Prospective study of progeny of inapparent equine carriers of equine infectious anemia virus.

Progeny of a band of horses, positive by the agar-gel immunodiffusion (AGID) test for equine infectious anemia (EIA) antibody, were observed through their weaning over a 4-year period. Sentinels (AGID test-negative) were allowed to mingle with EIA-infected mares and their foals in pasture situations in an area with high populations of potential vectors. Of 27 adult sentinels, 8 (30%) seroconverted in annual rates ranging from 0% to 75%. In contrast, only 2 of 31 (6%) foals weaned became infected. Difference in infection rates between adult sentinels and foals was significant (chi 2, P less than 0.05). Possible explanations for differences included protective value of colostral immunity and differences in attractiveness to blood feeding vectors. Detectable colostral immunity to EIA virus in the AGID test persisted for 25 to 195 days, with a mean of 124 days.