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

The virus is usually transmitted through large biting flies such as horseflies (Tabanid spp) or stable flies (Stomoxys calcitrans). Blood is transferred from infected horses when the flies feed, 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.

• Virus coats red blood cells and thrombocytes
• Type III Immune complex disease develops
• Non-neutralizing antibody and complement lyse RBC's, causing:
  • Jaundice
  • Vasculitis
  • Hemorrhage
  • Diarrhoea
  • Loss of condition
  • Oedema
  • High mortality

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 initial clinical signs may be vague or unnoticed. 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.

Control

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