Rhodococcus equi infection: Treatment and immunity in foals

Rhodococcus equi infection: Treatment and immunity in foals

Part 2 of this series explores a foal's innate defenses against a common bacterial pneumonia and how veterinarians can enhance natural immunity.
Sep 01, 2013

Foals are often exposed to Rhodococcus equi, a common cause of bacterial pneumonia occurring from a few weeks after birth up until 4 to 6 months of age.

"R. equi pneumonia rarely develops in older foals unless immunodeficient or immunocompromised," states a recent study by Dominic R. Dawson, DVM, DACVIM, CCRT, and other researchers from Cornell University's Equine Immunology Lab, which is headed by Maria Julia Felippe, Med. Vet., MS, PhD, DACVIM.1 The group is studying opsonization effect on the outcome of pathogen viability and the development of the immune system to help the foal in its fight. Though foals are affected at an early age, with their innate immunity and the help of a variety of equine practitioners' tools, they have a fighting chance to fend off the disease.

According to the study, passive immunoglobulin transfer via colostrum or plasma transfusion could help provide initial protection in the airways before R. equi reaches the intracellular environment and until a foal's own cellular and humoral immunity develops. Neutrophils are important cells of the innate immune system and inhabitants of the upper airways in horses. They're also the first cells recruited to the site of infection and have bactericidal qualities against opsonized R. equi, the group says.1

Phagocytic function in neonatal foals is thought to be important in controlling pathogen expansion and disease development until more complete acquired immunity develops. According to Dawson's study, "foal blood phagocytes are competent with phagocytic capacity and the expression of large amounts of inflammatory cytokines following R. equi infection." But the mechanisms underlying this response are poorly understood by researchers.1

The study investigated the effect of opsonization of R. equi and R. equi-specific antibodies in plasma on bacterial viability and phagocytic activation in a cell culture model of infection. "Results of the study confirmed that opsonization of R. equi with specific antibodies in a commercially available plasma product enhances phagocytic function of neutrophils and macrophages, including increases in oxidative burst activity and TNF-alpha production, and a decrease in bacterial viability," researchers state.1

In the context of the airways, it is possible that enriched plasma containing high concentrations of R. equi-specific antibody may limit the viability of bacteria on the mucosal surface and may increase the intracellular killing by airway phagocytes.

Although neonatal foals are born competent for the production of antibodies, endogenous serum immunoglobulins may take a long time to achieve protective concentrations, according to a study by Rachel B. Gardner, DVM, DACVIM, and other researchers, also at Cornell's Equine Immunology Lab.2 "Therefore, passively acquired maternal antibodies from colostrum play an important role in preventing infection in the initial two to three months of life."2