Influenza is among the most common upper respiratory diseases of horses. Foals are not usually clinically infected with influenza because the dam’s colostrum provides the newborn with protective maternal antibodies. As foals get older their maternal antibodies decay and they become susceptible to influenza, especially as yearlings and two-year-olds.

The current veterinary practice is to begin an immunization schedule for equine influenza between two and six months of age. However, researchers in Europe have questioned the effectiveness of this strategy due to possible interference from maternal antibodies. Recent work in our laboratory, presented by Dr. H.S. Conboy at the 1997 American Association of Equine Practitioners Convention, gauged the effectiveness of influenza vaccination starting at different ages of the foal to determine any maternal antibody interference. This information is needed by practitioners to determine an effective influenza program in foals.

The study sample included 187 foals whose dams were vaccinated with inactivated influenza vaccine approximately 30 days before parturition. Foals received maternal antibodies in the first 24 hours of life via colostrum, as verified by serology. The hemagglutination inhibition (HI) serological assay was used to test foals’ sera for antibodies to the A1 and A2 subtypes of equine influenza.

The 187 foals, divided into six groups, were vaccinated with the standard two doses of vaccine, one month apart, with treatment starting between two and seven months of age. The two dose course of vaccination did not induce detectable antibody production in any foal against A2 virus, the subtype presently found in nature. Even three doses of vaccine given at seven, eight and nine months of age induced seroconversion to A2 virus in only 25% (3/12) of foals