Rhodococcus equi in Foals

Rhodococcus equi pneumonia is a serious and often fatal disease in foals, typically affecting those between 1 to 6 months of age. This gram-positive facultative intracellular bacterium causes chronic suppurative bronchopneumonia and pulmonary abscessation, often accompanied by extrapulmonary disorders such as diarrhea, uveitis, or septic arthritis.

Overview:

Rhodococcus equi is a Gram-positive, facultative intracellular pathogen primarily affecting foals between 1 to 6 months of age. It is a significant cause of bronchopneumonia, leading to high morbidity and mortality rates in affected animals. The pathogen is commonly found in the soil of horse farms and can cause both pulmonary and extrapulmonary infections.

Clinical Signs:

Early signs of R. equi infection are often subtle and include a slight increase in respiratory rate and mild fever. As the disease progresses, more pronounced symptoms such as cough, nasal discharge, lethargy, fever, tachypnea, and respiratory distress become evident. In advanced cases, abscess formation within the lungs is common.

Diagnosis:

Diagnosis is typically based on a combination of clinical signs, thoracic imaging, and microbiological culture. Thoracic ultrasound and radiography are key tools, with ultrasound being particularly effective in identifying peripheral lung lesions. Transtracheal wash samples can be used for cytology and culture, revealing intracellular coccobacilli. Serologic tests are not recommended for diagnosis due to their poor correlation with disease severity.

Treatment:

The standard treatment involves a combination of a macrolide antibiotic (e.g., clarithromycin or azithromycin) and rifampin. This combination is preferred due to the synergistic action of these drugs and their ability to penetrate abscesses. Treatment duration ranges from 2 to 12 weeks, depending on the severity of the infection. Monitoring is essential to adjust therapy based on the foal’s clinical response and radiographic findings.

Prevention:

Preventive measures include environmental management to reduce dust and exposure to contaminated soil, as well as early detection and treatment of subclinical infections. Vaccination strategies are still under research, with some promising developments.

Prognosis:

The prognosis for foals with R. equi pneumonia ranges from guarded to good, with survival rates between 60% and 90% if treated early and appropriately.

References:

• Merck Veterinary Manual. Rhodococcus equi Pneumonia in Foals.

• dvm360. Rhodococcus equi infection in foals: diagnosis and treatment (Proceedings)

• American Association of Equine Practitioners (AAEP). Rhodococcus Equi.