Equine Infectious Anaemia (EIA) occurs in most countries around the world, including Australia, where it occurs in small pockets along the river systems in central and western Queensland. The incidence of EIA in Queensland tends to increase after flooding. The disease is called “swamp fever” in some areas. EIA is caused by a lentivirus within the retrovirus family of viruses. It primarily infects macrophages. Natural transmission occurs through the bites of flying insects, such as horseflies (Tabanus species) or deerflies. A fly will feed on an infected horse and if the feeding is interrupted (horsefly bites are painful) the fly will leave that horse in search of another to continue feeding. The virus is carried on the mouthparts of biting insects and will not survive more than 2 hours. The virus can also be transmitted through contaminated needles, syringes, dental tools and surgical instruments. EIA can infect developing foals in utero or newborn foals through colostrum.
Signs of Infection
There are three clinical syndromes seen in horses exposed to equine infectious anaemia virus. Initially signs appear about 30 days after exposure, and a test for antibodies will be positive 16-42 days after infection.
In the Acute syndrome there is high fever (up to 40.5 degrees C) and a loss of red blood cells (extravascular haemolysis) and platelets (thrombocytopaenia). This leads to pale icteric membranes with petechial (pin point) haemorrhages. Horses are depressed and inappetant, particularly when fevers are present. The heart and respiratory rates are elevated and there is oedema under thorax and in the limbs. Less commonly horses may develop neurologic signs. Pregnant mares may abort.
The Chronic syndrome repeated episodes of fever, anaemia and thrombocytopaenia. Fatality is uncommon but is most likely to occur in the acute or early chronic phases of infection. Chronically affected horses are typically thin and have oedema on ventral thorax and abdomen and distal limbs.
Most horses recover and either continue to have less frequent episodes or show no clinical signs. These horses become Inapparent carriers and serve as a source of on-going infections in close contacts.
Diagnosis
The classical blood test is called the Coggins Test. It is an agar gel immunodiffusion test that has excellent sensitivity and specificity, but does require a minimum of 24 hours. There is a C-ELISA test available which can return a result within an hour. This is very important for veterinary hospitals in order to rule out infection before mixing with other hospital patients. Clinical episodes are characterized by anaemia and thrombocytopaenia.
Treatment and Prevention
There is no treatment for EIA and there is no vaccine available. Prevention is through identification and euthanasia or lifelong quarantine of positive animals. In the US all horses in endemic states are required to be tested annually. Interestingly in some states in the US quarantine is achieved by maintaining a minimum distance of 200 yards between infected horses and susceptible populations. Some states also require screening in addition to physical distance. There are retirement farms specifically for EIA positive horses.
In Australia EIA infection is a reportable condition, although there is no monitoring program in place.
Tags: Infectious diseases; Haemopoietic system