Immunity and Vaccination in Foals

Maternal antibodies passively transferred in colostrum confer excellent immunity to foals. Levels of maternal antibodies decrease through normal catabolic processes until the foal be­comes susceptible to diseases indigenous to its relatively new environment. Vaccination should commence at this point. Ideally, the duration of immunity conferred by colostral antibodies should be assessed before initial vaccination against specific diseases in foals. This is partic­ularly important since passively acquired an­tibodies often interfere with the response to active immunization. However, it is impracti­cal to assess the persistence of passively ac­quired antibodies in individual foals to determine the optimal time for initial vacci­nation. The persistence and rate of decline of passively acquired antibodies depends upon the concentration of colostral antibodies ab­sorbed and on the amount of antigenic stimu­lation during this period. Interference with the response to immunization by passively ac­quired antibodies is thought to occur with ad­ministration of vaccines for strangles, equine viral arteritis, equine encephalomyelitis and African horse sickness.^20-23 These studies indi­cate that the practice of immunizing foals in the first few weeks of life is neither logical nor advantageous unless failure of passive transfer has occurred.

The persistence of maternal antibodies in foals has been investigated.⁹ Serum-neutrali­zation tests for colostral EHV-1 antibodies in 14 foals showed adequate protection against EHV-1 infection within a few days after birth. Passively derived antibody levels decreased to marginally protective levels by 2 months of age and to nonprotective levels by 3 months of age.

Similar results were obtained in a study of the duration of passive immunity to equine viral arteritis.²¹ Protective titers present 24 hours after birth steadily declined to marginal or nonprotective levels. By 4 months of age, an­tibodies against equine viral arteritis were not detectible in 6 of 8 foals; the 2 remaining foals had marginally protective antibody levels.

Recent investigations of antibodies pas­sively acquired from administration of tetanus antitoxin indicated that 60% of foals lost their protective titers by 2 months of age.²⁴ By 3 months of age, 77% lost their protective titers against tetanus. Other investigations on the duration of passive immunity to tetanus in foals revealed protective antibody levels for up to 6 months and up to 12-18 months after ad­ministration of tetanus antitoxin.^20.25 More studies are needed to clarify these differences.

Results of these studies in foals suggest ini­tiation of vaccination against certain diseases at 2-3 months of age rather than at 4-6 months of age. Evidence supporting this view is that combined-immunodeficient (CID) foals that cannot respond immunologically remain healthy until 2 months of age, providing trans­fer of passive immunity is complete. At 2 months of age, however, such foals become re­markably susceptible to infection once mater­nal antibodies are catabolized.

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From the information available, initial vac­cination at approximately 3 months of age, fol­lowed by revaccination 2-3 months later, offers the best protection up to 7-9 months of age.