Rhodococcus equi is one of the most frequent causes of pneumonia in foals between the ages of 1 and 6 months. Although most horse farms are likely to be infected, clinical disease is devastating on some farms while sporadic on others. This reflects differences in environmental (temperature, dust, soil pH, and soil type) and management conditions. On farms where the disease is widespread, it leads to significant financial loss due to the cost of therapy and occasional death of foals. Rhodococcus is known to cause severe pneumonia in young foals. In addition, young foals may develop extra-pulmonary disease such as septic arthritis (joint infection), osteomyelitis (bone infection), neonatal diarrhea, inflammation of lymph nodes, abdominal and spinal cord abscesses, liver disease, and sudden death. It is thought that Rhodococcus equi bacterium is present in soil and the feces of horses. It appears that it is actually concentrated in the feces. Foals are thought to become infected when they ingest or breath in soil, dust, and fecal particles harboring the bacteria within the first few days of life. The bacteria then multiply inside white blood cells and become clinically apparent at 30-60 days of age or later. While most foals are exposed to the disease, not all foals develop the disease. We do not know why some develop it while others do not. It is most likely a combination of the foal’s immune status, environmental factors, and farm management practices. Clinical Signs The most common clinical signs of Rhodococcus equi pneumonia include a cough, thick yellow-green nasal discharge, high fever, lethargy, depression, weight loss or poor weight gain, and increased respiratory effort.

Presently the gold standard test for Rhodococcus equi includes bacterial culture or PCR and analysis of transtracheal aspirate. This involves passing a small plastic sterile tube down the trachea to the lungs and obtaining a sample. We can also make a tentative diagnosis of Rhodococcus equi pneumonia based on clinical signs and radiographic images or ultrasound images of the chest. Diagnosis Early diagnosis before development of clinical signs could reduce losses and decrease treatment related costs. We recommend every other week visits by the veterinarians to visually inspect the foals as well as collection of blood for fibrinogen measurements. We recommend ultrasound examinations in foals be performed every 2-4 weeks. This will allow us to identify subclinical cases of Rhodococcus equi pneumonia before they are clinically apparent. In addition the owner and farm managers can take daily temperature and respiratory rate recordings to identify affected foals. Treatment Treatment of choice is to place all affected foals on long-term antibiotic therapy, typically 30- 60 days. We can monitor blood work and thoracic images to determine when to stop therapy. There have been reports of antibiotic associated complications such as hyperthermia (markedly elevated body temperature) and mild diarrhea. We highly recommend keeping the foals being treated out of the sun while on the medication. Prevention The Rhodococcus equi bacterium is relatively difficult to control. There is no proven vaccination for prevention of the disease. However, we feel that adequate immunization of the mare pre-foaling will help reduce the incidence of viral respiratory pathogens in the foal, thereby reducing the incidence of respiratory compromise leading to opportunistic Rhodococcus equi infections. One of the key steps in determining adequate immunity is to run an IgG on all foals after 24 hours of birth to determine passive immunity. If the foal had failure of passive transfer then administration of hyperimmune plasma can be performed. The only proven preventative is to administer Rhodococcus equi hyperimmune plasma to provide immunity to foals and reduce the incidence of pneumonia. A good, effective parasite program is also important in the prevention of respiratory disease in foals. The immature parasites have been documented to migrate through the lungs leading to damage and opportunistic infection with Rhodococcus equi bacterium. Good farm management is pivotal to the control of Rhodococcus pneumonia on farms. We recommend foaling all mares in stalls without dirt floors. We also recommend picking the manure out of the stalls as often as possible thereby reducing the foals exposure to concentrated Rhodococcus equi bacterium. Once the foals are several weeks old we recommend moving them to pasture with good grass, rather than dry, dusty paddocks to avoid ingestion or inhalation of the soil, dust, or fecal particles. Before another mare is placed in the stall, the stall should be stripped and adequately disinfected. Some farms place sprinklers in their pastures to reduce the dry dusty conditions. Rhodococcus equi infections can be successfully controlled by a management strategy that involves the practices listed above. Conscientiously applied, this will reduce the spread of infection and therefore the costs associated with the disease.