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To evaluate the ability of nucleic acid amplification techniques to detect Rhodococcus equi in equine buffy coat, blood, and tracheal wash fluid and to differentiate between virulent and avirulent strains of the bacteria.

Sample Population

Blood anticoagulated with EDTA and tracheal wash fluid from healthy horses.


Logarithmic dilutions of virulent and avirulent strains of R equi were added to equine buffy coat and tracheal wash fluid samples. The DNA was extracted and amplified by polymerase chain reaction (PCR), using primers specific for the 16S ribosomal subunit gene and the virulence plasmid of R equi.


PCR with 16S ribosomal subunit primers amplified a 441-bp segment of DNA from virulent and avirulent strains of R equi, but not from samples containing other species of bacteria. The virulence plasmid primers amplified an 875-bp segment of DNA from virulent strains of R equi, but not from avirulent R equi, or from other species of bacteria. Virulent strains of R equi could be identified by PCR and differentiated from avirulent strains within 12 to 24 hours after sample collection, with as few as 10 to 100 organisms present.


PCR can be used to rapidly and accurately identify R equi in equine blood and tracheal wash fluid samples and can differentiate between virulent and avirulent strains of the organism.

Clinical Relevance

Because PCR can confirm a diagnosis of R equi infection in horses more rapidly and specifically than use of standard culture techniques, extrapolation of this assay to soil and fecal samples could be useful in epidemiologic studies and studies of environmental disinfection or decontamination. (Am J Vet Res 1997;58:1232–1237)

Free access
in American Journal of Veterinary Research


OBJECTIVE To describe the antimicrobial resistance patterns of Salmonella isolates obtained from horses in the northeastern United States and to identify trends in resistance to select antimicrobials over time.

SAMPLE 462 Salmonella isolates from horses.

PROCEDURES Retrospective data were collected for all Salmonella isolates obtained from equine specimens that were submitted to the Cornell University Animal Health Diagnostic Center between January 1, 2001, and December 31, 2013. Temporal trends in the prevalence of resistant Salmonella isolates were investigated for each of 13 antimicrobials by use of the Cochran-Armitage trend test.

RESULTS The prevalence of resistant isolates varied among antimicrobials and ranged from 0% (imipenem) to 51.5% (chloramphenicol). During the observation period, the prevalence of resistant isolates decreased significantly for amoxicillin—clavulanic acid, ampicillin, cefazolin, cefoxitin, ceftiofur, chloramphenicol, and tetracycline and remained negligible for amikacin and enrofloxacin. Of the 337 isolates for which the susceptibility to all 13 antimicrobials was determined, 138 (40.9%) were pansusceptible and 192 (57.0%) were multidrug resistant (resistant to ≥ 3 antimicrobial classes). The most common serovar isolated was Salmonella Newport, and although the annual prevalence of that serovar decreased significantly over time, that decrease had only a minimal effect on the observed antimicrobial resistance trends.

CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that current antimicrobial use in horses is not promoting the emergence and dissemination of antimicrobial-resistant Salmonella strains in the region served by the laboratory.

Full access
in American Journal of Veterinary Research



To investigate (1) variables associated with the likelihood of obtaining a positive culture, (2) commonly isolated microorganisms, and (3) antimicrobial resistance patterns of isolates from horses with presumptive synovial sepsis.


Synovial fluid, synovium, and bone samples from equine cases with presumptive synovial sepsis submitted to the Cornell University Animal Health Diagnostic Center from 2000 to 2020 for microbial culture and antimicrobial sensitivity testing.


Univariable and multivariable analyses were performed to determine the effect of variables on the likelihood of positive culture. Frequency distributions for isolated organisms and antimicrobial resistance were generated. Multidrug resistance patterns and associations were assessed with association rule mining.


The positive culture rate for all samples was 37.4%, while the positive culture rate among samples confirmed to be septic by a combination of clinical pathological variables and case details was 43%. Blood culture vial submissions were 1.7 times more likely to yield a positive culture compared to samples submitted in a serum tube. Structure sampled, tissue submitted, and horse age were associated with a positive culture. Staphylococcus spp (23.7%), Streptococcus spp (22.4%), and Enterococcus spp (9.67%) were commonly isolated. Multidrug resistance prevalence decreased from 92% (2000 to 2009) to 76% (2010 to 2020) of gram-negative isolates and 60% (2000 to 2009) to 52% (2010 to 2020) of gram-positive isolates.


The positive culture rate from synovial fluid submissions with traditional sampling and culture methods remains low and may be optimized by submitting samples in blood culture vials. Overall, antimicrobial resistance was frequently observed but did not increase from the first to second decade for most genera.

Open access
in American Journal of Veterinary Research