The EIAV is transmitted between horses by transfer of infected blood or blood products. This can occur in the following ways:
- By insect vectors such as biting flies (including horse, deer and stable flies) and (very rarely) mosquitoes.
- By administration of infected blood products (including plasma) and unauthorised blood-based veterinary medicinal products.
- By contaminated veterinary or dental equipment.
- By other equipment that may become contaminated by blood and act as a vector between animals, e.g. twitches and curry combs.
- From mare to foal via the placenta, or, rarely, via virus-contaminated colostrum or milk in newborn foals.
Transmission through semen is uncommon but is a potential risk.
Both clinically and sub-clinically affected horses can be a source of infection for other horses, although animals suffering acute disease or recurring bouts of chronic disease are likely to be the most highly infectious.