Maedi-Visna and Caprine Arthritis Encephalitis (CAE)

Maedi-Visna (MV) and Caprine Arthritis Encephalitis (CAE) are chronic, progressive, multisystemic diseases of sheep and goats caused by small ruminant lentiviruses (SRLVs) of the family Retroviridae, genus Lentivirus. These viruses are characterized by a long incubation period, persistent infection, and slow development of clinical disease. In sheep, MV primarily manifests as a progressive interstitial pneumonia (maedi) and a demyelinating leukoencephalomyelitis (visna), while in goats, CAE typically presents as chronic arthritis, indurative mastitis, and occasionally encephalomyelitis in young kids. The diseases are economically significant in intensive dairy goat operations and in sheep flocks, leading to premature culling, reduced milk production, and increased mortality. The term 'maedi' is Icelandic for 'dyspnea' and 'visna' for 'wasting', reflecting the clinical presentation. The diseases are notifiable in many countries and have no effective treatment, making control and prevention crucial.

The causative agents are small ruminant lentiviruses (SRLVs), which are enveloped, single-stranded RNA viruses. Two major genotypes are recognized: genotype A (Maedi-Visna virus, MVV) primarily infects sheep, and genotype B (Caprine arthritis encephalitis virus, CAEV) primarily infects goats. However, cross-species transmission can occur, and both viruses can infect both species. The viruses are closely related and share antigenic epitopes, but genetic diversity exists among strains. The virus is present in blood, milk, colostrum, respiratory secretions, and inflammatory exudates. The virus targets cells of the monocyte/macrophage lineage, integrating its proviral DNA into the host genome, leading to lifelong infection. The virus is relatively fragile in the environment and is inactivated by common disinfectants, heat, and drying.

MV and CAE have a worldwide distribution, with higher prevalence in intensive dairy goat herds and in sheep flocks with a history of importing infected animals. In goats, CAE is more common in dairy breeds (e.g., Saanen, Alpine, Nubian) than in meat or fiber breeds. In sheep, MV is more prevalent in older animals, with a higher incidence in housed flocks during winter. The primary route of transmission is through ingestion of infected colostrum and milk by neonates. Horizontal transmission can also occur through respiratory aerosols, especially in crowded housing, and through contaminated equipment (e.g., tattooing, ear tagging, needles). Iatrogenic transmission via blood-contaminated instruments is significant. The prevalence can be as high as 80% in some dairy goat herds, but clinical disease occurs in only a fraction of infected animals (10-20%). Morbidity and mortality vary, but economic losses result from reduced milk yield, increased culling, and premature death. The incubation period is long, often 2-3 years for clinical disease, which complicates control.

The virus infects cells of the monocyte/macrophage lineage, where it remains latent until the cells mature into tissue macrophages. Upon activation, the virus replicates, leading to chronic inflammation and tissue damage. In the lungs, interstitial pneumonia develops with thickening of alveolar septa, infiltration of mononuclear cells, and smooth muscle hyperplasia, resulting in progressive dyspnea. In the central nervous system, demyelinating leukoencephalomyelitis occurs, characterized by perivascular cuffing with lymphocytes and macrophages, leading to neurological signs. In the joints, chronic proliferative synovitis leads to arthritis with fibrosis and joint effusion. In the mammary gland, indurative mastitis results from lymphocytic infiltration and fibrosis, reducing milk production. The virus also causes a persistent viremia and antibody response, but the immune response is ineffective in clearing the infection. The pathogenesis is immune-mediated, with the host's inflammatory response contributing to tissue damage.

Intrinsic factors include age (clinical disease more common in animals >2 years), breed (dairy goats more susceptible), and genetic susceptibility. Extrinsic factors include management practices such as feeding pooled colostrum or milk from infected dams, high stocking density, poor ventilation, and lack of biosecurity. Stress, intercurrent disease, and poor nutrition may accelerate disease progression. In sheep, the practice of housing animals in winter increases the risk of respiratory transmission. In goats, the intensive dairy system with prolonged milking and high milk production may exacerbate mastitis. The use of contaminated needles, ear tags, and tattooing equipment can transmit the virus. The presence of other lentiviruses or immunosuppressive conditions may also increase susceptibility.

Clinical signs vary by species and organ system affected. In sheep with maedi, progressive dyspnea, tachypnea, and weight loss are prominent, with a dry cough. In goats with CAE, chronic arthritis is the most common sign, especially of the carpal joints, leading to lameness and joint swelling. Indurative mastitis is common in dairy goats, presenting as a hard, non-painful udder with reduced milk production. Encephalomyelitis occurs in kids aged 2-6 months, with progressive paresis, ataxia, and proprioceptive deficits. Neurological signs may also occur in adult goats. In sheep, visna is less common but can cause hindlimb paresis, facial nerve paralysis, and blindness. Systemic signs include weight loss, poor coat condition, and decreased milk yield. The disease is progressive, and animals may be culled due to debilitation.

Differential diagnoses for MV/CAE include: 1) Caseous lymphadenitis (CLA) caused by Corynebacterium pseudotuberculosis, which presents with peripheral lymph node abscesses and can also cause arthritis and mastitis; 2) Mycoplasma spp. infections, particularly Mycoplasma agalactiae, causing contagious agalactia with mastitis, arthritis, and keratoconjunctivitis; 3) Ovine progressive pneumonia (OPP) is the same as maedi, but other causes of pneumonia include bacterial bronchopneumonia (Mannheimia haemolytica, Pasteurella multocida) and parasitic pneumonia (Muellerius capillaris); 4) Arthritis due to bacterial infections (Erysipelothrix rhusiopathiae, Streptococcus spp.) or trauma; 5) Encephalomyelitis caused by Listeria monocytogenes, which typically presents with unilateral cranial nerve deficits and is more acute; 6) Scrapie, a prion disease, causing progressive neurological signs but with characteristic pruritus and behavioral changes; 7) Polioencephalomalacia (thiamine deficiency) which is acute and responds to thiamine; 8) Spinal cord abscesses or trauma. Definitive diagnosis relies on serology (AGID, ELISA) and PCR.

The diagnostic approach begins with a thorough flock history, including the presence of chronic progressive disease, arthritis, mastitis, or neurological signs. Physical examination should focus on respiratory effort, joint swelling, udder consistency, and neurological assessment. If MV/CAE is suspected, serological testing is the first step: agar gel immunodiffusion (AGID) or ELISA for antibodies against SRLV. Positive results confirm exposure, but false negatives can occur in early infection. PCR on blood or milk samples can detect proviral DNA and is more sensitive. For animals with clinical signs, necropsy and histopathology are confirmatory, revealing characteristic lesions. In live animals, joint radiography or ultrasonography may show chronic proliferative changes. Milk somatic cell counts may be elevated in mastitis. Differential diagnoses should be ruled out with appropriate tests (e.g., bacterial culture, serology for CLA, etc.).

Serology: Antibodies to SRLV can be detected by AGID or ELISA. PCR: Detection of proviral DNA in peripheral blood leukocytes or milk macrophages. Hematology: May show mild anemia and lymphopenia. Biochemistry: No specific changes, but may show hyperglobulinemia due to chronic inflammation. Cerebrospinal fluid (CSF) analysis in neurological cases may show mild lymphocytic pleocytosis and elevated protein. Milk somatic cell count (SCC) may be elevated (>200,000 cells/mL) in mastitis. Joint fluid analysis in arthritis cases shows increased neutrophils and mononuclear cells. Histopathology: Lung shows interstitial pneumonia with lymphoid hyperplasia; brain shows demyelination and perivascular cuffing; joints show chronic synovitis; mammary gland shows lymphocytic infiltration and fibrosis.

Radiography of affected joints may reveal soft tissue swelling, joint effusion, and periarticular osteophyte formation in chronic arthritis. Ultrasonography of the lungs may show diffuse interstitial changes, but is not specific. In cases of mastitis, ultrasonography of the udder may show increased echogenicity and fibrosis. Computed tomography (CT) or magnetic resonance imaging (MRI) of the brain may be used in neurological cases, but is rarely performed in practice due to cost and availability. Imaging is not diagnostic but can support the clinical suspicion.

Histopathology is the gold standard for diagnosis. In the lung, there is diffuse interstitial pneumonia with thickening of alveolar septa, infiltration of lymphocytes, macrophages, and plasma cells, and smooth muscle hyperplasia. In the brain, there is demyelinating leukoencephalomyelitis with perivascular cuffing, gliosis, and neuronal necrosis. In the joints, there is chronic proliferative synovitis with villous hypertrophy, lymphocytic infiltration, and fibrosis. In the mammary gland, there is interstitial mastitis with lymphocytic infiltration and fibrosis. Lymph node cytology may show reactive hyperplasia. These lesions are characteristic but not pathognomonic, and must be interpreted in conjunction with serology and PCR.

There is no effective antiviral treatment for MV/CAE. Management focuses on supportive care and prevention of secondary infections. For arthritis, non-steroidal anti-inflammatory drugs (NSAIDs) such as flunixin meglumine (1.1-2.2 mg/kg IV or IM, q24h) or meloxicam (0.5 mg/kg PO, q24h) may provide pain relief. For mastitis, antibiotics are not effective, but good milking hygiene and culling of affected animals are recommended. For neurological signs, corticosteroids (e.g., dexamethasone 0.1-0.2 mg/kg IV or IM) may temporarily reduce inflammation, but the prognosis is poor. Supportive care includes good nutrition, clean housing, and minimizing stress. Control measures include test-and-cull programs, segregation of seronegative animals, and feeding pasteurized colostrum and milk to kids/lambs. Vaccines are not available.

The prognosis for individual animals with clinical MV/CAE is poor, as the disease is progressive and incurable. Animals with arthritis may survive for months to years with supportive care, but their productivity is reduced. Animals with encephalomyelitis or severe pneumonia have a guarded to poor prognosis, with death or euthanasia often necessary. In flocks/herds, the prognosis is guarded, but with rigorous control programs, the prevalence can be reduced and eventually eradicated. Early detection and culling of seropositive animals can improve the long-term outlook.

For affected flocks/herds, a comprehensive control program should be implemented. This includes: 1) Regular serological testing (every 6-12 months) to identify infected animals; 2) Immediate culling of seropositive animals, especially those with clinical signs; 3) Segregation of seronegative animals from seropositive ones; 4) Feeding pasteurized colostrum (56°C for 60 minutes) and pasteurized milk (72°C for 15 seconds) to neonates; 5) Strict biosecurity measures, including disinfection of equipment and avoiding sharing needles; 6) Monitoring for new infections and re-testing. For individual animals, monitor clinical signs and provide supportive care. The flock/herd should be re-tested at intervals until two consecutive negative tests are obtained.

Pearls: 1) MV/CAE should be suspected in any adult goat with chronic arthritis, especially of the carpus, or in any sheep with progressive pneumonia. 2) Serology is the primary diagnostic tool, but false negatives can occur in early infection; PCR is more sensitive. 3) Pasteurization of colostrum and milk is critical for breaking the transmission cycle. 4) Control programs require a long-term commitment and may take years to achieve eradication. Pitfalls: 1) Misdiagnosis as bacterial arthritis or pneumonia, leading to inappropriate antibiotic therapy. 2) Relying solely on clinical signs, as many infected animals are asymptomatic. 3) Failure to implement biosecurity measures, leading to continued spread. 4) Using unpasteurized milk for feeding kids/lambs, perpetuating infection. 5) Not considering the economic impact of the disease, which can be significant.

There is no specific antiviral therapy. Supportive treatments include: 1) NSAIDs for arthritis: Flunixin meglumine (1.1-2.2 mg/kg IV or IM, q24h) or Meloxicam (0.5 mg/kg PO, q24h) for up to 3-5 days. 2) Corticosteroids for neurological inflammation: Dexamethasone (0.1-0.2 mg/kg IV or IM, once) or Prednisolone (1 mg/kg PO, q24h) for 3-5 days, with caution. 3) Antibiotics for secondary bacterial infections: Oxytetracycline (10 mg/kg IV or IM, q24h) or Procaine penicillin G (20,000-40,000 IU/kg IM, q24h) for 5-7 days. 4) Fluid therapy if dehydrated: Lactated Ringer's solution (20-40 mL/kg IV, then maintenance). 5) Nutritional support: High-quality feed and supplements. Withdrawal times must be observed for meat and milk. Always consult a veterinarian for specific protocols.

Research on MV/CAE has established the viral etiology, transmission routes, and pathogenesis. Serological surveys have shown high prevalence in many countries. Control programs based on test-and-cull and pasteurization of milk have been successful in eradicating the disease from some herds. Studies have demonstrated the economic impact, including reduced milk production and increased culling. There is ongoing research on vaccine development, but no effective vaccine is currently available. The AASRP and ECSRHM provide guidelines for diagnosis and control. Key references include 'Diseases of Sheep' by Martin & Aitken, 'Goat Medicine' by Smith & Sherman, and 'Veterinary Medicine' by Constable et al.