Rhodococcal Pneumonia in Foals
Dr. Philip Johnson
Although foals are challenged by and potentially infected with Rhodococcus equi during their first four weeks of life, signs of disease are not usually recognized until at least six weeks of age. Most affected foals are presented to veterinarians between 6 and 12 weeks of age for respiratory symptoms, although other symptoms could prompt evaluation in a foal with Rhodococcus equi.
Foals usually have some evidence of infection of the respiratory system including fever, increased respiratory rate, exercise intolerance, nasal discharge and coughing. However, it should be noted that, with this disease, both nasal discharge and coughing are often mild and infrequent. Some affected foals are mildly diseased, showing subtle signs such as a slightly reduced growth rate, slight lethargy and depression and a poor quality hair coat. However, most foals are in excellent body condition and appear to thrive.
Many affected foals develop clattering breath sounds, due to the presence of inflammatory material in their airway, hence the name rattles. In many respects, Rhodococcus equi pneumonia is similar to tuberculosis in people, which used to be known as consumption. Consumptive lung disease causes insidious, slowly-progressive, extensive and devastating destruction of the lungs over a long period of time. As the lungs gradually become more affected with this disease process, the foal's ability to obtain oxygen from the air is eventually diminished.
Some infected foals also develop remote immune-mediated signs such as joint-distention, limb swelling and uveitis (inflammation in the eyes). In some of these cases, a veterinarian has been called to examine and treat a young foal for an eye problem or to make a diagnosis of joint distention. However, following a complete physical examination, the veterinarian will recognize that the major problem is in the lungs.
It is particularly interesting that affected foals are commonly presented to veterinarians in a state of severe, life-threatening respiratory distress but that the owners recall that the foal had been unremarkable on the preceding day. Sudden-onset respiratory distress in the face of progressive consumption of the lung occurs when the foal's remarkable compensatory mechanisms have been finally depleted. Often, these foals collapse if they are exercised or if the ambient temperature and humidity are very high. Infection by Rhodococcus equi does sometimes also cause diarrhea and, less commonly, joint and bone infections (osteomyelitis).