Identification of carriers of Streptococcus equi in a naturally infected herd

During an outbreak of strangles in a population of research horses, 4 mares were identified as carriers of Streptococcus equi. Three of the mares had typical signs of strangles (severe regional lymphadenitis with or without rupture of abscessed lymph nodes). The 4th mare experienced episodes of serous to mucopurulent nasal discharge, but never had more than a mild degree of lymph node enlargement. Streptococcus equi was isolated from the abscessed lymph nodes and from nasopharyngeal swab specimens from the first 3 mares from 6 to 19 weeks after rupture of involved nodes. Streptococcus equi was isolated from the nasopharynx of the 4th mare on introduction into the herd and intermittently over the ensuing 6 months. During the 7th month, mare 4 was placed in isolation, where she continued to shed S equi for 4 more months. A complete physical examination during the 10th month, including radiography of the head and thorax, did not reveal any relevant abnormalities, but a pharyngeal swab specimen was culture-positive for S equi. This isolate was used to inoculate 2 yearling colts, which developed strangles and from which S equi was reisolated. Shedding of S equi by mare 4 ceased in the 11th month, and at necropsy 2 months later, S equi was not recovered from any organ or tissue. Corticosteroid administration 3 weeks prior to necropsy had induced neither shedding of the organism nor clinical signs of strangles. The study provided clinical, epidemiologic, and bacteriologic evidence to support the existence of a carrier state following natural infection with S equi.