Epidemiology of Rhodococcus equi Pneumonia in Foals

CHAPTER 29 Epidemiology of Rhodococcus equi Pneumonia in Foals



Noah D. Cohen


Rhodococcus equi pneumonia remains an important cause of disease and death in foals at breeding farms. Although treating affected foals is necessary and compelling, the burden of infectious diseases is most effectively reduced by methods for control and prevention. Understanding the epidemiology of R. equi infection is vital to the control and prevention of associated disease, yet our understanding remains rudimentary. A complete review of all aspects of the epidemiology of R. equi is beyond the scope of this chapter, but some key aspects of clinical importance are considered.



PREVALENCE


Disease caused by R. equi is uncommon among adult horses, although serologic and ecologic evidence indicates that exposure is widespread. Although pneumonia and infection in sites other than the lung appear to be primarily problems of foals, there are few descriptions of the prevalence or incidence of R. equi pneumonia in that population. The disease appears to be absent from some breeding farms, but at others, hereinafter referred to as endemic farms, it may occur either sporadically or on a regular, recurrent basis. Because the burden of disease is often greatest on endemic farms, summary statistics must be farm-specific. Another complication in estimating the frequency of occurrence of disease is variability in diagnostic criteria and practices. Because diagnostic procedures are invasive and are labor intensive in young foals, many veterinarians eschew performing transendoscopic or transcutaneous tracheobronchial aspiration to obtain a sample for definitive diagnosis of R. equi. Thus, estimated prevalences of R. equi pneumonia are imprecise. Furthermore, the sensitive identification of subclinical or early infections by use of ultrasonography has also likely resulted in an increased prevalence of presumptively diagnosed disease.


These caveats should be considered when reviewing the data from the few studies from Australia and the United States that have reported the proportion of affected foals at more than a handful of breeding farms. Generally, the proportion of foals with clinical signs of R. equi pneumonia ranges from 7% to 14%, but proportions of more than 20% are not unusual. Cumulative evidence is considerably higher when ultrasonography is used to screen for pulmonary lesions that are then attributed to R. equi. In farm-based studies, case fatality rates generally range from 10% to 30% but are lower when ultrasonographic or other screening methods are employed for earlier detection of disease. To my knowledge, there are no reports on the economic impact of this disease.



AGENT, ENVIRONMENT, AND HOST


The epidemiology of an infectious disease is generally determined by the roles of disease agent, environment, and host, as well interactions among these factors. With respect to the agent, disease is associated with R. equi isolates carrying an 85- to 95-kb plasmid that includes a pathogenicity island whose gene products include the virulence-associated protein A (referred to as vapA). Although there is some regional variation in the genotypes of plasmids, molecular epidemiologic studies of chromosomal DNA conducted to date show marked variation among isolates by geographic region and year. Isolates obtained from affected foals or soil samples of endemic farms from the same year can appear molecularly different, indicating that infection with a particular strain does not explain recurrences or the endemic infections.


The presence of virulent isolates of R. equi appears to be widespread in the environment of foals, and virulent organisms can be isolated from the soil of farms regardless of their history of R. equi pneumonia. Although some studies have demonstrated higher concentrations of virulent R. equi

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May 28, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Epidemiology of Rhodococcus equi Pneumonia in Foals

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