Comparative studies have revealed that EAV strains differ in virulence. The most conspicuous gross lesions observed in fatal cases of infection with the Bucyrus strain are oedema, congestion, and haemorrhage in many tissues. The Italian ITA92 strain has been associated with disease of moderate clinical severity, but no mortality. There is also evidence to suggest that different EAV strains may differ in abortigenicity. Experience with other viruses has shown that a number of factors can affect clone stability and utility, and that different clones are not all equivalent.
Commercial considerations make access to clones generated by other groups an unlikely prospect.
Laboratory tests are required to diagnose outbreaks of clinical disease, to screen for subclinical infection and to identify carrier stallions. Horses are often moved over long distances both nationally and internationally, especially to attend races, and for breeding. Other members of the Equidae , including mules, donkeys and zebras are also being transported around the world in ever increasing numbers. This is especially so where countries differ markedly in their attitudes to the control of infection. The presence of specific antibodies in the blood indicates past infection or vaccination.