Rhodococcus equi

Test codes:
B0026 - Ultrasensitive qualitative detection of Rhodococcus equi by real time polymerase chain reaction
P0013 - Equine respiratory panel (includes Rhodococcus equi and other pathogens)

Rhodococcus equi is a gram-positive, pleomorphic coccobacillus that is a frequent cause of pneumonia and enteritis in foals, especially before 6 months of age. It has also been linked to a variety of suppurative processes in immune-suppressed humans (Prescott, 1991). The organism has a worldwide distribution and can easily be isolated from soil and environmental samples (Barton and Hughes, 1984; Debey and Bailie, 1987; Takai et al., 1991). Pathogenic R. equi isolated from sick foals uniformly contain an 85- to 90-kb plasmid known as vapA, which carries a gene responsible for expression of a 15- to 17-kDa antigen of undetermined function (Takai et al., 1991, 1993). Environmental strains of R. equi not associated with equine disease do not contain this plasmid.

The onset of clinical signs of R. equi pneumonia in foals is often subtle, and the infection is usually not recognized until severe abscessation has occurred. By that time, prognosis is poor. Culture of the organism from tracheal wash was previously considered the "gold standard" for diagnosis. However, it can be difficult to reliably culture R. equi from a tracheal wash sample, possibly because of prior antibiotic administration or the presence of multiple pathogenic bacterial species. A recent study has reported that only 62% of horses with positive R. equi cultures at necropsy, and 64% with radiographic evidence of lung abscessation, yielded R. equi on culture of tracheal wash (Hillidge, 1987), indicating an unacceptably high false negative rate using culture alone.

PCR detection of R. equi can offer rapid detection of this pathogenic bacterium. A recent study has shown that PCR of tracheal wash using primers that recognized the vapA virulence plasmid of R. equi has a diagnostic sensitivity of 100% and specificity of 90.6%. In contrast, sensitivity and specificity are only 57.1% and 93.8%, respectively, for standard microbiologic culture of tracheal wash and only 62.5% and 75.9%, respectively, for serology (Sellon et al, 2001).

Thus, PCR detection of R. equi in tracheal wash fluid is more sensitive and specific for diagnosis of R. equi pneumonia than are other available diagnostic tests. This PCR assay detects only pathogenic strains of Rhodococcus equi; non-pathogenic environmental strains are not detected.

Utilities:

• Confirm the disease causing agent
• Shorten the time required to confirm a clinical diagnosis of R. equi infection.
• Ensure that horse populations are free of R. equi
• Early prevention of spread of this bacterium
• Minimize personnel exposure to this bacterium
• Safety monitoring of biological products that derive from horses

References:
Barton, M. D., and Hughes, K.L. (1984) Ecology of Rhodococcus equi. Vet. Microbiol. 9:65-76.
Debey, M. C., and Bailie, W.E. (1987) Rhodococcus equi in fecal and environmental samples from Kansas horse farms. Vet. Microbiol. 14:251-257.
Hillidge, C. J. (1987) Use of erythromycin-rifampin combination in treatment of Rhodococcus equi pneumonia. Vet. Microbiol. 14:215-224.
Prescott, J. F. (1991) Rhodococcus equi: an animal and human pathogen. Clin. Microbiol. Rev. 4:20-34.
Sellon, D.C., Besser, T.E., Vivrette, S.L. and McConnico, R.S. (2001) Comparison of nucleic acid amplification, serology, and microbiologic culture for diagnosis of Rhodococcus equi pneumonia in foals. J. Clin. Microbiol. 39:1289-1293.
Takai, S., Ohbushi, S., Koike, K., Tsubaki, S., Oishi, H. and Kamada, M. (1991) 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. 29:2887-2889.
Takai, S., Sekizaki, T., Ozawa, T., Sugawara, T., Watanabe, Y. and Tsubaki, S. (1991) Association between a large plasmid and 15- to 17-kilodalton antigens in virulent Rhodococcus equi. Infect. Immun. 59:4056-4060.
Takai, S., Watanabe, Y., Ikeda, T., Ozawa, T., Matsukura, S., Tamada, Y., Tsubaki, S. and Sekizaki, T. (1993) Virulence-associated plasmids in Rhodococcus equi. J. Clin. Microbiol. 31:1726-1729.

Specimen requirements: Nasopharyngeal swab, or 1 ml tracheal wash, or 1 ml whole blood in EDTA (purple top) or ACD (yellow top) tube, or 1 ml feces, shipped overnight at room temperature.

For specimen types other than those listed here, please call to confirm specimen acceptability and shipping instructions.

For all specimen types, if there will be a delay in shipping, or during very warm weather, refrigerate specimens until shipped and ship with a cold pack unless more stringent shipping requirements are specified. Frozen specimens should be shipped so as to remain frozen in transit. See shipping instructions for more information.

Turnaround time: 2 business days

Methodology:
B0026 - Qualitative real time PCR
P0013 - Qualitative PCR, qualitative reverse transcription PCR and qualitative real time PCR

Normal range: Nondetected