When foals get sick, horse owners can sometimes face many sleepless nights, as well as weeks or months of intensive management, to get these babies through the rough spots. One of the most common problems in the ill foal is pneumonia, caused by a bacteria called Rhodococcus equi, which attacks foals between one and six months of age. Most foals will tend to show clinical signs, including coughing, abnormal breathing patterns and, less commonly, diarrhea, before the age of four months. Most farms are susceptible to the disease, but some farms are considered enzootic or particularly prone to have the problem continually. On these enzootic farms, the disease can lead to significant financial loss because of the cost of therapy and the occasional death of foals.

While adult horses can carry R. equi in small numbers in their intestine and not suffer any ill effects, in the foal, the organism can actually multiply in the intestine for up to three months of age so that the presence of foals inevitably contributes to the development of the infection on the farm.

Foals with R. equi pneumonia swallow the mucous that accompanies the illness; the bacteria ends up back in their intestines. Therefore, their manure is likely a major source of contamination. Under suitable conditions of high summer temperatures, R. equi can multiply in the environment by 10,000-fold in only two weeks. A single gram of soil contaminated with foal manure could contain millions of virulent R. equi. The inhalation of dust particles laden with R. equi is the major route of pneumonic infection in foals.

The successful control of R. equi infections on the enzootic farm depends on decreasing the size of the infective challenge, earlier recognition of the disease, and passive immunization