Although about 30% of foals that develop Rhodococcus equi foal pneumonia do not survive, the majority of foals can be treated successfully and proceed with a normal life. But why is it that only foals are affected by this respiratory disease? According to M. Julia B. Felippe, DVM, MS, PhD, Dipl. ACVIM, of Cornell University's College of Veterinary Medicine, one of the contributing factors is that some aspects of the foal's immune system take time to develop and, thus, contribute to susceptibility to disease.
Felippe summarized research findings on the topic at the 2010 American Association of Equine Practitioners Convention, held Dec. 4-8, 2010, in Baltimore, Md.
"The unique susceptibility of young foals to R. equi disease is still puzzling, despite many studies investigating their innate and acquired immune systems," Felippe said. Part of the puzzling aspect is why only certain foals contract the disease, while other foals remain healthy.
Despite the fact that many foals with naive immune systems (they haven't been exposed to the pathogen in question before) are exposed to R. equi, only some of them develop disease in the face of that exposure. According to Felippe, this points towards some foals having individual risk factors that make them more susceptible to the R. equi bacterium.
Felippe explained that the foal must have both innate and acquired immunity for complete protection from R. equi bacteria. The innate immune system recognizes the presence of organisms without the need of previous exposure. Some cells of the innate immune system of the foal are very effective in killing extracellular (outside cells) pathogens, including R. equi. But the interaction of the innate with the acquired immune system seems necessary for protection against disease.
She added that acquired immune responses develop after R. equi exposure, and are carried out by a variety of "T-helper" cells, which scientists call "Th cells." Each Th cell plays a different role in fighting diseases. For example, in the horse Th-1 cells support an immune response to fight intracellular (within cells) infections, while Th-2 cells promote antibody production for protection against extracellular organisms.
According to Felippe, recent studies have indicated that foals can develop Th-1 immunity as a defense against R. equi. She said this is the type of immune response that adult horses produce when challenged with experimental infection in scientific studies; such a response indicates that some foals can defend against R. equi. The question remains if, for some foals, this type of response takes longer to become effective, creating the window of susceptibility to the organism.
Additionally, she examined the possibility of antibodies providing immunity for the foal against R. equi. Although a protective effect of antibodies on the bacterium has been shown in vitro (in the laboratory, not in the live horse), studies evaluating the use of plasma products enriched with antibodies against R. equi have contrasting results in the field. Nevertheless, colostrum and plasma products supply at birth essential antibodies for the optimal function of the cells of the innate immune system, until the foal can produce its own antibodies through the acquired immune system.
Felippe concluded that while scientists have made strides in understanding how R. equi affects foals, further studies are required to uncover how--and why--it only affects certain foals.
"Many current studies (are investigating) the pathogenic mechanisms in the early stages of infection," she said, adding that the goal is to develop better preventive methods, including immunomodulators (drugs that alter the immune system), vaccines, and improved herd management techniques.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.