caprine arthritis encephalitis

Caprine arthritis-encephalitis virus (CAEV) transmits between goats mainly through colostrum and milk secretions (Adams et al., 1983; Rowe et al., 1992b). In sheep, maedi-visna virus (MVV) transmits by colostrum/milk and/or respiratory secretions (Sigurdsson et al., 1953; Houwers et al., 1983). Studies focusing on genomic and phylogenetic analysis of small ruminant lentivirus (SRLV) suggested that CAEV can naturally infect sheep and MVV can naturally infect goats (Leroux et al., 1997; Zanoni 1998; Rolland et al., 2002; Shah et al., 2004). CAEV can infect sheep, and MVV can infect goats in an experimental setting (Banks et al., 1983), and lambs can be infected with milk from CAEV-infected does (Oliver et al., 1985). Given similar transmission sources and the results of experimental transmission, it is plausible that CAEV could infect sheep naturally. In naturally infected goats, persistent infections with both CAEV and MVV have been described, and viral chimeras generated by recombination between these variants have been detected (Pisoni et al., 2007; Cruz et al., 2013). In cases where sheep and goats are commonly housed together, a combination of CAEV and MVV/OPP prevention and control methods are advised.

The global distribution of caprine arthritis-encephalitis virus (CAEV) is reported by OIE and is based upon serological and/or virological evidence of CAEV.

CAEV is typically found in countries that routinely serologically test for the virus. Countries that are not listed as having CAEV present do not routinely serologically test for CAEV. In 1984, a global CAEV survey was performed where serum samples came from 112 different locations around the world. At that time, 90% of the positive CAEV AGID tests came from Canada, France, Norway, Switzerland and the USA (Adams et al., 1984). In 2003, these same countries reported the presence but not the percentage of CAEV infection in their goatherds.

Regional seroprevalence studies of CAEV have been performed in the USA and Australia. In 1992, 3790 goats from 28 states in the USA were tested for CAEV using AGID, and 31% tested positive (Cutlip et al., 1992). In Australia in 1995, seroprevalence of CAEV in New South Wales was examined in 1484 goats in 14 dairy herds and 56.8% were CAEV-positive using an ELISA (Greenwood et al., 1995).

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

Clinical signs and the detection of antibodies to caprine arthritis-encephalitis virus (CAEV) in serum are the main techniques for CAEV diagnosis. Differential diagnosis exists for clinical signs caused by CAEV infection.

Clinical diagnosis

Clinical characterization of CAEV progressor status (chronic arthritis goats) and non-progressor status (asymptomatic goats) is subjective and depends primarily on the severity of arthritis. Adult CAEV progressor goats may exhibit debilitating lameness or severe arthritis and may show swelling of the carpel bursa, middle carpel and distal carpal joints or ‘water knee’. Other clinical signs associated with arthritis include lameness, reluctance to move and general recumbency. Radiographs can be very useful for identifying soft tissue mineralization osteoarthritis accompanied by osteolysis. There is a good correlation between radiographic abnormalities and the severity of the joint lesions (Woodard et al., 1982). A ratio of the circumference of the carpel and metacarpal (C/MC) regions is greater than 2.0 is a good clinical indicator of inflammatory radiocarpel lesions caused by CAEV infection and has been used to define progressor goats. However, in studies involving experimental infection C/MC ratios of greater than 2.0 typically occur 1-2 years after inoculation (Cheevers et al., 1988; Wilkerson et al., 1995b; Trujillo et al., 2004a).

Other clinical disease syndromes caused by CAEV infection in adult goats can occur singularly or in conjunction with CAEV-induced arthritis. These include mastitis or ‘hard-bag’ and chronic pneumonia characterized by dyspnoea. Kids experiencing encephalomyelitis typically display progressive hind-limb ataxia, paresis and paralysis; however, numerous non-specific clinical signs associated with white matter destruction and motor dysfunction are reported. Pneumonia presented as dyspnoea has been reported to occur with the neurological form of CAEV.

Lesions

If only postmortem investigation is available for diagnosis, gross and histological evaluation of target tissues (synovial lining of the carpal joints and bursa of the extensor carpal radialis tendon, central nervous system, mammary gland, lung, and lymph nodes) can be used to support the diagnosis of CAEV. Arthritis is characterized by mononuclear inflammation within the synovial lining and surrounding supportive tissue, proliferation of the synovium accompanied by oedema and necrosis in severe lesions. A semi-quantitative grading system for lesion severity in the radiocarpel joint has been developed for research studies (Cheevers et al., 2003). CNS lesions occur in the brain and spinal cord and include mild to moderate mononuclear meningoencephalitis including involvement of the choroid plexus, white matter gliosis, demyelination, oedema and hyperemia. Mammary gland lesions consist of minimal to severe infiltrates of mononuclear inflammatory cells in the periductal and periacinar stroma where severe lesions consist of coalescing infiltrates of inflammatory cells, lymphoid follicle formation, stroma oedema and fibrosis and inspissation or mineralization of duct contents (Cheevers et al., 1988). Lung lesions are generally mild to moderate and include interstitial infiltrates of mononuclear inflammatory cells predominately around airways and the pulmonary vasculature accompanied by variable hyperplasia of the respiratory epithelium. Typically in progressor goats there is marked hyperplasia of the lymphoid cells in the axillary and prescapular lymph nodes, which in severe cases can efface the normal nodal architecture.

Differential diagnosis

Differential diagnoses for CAEV-induced disease were reported by East (2002). Differential diagnoses for the neurological form of CAEV in goat kids include, but are not limited to, copper deficiency, lumbar cord abscesses, selenium and vitamin E deficiency, bacterial septicemia, septic arthritis or spinal cord injury. Differential diagnoses of the arthritic form of CAEV include septic arthritis, Chlamydia- and Mycoplasma-induced arthritis, and trauma. On occasion, the respiratory and cachexia forms of CAEV will manifest without clinical arthritis or encephalitis. The chronic form of pneumonia needs to be differentiated from lungworm infection, chronic bacterial pneumonia and pulmonary abscesses. The mastitis form of CAEV is often observed with the arthritic form of CAEV. The two primary differentials for mastitis in goats include bacterial infections and CAEV.

Laboratory diagnosis

Serological tests, PCR tests, immunohistochemical detection of CAEV antigens, and virus isolation are the four laboratory diagnostic tests used to confirm CAEV.

The presence of antibodies to CAEV in the serum of infected adult goats confirms infection and is the most reliable laboratory test. Serological diagnosis can be made using a variety of tests including Western blot analysis, immunoprecipitation (IP) of [³⁵S] methionine/cysteine-labeled CAEV, agar gel immunodiffusion assay, and ELISA. Western blot analysis and IP are typically used as confirmatory serological tests and are performed in a research laboratory setting. The CAEV AGID tests for the presence of antibody to the CAEV capsid protein, p28, and surface envelope glycoprotein, gp135; however, if the ovine progressive pneumonia virus (OPPV) AGID is used instead of the CAEV AGID with goat sera, approximately 40% of CAEV-positive goats will show negative (Knowles et al., 1994). ELISA tests are most commonly used for serological diagnosis, but some have better sensitivities and specificities than others do. Many ELISAs have been produced using whole CAEV and recombinant CAEV proteins (Simard et al., 2001; Clavijo and Thorsen, 1995; Rimstad et al., 1994; Celer et al., 1993; Heckert et al., 1992; Zanoni et al., 1991; Archambault et al., 1988). Indirect ELISAs are more problematic due to the dilutions required to lower background effects, which also increases the number of false negatives. The CAEV competitive inhibition ELISA (cELISA) does not require dilution of the sera before testing and since it depends on the presence of serological antibodies to bind to CAEV envelope glycoprotein (SU) and inhibit the binding of a monoclonal antibody to SU, the CAEV cELISA has a sensitivity of 100% and specificity of 96.4% when compared with IP (Herrmann et al., 2003). The CAEV cELISA offers a higher sensitivity than CAEV AGID when compared to IP (Knowles et al., 1994; Herrmann et al., 2003). However, the CAEV cELISA (96.4%) has a slightly lower specificity than CAEV AGID (100%), but this may be due to the increased sensitivity of the CAEV cELISA. Overall, results of CAEV serological tests are dependent on the infecting strain of CAEV; goats infected with a similar strain as the strain used in the serological test will test positive while goats infected with a markedly different strain of CAEV will test negative. If the animal has clinical signs of CAEV but tests negative in initial serological testing, different serological tests and other ancillary tests are advised. Moreover, a CAEV-seropositive kid goat, less than 1 year in age may be affected by passive transfer of material antibody, but may not necessarily be infected. Therefore, seropositive kid goats not displaying neurological disease or pneumonia should be tested by other means such as the PCR method described below.

PCR-based laboratory diagnostic tests for the detection of provirus in peripheral blood leukocytes (PBL) or peripheral blood mononuclear cells (PBMC) are becoming more common (Reddy et al., 1993; Rimstad et al., 1993; Barlough et al., 1994; Leroux et al., 1997; Wagter et al., 1998; Konishi et al., 2004). In addition, tissues and fluids such as brain, bone marrow, mammary gland, uterus, oviduct, seminal fluid, synovial membranes, lung, spinal cord, and lymph nodes have been found PCR-positive for CAEV in CAEV-infected goats (Barlough et al., 1994; Leroux et al., 1995; Travassos et al., 1998; Sanna et al., 1999; Fieni et al., 2003). At this time, PCR tests are considered secondary tests to serology. Some have found that PCR detection of CAEV provirus in peripheral blood mononuclear cells may precede seroconversion or antibody responses (Rimstad et al., 1993; Reddy et al., 1993); however, since false PCR positives are common, precautions are needed to reduce contamination and positives should be sequenced until the assay is optimized and validated (Zanoni et al., 1996). PCR diagnostic tests for CAEV have yet to be validated and, like serologic diagnostic tests, the sensitivity and specificity of PCR detection of CAEV depends upon sequence similarity of the infective provirus and the sequence used to develop primers. Therefore, primers for PCR amplification of CAEV provirus needs to be designed to amplify regions of the provirus with high similarity or regions conserved within all known small ruminant lentivirus (SRLV) provirus sequences. Although once optimized and validated, PCR tests for provirus in PBL or PBMC show promise in becoming an early indicator of CAEV-infection and may prove useful as a live-animal, confirmatory laboratory test.

Few studies have been published regarding immunohistochemical (IHC) detection of CAEV-infection in goats. CAEV-infected synovial cells in synovial fluid of CAEV-infected goats have been identified by immunohistochemical detection using monoclonal antibody 10A1, which binds to a capsid protein (p28) of CAEV (McGuire et al., 1987; Cheevers et al., 1994). Results of this study suggest that IHC could be used as a confirmatory test for CAEV-induced arthritis in live animals. Storset et al. (1997) identified CAEV-infected cells in brain, spinal cord, lung, mammary gland and lymph node tissues embedded in paraffin wax, followed by immunohistochemical detection using rabbit antiserum to recombinant capsid and matrix proteins. Results of this study suggest that IHC could be used as a postmortem test for CAEV-induced encephalitis, pneumonia, and mastitis. At present, there have been no dual immunohistochemical or immunofluorescence studies performed in situ identifying specific cell types in the joint, mammary gland, lung, lymph nodes, brain, or spinal cord that accumulate CAEV. This is presumably due to the low levels of replicating virus within macrophages, or the lack of adequate sampling in tissues.

Virus isolation is often performed in a research setting due to the high degree of technical difficulty and lack of sensitivity. Free viremia rarely or never occurs in CAEV-infected goats and isolation of virus from synovial fluid is sporadic. Therefore, suspected CAEV-infected cells or tissues are co-cultured with newborn goat synovial membrane cells for up to 6 weeks (Crawford et al., 1980). Synovial tissue from CAEV-infected goats can be cultured directly for up to 6 weeks. Mammary and peripheral blood mononuclear cells can be successfully co-cultured with goat synovial membrane cells (Cork and Narayan, 1980; Klevjer-Anderson et al., 1984; Kennedy-Stoskopf et al., 1985).

Immunization and vaccines

There are no commercially available vaccines for caprine arthritis-encephalitis virus (CAEV) infection. In research settings, various vaccination strategies have been tested using niaive goats as an animal model for HIV vaccine development. These studies used inactivated CAEV, vaccinia-based CAEV genes with and without vaccinia-based cytokine genes, and plasmid-based CAEV genes with and without plasmid-based cytokine genes followed by challenge using CAEV molecular clones or CAEV field isolates (McGuire et al., 1986; Russo et al., 1993; Vitu et al., 1993; Cheevers et al., 1994; Cheevers et al., 2003). Most of these vaccination strategies have failed to protect against challenge. However, plasmid DNA vaccinations encoding CAEV SU followed by a boost with viral SU in Freund's incomplete adjuvant (SU-FIA) provided protection in 4 of 4 goats based upon the absence of severe radiocarpel joint lesions and lower provirus loads in the radiocarpel joint (Cheevers et al., 2003). More recently, a protein-based vaccination strategy using an engineered SU mutated at two sites E542N and R539S (SU-M) and administered subcutaneously with saponin adjuvant produced high titres of type-specific and cross-reactive neutralizing antibodies (Trujillo et al., 2004b). The production of long-lived cross-reactive neutralizing antibodies is a major goal of lentivirus vaccine strategies. Reina et al. (2013) review immunization strategies against small ruminant lentiviruses and progress that has been made towards the development of an effective vaccine.

Husbandry methods and good practice

Control and prevention methods for CAEV in goat herds have focused primarily on serological testing and isolating or culling infected goats, and heat inactivation of colostrum (56ºC for 1 h) and supplementation with bovine colostrum for kids born to CAEV-infected nannies (Adams et al., 1983; Rowe and East, 1997). Separating CAEV seropositive goats from seronegative goats has significantly helped the goat dairy industry by reducing the risk of transmission (Rowe et al., 1992b). These prevention and control strategies have not completely eradicated CAEV; however, utilizing a more sensitive serological diagnostic test such as cELISA to identify and eliminate seropositive goats annually can virtually eliminate the herd presence of CAEV over the course of 6 years in a commercial dairy goatherd (Herrmann et al., 2003). There are two reasons for incomplete eradication of CAEV. One is that there is delayed seroconversion where essentially seronegative goats are transmitting virus, and second, the serological test lacks sensitivity to detect all infected goats. Seropositive progressor goats transmit virus to niaive goats in a natural herd situation (Woodard et al., 1982); however, the transmission contribution of seropositive non-progressor goats remains to be determined. Furthermore, since lentiviruses and other infectious diseases can be transmitted through blood, re-use of needles should be discouraged.

Genetics

There is evidence that host genetics play a role in determining susceptibility/resistance to SRLV infection and disease progression, but little work has been performed in small ruminants (Herrmann-Hoesing et al., 2008; Larruskain and Jugo, 2013). A number of genes implicated in SRLV infection and disease have been identified, but more research is necessary to understand the host-SRLV interaction. A better understanding of loci involved in SRLV infection and pathology and their polymorphism could lead to the identification and selective breeding of naturally resistant animals (Heaton et al., 2012; White and Knowles, 2013).