Rats and mice and animal feed--a risk too far?

On cattle, sheep and pig farms, it is common to see house mice and not uncommon to see brown rats, while evidence of their presence in the form of numerous small dark droppings is rarely hard to find. These two species of rodents specialise in exploiting agricultural and urban environments and are particularly attracted by the easy availability of animal feed and by safe harbourage in and around farm buildings. Dairy farmers increasingly feed straights, stored in open barns where tractors can load the feed into mixer wagons, but where rodents can also gain access. Compound feeds are more often stored in sealed rodent-proof hoppers, but the parlour feeders and any spilled feed are often accessible. In addition to the damage they cause, rodents can contaminate feeds with bacteria, such as Salmonella or Cryptosporidium species, in addition to other pathogens of veterinary and human importance such as tapeworms and Toxoplasma gondii (Anon, 2001a; b). Over 1000 incidents of Salmonella in cattle are reported each year. About 1500 incidents of Cryptosporidium in cattle were reported in 1999, and about 5500 cases in human beings. One-third of abortions in sheep in which a pathogen is identified are associated with Toxoplasma. Surveys of UK farms indicate that rodents are significant carriers of these diseases and may present a serious risk to the health of domestic animals as well as farm workers. For example, Cryptosporidium parvum has been found in 24% (Quy et al., 1999) or 63% (Webster & Macdonald, 1995) of rats and in 22% of house mice (Chalmers et al., 1997). Rats may also be capable of carrying Foot and Mouth virus (Capel-Edwards, 1971) while Webster et al. (1995) reported that 14% of rats in UK farms carry Leptospira spp, transmitted through their urine. The main route of transmission is likely to be through ingestion of faeces in contaminated feed ± each rat or mouse produces around 40 to 55 faecal pellets per day. Mice simply deposit their faeces