Voluntary Biosurveillance of Streptococcus equi Subsp. equi in Nasal Secretions of 9409 Equids with Upper Airway Infection in the USA

SIMPLE SUMMARY: Strangles caused by Streptococcus equi subsp. equi (S. equi) is considered one of the most prevalent and widely distributed infectious diseases in equids. Large epidemiological studies looking at prevalence factors associated with clinical disease are seldom reported. The present study aimed at describing selected prevalence factors of 9409 equids with acute onset of fever and respiratory signs tested for S. equi by qPCR and to determine the impact of vaccination on S. equi detection. A total of 715 horses (7.6%) tested qPCR-positive for S. equi, of which 226 horses had coinfections with common respiratory viruses (EIV, EHV-1, EHV-4, ERBV). Various prevalence factors were associated with S. equi qPCR-positive status, including season (winter and spring), use (competition and ranch/farm use), and clinical signs (nasal discharge, fever, lethargy, anorexia, and ocular discharge). Vaccination against S. equi was associated with a lower frequency of S. equi qPCR-positive index cases. ABSTRACT: This study aimed to describe selected epidemiological aspects of horses with acute onset of fever and respiratory signs testing qPCR-positive for S. equi and to determine the effect of vaccination against S. equi on qPCR status. Horses with acute onset of fever and respiratory signs from all regions of the United States were included in a voluntary biosurveillance program from 2008 to 2020 and nasal secretions were tested via qPCR for S. equi and common respiratory viruses. A total of 715/9409 equids (7.6%) tested qPCR-positive for S. equi, with 226 horses showing coinfections with EIV, EHV-1, EHV-4, and ERBV. The median age for the S. equi qPCR-positive horses was 8 ± 4 years and there was significant difference when compared to the median age of the S. equi qPCR-negative horses (6 ± 2 years; p = 0.004). Quarter Horse, Warmblood, and Thoroughbred were the more frequent breed in this horse population, and these breeds were more likely to test qPCR-positive for S. equi compared to other breeds. There was not statistical difference for sex between S. equi qPCR-positive and qPCR-negative horses. Horses used for competition and ranch/farm use were more likely to test qPCR-positive for S. equi (p = 0.006). Horses that tested S. equi qPCR-positive were more likely to display nasal discharge, fever, lethargy, anorexia, and ocular discharge compared to horses that tested S. equi qPCR-negative (p = 0.001). Vaccination against S. equi was associated with a lower frequency of S. equi qPCR-positive status.