By William Gilsenan, VMD, Dipl . ACVIM-LAIM
Rhodococcus equi is a ubiquitous environmen-tal bacterium that causes one of the costliest diseases on equine breeding farms. The most com-mon disease manifestation is pneumonia in young foals. Infection with R. equi appears to occur in the first few days of life and remains subclinical for several weeks. Many foals eliminate infection on their own, while others develop respiratory disease. Management of rhodococcosiscan be further complicated by the development of extra-pulmonary disorders (EPDs). Extra-pulmonary disorders refer to any clini-cal manifestation of rhodococcosisother than pneumonia.
A large study estimated that 74% of infected foals had at least one EPD. In many cases, EPDs were not diagnosed until postmortem ex-amination. In this study, survival was significantly lower in foals with EPDs. Factors that influence development of EPDs are not completely under-stood, but it is generally accepted that EPDs are indicative of a more severe infection. EPDs can pose a major diagnostic challenge and warrant thorough evaluation of any sick juvenile foal. Identification of EPDs can aid in selection of ap-propriate therapy, monitoring response to therapy, and prognostication.
Clinical signs of R. equi pneumonia rarely devel-op before three to four weeks of age. While blood work and ultrasonography can increase suspicion of infection, a tracheal wash provides a definitive diagnosis. Suspicion of R. equi pneumonia should prompt investigation for EPDs elsewhere in the body. Intra-abdominal abscessation is one of the most common EPD. It is suspected that R. equi spreads hematogenously or is ingested in sputum to cause abdominal lesions. Abscesses are best diagnosed via ultrasonography, highlighting the importance of abdominal ultrasonography in foals with pneumo-nia. Alternatively, abdominal abscessation might only manifest as a recurrent fever or unexplained high white blood cell count.
Generally, a sample cannot be obtained from abscesses to confirm R. equi involvement. Presumptive diagnosis is made based on the foal’s age and concurrent pneumonia. Successful treatment of abscessation is possible, but foals with intra-abdominal abscesses are at a significantly greater mortality risk. Surviv-ability seems to decrease with increasing size of abdominal abscesses. Treatment of abdominal R. equi infection includes provision of a long course of antimicrobials. Surgical debulking or resection typically is not possible due to attachments of the abscess to adjacent structures.R. equi can spread hematogenously and result in osteomyelitis.
Clinical signs of affected foals in-clude lameness, ataxia, and recumbency. Retrieving diagnostic samples from affected sites for disease confirmation can be difficult. Osteomyelitis may be best diagnosed via radiography or computed to-mography. Successful treatment has been reported, but prognosis is typically considered poor. Other documented sites of R. equi infection include the central nervous system, kidney, and liver.Polysynovitis is perhaps the most commonly observed EPD. It is characterized by symmetric joint effusion without lameness. R. equi is rarely cultured from affected joints; this finding has long-supported a conclusion that this phenomenon is immune-mediated.
Recently, experimental infec-tion of foals resulted in the isolation of R. equi from synovial fluid 14 days post-infection. This lends support to a newer theory that polysynovitis is a septic complication of infection that might actually result from hematogenous spread. In the same study, experimental infection also resulted in the isolation of R. equi from ocular fluid. Uve-itis has also been considered immune-mediated. Unlike polysynovitis, R. equi uveitis is strongly associated with nonsurvival in foals. With both of these EPDs, a suspected hematogenous origin is an interesting development in our understanding of the disease as many other EPDs appear to pre-cipitate from bacteremia.
The role of the immune response in the development of EPDs should not be discounted yet, as several reports exist of foals affected by immune-mediated hemolytic anemia associated with R. equi infection.Extra-pulmonary disorders are frequently encountered during the management of foals infected with Rhodococcus equi. As current literature suggests a septic process in the pathogenesis of polysynovitis and uveitis, it is logical to speculate that the precipitation of most EPDs is a function of bacteremia and the host’s immune response to systemic infection.