Infectious respiratory diseases constitute one of the major causes of death in the horse industry. In addition to the sorrow caused, they also are major economic threats. One of the most common diseases in foals six months and younger is pneumonia. Although many different organisms can cause foal pneumonia, Rhodococcus equi (R. equi) is considered the most common culprit in a severe case. A nationwide survey indicated that respiratory disease is the third-leading cause of disease in foals and ranks second as a cause of death, following injury or wounds.

Because foals’ active immune systems are still developing, they are susceptible to a number of infectious diseases. A Texas A&M study found that in foals, respiratory disease was the leading cause of disease and death. During a 2004-2005 study at a Kentucky breeding farm, 30% of foals developed R. equi pneumonia.

No horse breed or geographical region in the U.S. is exempt from foal pneumonia, which can be caused by a bacterium that is often present in horse farm soil and grows in the manure of grazing animals. The R. equi bacterium is similar to the one that causes tuberculosis (TB). Like TB, foal pneumonia caused by R. equi develops slowly and by the time clinical symptoms are detectable, the disease is in a relatively advanced stage. Indeed, some foals don’t show signs of respiratory distress until the disease is irreversibly severe. Although effective treatments exist, such as the oral administration of azithromycin and rifampin, waiting until signs develop can result in a therapeutic course that is prolonged with lower chances for success. Thus, there is a critical need to discover methods for preventing foal pneumonia.

Investigators at several institutions have been working, often collaboratively, to further our understanding of the bacterium and the disease. They have had important breakthroughs, such as the demonstration that transfusion of hyperimmune plasma can reduce the frequency and the severity of pneumonia caused by R. equi, the identification and characterization of disease-causing factors of the bacterium, and the dissection of some aspects of the immune response of horses and foals to R. equi. Hyperimmune plasma provides passive immunity, but it is time consuming to administer and expensive to produce or buy. A vaccine to provide active immunity would be more convenient, but development of an effective one has proven elusive