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Due to the variability and possible absence of outward signs of EIA, clinical diagnosis is not always possible. Laboratory diagnosis, through blood testing, is essential.

The laboratory tests the blood sample for the presence of antibodies against EIAV proteins. Detectable antibodies are usually present in the blood 7-14 days after infection and remain present for the rest of the horse’s life. Diagnosis should be by means of the Coggins test (also known as the Agar Gel Immunodiffusion test, AGID). The Coggins test is currently the only test recognised officially for the purpose of international movement of horses.

An ELISA test for EIA has recently been developed. As this test can provide results more quickly and economically than the Coggins test, it is widely used for routine screening in populations where EIA is not suspected, e.g. pre-breeding, pre-sales and pre-sporting events. Greater sensitivity means that the ELISA test can produce occasional false positive results and positive results must therefore be clarified by the Coggins test. The Coggins test should always be used to test horses with clinical signs, to test horses that have been in contact with others who have or are at risk of having EIA and for official export certification. In such cases, samples for EIA (Coggins) blood testing must be sent to the APHA Weybridge (tel: 01932 357335).