The epidemiology of R. equi remains ill-defined and continues to evolve. Two epidemiologic questions of clinical importance regarding Rhodococcus equi foal pneumonia are "Why are some foals affected while others in the same environment remain unaffected?", and "Why does the disease occur recurrently at some farms but not at others?". To answer these questions, studies performed at the level of the foal and farm, respectively, are needed.
The epidemiology of R. equi remains ill-defined and continues to evolve. Two epidemiologic questions of clinical importance regarding Rhodococcus equi foal pneumonia are "Why are some foals affected while others in the same environment remain unaffected?", and "Why does the disease occur recurrently at some farms but not at others?". To answer these questions, studies performed at the level of the foal and farm, respectively, are needed. The evidence from foal-level and farm-level studies will be reviewed in this presentation.
Rhodococcus equi is widespread in the environment of mares and foals. This is reflected by high seroprevalence, and the fact that R. equi can be isolated from feces, soil, air, and feed at horse breeding farms. Consequently, most (if not all) foals are exposed to virulent organisms, yet only some foals develop disease. These findings imply that some foals are more susceptible to infection than others, are exposed to higher concentrations of virulent organisms, or both. One study has examined fecal concentrations of virulent R. equi of dams in relation to subsequent disease in their foals. Although the fecal concentrations of individual mares did not explain the occurrence of disease in their particular foals, the finding that all mares at a breeding farm were shedding virulent R. equi in their feces during the periparturient period indicates that mares are a source of virulent R. equi for the environment into which foals are born. A subsequent study has demonstrated that the high prevalence of fecal shedding of virulent R. equi can be generalized to other breeding farms in central Kentucky. To date, only 1 study has specifically attempted to address the association between signalment, history, and management factors for foals and R. equi pneumonia. That study identified significant differences in disease incidence between farms and between years, but no foal-level factors were significantly associated with the disease.
Anecdotally, some mares are described as being more likely to have affected foals. These anecdotes along with aforementioned evidence that some foals are predisposed to developing disease suggest the possibility of genetic contributions to susceptibility to R. equi pneumonia. Polymorphisms in microsatellites, and the transferrin, IL1-receptor, and NRAMP1 genes have been associated with susceptibility to R. equi pneumonia, but the strength of these associations have not been compellingly strong. Mice lacking the galectin-3 gene are more resistant to R. equi infection than mice with that gene. Thus, genetic factors influence host susceptibility to R. equi, but the trait is likely complex and polygenic in nature, and is undoubtedly modulated by environmental factors such as the density of mares and foals at farms.
Both innate and adaptive immune responses of the host play a critical role in the pathogenesis of R. equi pneumonia. The concentrations of neutrophils (cells that play a key role in innate immune responses) and the ratio of CD4+:CD8+ lymphocytes (reflective of adaptive immunity) were significantly lower at 2 weeks of age among foals that subsequently developed R. equi pneumonia than among age-matched controls from the same environment that failed to develop the disease.
In summary, few studies have examined foal-level risk factors, and these studies have not yielded strong evidence for any particular causal factor(s).
A number of recent studies have documented that the presence or concentration of R. equi in soil fails to explain the incidence of R. equi at breeding farms. There is great diversity in the genotypes (DNA "fingerprints") of isolates within and among farms. To date, evidence that soil geochemistry influences the risk of R. equi pneumonia has not been reported.2 The proportion of virulent isolates of R. equi in soil at breeding farms in central Kentucky was increased later in the breeding/foaling season. Airborne concentrations of R. equi have been correlated with the incidence of R. equi pneumonia in Australia. Factors that were associated with increased airborne concentration of R. equi included: site (holding pens/lanes relative to paddocks); warmer ambient temperature; less soil moisture; reduced grass height; and, later date during the breeding/foaling season. One needs to be careful in differentiating significant statistical associations with causal associations in environmental/ecological studies of R. equi. Causes must precede effects, and sorting out whether factors associated with disease preceded infection or disease development can be very difficult because of the insidious nature of R. equi pneumonia.
Some evidence exists that foaling in pasture may reduce the risk of R. equi pneumonia.5 The density of mares and foals appears to be positively correlated with incidence of the disease.2,3,5 The disease appears to be associated with well managed farms that use management practices generally deemed to be desirable for preventing infectious diseases of foals.2 This association is not likely causal, but does indicate that practices effective for preventing other infectious diseases of neonates are of limited benefit against R. equi.
SUMMARY: Relatively few studies have addressed the fundamental epidemiological issues that are of greatest clinical importance regarding R. equi pneumonia in foals. Much work remains to be done to elucidate these factors. Our laboratory continues to seek support support and resources – including veterinarians and farms willing to collaborate on projects - to address these aims.
Chaffin MK, Cohen ND, Martens RJ. Foal-related risk factors associated with development of Rhodococcus equi pneumonia on farms with endemic infection. J Am Vet Med Assoc 2003;222:476-485.
Chaffin MK, Cohen ND, Martens RJ. Evaluation of equine breeding farm characteristics as risk factors for development of Rhodococcus equi pneumonia in foals. J Am Vet Med Assoc 2003;222:467-475.
Cohen ND, Carter CN, Scott HM, et al. Association of soil concentrations of Rhodococcus equi and incidence of pneumonia attributable to Rhodococcus equi in foals on farms in central Kentucky. Am J Vet Res 2008 69; 385-95.
Cohen ND, Chaffin MK, Martens RJ. Control and prevention of Rhodococcus equi pneumonia in foals. Compend Contin Educ Pract Vet 2000; 22:1062-1070.
Cohen ND, O'Conor MS, Chaffin MK, Martens RJ. Farm characteristics and management practices associated with development of Rhodococcus equi pneumonia in foals. J Am Vet Med Assoc 2005;226:404-413.
Cohen ND, Smith KE, Ficht TA, et al. Epidemiologic study of results of pulsed-field gel electrophoresis of isolates of Rhodococcus equi obtained from horses and horse farms. Am J Vet Res 2003;64:153-16.
Ferraz LC, Bernardes ES, Oliveira AF, et al. Lack of galectin-3 alters the balance of innate immune cytokines and confers resistance to Rhodococcus equi infection. Eur J Immunol 2008;38:2762-2775.
Giguere S, Prescott JF. Clinical manifestations, diagnosis, treatment, and prevention of Rhodococcus equi infections in foals. Vet Microbiol 1997; 56:313-334.
Grimm MB, Cohen ND, Slovis NM, et al. Evaluation of fecal samples from mares as a source of Rhodococcus equi for their foals by use of a quantitative bacteriologic culture and colony immunoblot analyses. Am J Vet Res 2007;68:63-71.
Halbert ND, Cohen ND, Slovis NM, et al. Variations in equid SLC11A1 (NRAMP1) genes and associations with Rhodococcus equi pneumonia in horses. J Vet Int Med 2006;20:974-979.
Horin P, Smola J, Matiasovic J, et al. Polymorphisms in equine immune response genes and their associations with infections. Mamm Genome 2004;15:843–850.
Horin P, Osickova J, Necesankova M, Matiasovic J, et al. Single nucleotide polymorphisms of interleukin-1 beta related genes and their associations with infection in the horse. Dev Biol (Basel) 2008;132:347-351.
Horowitz ML, Cohen ND, Takai S, et al. Application of Sartwell's model (lognormal distribution of incubation periods) to age at onset and age at death of foals with Rhodococcus equi pneumonia as evidence of perinatal infection. J Vet Intern Med 2001;15:171-175.
Lavoie JP, Fiset L, Laverty S. Review of 40 cases of lung abscesses in foals and adult horses. Equine Vet J 1994;26:348–352.
Malschitzky E, Neves AP, Gregory RM, et al. Reduzir o uso da cocheira a incidencia de infeccoes por Rhodococcus equi em potros. A Hora Veterinaria 2005;24:27-30.
Martens RJ, Takai S, Cohen ND, et al. Association of disease with isolation and virulence of Rhodococcus equi from farm soil and foals with pneumonia. J Am Vet Med Assoc 2000;217:220–225.
Morton AC, Baseggio N, Peters MA, et al. Diversity of isolates of Rhodococcus equi from Australian thoroughbred horse farms. Antonie van Leeuwenhoek 1998;74:21–25.
Morton AC, Begg AP, Anderson GA, et al. Epidemiology of Rhodococcus equi strains on Thoroughbred horse farms. Appl Environ Microbiol 2001;67:2167–2175.
Mousel MR, Harrison L, Donahue JM, Bailey E. Rhodococcus equi and genetic susceptibility: assessing transferrin genotypes from paraffin-embedded tissues. J Vet Diagn Invest 2003;15:470-472.
Muscatello G, Anderson GA, Gilkerson JR, Browning GF. Associations between the ecology of virulent Rhodococcus equi and the epidemiology of R. equi pneumonia on Australian thoroughbred farms. Appl Environ Microbiol 2006; 72:6152-6160.
Muscatello G, Gerbaud S, Kennedy C, et al. Comparison of concentrations of Rhodococcus equi and virulent R. equi in air of stables and paddocks on horse breeding farms in a temperate climate. Equine Vet J 2006;38:263-5.
Takai S, Ohbushi S, Koike K, et al. Prevalence of virulent Rhodococcus equi in isolates from soil and feces of horses from horse-breeding farms with and without endemic infections. J Clin Microbiol 1991;29:2887–2889.
Takai S. Epidemiology of Rhodococcus equi infections: a review. Vet Microbiol 1997; 56:167-176.
Venner M, Meyer-Hamme B, Vespohl J, et al. Genotypic characterization of VapA positive Rhodococcus equi in foals with pulmonary affection and their soil environment on a warmblood horse breeding farm in Germany. Res Vet Sci 2007;83:311-317.
Buntain S, Carter C, Kuskie K, Smith JS, Stepusin R, Chaffin MK, Takai S, Cohen ND. Frequency of Rhodococcus equi in feces of mares in central Kentucky. J Equine Vet Sci 2010;30(4):191-195.
Chaffin MK, Cohen ND, Martens RJ, Edwards RF, Nevill M, Smith R 3rd. Hematologic and immunophenotypic factors associated with development of Rhodococcus equi pneumonia of foals at equine breeding farms with endemic infection. Vet Immunol Immunopath 2004;100:33-48.
Martens RJ, Cohen ND, Chaffin MK, Waskom JS. Association of Rhodoccus equi foal pneumonia with farm soil geochemistry. Am J Vet Res 2002;63:95-98.
Chaffin MK, Cohen ND, Martens RJ. Evaluation of equine breeding farm management and preventative health practices as risk factors for development of Rhodococcus equi pneumonia in foals. J Am Vet Med Assoc 2003;222:476-484.