Equine Infectious Anemia

Also known as: EIA, Swamp Fever

Description

Equine Infectious Anemia (EIA) is an exotic viral disease affecting horses, donkeys and mules characterised by immuned-mediated haemolytic anemia. It is caused by Equine Infectious Anemia Virus (EIAV), an equid-specific lentivirus in the retrovirus family that is closely to related to HIV in humans.

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 when a nursing foal ingests milk from a carrier mare or iatrogenically via contaminated needles.

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. EIA may be induced by stress, disease or immunosuppressive drugs.

Epidemiology

• Notifiable in UK but endemic in parts of the US
• Animals can be carriers for years despite being antibody-positive
• Transfer is mechanical via vectors:
  • Mosquitoes transfer infected macrophages
• Also transferred via infected needles, semen and milk
• Infected horses have lifetime latent infection of macrophages with recurring bouts of virus when immunocompromised

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 haemorrhages, depression, 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, 'spiking' fever 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. 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 in order to increase the test sensitivity.

Treatment

No specific treatment or vaccine for EIA is available and treatment is usually supportive. NSAIDs may be used to control pyrexia and inflammation and

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.

Control

• Slaughter of infected animal
• 2 clear Coggins tests of all horses before movement allowed
• Vector control: stabling, etc.

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
• Lavoie, J. P., Hinchcliff, K. W. (2009) 'Blackwell's Five-Minute Veterinary Consult: Equine