Immunology of Foals: Environmental Effects on Immune Maturation

Newborn foals can be susceptible to many viral and bacterial infections. One example is the unique susceptibility of young foals to Rhodococcus equi, a common cause of bronchopneumonia.

Foals' increased susceptibility appears to be due to their immune systems not being as developed as those of adult horses. While maternal antibodies obtained through colostrum provide some degree of protection, immunity against intracellular pathogens, such as viruses and bacteria (Rhodococcus equi, for example), is dependent upon the generation of cell-mediated immune (CMI) responses. It is this component of the immune system that appears to be most deficient in foals and leads to their infection susceptibility.

The focus of the immunology research program at the Gluck Equine Research Center is to understand the underlying mechanism responsible for this deficiency, in order to develop therapeutic strategies that might increase a foal's resistance to Rhodococcus equi and other infectious disease-causing pathogens.

Vaccines are typically used to protect horses from infectious diseases, but there are a number of difficulties associated with vaccinating young animals, due to their undeveloped immune systems. Newborn foals often produce sufficient antibodies, but they have a limited CMI response to vaccines. Such biasing toward an antibody response fails to provide protection against viral and bacterial infections, such as Rhodococcus equi. Horses must develop potent CMI responses to have complete immunity against these pathogens. The factors in newborn foals that are responsible for this are not known, but could include maternal environment during pregnancy.

CMI response of foals

Mares produce factors during pregnancy that inhibit CMI responses, possibly as a means of preventing fetal rejection. These maternal influences might persist in the neonate, leading to delayed CMI responses.

The process whereby the immune response of the neonate eventually matures, leading to increased resistance to viruses and bacteria, remains unclear. Current opinion is that encounters with microbial products in the environment might be required for immune system maturation of newborn foals. The Gluck Center's immunology group recently presented a paper at the Conference of Research Workers in Animal Disease, describing how foals that spent four hours a day in the barn had elevated CMI responses compared to foals kept in pastures only (Figure 2).

While the precise mechanism involved in increased CMI responses is unknown, it likely involves stimulation of the immune system by microbial products present in the barn environment. Current research efforts are directed toward understanding this process of the foal's immune system maturation.

David W. Horohov, PhD, is the William Robert Mills Chair in Equine Immunology at the Gluck Center.

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