Epizootiology of Listeric Infection in Sheep

Abstract

SUMMARY The epizootiology of enzootic listeric infection in a flock of approximately 1,000 sheep at the Dixon Springs Agricultural Center (DSAC) in southern Illinois was investigated by bacteriologic cultural isolation and serologic procedures. Listeria monocytogenes, predominantly serotype 4b, was isolated from the brains of sheep with listeric encephalitis and from nasal mucosa and feces of healthy sheep. The organism was isolated also from the liver and spleen of mice caught near the sheep feed bunks and from feces of deer shot in the adjacent forest areas. The organism was recovered from poor-quality silage from silos from which sheep were fed and from corn that had overwintered in the field at DSAC. In a group of Suffolk and a group of Targhee sheep studied at stages of life from lambs through first parturition, bacteriologic and serologic evidence indicated a low incidence of listeric carriers in lambs and yearlings but an increase in nasal listeric carriers when the ewes were pregnant, closely confined during the winter in a barn, and fed silage. The strains of L. monocytogenes isolated from the brains of sheep with encephalitis were virulent for mice. Many of the isolants obtained from the nasal mucosa of sheep, silage, and corn were avirulent for mice. It was concluded that the relationship of avirulent strains of L. monocytogenes to the epizootiology of listeriosis requires further investigation.