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Veterinarian Technician January 2008 (Vol 29, No 1)

Rhodococcus equi Pneumonia

by Debra A. Hembroff, RVT

    Foal pneumonia, particularly that caused by Rhodococcus equi (also known as Corynebacterium equi), is a devastating disease that can become endemic to some breeding facilities. Technicians with a basic understanding of R. equi pneumonia and the environmental factors necessary for R. equi to thrive and spread can help clients develop husbandry practices that aid in the prevention of this disease.

    Causative Agent

    R. equi is a robust, soilborne, gram-positive, facultative, intracellular bacterium that is widespread within the environment. Optimal environmental growth is achieved at 86°F (30°C), and the organism's simple nutritional needs are easily met by common barnyard manure, such as that of swine, cattle, poultry, and horses.1 R. equi is commonly considered the most deadly respiratory pathogen to infect foals. All horse facilities are likely to be contaminated with this pathogen to some degree; however, healthy horses develop immunity to it by the time they reach maturity.1 Several strains of R. equi exist, but the strain that carries virulence-associated protein A has been found to be particularly pathogenic.2

    During the first 12 weeks of a foal's life, as its immune system develops, ingestion of low concentrations of R. equi likely stimulates the production of natural immunity.1 However, R. equi can replicate exponentially within the intestines of immunologically naive foals, ultimately increasing contamination of the environment. This is likely the reason why some breeding facilities become heavily contaminated by R. equi.1 Exposure to heavily contaminated dust, particularly through inhalation, can overwhelm a foal's developing immune system, allowing infection to occur. This is a particular risk in foals exposed to the virulent form of R. equi, which invades only the macrophages of the lung tissue, creating isolated abscesses that initially develop slowly. In this way, R. equi effectively "hides" itself from the developing immune system of the foal. Diagnosis can be difficult during the early stages of the disease because the foal is initially able to compensate for the loss of lung function and shows few (if any) clinical signs.3,4 R. equi pneumonia is most commonly diagnosed in foals aged 6 to 12 weeks, highlighting the fact that the foal is especially susceptible to infection during the first few weeks of life.1

    Clinical Signs

    R. equi infection is initially suspected on farms where several foals between 4 and 6 weeks of age develop low-grade fever and cough. As mentioned above, the pathogen enters and infects the alveolar macrophages of the lungs, and affected foals develop chronic suppurative bronchopneumonia with extensive abscessation. The lung infection develops slowly and, in some instances, is complicated by diarrhea or colic as a result of concurrent R. equi intestinal infection.3,4 Once clinical signs appear, a foal's condition often worsens so rapidly that the foal can be critically ill before barn staff are aware of the illness; therefore, careful early monitoring is essential.1 Common clinical signs include fever, increased respiratory effort and bronchial sounds, coughing, and wheezing. Respiratory rates may increase to more than 40 breaths/min.1

    Clinical signs of R. equi pneumonia are very similar to those of other respiratory diseases, and a number of diagnostic tests are available to help veterinarians make a definitive diagnosis.


    Complete blood count, fibrinogen concentration, serologic study, and radiography can all be used to diagnose respiratory diseases in foals. However, a definitive diagnosis of R. equi pneumonia requires bacterial culture or polymerase chain reaction amplification, combined with cytologic examination of tracheobronchial aspirate.3


    Although R. equi is susceptible to a wide variety of antimicrobial agents, effective treatment can be difficult. The large abscesses created by R. equi are encased with thick, caseous material that forms a barrier to many antibiotics. However, some lipid-soluble antimicrobials, such as erythromycin and rifampin, are able to penetrate this caseous barrier. For this reason, the combination of erythromycin and rifampin has been described as the treatment of choice for R. equi infection.3

    When an affected foal is identified, it should be immediately isolated and treated, contaminated manure should be removed from the environment, and stalls and equipment should be disinfected. Any other foals on the property should be considered especially at risk for infection and carefully monitored. Attempts to predict the likelihood of exposure based on barnyard cultures have had mixed results because R. equi can be isolated on affected and unaffected farms.5 To complicate matters further, R. equi thrives in soil, which constitutes a large part of the equine environment and is exceptionally difficult, if not impossible, to disinfect.


    R. equi infection is endemic on some farms and sporadic on others, but fortunately, it does not occur on most, despite widespread environmental distribution.3,4 Although researchers have found most barnyards to be contaminated with R. equi, they have not been able to definitively predict areas of increased virulence.4,5 Because foals spend a lot of time lying down or exploring (sniffing) their environment, they are likely to inhale bacteria-laden dust. Therefore, young foals, especially those up to 4 to 6 weeks of age, should be considered vulnerable. Neonatal foals may become infected in the first few days of life. As research continues into developing a vaccine that is compatible with the developing immune system of very young foals, the following are commonly recommended prevention strategies:

    • Minimize the risk of infection. Infected foals can shed up to 10,000 organisms per gram of manure.1 Practicing good hygiene (i.e., removing manure), decreasing population density, avoiding the use of dusty loafing pens, and foaling during cold seasons (when the organism remains frozen in the ground) can help protect developing lungs.1,3
    • Recognize and treat infection at an early stage. R. equi pneumonia is often not recognized until it is well advanced; initially, affected foals are often bright, alert, and vigorous and are sucking well. It is important to critically assess the condition of foals by conducting a thorough physical examination as often as twice weekly on farms with enzootic problems.1,2 Accurate records of information such as temperature, pulse, respiratory rate, and even behavior can be essential tools in the early identification of sick animals. For example, even a sign as subtle as unusual shortness of breath after mild exertion warrants closer evaluation of a foal's respiratory health status.
    • Implement passive immunization. Vaccination of pregnant mares to increase foals' passive immunity derived via colostrum has been found to be ineffective.6 Hyperimmune plasma can be used to boost the immune system of foals in endemic areas, but this treatment is expensive and time-consuming and may not supply complete protection. Therefore, if used, passive immunization with hyperimmune plasma should be combined with other preventive strategies.3,7


    Several pathogens can cause pneumonia in foals. R. equi pneumonia, one of the most deadly forms of foal pneumonia, all too often has a significant economic impact on breeding facilities because of its high mortality rate, the loss of athletic potential in affected foals, and the expense of treatment. A basic understanding of the virulence and life cycle of R. equi will help the veterinary team work with clients to prevent the spread of this deadly pathogen.

    1. Wright B: Rhodococcus equi Pneumonia of Foals. Accessed June 2007 at www.omafra.gov.on.ca/english/livestock/horses/facts/90-056.htm.

    2. Jain S, Bloom BR, Hondalus MK: Deletion of vapA encoding virulence associated protein A attenuates the intracellular actinomycete Rhodococcus equi. Mol Microbiol 50(1):115-128, 2003.

    3. Giguère S: Rhodococcus equi Pneumonia. Accessed June 2007 at www.ivis.org/proceedings/AAEP/2001/91010100456.pdf.

    4. Hondalus MK: Rhodococcus equi: Pathogenesis and Virulence. Accessed June 2007 at www.ivis.org/proceedings/AAEP/1997/Hondalus.pdf.

    5. Martens RJ, Takai S, Cohen ND, et al: Prevalence and Virulence of Rhodococcus equi in Sick Foals and Soil of Horse-Breeding Farms in Texas. Accessed June 2007 at www.ivis.org/proceedings/AAEP/1999/53.pdf.

    6. Cohen ND, Chaffin MK, Martens RJ: How to Prevent and Control Pneumonia Caused by Rhodococcus equi at Affected Farms. Accessed November 2007 at www.ivis.org/proceedings/AAEP/2002/910102000295.PDF.

    7. Lunn DP: Practical Foal Vaccination Strategies. Accessed June 2007 at www.ivis.org/proceedings/AAEP/1997/Lunn.pdf.

    References »

    NEXT: State News: Arizona (January 2008)


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