New Research Examines Foal Influenza Vaccine

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.

Vaccination against A1 virus was slightly more efficient although three doses were still required to produce a seroconversion rate greater than 50%. therefore, during approximately the first seven months of life, conventional equine influenza vaccines fail to produce a protective antibody response in foals from vaccinated dams.

In another study, foals which lacked maternal antibodies to either A1 or A2 influenza would seroconvert specifically to that strain after vaccination at just 3 to 4 months of age. This is the first direct evidence that maternal antibodies are responsible for the failure of foal vaccinations. Since virtually every dam in the U.S. has prior exposure to influenza, thereby having some level of maternal antibody to it, we expect that most foals will fail to seroconvert to influenza vaccination.

At present the only known solution is to wait until foals are old enough, i.e., weanlings of at least 8 months age, before beginning a vaccination program for influenza. This information should not be extrapolated to other vaccinations until specific studies are completed to determine the influence of maternal antibody on specific vaccine antigens.

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