Zoonotic risks of pathogens from dairy cattle and their milk-borne transmission

Dairy products are major sources of high-quality protein and bioavailable nutrients and dairy production contributes to local, regional and national-level economies. Consumption of raw milk and raw milk products does, however, carry a zoonotic risk, as does direct contact with cattle by farm husbandry staff and other employees. This review will mainly focus on the latter, and deal with it from the standpoint of a well-developed dairy industry, using the example of the Netherlands. With regard to dairy cattle, the main bacterial pathogens are Salmonella spp., Listeria monocytogenes and Leptospira hardjo as well as Brucella abortus and Chlamydia abortus. The main viral pathogens associated with dairy are Rift Valley fever virus, rabies virus, cowpox virus and vaccinia virus. The main parasitological infections are Echinococcus granulosis, Cryptosporidium parvum and Giardia duodenalis, however, the last mentioned have mainly swimming pools as sources of human infection. Finally ectoparasites such as lice and mites and Trichophyton verrucosum may affect employees. Some pathogens may cause health problems due to contamination. Bacterial pathogens of importance that may contaminate milk are Campylolobacter jejuni, Escherichia coli, Mycobacterium avium subsp. paratuberculosis, Leptospira hardjo and Salmonella typhimurium. Excretion of zoonotic viruses in milk is negligible in the Netherlands, and the endoparasite, Toxocara vitulorum is mainly found in suckling and fattening calves, whilst the risk in dairy cattle is limited. Excretion of transmissible spongiform encephalopathies (TSEs) or mycoses in milk are not expected and are, therefore, not of importance here.Being aware of the risks and working according to hygiene standards can substantially limit zoonotic risks for employees. Additionally, diseased employees are advised to limit their contact with cattle and to indicate that they work with cattle when consulting a physician. To prevent zoonotic risks through excretion of pathogens in milk, standard hygiene measures are necessary. Further, using only pasteurised milk for consumption and/or processing of milk can considerably limit the risks. If these measures are not possible, well-constructed monitoring can be followed. Monitoring programmes already exist for pathogens such as for Salmonella spp., Leptospira hardjo and Mycobacterium avium subsp. paratuberculosis. For others, like Campylobacter jejuni and E. coli, programmes are not available yet as far as we know.