Transmission of equine infectious anaemia usually occurs through blood-sucking mosquitoes, midges, biting houseflies (stable flies) and horse flies. EIAV survives only 30 minutes to a few hours on the mandibles and does not reproduce inside the insect. Transmission over longer distances (more than 200 m) has not been observed. The transmission rate increases in late spring, peaks in late summer and early autumn and is at its lowest in winter. The accumulation of infections is at its peak when grazing in river valleys, marshes and on forest land, and lower on mountain pastures and in stable feeding. The regional and seasonal distribution patterns are explained by the appearance and population density of the insects.
Transmission from horse to horse requires very close contact between the animals. The virus reservoir is chronically ill horses that are lifelong virus carriers. The virus is mainly imported into disease-free livestock via chronically infected animals. Transmission can occur in the course of mating; pregnant mares can transmit the disease to the foetus via their placenta; mother’s milk and the secretion of saliva and urine.
Transmission is also possible via grooming equipment contaminated with blood and secretions, such as mouthpieces, but also via syringes, surgical instruments and non-licensed blood and plasma products.
The progression of the disease depends on the animal’s general health. The incubation period is 1-3 weeks, in rare cases more than three months.
This is followed by an increase in body temperature to over 40 °C; the fever may last only one day, but can also last several days. The animals are lethargic, become anaemic and show thick, sometimes icteric mucous membranes as the result of accompanying heart failure. Losses in stamina and weight follow. The hind legs of infected animals may sway and animals exhibit oedemas on their extremities, abdomen and lower thorax. EIA should be clarified using differential diagnosis if the animals display therapy-resistant fever or in cases of thrombocytopenia (= lack of thrombocytes).
Equine infectious anaemia may occur in peracute (sudden death), acute or chronic form. The acute form of the disease is mostly terminal with the animals dying within a few days. The chronic form may last for several years.
Symptoms of the acute form are fever, apathy, weakness, defunct movement coordination (= ataxia), jaundice (= icterus), heart palpitations, cardiac arrhythmia and bleeding from the capillaries of the mucous membranes (= petechia) found predominantly on the underside of the tongue, on mucous membranes and the connective tissue of the eyes. Pregnant mares may experience miscarriages or give birth to weak foals.
The chronic form is characterised by recurring flare-ups of the disease featuring fever, tiredness and oedemas on the lower abdomen. The recurring flare-ups appear in intervals of 4-6 weeks and last for 2-5 days. Afterwards, the animals show healthy behaviour until the next flare-up. The animals begin to develop anaemia and hypergammaglobulinemia (=inceased levels gammaglobulin in the blood) with the duration of the disease. Finally, the animals get so weak that they cannot stand up anymore. The animals also lose weight despite eating.
An infected animal must be considered a lifelong virus carrier (persistant viremia) and potential virus excretor, notwithstanding whether a clinical manifestation has been observed. A good 30 to 90 % of infections progress without showing any visible symptoms for a very long time. The anaemia that lends its name to the disease is often not observed – it develops through the immunopathologic destruction of red blood cells.
To date, there is no treatment or vaccine for the disease. EIA must be reported to the authorities in all EU Member States and is among animal epidemics that must be notified to World Organisation for Animal Health (OIE). An antibody detection test should be carried out before acquiring horses from EIA regions to exclude EIA.
Pathologic Symptoms, Detection Methods
EIA diagnosis is based on serologic detection methods (Coggins Test or ELISA), as the PCR method does not have the required sensitivity and specificity of serologic methods and is not a routine diagnosis method. The antibodies can be detected using the Coggins Test in 2 to 3 weeks, in rare cases more than 60 days, after the initial infection.
Equine viral arteritis, African horse sickness, equine piroplasmosis, babesiosis, ehrlichiosis, leptospirosis, petechial fever, kidney or heart disorders, chronic bacterial infections, severe parasitic infestation or gastroenteritis should also be taken into consideration from a differential diagnostic perspective.