In acute cases, lymph nodes, spleen and liver are hyperaemic and enlarged. Histologically these organs are infiltrated with nests of immature lymphocytes and plasma cells. Kupffer cells in the liver often contain haemosiderin or erythrocytes. The enlarged spleen may be felt on rectal examination. Once a horse is infected with EIA virus, its blood is liable to remain infectious for the remainder of its life.
Transmission occurs by transfer of blood cells from an infected horse.
In nature, spread of the virus is most likely via interrupted feeding of bloodsucking horseflies on a clinically ill horse and then on susceptible horses, or from the use of con-taminated needles. Nucleic acid sequence compar-isons have demonstrated a marked relatedness. Identification of the agent Virus isolation is usually not necessary to make a diagnosis. Isolation of virus from suspect horses may be made by inoculating their blood onto leukocyte cultures prepared from horses free of infection. Virus isolation is rarely attempted because of the difficulty of growing horse leukocyte cultures.