOIE, 2009
Congo Democratic RepublicDisease not reportedOIE Handistatus, 2005
Côte d'IvoireDisease not reportedOIE Handistatus, 2005
DjiboutiDisease never reportedOIE, 2009
EgyptDisease not reported1997OIE, 1999; OIE, 2009
EritreaNo information availableOIE, 2009
EthiopiaNo information availableNULLHeinonen and Assefa, 1995; OIE, 2009
GabonDisease never reportedOIE, 2009
GambiaNo information availableOIE, 2009
GhanaNo information availableOIE, 2009
GuineaNo information availableOIE, 2009
Guinea-BissauNo information availableOIE, 2009
KenyaDisease never reportedOIE, 2009
LesothoDisease never reportedOIE, 2009
LibyaDisease never reportedOIE Handistatus, 2005
MadagascarDisease never reportedOIE, 2009
MalawiPresentOIE, 2009
MaliNo information availableOIE, 2009
MauritiusDisease not reportedOIE, 2009
MoroccoNo information availableOIE, 2009
MozambiqueNo information availableOIE, 2009
NamibiaDisease not reported1993OIE, 1999; OIE, 2009
NigerPresentOyejide et al., 1987
NigeriaNo information availableOIE, 2009
RéunionReported present or known to be presentOIE Handistatus, 2005
RwandaNo information availableOIE, 2009
Sao Tome and PrincipeDisease not reportedOIE Handistatus, 2005
SenegalNo information availableOIE, 2009
SeychellesDisease not reportedOIE, 1999; OIE Handistatus, 2005
SomaliaNo information availableOIE Handistatus, 2005
South AfricaDisease not reported200507OIE, 1999; OIE, 2009
SudanDisease never reportedOIE, 2009
SwazilandNo information availableOIE, 2009
TanzaniaNo information availableOIE, 2009
TogoNo information availableOIE, 2009
TunisiaDisease not reported200507OIE, 1999; OIE, 2009
UgandaNo information availableOIE, 2009
ZambiaDisease not reportedOIE, 2009
ZimbabweDisease not reportedNULLOIE, 1999; OIE, 2009

North America

BermudaDisease not reportedOIE Handistatus, 2005
CanadaPresentNULLOIE, 1999; OIE, 2009
GreenlandDisease never reportedOIE, 2009
MexicoPresentNULLOIE, 1999; OIE, 2009
USAPresentNULLOIE, 1999; OIE, 2009
-AlabamaPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-ArkansasPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-CaliforniaPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-ColoradoPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-FloridaPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-GeorgiaUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-IdahoPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-IllinoisPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-IowaPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-KansasPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-KentuckyPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-MichiganPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-MinnesotaPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-MississippiPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-MissouriPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-MontanaPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-NebraskaPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-New MexicoPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-New YorkPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-North DakotaPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-OhioPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-OklahomaPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-OregonPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-PennsylvaniaPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-South DakotaPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-TennesseePresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-TexasPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-VermontPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-VirginiaPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-WashingtonPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999
-WisconsinPresentUSDA/APHIS/Veterinary Services, 1997; USDA/APHIS/Veterinary Services, 1999

Central America and Caribbean

BarbadosSerological evidence and/or isolation of the agentOIE Handistatus, 2005
BelizeDisease not reportedOIE, 2009
British Virgin IslandsDisease never reportedOIE Handistatus, 2005
Cayman IslandsDisease not reportedOIE Handistatus, 2005
Costa RicaPresentNULLOIE, 1999; OIE, 2009
CubaPresentNULLOIE, 1999; OIE, 2009
CuraçaoDisease not reportedOIE Handistatus, 2005
DominicaDisease not reportedOIE Handistatus, 2005
Dominican RepublicPresentNULLOIE, 1999; OIE, 2009
El SalvadorNo information availableOIE, 2009
GuadeloupeNo information availableOIE, 2009
GuatemalaDisease not reportedOIE, 2009
HaitiDisease never reportedOIE, 2009
HondurasPresentOIE, 2009
JamaicaDisease not reportedOIE, 2009
MartiniquePresentNULLOIE, 1999; OIE, 2009
NicaraguaPresentOIE, 2009
PanamaPresentNULLOIE, 1999; OIE, 2009
Saint Kitts and NevisNo information availableOIE Handistatus, 2005
Saint Vincent and the GrenadinesDisease never reportedOIE Handistatus, 2005
Trinidad and TobagoLast reported1996OIE, 1999; OIE Handistatus, 2005

South America

ArgentinaPresentNULLOIE, 1999; OIE, 2009
BoliviaNo information availableNULLOIE, 1999; OIE, 2009
BrazilPresentNULLOIE, 1999; OIE, 2009
-ParaPresentMolnár et al., 1999b
-ParanaPresentCarvalho et al., 1996
-Sao PauloPresentSamara et al., 1997; D'Angelino et al., 1998
ChilePresentNULLOIE, 1999; OIE, 2009
ColombiaPresentNULLOIE, 1999; OIE, 2009
EcuadorPresentNULLOIE, 1999; OIE, 2009
Falkland IslandsDisease never reportedOIE Handistatus, 2005
French GuianaDisease not reportedOIE, 2009
GuyanaDisease not reportedOIE Handistatus, 2005
ParaguayOIE, 1999; OIE Handistatus, 2005
PeruRestricted distributionNULLOIE, 1999; OIE, 2009
UruguayRestricted distributionNULLOIE, 1999; OIE, 2009
VenezuelaPresentOIE, 2009

Europe

AlbaniaNo information availableOIE, 2009
AndorraLast reported1994OIE, 1999; OIE Handistatus, 2005
AustriaDisease not reported2001OIE, 1999; OIE, 2009
BelarusDisease not reportedNULLOIE, 1999; OIE, 2009
BelgiumDisease not reportedNULLOIE, 1999; OIE, 2009
Bosnia-HercegovinaReported present or known to be presentOIE Handistatus, 2005
BulgariaPresentNULLOIE, 1999; OIE, 2009
CroatiaPresentNULLOIE, 1999; OIE, 2009
CyprusDisease not reported1995OIE, 1999; OIE, 2009
Czech RepublicPresentNULLOIE, 1999; OIE, 2009
DenmarkDisease not reported1990OIE, 1999; OIE, 2009
EstoniaPresentNULLOIE, 1999; OIE, 2009
FinlandPresentNULLOIE, 1999; OIE, 2009
FranceNo information availableNULLOIE, 1999; OIE, 2009
GermanyPresentNULLOIE, 1999; OIE, 2009
GreecePresentNULLOIE, 1999; OIE, 2009
HungaryPresentNULLOIE, 1999; OIE, 2009
IcelandDisease never reportedOIE, 2009
IrelandDisease not reported1999OIE, 1999; OIE, 2009
Isle of Man (UK)Last reported1991OIE Handistatus, 2005
ItalyRestricted distributionNULLOIE, 1999; OIE, 2009
JerseyDisease never reportedOIE Handistatus, 2005
LatviaPresentNULLOIE, 1999; OIE, 2009
LiechtensteinDisease not reportedOIE, 2009
LithuaniaPresentNULLOIE, 1999; OIE, 2009
LuxembourgDisease not reportedOIE, 2009
MacedoniaPresentOIE, 2009
MaltaPresentNULLOIE, 1999; OIE, 2009
MoldovaSerological evidence and/or isolation of the agentOIE Handistatus, 2005
MontenegroPresentOIE, 2009
NetherlandsPresentNULLOIE, 1999; OIE, 2009
NorwayDisease not reported2002OIE, 1999; OIE, 2009
PolandPresentNULLOIE, 1999; OIE, 2009
PortugalPresentOIE, 2009
RomaniaRestricted distributionNULLOIE, 1999; OIE, 2009
Russian FederationPresentOIE, 2009
SerbiaPresentOIE, 2009
SlovakiaPresentNULLOIE, 1999; OIE, 2009
SloveniaDisease not reported200606OIE, 1999; OIE, 2009
SpainDisease not reportedNULLOIE, 1999; OIE, 2009
SwedenDisease not reported200703OIE, 1999; OIE, 2009
SwitzerlandDisease not reported200505OIE, 1999; OIE, 2009
UKDisease not reportedOIE, 2009
-Northern IrelandDisease never reportedOIE Handistatus, 2005
UkrainePresentNULLOIE, 1999; OIE, 2009
Yugoslavia (former)No information availableOIE Handistatus, 2005
Yugoslavia (Serbia and Montenegro)Reported present or known to be presentOIE Handistatus, 2005

Oceania

AustraliaRestricted distributionNULLOIE, 1999; OIE, 2009
-New South WalesPresentAustralia's Nat Animal Health Info System, 1998
-QueenslandPresentAustralia's Nat Animal Health Info System, 1998
-South AustraliaPresentAustralia's Nat Animal Health Info System, 1998
-TasmaniaPresentAustralia's Nat Animal Health Info System, 1998
-VictoriaPresentAustralia's Nat Animal Health Info System, 1998
French PolynesiaNo information availableNULLOIE, 1999; OIE, 2009
GuamReported present or known to be presentOIE, 1999
New CaledoniaDisease never reportedOIE, 2009
New ZealandDisease not reportedNULLOIE, 1999; OIE, 2009
SamoaDisease never reportedOIE Handistatus, 2005
VanuatuDisease never reportedOIE Handistatus, 2005
Wallis and Futuna IslandsNo information availableOIE Handistatus, 2005

Pathology

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Gross Pathology


Enzootic bovine leukosis leads to a multi-centric lymphosarcoma in cows that usually are at least 2 years of age (Parodi, 1987). Multi-centric lymphosarcoma is characterized by infiltrating tumours of the lymph nodes and non-haematopoietic organs. There are three groups of lymphosarcomas: (1) generalized lymphosarcomas with lesions involving 76 to 100% of the lymph nodes, (2) disseminated lymphosarcomas involving 26 to 75% of lymph nodes, and (3) localized lymphosarcomas involving 15 to 25% of lymph nodes. The most frequently affected lymph nodes are the iliac, intra-thoracic, and mesenteric nodes. These nodes are enlarged to several centimetres. The incised surface homogenous tumour tissue is soft and grey to white. Foci of haemorrhage and yellow necrotic areas may be visible. The spleen is enlarged diffusely, and reddened splenic pulp may be interrupted by grey granulations. The liver is enlarged, light tan to yellow, and has marbled-grey tissue due to infiltration of tumours into the connective tissue spaces. Tumours in the heart originate in the right atrial wall primarily, then spread to the ventricles and the septum. Neoplastic nodules create ulcerated, haemorrhagic lesions in the pyloric region of the abomasum. Tumours may infiltrate the endometrium and myometrium as well as the placenta in pregnant cows. Diffuse and nodular tumours infiltrate the renal cortex, medulla, and pelvis. Pulmonary, bladder, bone marrow and skeletal muscle lesions are rare.


Histopathology


At least eight different cytological types of tumours have been described in multi-centric lymphosarcoma (Parodi, 1987). These are (1) lymphoplasmacytic/lymphoplasmacytoid, (2) centroblastic / centrocytic; (3) centroblastic, monomorphous subtype, (4) centroblastic, polymorphous subtype, (5) immunoblastic without plasmablastic differentiation, (6) immunoblastic with plasmablastic differentiation, (7) lymphoblastic, convoluted cell-type, (8) lymphoblastic, non-convoluted cell-type, and (9) undetermined. Most multi-centric lymphosarcoma are derived from B-cells.

Diagnosis

Top of page Clinical Diagnosis

The clinical signs associated with the deposition of solid tumours in cattle with enzootic bovine leukosis (EBL) have been reviewed (Reed, 1981; Stober, 1981). Although the tumours originate from lymphoid tissue, they may be located in virtually every organ. Whether clinical signs are present is determined largely by the location of the tumours. Although enlargement of the external lymph nodes is common, the internal nodes may also be enlarged in the absence of external involvement. The sublumbar and deep inguinal lymph nodes are common sites for tumours. The tumours very often invade the gastrointestinal tract, especially the abomasum, thus causing obstructions or ulcers that lead to anorexia and weight loss. Tumours in the spinal cord give rise to neurological disturbances. Clinical signs of heart failure are often associated with tumours of the myocardium. Retrobulbar tumours, which occur frequently, are clinically evident as protrusions of the eyeballs.

Differential Diagnosis

EBL is a neoplastic disease that can be confused with numerous diseases because of metatases to numerous organ systems. A non-exhaustive list of differential diagnoses for EBL are abomasal displacement, abomasal ulcers, traumatic reticulo-peritonitis, vagal indigestion, hypocalcaemia, hypokalaemia, fractured vertebrae, spinal cord abscess, peroneal nerve injury, downer cow syndrome, botulism, keratoconjunctivitis, ocular squamous cell carcinoma, pyelonephritis, and endocarditis.

Laboratory Diagnosis

The four most commonly used serologic assays for the bovine retroviruses are the agar gel immunodiffusion (AGID) assay; enzyme linked immunosorbent assay (ELISA-Ab); western blot (WB); and indirect fluorescent antibody (IFA), but not all are readily available commercially (Evermann and Jackson, 1997). The AGID test continues to be one of the most reliable indicators of bovine leukosis virus (BLV) infection. This test correlates well with infection, and it has a high degree of specificity, due in part to the relative stability of the BLV genome. Although the AGID test is unable to detect low levels of BLV antibodies soon after infection, it has been shown to reliably detect antibodies within 2 to 4 weeks following experimental infection. In natural infection, where the dose of virus-infected cells may be low, it may take up to 12 weeks to detect antibodies.

Although the AGID test will detect BLV-infected cattle, it cannot distinguish between passively acquired (colostral) antibodies and antibodies acquired through natural infection, and depending on the level of virus-infected lymphocytes, it may not detect infected cattle until several months after infection (Evermann and Jackson, 1997). The AGID test is also less sensitive than several other serological assays, including ELISA and radioimmunoassay (RIA), and especially those assays that use monoclonal antibodies. The AGID test does have the advantages of being inexpensive and easy to perform, and it yields quick, clear, and easily interpreted results. These attributes outweigh some of the limitations of the test’s sensitivity. Because of the diverse serological assays available, a recommendation has been made to standardize BLV serological assays using a known set of reference sera.

Although immunoassays have relied upon serum for the detection of antibodies, there would be advantages to testing other body secretions, such as milk and urine (for example, ease of collection and a larger volume of substrate available for testing) (Evermann and Jackson, 1997). The disadvantages of assaying milk for BLV antibodies would be the large dilution factor, the interference by lactogenic proteins, and the instability of antibodies in stored milk due to bacterial growth. Despite these problems, there are good reasons to utilize a milk-based antibody assay for BLV. For one thing, such a test would allow for cow-side testing, which would be convenient and save time and money.

List of Symptoms/Signs

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SignLife StagesType
Cardiovascular Signs / Arrhythmia, irregular heart rate, pulse Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Cardiovascular Signs / Heart murmur Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Cardiovascular Signs / Jugular pulse Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Cardiovascular Signs / Muffled, decreased, heart sounds Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Cardiovascular Signs / Peripheral venous distention, jugular distention Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Cardiovascular Signs / Tachycardia, rapid pulse, high heart rate Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Cardiovascular Signs / Ventricular premature beat, multifocal or unifocal Sign
Digestive Signs / Abdominal distention Cattle & Buffaloes:All Stages Sign
Digestive Signs / Absence of tooth, decreased number of teeth Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Digestive Signs / Anorexia, loss or decreased appetite, not nursing, off feed Cattle & Buffaloes:All Stages Sign
Digestive Signs / Bilateral ping, auscultable gas filled viscus both sides Cattle & Buffaloes:All Stages Sign
Digestive Signs / Bloat in ruminants, tympany Cattle & Buffaloes:All Stages Sign
Digestive Signs / Decreased amount of stools, absent faeces, constipation Cattle & Buffaloes:All Stages Sign
Digestive Signs / Diarrhoea Cattle & Buffaloes:All Stages Sign
Digestive Signs / Difficulty in prehending or chewing food Cattle & Buffaloes:All Stages Sign
Digestive Signs / Dysphagia, difficulty swallowing Cattle & Buffaloes:All Stages Sign
Digestive Signs / Excessive salivation, frothing at the mouth, ptyalism Cattle & Buffaloes:All Stages Sign
Digestive Signs / Fecal incontinence Cattle & Buffaloes:All Stages Sign
Digestive Signs / Grinding teeth, bruxism, odontoprisis Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Digestive Signs / Hepatosplenomegaly, splenomegaly, hepatomegaly Cattle & Buffaloes:All Stages Sign
Digestive Signs / Loose teeth, tooth Cattle & Buffaloes:All Stages Sign
Digestive Signs / Melena or occult blood in faeces, stools Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Digestive Signs / Ping left side, auscultable gas filled viscus Cattle & Buffaloes:All Stages Sign
Digestive Signs / Ping right side, auscultable gas filled viscus Cattle & Buffaloes:All Stages Sign
Digestive Signs / Rumen hypomotility or atony, decreased rate, motility, strength Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Digestive Signs / Tongue protrusion Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
General Signs / Abnormal proprioceptive positioning, knuckling Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
General Signs / Ataxia, incoordination, staggering, falling Cattle & Buffaloes:All Stages Sign
General Signs / Back swelling, mass back region Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
General Signs / Dehydration Cattle & Buffaloes:All Stages Sign
General Signs / Dysmetria, hypermetria, hypometria Cattle & Buffaloes:All Stages Sign
General Signs / Exercise intolerance, tires easily Cattle & Buffaloes:All Stages Sign
General Signs / Fever, pyrexia, hyperthermia Cattle & Buffaloes:All Stages Diagnosis
General Signs / Forelimb weakness, paresis, paralysis front leg Sign
General Signs / Generalized lameness or stiffness, limping Sign
General Signs / Generalized weakness, paresis, paralysis Sign
General Signs / Generalized weakness, paresis, paralysis Sign
General Signs / Haemorrhage of any body part or clotting failure, bleeding Cattle & Buffaloes:All Stages Sign
General Signs / Head, face, ears, jaw, nose, nasal, swelling, mass Cattle & Buffaloes:All Stages Sign
General Signs / Hemiparesis Cattle & Buffaloes:All Stages Sign
General Signs / Hindlimb atrophy, wasting Cattle & Buffaloes:All Stages Sign
General Signs / Hindlimb lameness, stiffness, limping hind leg Cattle & Buffaloes:All Stages Sign
General Signs / Icterus, jaundice Cattle & Buffaloes:All Stages Sign
General Signs / Inability to stand, downer, prostration Cattle & Buffaloes:All Stages Sign
General Signs / Internal abdominal mass, swellings, adhesions abdomen Cattle & Buffaloes:All Stages Diagnosis
General Signs / Intraocular mass, swelling interior of eye Sign
General Signs / Intraocular mass, swelling interior of eye Sign
General Signs / Lack of growth or weight gain, retarded, stunted growth Cattle & Buffaloes:Calf Sign
General Signs / Laryngeal, tracheal, pharyngeal swelling, mass larynx, trachea, pharynx Sign
General Signs / Lymphadenopathy, swelling, mass or enlarged lymph nodes Cattle & Buffaloes:All Stages Diagnosis
General Signs / Neck swelling, mass cervical region Cattle & Buffaloes:Calf,Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
General Signs / Oral cavity, tongue swelling, mass in mouth Sign
General Signs / Orbital, periorbital, periocular, conjunctival swelling, eyeball mass Cattle & Buffaloes:All Stages Diagnosis
General Signs / Pale mucous membranes or skin, anemia Sign
General Signs / Paraparesis, weakness, paralysis both hind limbs Cattle & Buffaloes:All Stages Sign
General Signs / Sudden death, found dead Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
General Signs / Swelling mass cervix uteri Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
General Signs / Swelling mass ovary Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
General Signs / Swelling mass uterus Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
General Signs / Swelling skin or subcutaneous, mass, lump, nodule Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
General Signs / Swelling, mass external abdomen Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
General Signs / Tail weakness, paresis, paralysis sacrococcygeal region Cattle & Buffaloes:All Stages Sign
General Signs / Tenesmus, straining, dyschezia Cattle & Buffaloes:All Stages Sign
General Signs / Tetraparesis, weakness, paralysis all four limbs Cattle & Buffaloes:All Stages Sign
General Signs / Thoracic swelling, mass, thorax, chest, ribs, sternum Cattle & Buffaloes:All Stages Diagnosis
General Signs / Underweight, poor condition, thin, emaciated, unthriftiness, ill thrift Cattle & Buffaloes:All Stages Sign
General Signs / Weakness of one hindlimb, paresis paralysis rear leg Sign
General Signs / Weight loss Cattle & Buffaloes:All Stages Sign
Musculoskeletal Signs / Head or neck crepitation, crepitus Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Nervous Signs / Abnormal anal, perineal, tail reflexes, increased or decreased Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Nervous Signs / Abnormal forelimb reflexes, increased or decreased Sign
Nervous Signs / Abnormal forelimb reflexes, increased or decreased Sign
Nervous Signs / Abnormal hindlimb reflexes, increased or decreased Sign
Nervous Signs / Abnormal hindlimb reflexes, increased or decreased Sign
Nervous Signs / Back hypoesthesia, anesthesia Cattle & Buffaloes:All Stages Sign
Nervous Signs / Circling Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Nervous Signs / Dullness, depression, lethargy, depressed, lethargic, listless Cattle & Buffaloes:All Stages Sign
Nervous Signs / Forelimb hypoesthesia, anesthesia front leg Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Nervous Signs / Head pressing Sign
Nervous Signs / Head tilt Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Nervous Signs / Head, face, neck, tongue hypoesthesia, anesthesia Cattle & Buffaloes:All Stages Sign
Nervous Signs / Hindlimb hypoesthesia, anesthesia rear leg Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Nervous Signs / Tail, anal hypoesthesia, anesthesia perineum, anus Sign
Nervous Signs / Tail, anal hypoesthesia, anesthesia perineum, anus Sign
Nervous Signs / Thoracic or abdominal hypoesthesia, anesthesia chest, ribs, abdomen Sign
Nervous Signs / Thoracic or abdominal hypoesthesia, anesthesia chest, ribs, abdomen Sign
Ophthalmology Signs / Abnormal pupillary response to light Sign
Ophthalmology Signs / Anisocoria Cattle & Buffaloes:All Stages Sign
Ophthalmology Signs / Blindness Cattle & Buffaloes:All Stages Sign
Ophthalmology Signs / Buphthalmia, enlarged globe eye, glaucoma Cattle & Buffaloes:All Stages Sign
Ophthalmology Signs / Chemosis, conjunctival, scleral edema, swelling Cattle & Buffaloes:All Stages Sign
Ophthalmology Signs / Conjunctival, scleral, injection, abnormal vasculature Cattle & Buffaloes:All Stages Sign
Ophthalmology Signs / Conjunctival, scleral, redness Cattle & Buffaloes:All Stages Sign
Ophthalmology Signs / Corneal edema, opacity Cattle & Buffaloes:All Stages Sign
Ophthalmology Signs / Corneal neovascularization, pannus Cattle & Buffaloes:All Stages Sign
Ophthalmology Signs / Corneal swelling, mass, nodule Cattle & Buffaloes:All Stages Sign
Ophthalmology Signs / Corneal ulcer, erosion Cattle & Buffaloes:All Stages Sign
Ophthalmology Signs / Enophthalmos, sunken eyes Cattle & Buffaloes:All Stages Sign
Ophthalmology Signs / Exophthalmos, eyes protruding, proptosis Cattle & Buffaloes:All Stages Sign
Ophthalmology Signs / Hypopyon, lipid, or fibrin, flare, of anterior chamber Cattle & Buffaloes:All Stages Sign
Ophthalmology Signs / Lacrimation, tearing, serous ocular discharge, watery eyes Cattle & Buffaloes:All Stages Sign
Ophthalmology Signs / Mydriasis, dilated pupil Cattle & Buffaloes:All Stages Sign
Ophthalmology Signs / Ptosis, lid droop Cattle & Buffaloes:All Stages Sign
Ophthalmology Signs / Purulent discharge from eye Sign
Ophthalmology Signs / Purulent discharge from eye Sign
Pain / Discomfort Signs / Ocular pain, eye Cattle & Buffaloes:All Stages Sign
Pain / Discomfort Signs / Pain, head, face, jaw, ears Cattle & Buffaloes:All Stages Sign
Reproductive Signs / Absence of uterus and / or ovaries Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow Sign
Reproductive Signs / Agalactia, decreased, absent milk production Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow Sign
Reproductive Signs / Anestrus, absence of reproductive cycle, no visible estrus Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow Sign
Reproductive Signs / Enlarged firm ovary, ovaries Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow Sign
Respiratory Signs / Abnormal breathing sounds of the upper airway, airflow obstruction, stertor, snoring Sign
Respiratory Signs / Abnormal breathing sounds of the upper airway, airflow obstruction, stertor, snoring Sign
Respiratory Signs / Coughing, coughs Sign
Respiratory Signs / Coughing, coughs Sign
Respiratory Signs / Decreased, muffled, lung sounds, absent respiratory sounds Cattle & Buffaloes:All Stages Sign
Respiratory Signs / Dyspnea, difficult, open mouth breathing, grunt, gasping Cattle & Buffaloes:All Stages Sign
Respiratory Signs / Increased respiratory rate, polypnea, tachypnea, hyperpnea Cattle & Buffaloes:All Stages Sign
Skin / Integumentary Signs / Skin edema Sign
Skin / Integumentary Signs / Skin edema Sign
Urinary Signs / Enlarged, distended, urinary bladder Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Urinary Signs / Oliguria or anuria, retention of urine Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Urinary Signs / Palpable enlarged kidney ,renomegaly Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Urinary Signs / Proteinuria, protein in urine Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign
Urinary Signs / Urinary incontinence, dribbling urine Cattle & Buffaloes:Heifer,Cattle & Buffaloes:Cow,Cattle & Buffaloes:Steer,Cattle & Buffaloes:Bull,Cattle & Buffaloes:Ox Sign

Disease Course

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Cattle infected with bovine leukaemia virus (BLV) become permanent carriers. Most cattle older than 6 months and with a positive AGID test result should be considered permanently infected. Antibodies become detectable as early as two weeks after exposure to BLV. Thus, a negative agar gel immunodiffusion test (AGID) test indicates freedom from exposure to BLV for 3 to 12 weeks before collection of the serum sample. During the years after infection, as many as 30% of infected cattle develop persistent lymphocytosis. Persistent lymphocytosis is an increase in the absolute lymphocyte count of at least three standard deviations above the normal mean count for a specific breed and age group of cattle from enzootic bovine leukosis (EBL)-free herds. The lymphocytosis must persist for at least three consecutive months, and the cattle should have no detectable clinical manifestations of neoplastic lymphoproliferative lesions. Persistent lymphocytosis has been defined as a benign, polyclonal proliferation of B lymphocytes with CD5, CD11B and CD11C markers (Letesson et al., 1991). The prevalence of persistent lymphocytosis in BLV-infected cattle was 33% according to one investigator (Lewin and Harris, 1989). Seropositivity cannot be correlated with present or even subsequent tumour formation.

Tumours may develop in as few as 5% of infected animals, so tumour formation is much an exception rather than the rule during BLV infection. Although the pathogenesis of BLV infection has been studied for decades, it still remains somewhat elusive. BLV replicates in vivo only rarely. Viral mRNAs are rare, or are expressed weakly (Jensen et al., 1990) which is consistent with poorly detectable virus-expressing cells. The mechanism for BLV latency is poorly understood. Culturing lymphocytes from cattle and sheep with lymphocytosis does activate viral synthesis (Cornil et al., 1988). Activation of BLV synthesis is stronger if the lymphocytes are stimulated with concanavalin A and phytohaemagglutinin A. Likewise, the supernatants from cultured lymphocytes from infected animals activate viral synthesis, indicating a role for lymphocytokines in activation of BLV synthesis. The mechanism by which tumours develop in BLV-infected animals is almost as elusive as viral synthesis. Rearrangement of an oncogene(s) and insertational mutagenesis mechanisms are unlikely (Kettmann et al., 1983). The BLV Tax protein behaves as an oncogene, and mRNAs that encode BLV Tax are detectable in animals with persistent lymphocytosis and/or tumours (Willems et al., 1990). mRNAs that encode BLV structural proteins are not detectable in these animals (Haas et al., 1992). Thus, Tax may be the viral protein that initiates lymphocyte transformation (Sagata et al., 1985).

Epidemiology

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Numerous potential modes of transmission of bovine leukaemia virus (BLV) infection have been established (Johnson and Kaneene, 1991a, b; Hopkins and DiGiacomo, 1997). The proportion of BLV infections attributable to specific routes of transmission in an individual herd varies substantially with management practices in the herd and the extent to which infection control measures are practised. An evaluation of risk factors for transmission by Hopkins and DiGiacomo (1997) is based on four aspects of bovine husbandry: prenatal and periparturient period, calf management, breeding and reproductive management, and conditions of housing and confinement. It is suggested that a systematic evaluation of each of these aspects of a dairy or beef operation can assist veterinarians and producers in identifying modes of transmission and tailoring an effective BLV control programme.

Vertical transmission, a mode of transmission that is more likely to occur in utero than via contaminated colostrum and milk, accounts for a small proportion of all BLV infections, probably less than 8.0%. The prevalence of BLV in first-lactation cows in US dairy cattle was 35.3%, and BLV prevalence continued to increase in correlation with age of dairy cattle (USDA/APHIS/Veterinary Services, 1997). Thus, most infections must be acquired post-natally. Iatrogenic horizontal transmission through procedures that augment the transfer of contaminated blood among cattle is a major route of transmission in many settings. This has been illustrated by high rates of BLV infection due to gouge dehorning with an unsanitized common instrument. The same may be true for invasive surgical procedures, vaccinations, injections, and rectal palpations. Contact transmission probably stems from a combination of natural sources of blood, exudates, and tissues that are present during parturition and which enter the susceptible hosts’s body through mucosal surfaces or broken skin. Various aspects or periods of husbandry, the hypothesized risk factors of concern during the respective periods, and the qualitative and quantitative risks associated with each factor were compiled by Johnson (1999). Factors that are qualitatively and/or quantitatively associated with increased risk of BLV infection are seropositive dam with persistent lymphocytosis, gouge dehorning, injection of a vaccine that was contaminated with BLV, rectal palpation with a common sleeve, close physical contact in high prevalence herds, open herd, and diagnosis of malignant lymphoma in the herd.

Impact: Economic

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Estimates of the economic impact of BLV should include its impact on veterinary services, milk production, malignant lymphoma and restrictions on exports (Pelzer, 1997). An estimate of the cost of a single farm visit to diagnose malignant lymphoma was US $75 to US $90. An estimate of the total cost of one case of malignant lymphoma itself was as much as US $1453. A conservative estimate of the annual replacement costs due to malignant lymphoma in the USA in 1985 was more than US $16 million. The costs due to reduced exports (animals, semen, embryos) will vary with the value of these exports. An estimate of the value of beef and dairy animals and of semen that were exported from the USA in 1995 exceeded US $100 million. Thirteen studies of the economic impact of BLV infection on production were published between 1978 and 1998 (see Brenner et al., 1989; Emanuelson et al., 1992; Heald et al., 1992; Jacobs et al., 1991; Sargeant et al., 1997). Seven of the studies were "single-herd" studies, or studies in which all data for each study were collected from cows in one herd. The remaining six studies were "multiple-herd" studies. Data from each multiple-herd study were collected from as few as 12 herds and as many as 14,424 herds. The studies also differed according to the country, the study design, the number of cows, cow age, diagnostic tests, and laboratory specimens. The production variables were total milk, 305-day mature-equivalent fat corrected milk, actual fat-corrected milk, days milked, fat-corrected milk per day, total fat, fat percentage, somatic cell count, incidence of mastitis, culling rates and culling age. However, not every production variable was measured in all studies. There were increases, decreases, and no changes in these variables, depending on the study and on the production variable. The true impact of BLV infection on production should be examined further, due to the inconsistent conclusions from these published studies.

Zoonoses and Food Safety

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Several epidemiological studies have tried to show a relationship between human and animal leukaemia or lymphoma (Johnson and Kaneene, 1991a, b; DiGiacomo and Hopkins, 1997). In many of the studies the actual exposure to retroviruses is unknown. Based on the reported studies, retroviruses are not likely to be responsible for any significant occurrence of human disease, especially lymphoid malignancies. Although a definitive statement of no risk to human health is unwarranted, the evidence to date indicates that the risk to humans is low and perhaps non-existent.

Disease Treatment

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Bovine leukaemia virus (BLV) infection is always persistent, and the tumours are fatal eventually. No anti-retroviral drugs have been shown to be effective against BLV infection. No anti-tumour drugs have been shown to be effective against the tumours associated with EBL. Even if the drugs are shown to be effective against BLV, they will probably never be cost-effective for most animals in food animal practice.

Prevention and Control

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Efforts to develop a vaccine against bovine leukaemia verus (BLV) have not been successful. A BLV vaccine must meet the requirements of being non-infectious, non-oncogenic, and it should not interfere with the serological tests commonly used to detect cattle infected with BLV.

In the initial effort to develop a vaccine, calves were immunized with the BLV glycoprotein antigen (Miller et al., 1983). Seventy five percent of the calves (N=4) were protected. In a second study the challenge dose was increased 1600-fold, and only 16% (N=12) of the cattle were protected (Patruscu et al., 1980). A similar viral immunogen underwent extensive inactivation, and vaccine failure occurred in only 10% of 20 vaccinated animals (Onuma et al., 1984).

The immunogenic abilities of gp51 and p24 antigens, BLV-infected fetal lamb kidney (FLK) cells and also sheep fibroblasts transformed by BLV all were compared (Kono et al., 1986). Both gp51 and FLK immunogens induced gp51 antibodies and were protective, whereas p24 and SF28 immunogens induced only anti-p24 and anti-SF28 antibodies, respectively. Unlike gp51 and FLK, p24 and SF28 did not inhibit BLV infection after challenge.

The relationship between serum antibody titre and protection against infection by BLV was examined in sheep (Ohishi et al., 1990). The sheep had developed titres greater than 1:64 by two weeks after exposure. The titres persisted for 44 weeks, and the tests for infection remained negative.

The effect of a recombinant vaccinia virus expressing the envelope glycoprotein was investigated in sheep (Miller and Saltzman, 1986). Vaccination of animals with recombinant vaccinia viruses suppressed the growth of BLV in peripheral blood lymphocytes that became infected after challenge, but did not completely prevent infection. Unfortunately, the immunogen which generally protected against challenge (gp51), also produces antibodies that are detected with the AGID and ELISA diagnostic tests. Thus, the tests would not discriminate between naturally acquired and artificially acquired active antibodies in cattle. A battery of diagnostic tests were used by some of the investigators to detect BLV infection in the vaccinates, whereas others used only one relatively insensitive test.

One alternative which warrants exploration is to develop a gp51 subunit vaccine and replace the gp51 diagnostic tests with a p24 diagnostic test (Willems et al., 1989). Recombinant vaccinia virus containing gp51, or gp51 and gp30 prevented contact transmission of BLV in rabbits and sheep. The general conclusions of vaccine studies are that high titres appear to be correlated with protection against BLV infection. Few studies of BLV vaccines have progressed beyond the experimental phase, versus phase II and phase III trials (Portetelle et al., 1993). Thus, the development of a BLV vaccine seems rather unlikely at this time.


Husbandry Methods and Good Practice

The high individual animal prevalence in seropositive herds indicate that culling alone will not be a cost-effective method for reducing BLV seroprevalence in those herds. Thus, control strategies in which culling and risk factor management are combined may be the only cost-effective methods for reducing the incidence of the infection in high-prevalence herds (Johnson and Kaneene, 1991a,b). The husbandry methods and good practices are based on risk factors for the transmission of BLV infection. Good practices recommended by the New York State Bovine Leukosis Virus Eradication and Certification Program (NYSBLVECP) in 1985 are described below (Brunner et al., 1997). These include mandatory and non-mandatory management practices for controlling BLV infection in the production environment.


Mandatory Practices


1. Sterile, disposable needles must be used on a single animal and then discarded in biohazard containers.

2. A different disposable obstetrical sleeve must be used to palpate each cow, or at least for each cow that was seronegative on the last test.

3. Wash first and then disinfect any instruments that may be contaminated with blood. Some practitioners now avail themselves of at least two of the same instrument, first washing then soaking one set in a disinfectant while the other set is being used.

4. Use an electric dehorner rather than a gouge or a saw. Gouges and saws cause profuse haemorrhage and are difficult to sanitize.


Discretionary Practices


5. Develop a thorough insect vector control program.

6. Use artificial insemination.

7. Use a separate calving pen for each cow to reduce the exposure of susceptible animals to blood from infected cows.

8. Remove calves from their dams and raise them in hutches, or a similar facility.

9. Calves from infected dams should not be permitted to contact other calves until its infection status has been determined. A pre-colostral serum sample from highly valued offspring of infected dams should be tested. A positive test means that infection has already been acquired in utero.

10. If the prevalence of infection in the most recently-tested 6-month to 8-month-old calves is low (e.g. equal to in utero prevalence of 4 to 8%), colostrum probably is not a significant source of infection, and changes are not recommended.

11. If the prevalence of infection in the adult herd is low, feeding readily-available colostrum from non-infected cows minimizes the risk of infection from colostrum. Thus, the calves can be tested much sooner than the usual 6 to 8 months of age, and their disposition can be determined at an early age, before large economic investments in them by the producer.

12. If the prevalence of infection in the adult herd is high (>60%), all calves should be fed frozen colostrum from infected cows. The purpose of freezing the colostrum is to reduce the infectivity of lymphocytes. The anti-BLV antibodies are intended to protect calves against post-natal infection.

13. Separate infected and uninfected animals to reduce close contact between them. Separation also facilitates processing of uninfected animals before infected animals are processed.


Farm-level Control

The NYSBLVECP recommended that herds be tested for three reasons: (1) to distinguish infected animals from the population-at-risk of becoming infected during the initial phase of a control/eradication program, (2) to monitor progress towards the goal of eradication, and (3) to determine where there is horizontal transmission within the herd (for example bred heifers). In the latter case especially, additional management changes may be necessary to reduce the incidence of infection. The NYSBLVECP recommends that the entire herd be tested semi-annually. Assuming that an animal is not incubating BLV infection, the most that a producer can be assured of is that an animal was not infected on the date that the sample was collected. Thus, a 6-month test interval rather than a 1-year test interval, as an example, favours rapid and accurate identification of newly infected animals which can be followed by their separation from uninfected animals, and their subsequent removal from the herd, if warranted.


National and International Control Policy

"Standards for Certification of Cattle Herds as Bovine Leukosis Virus Free" was published by the Bovine Retrovirus Committee of the United States Animal Health Association (Miller and Lyle, 1998). This is a voluntary certification programme that requires that producers obtain the services of accredited veterinarians to collect and submit laboratory specimens for analysis using a diagnostic test that has been approved by USDA Animal and Plant Health Inspection Service. A rather detailed BLV control programme described in Council Directive 80/1102/EEC was established in the European Union (EU, formerly the "EEC") in 1980 (Bendixen, 1987). According to Bendixen, the seroprevalence in the entire EU cattle population rarely exceeded 0.5% to 1.5% when the report was published. A BLV eradication programme that was established in Sweden in 1988 by the Swedish Association for Livestock Breeding and Production showed that Sweden’s national seroprevalence was 20% (Emanuelson et al., 1992). Although three "outbreaks" involving 231 "cases" were reported by Sweden during 1998, Sweden considers itself to be in the "final phase of the national eradication programme."

The New Zealand dairy industry implemented a national eradication programme for BLV that began in 1996/1997 (Burton et al., 1997). The prevalence of infection in 14,561 herds was 7.2% in August 1998. The within-herd prevalence was only 2.6%. Although the original goal was for New Zealand to be free of BLV after 8.0 years of programme operation, all infected herds were expected subsequently to be free of BLV by April 1999, based on early test results (Hayes and Burton, 1998). The rapid progress in New Zealand has been attributed in part to: (1) efficient tracing of infected and suspect animals, and (2) demands by producers that newly acquired animals must be BLV seronegative before being admitted to their herds. An accreditation programme to eradicate BLV from Queensland, Australia was established in 1983, and the remaining states followed suit. The herd prevalence in Western Australia in 1998 was 0.0%, and the herd prevalence was low in most of the remaining states (Australia's Nat Animal Health Info System, 1998; OIE, 1999).

One study of BLV in Canada showed that 40% of its dairy herds and 11% of its beef herds were infected (Samagh and Kellar, 1982). The within-herd seroprevalence in dairy cattle was much higher than it was in beef cattle. In a second study in Ontario, the herd prevalence was 47%, and the within-herd seroprevalence was 24.2% (Heald et al., 1992). The herd prevalence in the most recent Canadian study was 69.6%, and the within-herd seroprevalence was 23.0% (Sargeant et al., 1997). The Canadian Health Accredited Herd (CHAH) Programme, created to protect animal and plant health in Canada, is an umbrella for BLV as well as other diseases (Agriculture Canada, 1989). CHAH-accredited laboratories test cattle for BLV infection in order to classify a herd as being a "CHAH Leucosis Free Herd" (Agriculture Canada, see listed weblinks). To reduce the incidence of BLV infection, the CHAH programme advocates physical separation of seropositive animals from seronegative animals. Membership in CHAH seems to be voluntary, indicating that there is no national effort to eradicate BLV from herds in Canada.

References

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Links to Websites

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Agriculture Canada, 2000. Inspection programs for animal and plant health 1997-1998.http://www.cfia-acia.agr.ca/Information contained here about the Canadian Health Accredited Herd (CHAH) Programme, created to protect animal and plant health in Canada, an umbrella for BLV as well as other diseases. Includes information on CHAH-accredited laboratories that test cattle for BLV infection in order to classify a herd as being a "CHAH Leucosis Free Herd".

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