R. Equi: Manifestations, Pathogenesis, and Diagnosis (AAEP 2010)

Rhodococcus equi is a dangerous pathogen that causes pneumonia in foals that are generally between the ages of 3 weeks and 5 months. In cases that caretakers and veterinarians catch early on, the foal can make a full recovery with proper treatment. However, in more serious cases the mortality rate is quite high, noted Steeve Giguère, DVM, PhD, Dipl. ACVIM, at the 56th Annual Convention of the American Association of Equine Practitioners, held Dec. 4-8, 2010. He discussed the importance of understanding the pathogenesis of R. equi (how it's acquired and how it causes disease), the associated clinical signs, and the diagnostic methods used to detect infection.

"The most common manifestation of R. equi  infections in foals is a chronic suppurative (pus-producing) bronchopneumonia with extensive abscessation," Giguère said. "The slow spread of the lung infection combined with the remarkable ability of foals to compensate for the progressive loss of functional lung makes early clinical diagnosis difficult."

Clinical Manifestations
Giguère closely analyzed the different clinical manifestations of R. equi pneumonia that the foals presented.

He noted that many foals with R. equi pneumonia also have extrapulmonary (outside the lungs) disorders, such as immune-mediated polysynovitis (arthritis of multiple joints), ulcerative enterotyphlocolitis (small erosions and abscesses in the wall of the small or large intestines), intra-abdominal abscessation, abdominal lymphadenitis (inflammation in one or more lymph nodes), uveitis (moon blindness), and pyogranulomatous hepatitis (microscopic abscesses in the liver).

Intestinal lesions were one of the most common extrapulmonary disorders, evident in about 50% of foals with R. equi  pneumonia that were presented for necropsy, Giguère noted. However he added that most foals that contract R. equi pneumonia do not show signs of intestinal disease while undergoing treatment for the pneumonia. "Foals with abdominal abscesses typically have a high fatality rate," Giguère said.

Giguère also found that polysynovitis (a condition in which multiple joint surfaces are inflamed) was present in about 25%-30% of live foals undergoing treatment for R. equi pneumonia. He added that in many of these cases, lameness was mild, if present at all.

Giguère added that immune-mediated processes might aid in the development of other extrapulmonary conditions seen in these cases, such as uveitis, anemia, and thrombocytopenia (low platelet counts) in some foals.

Pathogenesis
According to Giguère, little was known about the virulence of R. equi before scientists identified a virulent plasmid in 1991 that is essential for the bacterium’s ability to cause disease.

"With the presence of the plasmid, Rhodococcus can replicate in macrophages (white blood cells) and cause disease," Giguère said. "Without it, it loses its ability to cause disease in foals. Without the plasmids, the foals cleared the Rhodococcus within two weeks."

Researchers sequenced the plasmid’s nucleotide and "the genes for a family of eight closely related virulence-associated proteins," targeting one that they designated as VapA. After further research, Giguère said, scientists discovered that while VapA aided reproduction of the bacteria in the foal, it wasn't able to produce an R. equi  infection on its own. Other proteins were evaluated as well, although the role that each plays in the pathogenesis of R. equi is unknown, Giguère concluded.

Giguère also relayed that inhaling R. equi  bacteria is one avenue for the foal contracting pneumonia. He added that R. equi pneumonia could be detected as little as three days after a foal inhaled a large concentration of bacteria.

Diagnosis
Giguère said that several tests are available for diagnosing foal pneumonia, but the key is determining whether R. equi  pathogens caused the pneumonia, or whether another issue is to blame.

There are several tests that are able to detect R. equi, including complete blood counts, the measurement of fibrinogen concentrations, and ultrasonographic and radiographic exams. But Giguère cautions that veterinarians should use a bacteriologic culture or a polymerase chain reaction (PCR) test for the VapA protein, along with a cytological exam of the tracheobronchial aspirate, to definitively diagnose pneumonia caused by R. equi bacteria.

"A PCR test is slightly more sensitive than the other tests, however the culture offers the advantage of detecting other bacterial pathogens present and permits in vitro susceptibility testing of the recovered pathogens," Giguère said. "As a result, PCR amplification of the VapA gene may be done in association with, but should not replace, bacterial culture."

Giguère added that foals should be tested for R. equi  infections at the first sign that they might be ill: "Early diagnosis is of paramount importance for successful therapy of infected foals."

About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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