Equine Infectious Anemia

Also known as: EIA, Swamp Fever

Description

Equine Infectious Anaemia (EIA) is an exotic viral disease affecting horses, donkeys and mules characterised by immuned-mediated haemolytic anemia. It is caused by Equine Infectious Anaemia Virus (EIAV), an equid-specific lentivirus in the retrovirus family that is closely to related to HIV in humans. EIA occurs in acute and subacute forms in susceptible horses but more commonly assumes a chronic course. Affected horses that recover from EIA may become lifelong carriers, and relapse of the disease may occur during times of stress or illness. Cases of EIA in the UK are extremely rare and have previously occurred in horses that have travelled abroad or have been imported from areas of endemic disease.

Signalment

All equids are susceptible to EIA but donkeys and mules appear to be less severely affected. No breed, age or sex predilection has been reported.

Pathogenesis

EIA occurs worldwide but most commonly in countries with warm climates. The virus is usually transmitted via mechanical innoculation of blood from large biting flies such as horseflies (Tabanid spp) or stable flies (Stomoxys calcitrans). Transmission of the virus can also occur through saliva, milk, body secretions or via contaminated needles and blood products.

Following coating of erthrocytes and thrombocytes by the virus and the development of a Type III immune complex reacion, lysis of erthrocytes occur. Infection with EIA results in an acute phase of infection followed by recurrent episodes that usually susbide after a period of time. Affected horses become persistently infected, lifelong carriers of the disease due to latent infection of macrophages. EIA may be induced by stress, disease or immunosuppressive drugs.

Clinical signs

The incubation period of the disease ranges from 10 to over 45 days and the clinical presentation is highly variable. A large number of affected horses do not display any clinical signs. Animals may be affected acutely, chronically or sub-clinically and clinical signs vary according to the stage of the disease. Inapparent carriers may be clinically normal. In the acute stages, clinical signs may include mucosal petechial and ecchymotic haemorrhages, depression, lymphadenopathy, fever, lethargy and inappetance. Horses that have been infected for thirty days or may display the characteristic signs of EIA including ventral and limb oedema, anemia, icterus, pyrexia and cachexia.

Diagnosis

The 'gold standard' for diagnostic confirmation is an agar gel immunodiffusion test (the Coggins test) which detects serum antibodies against the EIA virus. The test is 95% accurate for the diagnosis of EIA but false positives may be obtained from foals that have absorbed colostrum from affected mares and false negatives may occur during acute stages of EIA. The Coggins test may be performed in combination with an ELISA or PCR test in order to increase the test sensitivity. Additionally, haematology may reveal a moderate to marked anaemia, leukcopaenia and bilirubinaemia. Mild thrombocytopaenia is common in the acute phase of infection.

Treatment

No specific treatment or vaccine for EIA is available and treatment is usually supportive. Infected horses should be isolated from other horses due to the risk of spread of disease. NSAIDs may be used to control pyrexia and inflammation and/. Corticosteroids are contraindicated due to the possibility of exacerbating viral replication and clinical disease. In the UK, DEFRA policy is that horses with a confirmed (tested) case of EIA must be slaughtered under Section 32 of the Animal Health Act 1981. Horses that have been in close contact with diseased animals must be kept in isolation for 90 days and tested on a monthly basis with regular veterinary assessment. After this period of testing, restrictions may be lifted if no disease is found.

Control

Although attenuated live vaccines are available in the United States, these are not in current use in the UK.

Prognosis

Foals exposed to EIAV have a high fatality rate but generally horses recover from the disease, becoming lifelong inapparent carriers. Although occasionally some horses die from EIA, the majority become lifelong inapparent carriers.

References

• Dwight, C., Hirsh, Y., Zee, Y. C. (1999) Veterinary Microbiology Wiley-Blackwell
• Higgins, A., Snyder, J. R. (2006) The Equine Manual Elsevier Health Sciences
• Jain, N. C. (1993) Essentials of Veterinary Haematology Wiley-Blackwell
• Lavoie, J. P., Hinchcliff, K. W. (2009) 'Blackwell's Five-Minute Veterinary Consult: Equine