Associations between the Ecology of Virulent Rhodococcus equi and the Epidemiology of R. equi Pneumonia on Australian Thoroughbred Farms

Abstract

The ecology of virulent strains of Rhodococcus equi on horse farms is likely to influence the prevalence and severity of R. equi pneumonia in foals. This study examined the association between the ecology of virulent R. equi and the epidemiology of R. equi pneumonia by collecting air and soil samples over two breeding seasons (28 farm-year combinations) on Thoroughbred breeding farms with different reported prevalences of R. equi pneumonia. Colony blotting and DNA hybridization were used to detect and measure concentrations of virulent R. equi. The prevalence of R. equi pneumonia was associated with the airborne burden of virulent R. equi (both the concentration and the proportion of R. equi bacteria that were virulent) but was not associated with the burden of virulent R. equi in the soil. Univariable screening and multivariable model building were used to evaluate the effect of environmental and management factors on virulent R. equi burdens. Lower soil moisture concentrations and lower pasture heights were significantly associated with elevated airborne concentrations of virulent R. equi, as were the holding pens and lanes, which typically were sandy, dry, and devoid of pasture cover. Few variables appeared to influence concentrations of virulent R. equi in soil. Acidic soil conditions may have contributed to an elevated proportion of virulent strains within the R. equi population. Environmental management strategies that aim to reduce the level of exposure of susceptible foals to airborne virulent R. equi are most likely to reduce the impact of R. equi pneumonia on endemically affected farms.

Different bacterial strategies to degrade taurocholate

“…However, the 16S-rRNA gene sequence of strain TAC-A1, with physiological data ( Table 1), indicated that the potential equine pathogen R. equi (100% sequence identity with the type strain) had been isolated. These strains, from arable soil, seemingly lacked the major marker for pathogenicity (vapA, assayed by PCR), which is typical of soil isolates (Lührmann et al 2004;Muscatello et al 2006), but they were still unsuitable for research in a non-veterinary laboratory. Isolate TAC-K3 (Table 1), from activated sludge, potentially represented the same metabolic trait (see below), and was identiWed as Pseudomonas sp.…”

Air Pollution and Domestic Animals

“…Expression of the first occurs when R. equi is cultured at 37 °C, but not at 30° C. Thus it is plausible that the majority of cases of R. equi pneumonia are seen during the summer months. The prevalence of R. equi pneumonia is further associated with the airborne burden of virulent R. equi, but unexpectedly it seems not directly to be associated with the burden of virulent R. equi in the soil (Muscatello et al, 2006). Only under special conditions of the soil, the virulent organisms may be a thread to foals.…”

“…Dry soil and little grass and holding pens and lanes which are sandy, dry, and lack sufficient grass cover are associated with elevated airborne concentrations of virulent R. equi. Hence, Muscatello et al (2006) consider that environmental management strategies aiming to reduce the level of exposure of susceptible foals to airborne virulent R. equi likely will reduce the impact of R. equi pneumonia on endemically affected farms. If contaminated dust is inhaled by foals of less than 5 month, pulmonary abscesses will develop (Fig.…”

Genetic Susceptibility to Rhodococcus equi

“…Many different strains of virulent (and avirulent) R. equi have been shown to be present in a common environment (ie, the same farm), and multiple genotypic virulent strains may exist even in an individual foal with R. equi pneumonia 5, 9, 10, 11. Although exposure to R. equi is widespread in farms where foals are affected, only a variable proportion of the foals will develop clinical disease at a given farm, whereas other foals at the same location will not develop disease 12, 13. In addition, anecdotal reports by veterinarians indicate that some mares recurrently have affected foals, whereas other mares from the same environment consistently have foals that do not develop R. equi pneumonia.…”