There is currently considerable interest in the occurrence of Escherichia coli in animal feeds following the association of the O157 type of these bacteria with human illness. In a recent United States study (Lynn et al., 1998), 30 percent of cattle feed samples obtained from commercial sources and farms contained E. coli, although none of the tests for E. coli O157 were positive. Replication of faecal E. coli, including the O157 type, was demonstrated in a variety of feeds under conditions likely to occur on cattle farms in the summer months. Since faecal contamination of feeds is widespread on farms, it is an important route for exposure of cattle to E. coli and other organisms. The potential for exposure to bacteria also exists when poultry litters are fed to cattle (in California, for example, two such poultry waste products are commercially available for use as cattle feed). However, providing the products are adequately heat-processed prior to distribution, the risks of contamination with E. coli, Salmonella spp. and Campylobacter spp. are likely to be minimized or even eliminated (Jeffrey et al., 1998). Nevertheless, it is worth noting that S. enterica commonly occurs in cattle feeds in the United States, Europe and South Africa, with contamination rates ranging from 5 to 19 percent (Krytenburg et al., 1998).
Listeria monocytogenes tends to occur in poor-quality silages and big-bale silage. When grass is ensiled under anaerobic conditions, the low pH regime ensures that Listeria is excluded from the resulting silage. However, in big-bale silage a degree of aerobic fermentation may occur, raising pH levels and allowing the growth of Listeria. These bacteria also survive at low temperatures and in silages with high levels of dry matter. Contamination of silage with Listeria is important as it causes abortion, meningitis, encephalitis and septicaemia in animals and humans. The incidence of various forms of listeriosis has been increasing in recent years.
