Does early antibiotic use in horses with ‘strangles’ cause metastatic Streptococcus equi bacterial infections?

Streptococcus equi Subspecies equi

Strangles, caused by the bacteria Streptococcus equi subsp equi, is a highly contagious disease of equids classically characterized by a high fever and enlarged lymph nodes of the head. Diagnostic sampling depends on the stage of the disease. The goal of treating strangles is to control transmission and to eliminate infection while providing future host immunity. Daily temperature checking and isolation of febrile horses is the key to controlling outbreaks. Eradication of this disease will not be possible until S equi carriers are eliminated from the equine population.

Antibiotic resistance in bacteria associated with equine respiratory disease in the United Kingdom

INTRODUCTION

Respiratory diseases account for the highest number of clinical problems in horses compared with other body systems. While microbiological culture and sensitivity testing is essential for certain cases, knowledge of the most likely bacterial agents and their susceptibilities is necessary to inform empirical antibiotic choices.

METHODS

A retrospective study of microbiological and cytological results from upper and lower respiratory samples (n=615) processed in a commercial laboratory between 2002 and 2012 was carried out. A further study of lower respiratory samples from horses with clinical signs of lower respiratory disease from May to June 2012 was undertaken.

RESULTS

Both studies revealed Streptococcus equi subspecies zooepidemicus, Pseudomonas aeruginosa, Pasteurella species, Escherichia coli and Bordetella bronchiseptica as the most frequently isolated species. S equi subspecies zooepidemicus and subspecies equi were susceptible to ceftiofur (100 per cent) and erythromycin (99 per cent). Resistance to penicillin (12.5 per cent of S equi subspecies equi from upper respiratory tract samples) and tetracycline (62.7 per cent) was also detected. Gram-negative isolates showed resistance to gentamicin, trimethoprim-sulfamethoxazole and tetracycline but susceptibility to enrofloxacin (except Pseudomonas species, where 46.2 per cent were resistant). Multiple drug resistance was detected in 1 per cent of isolates.

CONCLUSION

Resistance to first-choice antibiotics in common equine respiratory tract bacteria was noted and warrants continued monitoring of their susceptibility profiles. This can provide information to clinicians about the best empirical antimicrobial choices against certain pathogenic bacteria and help guide antibiotic stewardship efforts to converse their efficacy.

Streptococcus equi Infections in Horses: Guidelines for Treatment, Control, and Prevention of Strangles-Revised Consensus Statement

This consensus statement update reflects our current published knowledge and opinion about clinical signs, pathogenesis, epidemiology, treatment, complications, and control of strangles. This updated statement emphasizes varying presentations in the context of existing underlying immunity and carrier states of strangles in the transmission of disease. The statement redefines the “gold standard” for detection of possible infection and reviews the new technologies available in polymerase chain reaction diagnosis and serology and their use in outbreak control and prevention. We reiterate the importance of judicious use of antibiotics in horses with strangles. This updated consensus statement reviews current vaccine technology and the importance of linking vaccination with currently advocated disease control and prevention programs to facilitate the eradication of endemic infections while safely maintaining herd immunity. Differentiation between immune responses to primary and repeated exposure of subclinically infected animals and responses induced by vaccination is also addressed.

Predictor variables for and complications associated with Streptococcus equi subsp equi infection in horses

OBJECTIVE

To evaluate predictor variables for and complications associated with Streptococcus equi subsp equi infection (strangles) in horses.

DESIGN

Retrospective case-control study.

ANIMALS

108 horses with strangles (cases) and 215 horses without strangles (controls).

PROCEDURES

Medical records from January 2005 through July 2012 were reviewed. Cases were defined as horses with clinical signs of strangles (pyrexia, retropharyngeal lymphadenopathy, and mucopurulent nasal discharge) that were associated with a confirmed strangles outbreak or had positive results for S equi on PCR assay or bacteriologic culture. Controls were defined as horses with pyrexia that did not meet the other criteria for cases. Data compared between cases and controls included signalment, clinical signs, diagnostic test results, and disease complications and outcome. Logistic regression was used to identify variables associated with strangles and its complications.

RESULTS

Clinical signs of strangles were not evident in 12 of 25 cases classified as S equi carriers (infected > 40 days). Predictor variables associated with strangles included mucopurulent nasal discharge and external abscesses in the pharyngeal region. Strangles was more likely to be diagnosed in the spring than in the summer. Cases with anemia were more likely to develop purpura hemorrhagica than were cases without anemia. No risk factors were identified for the development of guttural pouch empyema or metastatic abscesses.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that not all horses infected with S equi develop clinical signs of strangles. We recommend that guttural pouch endoscopy and lavage with PCR assay of lavage fluid samples be performed to identify S equi carrier horses.