[Listeria strains isolated from sick animals and consumed silage]

Forty-eight strains of listeria isolated from sick animals (27 goats, 19 sheep, 2 cows) from 33 farms, and 40 isolated from the silage eaten by these animals were studied. Listeria were isolated more frequently from poor quality silages than from those of good quality, although they were found in 11 out of 31 excellent silages with a pH value comprised between 3.6 and 4. The 88 strains were characterized by biochemical and serological examinations, phage typing and measurement of their virulence. They showed a wide variety of characteristics and in only one case, three strains with the same characteristics were isolated one from a dead ewe with symptoms of encephalitis and the two others from the silage it had eaten. In four flocks, identical strains were found in several animals from each farm, but in five other flocks, strains isolated from animals were different. The listeria of the silage may be the direct cause of the disease, but the isolation of strain with different characteristics from animals of the same flock led us to think that other causes might favour the infection. So, it seems difficult to control the disease only by enhancement of the quality of the silage.