Environmental source of mycobacteriosis in a California swine herd

Between July 1985 and April 1986, mycobacterial lymphadenitis was diagnosed in 6 of 2407 slaughter pigs from a herd in which most animals were raised in confinement. All 6 affected pigs had access to earth-floored pens at least 81 days before slaughter. Infection rate was 9.4% in pigs kept on earth floors but was absent in pigs kept on concrete. In two litters 5 of 15 piglets kept on earth floors at 12 to 24 days of age, developed lesions, while none of 36 piglets (9 litters) kept on concrete was affected. Mycobacterium avium-complex bacteria were isolated from 9 of 15 piglets in affected litters but from none of the 9 unaffected litters. Serovars isolated included 1, 4, 8, 9, the dual serovar 4/8, and an untypable serovar. Incense-cedar bark (Calocedrus decurrens) used as flooring material was shown to be a potential source of M. avium-complex serovar 9. Serovar 4/8 and the untypable serovar were isolated from earth-floored pens. There was no evidence of cross-transmission of M. avium-complex infection. Infection in this herd probably resulted from infrequent exposure to a common environmental source.