Objective—To evaluate a real-time quantitative polymerase
chain reaction (QPCR) assay in the detection
and quantitation of virulent Rhodococcus equi.
Sample Population—1 virulent, 2 intermediately virulent,
and 2 avirulent strains of R equi and 16 isolates
of bacteria genetically related to R equi.
Procedure—The QPCR assay was evaluated for
detection and quantitation of the virulence-associated
gene (vapA) of R equi in pure culture and in samples
of tracheobronchial fluid, which were inoculated with
known numbers of virulent R equi. Results were compared
with those derived via quantitative microbial
culture and standard polymerase chain reaction
Results—The QPCR assay detected the vapAgene in
pure culture of R equi and in tracheobronchial fluid
samples that contained as few as 20 CFUs of virulent
R equi/mL and accurately quantitated virulent R equi
to 103 CFUs/mL of fluid. The assay was highly specific
for detection of the vapA gene of virulent R equi
and was more sensitive than standard polymerase
chain reaction for detection of R equi in tracheobronchial
Conclusions and Clinical Relevance—The QPCR
assay appears to be a rapid and reliable method for
detecting and quantitating virulent R equi. The accuracy
of the QPCR assay is comparable to that of
quantitative microbial culture. The increased sensitivity
of the QPCR method in detection of virulent
R equi should facilitate rapid and accurate diagnosis
of R equi pneumonia in foals. (Am J Vet Res
Objective—To determine the pharmacokinetics of gallium maltolate (GaM) after intragastric administration in healthy foals.
Animals—6 healthy neonatal foals.
Procedures—Each foal received GaM (20 mg/kg) by intragastric administration. Blood samples were obtained before (time 0) and at 0.25, 0.5, 1, 2, 4, 8, 12, 24, 36, and 48 hours after GaM administration for determination of serum gallium concentrations by use of inductively coupled plasma mass spectroscopy.
Results—Mean ± SD pharmacokinetic variables were as follows: peak serum gallium concentration, 1,079 ± 311 ng/mL; time to peak serum concentration, 4.3 ± 2.0 hours; area under the serum concentration versus time curve, 40,215 ± 8,420 ng/mL/h; mean residence time, 39.5 ± 17.2 hours; area under the moment curve, 1,636,554 ± 931,458 ng([h]2/mL); and terminal half-life, 26.6 ± 11.6 hours. The mean serum concentration of gallium at 12 hours was 756 ± 195 ng/mL.
Conclusions and Clinical Relevance—Gallium maltolate administered via nasogastric tube at a dose of 20 mg/kg to neonatal foals resulted in gallium serum concentrations considered sufficient to suppress growth or kill Rhodococcus equi in macrophages and other infected tissues.
Objective—To determine whether mares are a clinically important source of Rhodococcus equi for their foals.
Sample Population—171 mares and 171 foals from a farm in Kentucky (evaluated during 2004 and 2005).
Procedures—At 4 time points (2 before and 2 after parturition), the total concentration of R equi and concentration of virulent R equi were determined in fecal specimens from mares by use of quantitative bacteriologic culture and a colony immunoblot technique, respectively. These concentrations for mares of foals that developed R equi–associated pneumonia and for mares with unaffected foals were compared. Data for each year were analyzed separately.
Results—R equi–associated pneumonia developed in 53 of 171 (31%) foals. Fecal shedding of virulent R equi was detected in at least 1 time point for every mare; bacteriologic culture results were positive for 62 of 171 (36%) mares at all time points. However, compared with dams of unaffected foals, fecal concentrations of total or virulent R equi in dams of foals with R equi–associated pneumonia were not significantly different.
Conclusions and Clinical Relevance—Results indicate that dams of foals with R equi–associated pneumonia did not shed more R equi in feces than dams of unaffected foals; therefore, R equi infection in foals was not associated with comparatively greater fecal shedding by their dams. However, detection of virulent R equi in the feces of all mares during at least 1 time point suggests that mares can be an important source of R equi for the surrounding environment.
Objective—To determine the importance of iron for
in vitro growth of Rhodococcus equi, define potential
iron sources in the environment and mechanisms by
which R equi may obtain iron from the environment,
and assess expression and immunogenicity of
Sample Population—10 virulent and 11 avirulent
strains of R equi.
Procedure—In vitro growth rates and protein patterns
of R equi propagated in media with normal,
excess, or limited amounts of available iron were
compared. Immunoblot analyses that used serum
from foals naturally infected with R equi and monoclonal
antibody against virulence-associated protein
(Vap)A were conducted to determine immunogenicity
and identity of expressed proteins.
Results—Excess iron did not alter growth of any
R equi strains, whereas growth of all strains was significantly
decreased in response to limited amounts
of available iron. Virulent R equi were able to use iron
from ferrated deferoxamine, bovine transferrin, and
bovine lactoferrin. Only virulent R equi expressed an
iron-regulated, immunogenic, surface-associated protein
identified as VapA.
Conclusions and Clinical Relevance—Iron is
required for the growth and survival of R equi.
Sources of iron for R equi, and mechanisms by which
R equi acquire iron in vivo, may represent important
virulence factors and novel targets for the development
of therapeutic and immunoprophylactic strategies
to control R equi infection in foals. Expression of
VapA is substantially upregulated when there is a limited
amount of available iron. (Am J Vet Res 2003;64:1337–1346)