Identification of carriers of Streptococcus equi in a naturally infected herd.

This research identifies the presence of Streptococcus equi, the bacterium responsible for strangles disease, in four horses during an outbreak, suggesting the existence of a carrier state after natural infection.

Background

• The study was conducted during an outbreak of Strangles, a highly infectious disease of horses caused by the bacterium Streptococcus equi.
• Of the many horses that were part of the population, four mares were identified as potential carriers of the disease. Three of them showed severe symptoms of the disease, while one of the mares only showed mild symptoms.

Findings and Implications

• Strangles typically presents as severe regional lymphadenitis, or swelling of the lymph nodes, with or without rupture of abscessed lymph nodes. In this study, three of the horses exhibited such symptoms. S. equi was isolated from abscessed lymph nodes and from nasopharyngeal swab samples collected from these three horses approximately 6-19 weeks after the nodes ruptured.
• In contrast, the fourth mare displayed only mild symptoms (serous to mucopurulent nasal discharge with mild lymph node enlargement), did not exhibit node rupture, and did not develop severe lymphadenitis. The researchers did detect S. equi from this horse’s nasopharynx initially and sporadically over the next six months. Subsequent isolation for seven months showed continued shedding of S. equi bacteria, yet no significant abnormalities were detected via complete physical examination or radiography.
• Presence of S. equi in the fourth mare was further confirmed when two young horses (yearling colts) developed strangles after being inoculated with a swab specimen from her. Later, the bacterium ceased to be present in the mare’s system and was not found in any organ or tissue when she died.
• Results from the study suggest that horses can carry and shed S. equi even after they appear to have recovered, thus establishing a potential carrier state. Horses in such a carrier state can pose a serious risk of infection to other horses and are an important consideration during a strangles outbreak.

Conclusions

• The study provides clinical, epidemiological, and bacteriological evidence for the existence of a carrier state following an infection with S. equi. This emphasizes the need for rigorous follow-up testing and monitoring in horses that have recovered from strangles to prevent potential spread of the disease from carrier horses to the rest of the herd.
• The results will help inform control strategies for strangles in horse populations and highlight the importance of thorough and repeated testing during an outbreak.