Systematic Veterinary Virology

TRANSMISSION

• EIA is considered a classic bloodborne infection. The infection occurs in summer and autumn since the insects appear in large numbers in these seasons. The spread of EIA is usually slow although it can be quite fast where large groups of horses are gathered in a small area. EIA is very easily spread by the injection of small quantities of infected blood. 
• The EIAV most frequently is transmitted between horses in close proximity by large biting insects, such as horseflies and deerflies. The bites from these flies stimulate defensive movement by the horse, which often results in an interruption of the blood–feeding. When interrupted, the fly is motivated to complete the feeding as soon as possible. It then attacks the same or a second host and feeds to repletion.
• In this manner, any infective material from the blood of the first host that is present on the mouthparts of the insect can be mechanically transmitted to the second host. The virus can survive only less than 4 hours in the insect. 
• Insect transmission of EIAV is dependent on the number and habits of the insects, the density of the horse population, the number of times the insect bites the same and other horses, the amount of blood transferred between horses, and the level of virus in the blood of the infected horse from which the initial blood meal was obtained.
• Iatraogenic transmission is also possible during any manipulation that may involve the transfer of infected blood or saliva, for example through the use of unsterilised instruments or equipment. Vertical transmission (in utero or through the milk) is also possible