From Equine Disease Quarterly, a publication funded by Underwriters At Lloyd’s of London, Brokers, And Their Kentucky Agents

Although it seems counter to logic, acquiring equine infectious anemia (EIA) by being alongside an infected carrier horse may be reasonably rare for a foal.  Foals of EIA-positive dams with clinically inapparent infections have an excellent chance of being raised uninfected, even if they have been held together in pasture situations with high populations of mechanical vectors of EIA virus (EIAV).  A number of factors contribute to this.

Foals appear to be resistant to infection, but data to support this is not available.  In a study early fetuses succumbed to infection with a relatively avirulent strain of EIAV, while fetuses inoculated after 204 days of gestation produced antibodies in utero and were born virus- and antibody-positive.

What are the factors that make foals less likely to acquire EIA than adults?  The first candidate is passively acquired colostral antibody.  Studies suggest that passively transferred antibodies against EIAV may confer a level of protection against disease but do not protect against infection.