Equine Immunology: Past, Present and Future

The horse has played an important role in the history of immunology, dating back to Emil von Behring's description of curative antibodies in equine serum more than a century ago. Since then, the field of immunology has seen a tremendous increase in information regarding the role antibodies, lymphocytes and other cells play in a variety of diseases. While the mouse has replaced the horse as the predominant experimental animal in immunology research, efforts are still underway to characterize the immune response of horses. We have a better understanding of the horses’ immune system and the role it plays in a variety of infectious and non-infectious diseases, but we trail our colleagues working with mice and other domestic species in terms of both basic and applied information. Current efforts are focused on developing the reagents and techniques necessary for studying the horse’s immune system and the characterization of both protective and pathologic responses. As this knowledge base continues to expand, we can anticipate the translation of this basic information into practical application in the clinics, including improved diagnostic and therapeutic applications.

One example of this effort in my laboratory is a project investigating the immune responsiveness of very young foals. The development of immunological responses in newborn animals of all species, including the horse, is poorly understood. Clinical evidence suggests that very young foals are susceptible to a number of bacterial and viral infections despite the presence of maternal immunoglobulins. The unique susceptibility of young foals to infection with Rhodococcus equi is a well-recognized example of this age-dependent phenomenon. While resistance to R. equi infections likely involved multiple factors, it is known that cell-mediated immunity plays a critical role in immunity to this bacterium. We have recently shown that young foals have deficient cell-mediated immune responses when compared to older foals and adults. This defect is most apparent in foals less than 1 month of age, though foals older than 6 months can still be deficient when compared to adults. The underlying cause of this deficiency is unknown. The process whereby the young foal’s immune system matures to the point of being resistant to this bacterium is also unknown. Our efforts are directed towards identifying the nature of this immune defect, understanding the process involved in the attainment of full immunologic function in the foal, and identifying prophylactic or therapeutic strategies that may be employed to enhance the immune function of young foals.

A second project is directed towards characterizing the immune system of older horses. Horses over 20 years of age constitute about 15% of the equine population and many remain actively involved in equestrian sports and reproductive capacities as stallions and broodmares. Advancing age in horses, as with other species, is eventually associated with a decline in body condition, muscle tone and immune function. Nevertheless, there are no specific recommendations regarding the vaccination of older horses and ponies even though the best characterized effect of aging in horses is a reduced response to vaccination. We are in the process of characterizing the mechanism of the age-related decline in immune function in older horses. Once the underlying mechanism is understood, possible approaches for overcoming this effect may then be developed.

Dr. David Horohov, William Robert Mills Chair in Equine Infectious Diseases
(859) 257-4757, David.Horohov@uky.edu
Maxwell H. Gluck Equine Research Center, University of Kentucky.