Some Epidemiological Aspects of Rhodococcus equi Infection in Foals in Japan: A Review of 108 Cases in 1992-1998

Abstract

One hundred and eight foals were necropsied and examined for Rhodococcus equi infection from 1992 to 1998 in Hokkaido and Aomori, Japan. Fifty-four of 108 cases had been clinically diagnosed as having R. equi pneumonia, whereas the remaining 54 were diagnosed with non-R. equi pneumonia (23 foals), arthritis (6 foals) and other diseases (25 foals). Pathologically, 104 foals were shown to have suppurative pneumonia with abscesses, and the remaining 3 had intestinal involvement without pulmonary lesions and one had osteomyelitis. Based on the pathology and bacteriology, 108 were classified into 5 disease types: pneumonia (44 cases; suppurative bronchopneumonia), enteritis (3 cases; ulcerative enteritis associated with suppurative lymphadenitis of the intestines), mixed (25 cases; pneumonia, enteritis, and intestinal lymphadenitis), transitional (35 cases; pneumonia and intestinal lymphadenitis), and osteomyelitis (1 case). Eighty-three of the 108 foals (25 foals died due to non-infectious causes) were analyzed for age distributions at the first clinical examination and death. About 75% (62 foals) of the foals showed clinical signs within 2 months of age, peaking at 40 days, and the mean age at the clinical examination was 53.6 days. About 81% (67 foals) died within 3 months of age, peaking at 60 days, and the mean age at death was 77.2 days. About 57% (47 foals) died within 2 weeks after the first clinical examination, and mean days for treatment were 21.7. Mean ages at the clinical signs and death of the 83 foals were compared statistically according to breed, sex, birth month, and disease types, but the difference was not significant. Bacteriologically, virulent R. equi was isolated from lesions of all the 108 foals. Virulent isolates from the foals bred at Hokkaido and Aomori contained an 87-kb type II or a 90-kb plasmid with a different ratio. From this surveillance, it is clear that R. equi infection in foals is still one of the most important diseases in horse-breeding farms in Japan.