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Nairobi sheep disease

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Nairobi sheep disease

Summary

  • Last modified
  • 27 July 2017
  • Datasheet Type(s)
  • Animal Disease
  • Preferred Scientific Name
  • Nairobi sheep disease
  • Overview
  • Nairobi sheep disease (NSD) was identified as a problem in sheep and goats in Kenya in 1910 (Annual Report Agricultural Department, Kenya, 1913). The disease was characterized...

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Identity

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Preferred Scientific Name

  • Nairobi sheep disease

International Common Names

  • English: Kisanyi goat disease; nairobi sheep disease, kisanyi goat disease- exotic

English acronym

  • NSD

Overview

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Nairobi sheep disease (NSD) was identified as a problem in sheep and goats in Kenya in 1910 (Annual Report Agricultural Department, Kenya, 1913). The disease was characterized by the sudden onset of pyrexia with prostration, followed by a profuse foetid diarrhoea. NSD has a high mortality rate in susceptible sheep and goats. An important study was published in 1917 by R.E. Montgomery, who was working at the Veterinary Research Laboratory, Kabete, in Kenya, which showed that NSD was caused by a tick-transmitted virus (Montgomery, 1917). This was a classical early work, which demonstrated the trans-ovarial and trans-stadial transmission of the virus in the Ixodid tick, Rhipicephalus appendiculatus. He also showed that the pathogenicity of the virus appeared to be restricted to sheep and goats, and other domestic animals were refractory to infection with NSD.

The virus is a member of the genus Nairovirus of the family Bunyaviridae. Other members of the genus include Ganjam, Dugbe, Congo-Crimean haemorrhagic fever (CCHF) and Hazara viruses. These are transmitted by Ixodid ticks. Davies et al. (1978) observed that NSD appeared to be serologically identical with Ganjam virus isolated in several states in India, and subsequently in Sri Lanka. Ganjam virus is transmitted by Haemaphysalis intermedia and probably other tick species, and has caused disease in man and also produces an NSD-like syndrome in sheep (Dandawate et al., 1969). After molecular biological studies, it is proposed that Ganjam virus is a strain of NSD virus (Marezinke and Nihol, 2002). These are difficult to arrange due to the problems of intercontinental movement and study of pathogenic viruses.

There are reports of human disease in Uganda, which have been attributed to NSD on the basis of complement fixation and mouse virus-serum neutralization tests (Weinbren, 1959; Tukei, personal communication). The virus is not readily communicable to man (Davies, 1988), but human NSD infection may occur.

The range of NSD may be greater than that already described. It can persist in a state of enzootic stability with no manifestation of clinical disease problems until susceptible animals are introduced. NSD occurs in those eco-climatic zones where continuous life cycles of the tick are possible, and not in the drier zones, where only single generations of the tick can occur. The range of the principal tick vector Rh. appendiculatus includes parts of the southern Sudan, Botswana, Mozambique, Zambia and South Africa. In many of these places, Rhipicephalus zambeziensis (a closely related species) predominates, and Rh. appendiculatus is less common in these drier areas.

The disease causes periodic epizootics in most of the affected countries, which result in 50-75% losses in the flocks of the pastoralists involved. This has considerable socio-economic impact, for many groups rely almost entirely on the sale of small ruminants for their income. The epizootics appear to be triggered by climatic changes, which alter the microclimate in favour of increases in the tick vector populations.

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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The principal hosts for NSD are sheep and goats, and goats may be slightly less susceptible than sheep. There is considerable variation in the susceptibility of different breeds and strains of sheep and goats. The East African hair sheep breeds are highly susceptible to NSD virus and 75-95% mortality has been encountered in the field and after laboratory infections. The imported breeds of wool sheep, such as the Corriedale, have a lower mortality rate of 30-60% in laboratory studies. Terpstra (1969) observed an 88% mortality rate in goats in West Nile Province of Uganda, but in Kenya the Masai-type goats have a 10-40% mortality. All age groups remain highly susceptible to NSD. Nomadic pastoralists with their large flocks of small ruminants are at greatest risk from epizootic NSD. Settled agricultural and livestock farming in enzootic areas generally allows small ruminants to develop immunity at an early age and NSD presents no disease problems.

Weinbren (1959) isolated NSD virus from blue duikers at the Entebbe zoo. No disease syndrome has ever been identified elsewhere in wild ruminant species and these are remarkably free from antibody to NSD virus. This may be due to the origin of the samples tested, which were collected from the drier zones where NSD is not enzootic (Davies, 1978a, 1978b).

Cattle are not susceptible to NSD and neither are horses, donkeys, pigs, poultry, and dogs (Montgomery, 1917). Suckling mice are susceptible to NSD when inoculated by the intra-cerebral (ic) and intra-peritoneal (ip) routes, and older mice are susceptible to intra-cerebral inoculations. Rabbits, guinea pigs, hamsters and rats were refractory to intra-peritoneal and intra-cerebral inoculations. The wild rat Arvicanthus niloticus developed a viraemia of 105 mouse LD50 after intra-peritoneal inoculation.

There have been many reports of NSD and Ganjam infections in man (Weinbren, 1959; Tukei, personal communication). Reports of NSD have been entirely from Uganda and were reported at the East African Virus Research Institute at Entebbe. Some of the laboratory staff had antibody to NSD virus on mouse serum-virus neutralization tests. These tests are notoriously unreliable. No human NSD virus infections have been identified in Kenya, nor have any laboratory infections occurred. Staff who worked with NSD for more than 20 years and handled sick and dead sheep, NSD-infected mouse tissues and cultures, all remained sero-negative to the virus. Ganjam infections in man have been acquired in the laboratory. Human infections show a febrile reaction with headaches, shivering and muscle pains. These usually disappear after 3 days.

Distribution

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The disease has been found to coincide with the distribution of Rh. appendiculatus throughout Kenya, with virus and antibody occurring throughout the highland areas, the Lake Victoria basin and the coastal zone (Davies, 1978 a, 1978b). The disease is found wherever animal movements occur from the non-endemic to the endemic areas. These are typically when trade movement occurs from the semi-arid zones to the centres of population such as Nairobi and Mombasa. Wherever small ruminants are held outside the cities for any length of time, high mortality may follow. Sheep and goats moved from semi-arid areas into the Taita Hills of Kenya to feed troops during the First World War suffered high mortality from NSD. Similar disease problems have been encountered in Uganda with movements of small stock to Entebbe or to experimental farm units, and in Rwanda when they are moved to the shore of Lake Kivu, from the hill country.

The disease was reported clinically as early as 1931-33 in Entebbe and also in 1940. Virus was first isolated in Uganda from two blue duikers (Cephalophus caerulis) at the Entebbe zoo in 1957 (East African Virus Research Institute Reports, 1958) and subsequently encountered as a disease in sheep and goats in Entebbe in 1955 (Weinbren et al., 1958) and West Nile Province in 1963 (Terpstra, 1969), when exotic sheep were being introduced to improve production. Subsequently, NSD or antibody to the virus was found throughout most of the country apart from the arid and semi-arid zones to the north and north-east (Terpstra, 1969). The distribution was closely related to that of Rh. appendiculatus.

The disease was first identified in Somalia in 1938 by Pavalglia, in imported Karakul sheep (quoted by Pellegrini, 1950). Pellegrini (1950) made a study of the transmission of NSD in Somalia in 1950 and suggested that Rhipicephalus pulchellus was the likeliest vector in the Horn of Africa. It is by far the most common tick in this part of the Horn of Africa (Pegrum, 1976). Edelesten (1975) described an outbreak of NSD in the north of Somalia and also in the Ogaden of Ethiopia. All these workers identified a highly fatal clinical disease in sheep and goats, which was attributed to the virus. The Kenyan race of this tick was capable of transmitting NSD, but was shown to be an inefficient vector compared with Rh. appendiculatus.

Bugyaki (1955) has presented evidence of a tick-borne virus disease like NSD near Kisenyi, along the shores of Lake Kivu in Rwanda. Rh. appendiculatus was shown to be the vector and the disease was clinically the same. The disease has been reported from Tanzania and antibody to the virus has been found in sheep and goats throughout north Tanzania (Jesset, 1978). Virus was isolated from sick sheep in north Tanzania in 1977. The distribution of antibody to NSD in Tanzania is closely related to that of the tick vector, Rh. appendiculatus.

There is no evidence for NSD occurring as a disease in the south of Africa. Only very few sera with low titres of antibody have been found in limited serological surveys in Zambia and Botswana. Some 450 sera were examined from Botswana and 300 from Zambia. These were considered likely to be due to cross relationships with other Nairoviruses such as Crimean-Congo haemorrhagic fever. No clinical disease syndrome that might be NSD has ever been identified south of Tanzania. It is unlikely that the Veterinary Services in countries such as Zambia, Zimbabwe, Malawi, Mozambique, Botswana and South Africa would have failed to identify the presence of such a highly pathogenic virus.

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 never reportedOIE, 2009
BangladeshDisease never reportedOIE, 2009
BhutanDisease never reportedOIE, 2009
Brunei DarussalamDisease not reportedOIE Handistatus, 2005
CambodiaNo information availableOIE, 2009
ChinaDisease never reportedOIE, 2009
-Hong KongNo information availableOIE, 2009
Georgia (Republic of)Disease never reportedOIE Handistatus, 2005
IndiaDisease never reportedOIE, 2009
IndonesiaDisease not reportedOIE, 2009
IranDisease never reportedOIE, 2009
IraqDisease never reportedOIE, 2009
IsraelDisease never reportedOIE, 2009
JapanDisease never reportedOIE, 2009
JordanDisease never reportedOIE, 2009
KazakhstanDisease never reportedOIE, 2009
Korea, DPRDisease not reportedOIE Handistatus, 2005
Korea, Republic ofDisease never reportedOIE, 2009
KuwaitDisease not reportedOIE, 2009
KyrgyzstanDisease never reportedOIE, 2009
LaosDisease never reportedOIE, 2009
LebanonDisease never reportedOIE, 2009
MalaysiaDisease never reportedOIE, 2009
-Peninsular MalaysiaDisease not reportedOIE Handistatus, 2005
-SabahDisease never reportedOIE Handistatus, 2005
-SarawakDisease never reportedOIE Handistatus, 2005
MongoliaNo information availableOIE, 2009
MyanmarDisease never reportedOIE, 2009
NepalNo information availableOIE, 2009
OmanDisease not reportedOIE, 2009
PakistanDisease not reportedOIE, 2009
PhilippinesDisease never reportedOIE, 2009
QatarNo information availableOIE, 2009
Saudi ArabiaNo information availableOIE, 2009
SingaporeDisease never reportedOIE, 2009
Sri LankaDisease never reportedOIE, 2009
SyriaDisease not reportedOIE, 2009
TaiwanDisease never reportedOIE Handistatus, 2005
TajikistanDisease not reportedOIE, 2009
ThailandNo information availableOIE, 2009
TurkeyNo information availableOIE, 2009
TurkmenistanDisease not reportedOIE Handistatus, 2005
United Arab EmiratesDisease never reportedOIE, 2009
UzbekistanDisease never reportedOIE Handistatus, 2005
VietnamDisease never reportedOIE, 2009
YemenNo information availableOIE, 2009

Africa

AlgeriaDisease never reportedOIE, 2009
AngolaNo information availableOIE, 2009
BeninDisease not reportedOIE, 2009
BotswanaNo information availableOIE, 2009
Burkina FasoNo information availableOIE, 2009
BurundiDisease never reportedOIE Handistatus, 2005
CameroonDisease never reportedOIE 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 RepublicNo information availableOIE Handistatus, 2005
Côte d'IvoireLast reported1993OIE Handistatus, 2005
DjiboutiDisease not reportedOIE, 2009
EgyptDisease never reportedOIE, 2009
EritreaNo information availableOIE, 2009
EthiopiaDisease never reportedNULLPegrum, 1976; OIE, 2009
GabonNo information availableOIE, 2009
GambiaNo information availableOIE, 2009
GhanaNo information availableOIE, 2009
GuineaNo information availableOIE, 2009
Guinea-BissauNo information availableOIE, 2009
KenyaDisease not reported2003Montgomery, 1917; OIE, 2009
LesothoDisease never reportedOIE, 2009
LibyaDisease never reportedOIE Handistatus, 2005
MadagascarDisease never reportedOIE, 2009
MalawiNo information availableOIE, 2009
MaliNo information availableOIE, 2009
MauritiusDisease never reportedOIE, 2009
MoroccoDisease never reportedOIE, 2009
MozambiqueDisease not reportedOIE, 2009
NamibiaNo information availableOIE, 2009
NigeriaDisease never reportedOIE, 2009
RéunionDisease not reportedOIE Handistatus, 2005
RwandaNo information availableNULLBugyaki, 1955; OIE, 2009
Sao Tome and PrincipeDisease not reportedOIE Handistatus, 2005
SenegalNo information availableOIE, 2009
SeychellesNo information availableOIE Handistatus, 2005
SomaliaNo information availablePellegrini, 1950; OIE Handistatus, 2005
South AfricaDisease never reportedOIE, 2009
SudanDisease never reportedOIE, 2009
SwazilandDisease never reportedOIE, 2009
TanzaniaNo information availableNULLDavies et al., 1977; Jesset, 1978; OIE, 2009
TogoNo information availableOIE, 2009
TunisiaDisease not reportedOIE, 2009
UgandaNo information availableNULLWeinbren, 1959; OIE, 2009
ZambiaDisease never reportedOIE, 2009
ZimbabweDisease never reportedOIE, 2009

North America

BermudaDisease not reportedOIE Handistatus, 2005
CanadaDisease never reportedOIE, 2009
GreenlandDisease never reportedOIE, 2009
MexicoDisease never reportedOIE, 2009
USADisease never reportedOIE, 2009

Central America and Caribbean

BarbadosDisease never reportedOIE Handistatus, 2005
BelizeDisease never reportedOIE, 2009
British Virgin IslandsDisease never reportedOIE Handistatus, 2005
Cayman IslandsDisease never reportedOIE Handistatus, 2005
Costa RicaDisease never reportedOIE, 2009
CubaDisease never reportedOIE, 2009
CuraçaoDisease not reportedOIE Handistatus, 2005
DominicaDisease not reportedOIE Handistatus, 2005
Dominican RepublicDisease never reportedOIE, 2009
El SalvadorDisease never reportedOIE, 2009
GuadeloupeNo information availableOIE, 2009
GuatemalaDisease never reportedOIE, 2009
HaitiDisease never reportedOIE, 2009
HondurasDisease never reportedOIE, 2009
JamaicaNo information availableOIE, 2009
MartiniqueNo information availableOIE, 2009
NicaraguaDisease never reportedOIE, 2009
PanamaDisease never reportedOIE, 2009
Saint Kitts and NevisDisease never reportedOIE Handistatus, 2005
Saint Vincent and the GrenadinesDisease never reportedOIE Handistatus, 2005
Trinidad and TobagoDisease never reportedOIE Handistatus, 2005

South America

ArgentinaDisease never reportedOIE, 2009
BoliviaDisease never reportedOIE, 2009
BrazilDisease never reportedOIE, 2009
ChileDisease never reportedOIE, 2009
ColombiaDisease never reportedOIE, 2009
EcuadorDisease never reportedOIE, 2009
Falkland IslandsDisease never reportedOIE Handistatus, 2005
French GuianaDisease not reportedOIE, 2009
GuyanaDisease never reportedOIE Handistatus, 2005
ParaguayDisease never reportedOIE Handistatus, 2005
PeruDisease never reportedOIE, 2009
UruguayDisease never reportedOIE, 2009
VenezuelaDisease never reportedOIE, 2009

Europe

AlbaniaDisease never reportedOIE, 2009
AndorraDisease never reportedOIE Handistatus, 2005
AustriaNo information availableOIE, 2009
BelarusDisease never reportedOIE, 2009
BelgiumDisease not reportedOIE, 2009
Bosnia-HercegovinaDisease not reportedOIE Handistatus, 2005
BulgariaDisease never reportedOIE, 2009
CroatiaDisease never reportedOIE, 2009
CyprusDisease never reportedOIE, 2009
Czech RepublicDisease never reportedOIE, 2009
DenmarkDisease never reportedOIE, 2009
EstoniaDisease never reportedOIE, 2009
FinlandDisease never reportedOIE, 2009
FranceDisease never reportedOIE, 2009
GermanyDisease never reportedOIE, 2009
GreeceDisease not reportedOIE, 2009
HungaryDisease never reportedOIE, 2009
IcelandDisease never reportedOIE, 2009
IrelandDisease never reportedOIE, 2009
Isle of Man (UK)Disease never reportedOIE Handistatus, 2005
ItalyNo information availableOIE, 2009
JerseyDisease never reportedOIE Handistatus, 2005
LatviaDisease never reportedOIE, 2009
LiechtensteinDisease not reportedOIE, 2009
LithuaniaDisease never reportedOIE, 2009
LuxembourgDisease never reportedOIE, 2009
MacedoniaDisease never reportedOIE, 2009
MaltaDisease never reportedOIE, 2009
MoldovaDisease never reportedOIE Handistatus, 2005
MontenegroDisease never reportedOIE, 2009
NetherlandsDisease never reportedOIE, 2009
NorwayDisease never reportedOIE, 2009
PolandDisease never reportedOIE, 2009
PortugalDisease never reportedOIE, 2009
RomaniaDisease never reportedOIE, 2009
Russian FederationDisease never reportedOIE, 2009
SerbiaNo information availableOIE, 2009
SlovakiaDisease never reportedOIE, 2009
SloveniaDisease never reportedOIE, 2009
SpainDisease never reportedOIE, 2009
SwedenDisease never reportedOIE, 2009
SwitzerlandDisease never reportedOIE, 2009
UKDisease never reportedOIE, 2009
-Northern IrelandDisease never reportedOIE Handistatus, 2005
UkraineDisease never reportedOIE, 2009
Yugoslavia (former)No information availableOIE Handistatus, 2005
Yugoslavia (Serbia and Montenegro)Disease not reportedOIE Handistatus, 2005

Oceania

AustraliaDisease never reportedOIE, 2009
French PolynesiaDisease never reportedOIE, 2009
New CaledoniaDisease never reportedOIE, 2009
New ZealandDisease never reportedOIE, 2009
SamoaDisease never reportedOIE Handistatus, 2005
VanuatuDisease never reportedOIE Handistatus, 2005
Wallis and Futuna IslandsNo information availableOIE Handistatus, 2005

Pathology

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The first laboratory evidence of NSD infection is a markedly depressed total white-cell-count to 1-2 million per cubic millimetre. Animals which die at the early febrile stage of the disease show no signs that might be identifiable as characteristic of NSD. There will be a congestion of the carcass with lymphadenitis; usually the pre-scapular lymph node on one side will be notably enlarged and oedematous, with subcapsular haemorrhages. There will be petechial and ecchymotic haemorrhages, particularly on the serosal surfaces and epicardium and throughout the carcass.

Later in the course of the disease, when deaths follow the onset of diarrhoea, there will be catarrhal mucoid enteritis to a severe haemorrhagic form with extensive ulceration in the folds of the abomasum, duodenum, ileocaecal valve, caecum and colon, often with zebra striping in the rectal folds. Oedema of the lungs with froth in the trachea and bronchi is found in many cases. Extensive petechial and ecchymotic haemorrhages occur, usually in the subcapsular or serosal areas of the parenchymatous organs and lymph nodes, which are enlarged and hyperaemic. There will be haemorrhages on the epicardium with some pericardial fluid. The kidney may show some subcapsular haemorrhages. The urogenital tract may show infection and haemorrhage after abortions, with some bladder haemorrhages. The spleen usually shows some subcapsular haemorrhage and may or may not be enlarged.

The pathology of NSD has been described by Daubney and Hudson (1931), Weinbren et al. (1958), and Mugera and Chema (1967). The lesions in most of the tissues and organs are those associated with the haemorrhage, together with some inflammatory changes with cellular infiltration. The spleen and lymph nodes usually show some hyperplasia of the germinal centres. The most significant and specific lesions of NSD are of a glomerular-tubular nephritis. There is severe congestion associated with the glomeruli, and peri-vascular cuffing with plasma and other inflammatory cells. The nuclei of the endothelial cells of the glomeruli become swollen and the cytoplasm swollen, vacuolated and granular. Degenerative changes occur in the glomeruli and convoluted and other tubules. Casts of desquamated cells appear in the lumen with much granular hyaline material. In the medullary area of the kidney, there is much congestion and swelling but the tubules themselves are not as severely affected as elsewhere. These kidney lesions are highly specific for NSD in enzootic areas, for no other virus disease produces anything similar. Recovered animals show severe interstitial fibrosis with greatly compromised kidney function.

Diagnosis

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Pathology


The clinical signs are similar in both sheep and goats. The onset of fever 2-8 days after either movement to a new area or introductions from elsewhere, is a common history. Ixodid ticks infest the animals. The fever is high, 40-41.5°C and persists for at least 3 days and often longer. The fever is accompanied by severe depression, partial or total anorexia, a disinclination to move and often some respiratory distress. The superficial lymph nodes will be enlarged. There is usually conjunctival injection and a sero-sanguineous nasal discharge. Diarrhoea occurs within 36-48 hours of the onset of fever and this may be accompanied by abdominal pain and colic, with tenesmus and grunting. The diarrhoea is profuse and watery at first and later may become mucoid and haemorrhagic. Abortion will occur in affected pregnant females. The total white-cell-count will be found to be below 1-2 million per cubic millimetre. All age groups may be affected in a totally susceptible population group. The disease occurs in dramatic epidemic form, if there has been movement of animals from non-enzootic to NSD-enzootic zones, where the vector ticks are present.

Post-mortem signs are not helpful in the early stages of the disease, when only non-specific signs of an acute septicaemic or viral disease may be seen. This is likely to be the case in many NSD outbreak situations. Later in the course of the disease, the alimentary signs with zebra striping in the large bowel and abomasal haemorrhage may be helpful.


Laboratory Diagnosis


Diagnosis may be made by identification of the NSD viral antigen by agar-gel diffusion tests, which can be performed at any laboratory or by isolation of the causal virus. Plasma taken from a sheep with high fever or spleen and/or mesenteric lymph node tissue may be used as the optimal samples for virus isolation (Davies et al., 1977). This may be accomplished readily by the inoculation of suckling mice of 2-4 days of age by the intra-cerebral route or by the direct inoculation of tissue cultures. Cultures of lamb kidney or testis, or the cell lines such as BHK 21 C 13 and Vero cells may be used for primary isolation. Blind passage is advisable in both systems. In tissue cultures, blind passage is necessary in most cases before cytopathic effects become obvious, but the viral antigen may be readily detected at 56-72 h post inoculation by fluorescent antibody staining of the infected cultures. Cover-slip or slide-well cultures are recommended for this purpose. Deaths occur in infant mice at 5-10 days after inoculation. The virus may be identified by the preparation of an antigen for agar gel or complement fixation tests (CFT), sub-inoculation into tissue cultures or by a capture ELISA.

Staining of infected cultures with haematoxylin and eosin shows the interesting form of the inclusion bodies produced by NSD. These are of pleomorphic eosinophilic bodies with spindle or other irregular shape or as an irregular ring surrounding the nucleus. Fluorescent staining shows small granules of fluorescent antigen in the cytoplasm.

Serological diagnosis may be made with acute and convalescent sera using any of the older assay methods, which have included CFT, indirect FAT or indirect haem-agglutination. Neutralization tests have proved to be of no value, for both pre- and post-inoculation sera give similar endpoints in mouse or tissue culture assay systems. It has been suggested that the neutralization may be complement dependent and the addition of complement to the virus/serum mixtures may give different results. ELISA systems have been developed and used in the laboratory (Munz et al., 1983), but have not been extensively used in the field. All serological systems suffer the disadvantage that there are cross relationships with other members of the Nairovirus group. These are, however, at a much lower level than the specific NSD response. In the fluorescent assays for example, endpoints of 1/2500-1/10,000 may be obtained with specific sera and less than 1/100 with the non-specific cross relationships. Similar results have been obtained with indirect IHA, CFT and other test systems and cut-off points determined. The ELISA test systems need to be evaluated against field sera that are positive to other Nairovirus group members.

Emphasis should be made again of the difficulty of making any diagnosis purely on post-mortem evidence. Suspicion should be high, if there are large numbers of deaths in small ruminants caused by a disease with high fever in animals obviously infested with Ixodid ticks. The AGDP test can be performed in remote laboratory situations and is a low-cost test system. It is valuable in many areas where NSD is enzootic. Accurate diagnosis requires more sophisticated systems, which are only available in the National Reference laboratories in NSD-enzootic countries and not always there. More simple pen-side tests would be most valuable for NSD.


Immunology of NSD


Recovered animals remain immune for life. The antibody response may be detected after 3-5 days and for 6-9 months by CFT and for at least 2-3 years by the fluorescent antibody and indirect haemagglutination tests (Davies et al., 1976). The ELISA test has not been systematically evaluated over long periods of time.

Several vaccines have been developed against NSD. A strain has been attenuated by passage in adult mouse brain, and this has proved to be too pathogenic for field use. A strain was developed by passage in suckling mouse brain and this gave promising results in Uganda (Terpstra, 1969). Later reports from Uganda suggested that it was not a good immunogen and it has not been widely used. An inactivated vaccine was prepared at Kabete, Kenya from high titre tissue culture fluids and this gave protection after two inoculations given with adjuvant (Davies et al., 1977). It was prepared for use in valuable imported stock but was little used. Most indigenous animals in NSD-enzootic areas are immune and this stable enzootic state ensures there is little call for any intervention.

List of Symptoms/Signs

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SignLife StagesType
Cardiovascular Signs / Pulse deficiency, deficit Sheep & Goats:All Stages Sign
Cardiovascular Signs / Tachycardia, rapid pulse, high heart rate Sheep & Goats:All Stages Diagnosis
Digestive Signs / Anorexia, loss or decreased appetite, not nursing, off feed Sheep & Goats:All Stages Diagnosis
Digestive Signs / Bloody stools, faeces, haematochezia Sheep & Goats:All Stages Diagnosis
Digestive Signs / Diarrhoea Sheep & Goats:All Stages Diagnosis
Digestive Signs / Excessive salivation, frothing at the mouth, ptyalism Sheep & Goats:All Stages Diagnosis
Digestive Signs / Grinding teeth, bruxism, odontoprisis Sheep & Goats:All Stages Sign
Digestive Signs / Hepatosplenomegaly, splenomegaly, hepatomegaly Sheep & Goats:All Stages Sign
Digestive Signs / Unusual or foul odor, stools, faeces Sheep & Goats:All Stages Diagnosis
General Signs / Dehydration Sheep & Goats:All Stages Sign
General Signs / Fever, pyrexia, hyperthermia Sheep & Goats:All Stages Diagnosis
General Signs / Generalized weakness, paresis, paralysis Sheep & Goats:All Stages Sign
General Signs / Haemorrhage of any body part or clotting failure, bleeding Sheep & Goats:All Stages Sign
General Signs / Inability to stand, downer, prostration Sheep & Goats:All Stages Diagnosis
General Signs / Lymphadenopathy, swelling, mass or enlarged lymph nodes Sheep & Goats:All Stages Diagnosis
General Signs / Pale mucous membranes or skin, anemia Sheep & Goats:All Stages Sign
General Signs / Reluctant to move, refusal to move Sheep & Goats:All Stages Diagnosis
General Signs / Sudden death, found dead Sheep & Goats:All Stages Sign
General Signs / Swelling mass vagina Sign
General Signs / Swelling mass, vulva, clitoris Other:Adult Female Sign
General Signs / Tenesmus, straining, dyschezia Sheep & Goats:All Stages Diagnosis
General Signs / Weight loss Sheep & Goats:All Stages Sign
Nervous Signs / Coma, stupor Sheep & Goats:All Stages Sign
Nervous Signs / Dullness, depression, lethargy, depressed, lethargic, listless Sheep & Goats:All Stages Sign
Ophthalmology Signs / Conjunctival, scleral, injection, abnormal vasculature Sheep & Goats:All Stages Diagnosis
Ophthalmology Signs / Lacrimation, tearing, serous ocular discharge, watery eyes Sheep & Goats:All Stages Diagnosis
Ophthalmology Signs / Purulent discharge from eye Sign
Pain / Discomfort Signs / Colic, abdominal pain Sheep & Goats:All Stages Diagnosis
Reproductive Signs / Abortion or weak newborns, stillbirth Sheep & Goats:Gimmer,Sheep & Goats:Mature female Diagnosis
Reproductive Signs / Agalactia, decreased, absent milk production Sign
Respiratory Signs / Abnormal lung or pleural sounds, rales, crackles, wheezes, friction rubs Sheep & Goats:All Stages Sign
Respiratory Signs / Dull areas on percussion of chest, thorax Sheep & Goats:All Stages Sign
Respiratory Signs / Dyspnea, difficult, open mouth breathing, grunt, gasping Sign
Respiratory Signs / Epistaxis, nosebleed, nasal haemorrhage, bleeding Sign
Respiratory Signs / Increased respiratory rate, polypnea, tachypnea, hyperpnea Sheep & Goats:All Stages Sign
Respiratory Signs / Mucoid nasal discharge, serous, watery Sheep & Goats:All Stages Sign
Respiratory Signs / Purulent nasal discharge Sign
Skin / Integumentary Signs / Warm skin, hot, heat Sheep & Goats:All Stages Sign
Urinary Signs / Glucosuria Sheep & Goats:All Stages Sign
Urinary Signs / Proteinuria, protein in urine Sheep & Goats:All Stages Sign

Disease Course

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The incubation period for NSD is of 1-5 days after parenteral inoculation and will vary with the genotype of the animal, amount of virus inoculum and route of inoculation. Tick infections usually follow within 3-7 days of application and attachment. The initial signs are of a febrile reaction from 40 to 41.5°C, which may persist for 3-8 days and occasionally longer. The viraemia is correlated with the febrile reaction and subsequently virus may be isolated from the spleen and carcass lymph nodes for 10-15 days. A lymphadenitis may be detected at this stage and usually one of the pre-scapular lymph nodes is enlarged and is related to the site of attachment and feeding of the infected tick. During this period, the animal is extremely depressed with anorexia and disinclined to move; typically it stands with its head held down. There may be a sero-sanguineous nasal discharge with injection of the conjunctiva and some lachrymation. A foul-smelling diarrhoea usually develops 1-3 days after the onset of fever. At first this is watery green in colour and later may become mucoid and blood-stained. Abdominal colic and tenesmus with groaning may accompany this. An increased respiratory rate develops progressively during the course of the disease and respiratory distress is a common terminal sign. Pregnant animals may abort at this stage or later. Deaths may occur early after the onset of the febrile reaction and then for up to 12 days afterwards.

Mortality rates vary with the genotype of the animals affected and may vary from 15-30% to 95% of those infected. General observations suggest that those sheep indigenous to East Africa may be more susceptible than the exotic imported breeds. This is different from the mortality to other African virus diseases such as Rift Valley fever, when the reverse is the case. Some goat breeds appear relatively resistant to NSD with low mortality rates of 15-30%, in others 90% mortality occurs (Terpstra, 1969).

Epidemiology

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In Enzootic Areas


NSD virus can remain cryptic in enzootic areas for years, in ongoing tick-host-tick cycles and be maintained by trans-ovarial transmission with no manifestation of any disease problem. Young animals are challenged when they have maternal antibody and develop an active immune response. The small-ruminant populations show high levels of immunity (Davies, 1978a). This situation prevails throughout the enzootic areas for NSD in East Africa. Disease is signalled whenever susceptible breeds of sheep and goats are introduced into the enzootic areas to improve the production potential of the indigenous breeds. This has happened in Uganda (Terpstra, 1969), in Kenya and in Somalia (Pavalglia, 1938; quoted by Pellegrini, 1950), when Karakul sheep were brought from Italy. Disease also follows the movement of sheep and goats from non-enzootic semi-arid zones to the centres of population for trade purposes. Any attempts at tick control in the small-ruminant populations may allow the development of susceptible population groups, which are at risk when the tick control activities cease or fail for any reason. No control strategy is necessary for NSD in enzootic areas. The levels of flock immunity remain high, wherever high levels of larval survival ensure persistent populations of the tick vectors. This happens wherever there are two wet, or continuous wet with only short dry seasons. It is possible that there are such enzootic areas in the south of Sudan, where Rh. appendiculatus occurs, in Uganda and in parts of the Congo.


In Epizootic Areas


Pastoralists in areas that are non-enzootic for NSD have described epizootics that kill most of their sheep (Lewis, 1934). This is in areas where the tick vector may be present, but in only extremely small numbers, which is typically in the Acacia savannahs of agroclimatic zone III in East Africa. These populations have, however, been seen to increase dramatically following the periodic changes in rainfall pattern, when for 6-18 months there may be prolonged and heavy rainfall with heavy cloud cover. This results in the development of more ground vegetation, which allows huge increases in the tick populations to occur. Where previously, one Rh. appendiculatus tick may be found after examining 10 animals, after these changes hundreds may be found on a single animal. In such conditions epizootics of NSD have been seen with high mortality rates in the non-immune sheep and goat populations. Such climatic conditions have occurred in 7-15 year cycles, but may become more frequent following the changes induced by the El Nino/Southern Ocean Oscillation effects (ENSO).

The history of epizootic NSD in Somalia suggests a similar epidemiological basis. The outbreaks have followed periods of severe drought, which resulted in much movement of livestock. The mortality was occurring in these migratory flocks, which moved, presumably into NSD-enzootic areas.


Environmental Factors


The enzootic areas for NSD are in the moist-forest-derived or natural grasslands and in the contiguous moist bushed and wooded grasslands. These are in Eco-climatic Zones II & III in most classifications for Africa. Zone IV, which include the drier Acacia grasslands, are generally too dry for the tick vector, other than when associated with the riverine woodland zones. These have a higher water table and allow the persistence of Rh. appendiculatus in Zone IV; these populations increase hugely in favourable conditions.

Temperatures of 15-30°C are optimal for the maturation of the various stages of this tick species. A relative humidity of greater than 45%RH is necessary for maturation and development of the tick vector. Lower levels have a negative effect upon the larval and nymphal stages; higher levels have a positive effect. Such conditions are available in most of the NSD-enzootic areas of East Africa, where continuous life cycles are possible.


Vectors


The principal vector of NSD in Kenya and Uganda has been shown to be Rhipicephalus appendiculatus (Montgomery, 1917; Daubney and Hudson, 1931; Terpstra, 1969; Davies, 1978a, 1978b). Amblyomma variegatum has also been shown to transmit NSD, but is a less efficient vector than Rh. appendiculatus. Amblyomma gemma, Rh. simus and Rh. pulchellus have been described as "occasional vectors" in Kenya Departmental reports.

In Somalia, Pellegrini (1950) showed that Rh. pulchellus was capable of trans-ovarial and trans-stadial transmission of NSD virus. Similar experiments with the Kenyan race of Rh. pulchellus were not successful (Daubney and Hudson, 1934; Davies, unpublished data). Most subsequent attempts to transmit NSD with Rh. pulchellus have also failed, although there was a report from Kabete in 1948 that a successful transmission of NSD had occurred. The virus has not been isolated from pools of this tick species. The distribution of the tick does not coincide with that of NSD virus or antibody in Kenya.

Bugyaki (1955) found that Rh. appendiculatus was the vector of the NSD-like disease he described at Kisenyi on the shores of Lake Kivu in Rwanda.

Rh. appendiculatus was originally a tick species found on wild Bovidae. It has adapted to most domestic ruminant species and will feed upon domestic bovids, sheep and goats. Small ruminants are, however, less favoured hosts than cattle. Bos indicus cattle carry fewer ticks than the exotic Bos taurus breeds. The tick feeds in large numbers, particularly upon buffalo and waterbuck, with lower infestation rates on other wild ruminants (Walker, 1967). Populations of the tick are greatest where the temperature and humidity are optimal and where suitable hosts are available in large numbers. Larval survival rates may be high in optimal conditions and large populations of ticks develop with ongoing life cycles throughout the year. In less favourable conditions, which are drier with perhaps a single short wet season and prolonged dry conditions with fewer available host species, larval survival rates and tick populations are much lower. The NSD virus has been shown to survive in a single adult for 871 days, in larvae for 144 and nymphs for 138 (Lewis, 1946).

Rhipicephalus pulchellus is a tick species most commonly found on black cotton soils in East Africa. It is by far the most common tick species infesting cattle, sheep and goats in those parts of Somalia, where NSD has been identified (Pegrum 1976). The populations have been found to increase "40 fold" after the rains in the "haud" of north-west Somalia.

Ganjam virus has been isolated in India from Haemaphysalis intermedia and this is considered to be the most important vector species. It has also been isolated from Haemaphysalis wellingtonii and once from mosquitoes Culex vishnui (complex). In Sri Lanka, Haemaphysalis ticks also appear to be the main vectors.


Life Cycles


A classical work by Dr Aneurin Lewis at the Veterinary Research Laboratory at Kabete, examined the tick/vector relationships in detail (Lewis, 1947). He was able to show that individual adult ticks could retain NSD infection for at least 871 days, and that larvae remained infective for 144 and nymphs at least 138 days. This, together with the original evidence from Montgomery (1917) and Daubney and Hudson (1931), who demonstrated the trans-ovarial transmission of the virus, explains the long-term persistence of NSD in the enzootic areas. This happens when no manifestation of disease has ever been recorded. Davies (1978a) found evidence of high antibody levels in sheep populations, where no disease had been reported for more than 50 years.

Daubney and Hudson (1931) suggested that feeding resulted in sterilization of infected ticks, which then lost the ability to transmit at a subsequent feed. This point was studied in a series of experiments by Davies and Mwakima (1982). They showed that feeding a vertically infected tick population upon susceptible, immune or unsusceptible hosts did not result in their sterilization with regard to NSD virus. This is important because Rh. appendiculatus populations feed predominantly upon unsusceptible host species such as cattle and wild bovids. Tick populations would gradually become free from NSD if each feed resulted in sterilization. Field evidence suggested that it was not happening and the laboratory experiments confirmed this.

Little work has been done on the infection rates in tick populations but Davies (unpublished work) found that this was below 1% of the population he examined. It is possible that the assay systems he used were insensitive. Work done at the NERC, Oxford has shown that guinea pigs could pick up a nairovirus from ticks, when no viraemia had been detected (Jones et al., 1987). Transmission experiments that suggest that Rh. appendiculatus is probably the most important vector, have been carried out in Kenya, Uganda and Rwanda. Rh. pulchellus appears to be the important vector in north Somalia and probably in region V of Ethiopia. Other ticks, notably Amblyomma varieagatum, can transmit the virus, but less efficiently than Rh. appendiculatus. Rh. simus, Amblyomma gemma and possibly other Ixodid ticks may also be able to transmit NSD on occasion (Kenya Veterinary Department Annual Reports, 1948).

Impact: Economic

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NSD is an extremely pathogenic virus disease for sheep and goats. It is the most pathogenic virus disease of small ruminants in East Africa, with mortality rates in indigenous animals of 70-95%. No other disease shows such dramatic mortality: hundreds of animals may die over a period of 2-6 weeks when introduced into an enzootic area from a non-enzootic zone. Losses in the quarantine area outside Nairobi were extremely high until the animals were transported straight for slaughter.

The extensions in the range of the tick vector, which follow the periodic periods of prolonged and heavy rainfall, also result in huge losses in these extension zones. The pastoralists have reported losing from 50 to 75% of their flocks during such periods, which has considerable socio-economic consequences. Similar losses have been sustained in these population groups in Somalia.

Introductions of exotic breeds are hazardous in enzootic areas, because losses due to NSD will be high. Vaccines have been developed for use in such circumstances, but these are not usually available. There is no treatment for the disease.

Zoonoses and Food Safety

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No human disease problems have been encountered amongst the pastoral and other populations in the NSD enzootic areas, which might be due to NSD. Small ruminants are being killed throughout the year and ticks are regularly removed from individual animals and killed by squeezing between finger and thumb. This can create an aerosol, which is infected with NSD if the ticks are contaminated with the virus. Such pastoral populations suffer from Rift Valley fever (RVF) contracted after slaughter of their livestock during RVF epizootics. They do not appear to identify a disease syndrome, which could be NSD, and which follows killing and butchering their stock. This is not the case with another Nairovirus, Crimean Congo haemorrhagic fever, which can be transmitted in an identical manner.

The virus may persist in a carcass until such time as the pH falls 8-12 h after death; the virus is destroyed at low pH. It is not considered to present a risk to personnel at any point in the food chain. NSD-infected carcasses in a slaughter-house may be condemned as fevered or diseased. They would not present any food safety hazard if they passed through the system at an early stage of incubation.

NSD is on List B of the Office International des Epizooties (OIE) and is therefore a reportable disease; it is on the list of exotic diseases of the USDA. Its occurrence in a new country should be followed by immediate slaughter of all affected and in-contact animals. Extensive tick surveys should then be carried out to eliminate the imported infected ticks and their hosts to prevent the disease becoming established.

There is a danger that NSD might be introduced to a new continent or country with the importation of animals infested with the vector tick and virus. Any importations of wild or domestic Bovidae and other species which are parasitized by the tick, should be treated on at least two occasions with acaricides and be subject to detailed inspection for ticks before export. Trade in wild ruminant and other species presents the greatest possibility for this to happen. The virus does not persist in infected animals for longer than the febrile period and no transmission is likely to occur from infected tissues. Most importing countries require a period of quarantine greater than the possible danger period for NSD.

There is a theoretical possibility that the virus could be transferred with Palearctic bird species in their migration from Africa north into Europe and Eurasia. This appears not to have occurred thus far, but remains a possibility. Birds are frequently seen to have Rhipicephalus larvae or nymphs when trapped on migration northward.

Disease Treatment

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No drug therapy is currently available that could be of any value in the treatment of NSD other than ribavirin. Ribavirin is used successfully in the treatment of other Bunyanvirus diseases in humans, such as Rift Valley fever. It has an important role in human disease control and there is no doubt that any case of human NSD would benefit from early administration of this anti-viral agent. It is not available for use in animals and in the present situation cannot be a cost-effective option.

Prevention and Control

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The use of vaccines for the prophylaxis of NSD has been suggested whenever valuable small ruminants are being imported into an enzootic area. Vaccines have been developed which showed considerable promise when tested in the laboratory, but there is no demand for these in the field. The steady enzootic state ensures that high levels of immunity are present in the small-ruminant populations at risk. No interventions such as vaccination, nor tick control are indicated.

This enzootic state can be altered by tick control practices, which may then lapse, and most importantly by extension in the range of the tick. The current ENSO activity (El Nino Southern Oscillation) results in periods of greater drought and greater rainfall. The latter is important as the cause of extensions in the range of the tick beyond the enzootic zones. Such changes can be predicted using serology and the CLIMEX data [see www.ento.csiro.au/climex/climex.htm; CSIRO Australia] for Rh. appendiculatus as a basis for defining enzootic areas and monitoring the data generated by ARTEMIS (African Real Time Environmental Monitoring Information Service). This provides data on the changes in vegetation, which develop following prolonged and heavy rainfall and favour extension and the development of large tick populations in hitherto clean areas. Short-term tick control or vaccination may then be instituted in these extension zones. The availability of 'pour on' acaricides (permethrin and cypermethrin) provide a cost-effective means of controlling the tick infestations during the critical periods, when these extensions occur.

The pastoralists themselves are aware of the dangers presented by areas infested with the vector tick, which also transmits a serious disease of cattle (Lewis, 1934). They are able to identify such problem areas and avoid grazing them at times when a danger is perceived.

The broad level of flock immunity which prevails throughout the enzootic areas for NSD in East African countries, and the awareness of the dangers of movement into enzootic areas from the arid and semi-arid zones, leads to a successful 'laissez faire' approach to NSD. Traders are aware of the problems and no national policies have been generated for control of the disease, for the scientific evidence supports a non-interventional approach. The dangers of extensions into agro-climatic zone IV following climatic changes remains and predictive epidemiology, as is now being used for Rift Valley fever, should be adopted to define and monitor the danger zones.

References

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