The summer pneumonia (rattles) usually affects foals aged 2 to 6 months. The respiratory problem is the most common disorder although there may be alimentary tract involvement. Abscesses form in the lungs accompanied by progressive signs of pneumonia. The disease is caused by Rhodococcus equi bacteria (previously known as Corynebacterium equi) that gains entry to the body through inhalation or ingestion. Foals rarely show definitive signs until abscesses and bronchopneumonia are well established. Signs include rapid and labored breathing, cough, nasal discharge, a persistent, slightly elevated temperature, poor appetite and weight loss as the disease progresses. The prognosis may be grave in cases with multiple lung abscesses and particularly if the abdomen is involved.
Rhodococcus equi is an intracellular organism which provokes an inflammatory reaction. Although the organism is sensitive to several antibiotics and a combination of them have proved successful in some cases, treatment is difficult. Affected foals need prolonged treatment because of the persistence of the bacteria within abscesses in the lung and because immunity to lung infection is poor, the disease tends to recur. The combination of erythromycin and rifampicin have improved the survival of foals infected with R. equi; however, erythromycin can cause adverse reactions in foals and mares, which has prompted the search for alternative therapies.
Rhodococcus equi is a robust soil organism widespread in the environment. The organism will potentially multiply wherever there is horse manure. Because the organism reaches the lung by inhalation, dusty manure-contaminated environments (such as are commonly found in loafing paddocks on horse breeding farms in the summer) are potentially lethal sources of infection. It is also important to ensure that loafing paddocks are well grassed, and not totally grazed, reducing them to dusty sandpits. Rhodococcus equi, a recognized bacteria in horses, is emerging as a human pathogen, especially in immunocompromized hosts with an overall mortality of 75%. Wound infections and skin abscess, have been reported in humans.