Streptococcus equi subsp. equi in Nasal Secretions 

Researchers found that vaccination against S. equi was associated with a lower frequency of qPCR-positive cases.
A veterinarian preparing to vaccinate a horse against Streptococcus equi
Researchers found that vaccination against Streptococcus equi was associated with a lower frequency of qPCR-positive cases. | Getty Images

Streptococcus equi is a highly infectious bacterial respiratory infection that can significantly impact a herd of horses with morbidity and forestall owner/rider participation in events and clinics off the premises. An evaluation of horses with acute onset fever and respiratory illness reviewed the effect of Streptococcus equi vaccination on qPCR status [Jaramillo-Morales, C.; James, K.; Barnum, S.; Vaala, W.; et al. Voluntary Biosurveillance of Streptococcus equi subsp. equi in Nasal Scretions of 9409 Equids with Upper Airway Infection in the USA. Veterinary Sciences 2023, 10, 78; https://doi.org/10.3390/vetsci10020078]. 

The authors noted that many owners forewent immunization with S. equi vaccines due to potential side effects such as purpura hemorrhagica or localized abscesses. The study described the prevalence of Streptococcus equi in nasal secretions of 9,409 horses with acute respiratory disease, and to determine the impact of S. equi vaccine on diagnostic identification of the bacteria. Horses were included in the voluntary surveillance program from March 2008 to December 2020 across 261 equine veterinary practices throughout the United States.  

A full physical exam and nasal swabs for PCR analysis were obtained in horses aged 3 months to 32 years. Guttural pouches were not sampled. Participating veterinarians answered a questionnaire about signalment, intended use, transport history, number of affected horses, clinical signs and S. equi vaccine history. Horses testing positive for S. equi had clinical signs of nasal discharge, fever, lethargy, anorexia, cough and ocular discharge. 

The results: 

  • qPCR positivity for Streptococcus equi occurred in 7.6% (n = 715) of 9409 horses. 
  • Nasal swab testing detected other respiratory infections: EHV-4 in 10.5%; EIV in 9.7%; ERBV (equine rhinitis B) in 3.3%; EHV-1 in 1.6%; and ERAV in 0.1%. 
  • Horses with single infections tested positive for S. equi 6.6% of the time. 
  • Horses with co-infections tested positive for S. equi 9.4% of the time. 
  • Horses less than 1 year old tested positive for S. equi less than other ages. 
  • Horses aged 5-9 years were more likely to test positive for S. equi compared to horses aged 10-14 and >20 years of age. 
  • Competition and farm/ranch horses had greater likelihood of testing positive. 
  • Of the 26% of horses that were transported within an 8-week period, a little more than 10% of those tested positive for S. equi, suggesting that transportation history did not significantly impact positive S. equi results. 

Geographic location had no effect on the percentage of horses testing positive for S. equi. Season showed a somewhat different disparity for qPCR-positive testing—more horses tested S. equi positive in winter/spring (27%) than in summer/fall (16%). 

Regarding vaccination status, 9% were immunized against Streptococcus equi, and of those, 9% tested positive for S. equi. Those immunized against S. equi were less likely to test positive in their nasal secretions. Because there are so many types and administration routes of S. equi vaccines, the authors could not form a conclusion on efficacy of vaccine type. Approximately one-third of the horses were immunized against EHV-1/4 and EIV, yet there was no statistical difference with those horses and positive testing for S. equi

In summary, the authors state that vaccination against Streptococcus equi was associated with a lower frequency of S. equi qPCR-positive cases.

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