Epidemiological Analysis of a Herd Infected with Strangles by Imported Carrier Horse

Abstract

During quarantine inspection, fever and enlarged submandibular lymph nodes were observed in 3 out of 9 quarter horses imported into Japan from the USA [date not given]. Because of negative results in bacterial examinations during the quarantine period, these horses were released. At the riding club that was their destination, however, Streptococcus equi subsp. equi was isolated from 3 of them. Then strangles spread to other horses in the riding club. Nasal swabs were obtained from all horses at the club weekly until the disease subsided 22 weeks later. S. equi was isolated from 25 of 58 horses (43.1%). The isolates formed mucoidal colonies and their colony type and SeM genotype differed from those of strains previously isolated in Japan. A decrease in the titre of the serum antibody against S. equi was observed in 4 of the 9 imported horses. In conclusion, the herd infection was caused by the imported carrier horses and spread throughout the riding club.