A new solution for control of respiratory infection by Streptococcus equi isolated from horses in northern Iran

Respiratory infections caused by Streptococcus equi are one of the most important diseases in the equine industry. The increasing rate of antibiotic resistance among this bacterial species necessitates new antimicrobial agents. In this research, nasopharyngeal samples were taken from horses (n=90) in some areas of the Golestan Province, Iran. After isolation and identification of S. equi strains, antibiotic resistance of the isolates was evaluated using the Kirby-Bauer method. The antibacterial effect of lavender essential oil was investigated by broth microdilution assay. The frequency of streptococcal strains was 27.8%. The most frequent isolates were S. equi subsp. equi (72%) and S. equi subsp. zooepidemicus (28%). The frequency of S. equi was highest in horses aged less than 2 years (69.3%) and thoroughbred Turkmen horses (61.5%). The highest rate of resistance was observed against amoxicillin, while the highest rate of sensitivity was to ceftriaxone. Furthermore, the essential oil of lavender inhibited 90% of the isolates at concentrations ≥2048 μL/mL. Our study revealed that the frequency of antibiotic-resistant S. equi isolates was relatively high in the studied population of horses in Iran. It was concluded that essential oil of lavender possessed more potent in vitro activity than the common antibiotics for treatment of infections caused by S. equi subspecies.