Vol 59, No 2 May 2006 International Journal of Dairy Technology CONFERENCE CONTRIBUTION *Author for correspondence. E-mail: i.grant@qub.ac.uk © 2006 Society of Dairy Technology 112 Blackwell Publishing Ltd Mycobacterium avium ssp. paratuberculosis in foods: current evidence and potential consequences IRENE R GRANT* Department of Food Microbiology, Queen’s University of Belfast, Agriculture and Food Science Centre, Newforge Lane, Belfast BT9 5PX, Northern Ireland, UK Mycobacterium avium ssp. paratuberculosis (MAP), the cause of Johne’s disease in cattle, sheep and goats, may have a role in Crohn’s disease in humans. Animals with Johne’s disease shed viable MAP in their milk and faeces. The organism is also widely disseminated in the blood and tissues of infected animals. Consequently, transmission to humans via consumption of animal-derived foods is a distinct possibility. Milk, other dairy products, beef and water have been identified as possible food vehicles of transmission. To date, viable MAP has been cultured from raw cows’, sheep and goats’ milk, retail pasteurized cows’ milk, and some retail cheeses in several countries during recent studies. MAP has not been isolated from retail beef to date, although limited testing has been carried out. The public health consequences, if any, of low numbers of viable MAP being periodically consumed by susceptible individuals are uncertain. An association between MAP and Crohn’s disease is not proven, but neither can it be discounted on the basis of current evidence. A precautionary approach is therefore warranted in relation to the existence of MAP in food, and action is needed to reduce the prevalence of Johne’s disease in the cattle population worldwide, in order to minimize public exposure to this potential human pathogen. Keywords Crohn’s disease, Milk, Mycobacterium avium ssp. paratuberculosis, Pasteurization, Vehicles of transmission. INTRODUCTION Mycobacterium avium ssp. paratuberculosis (MAP) is the causative agent of Johne’s disease (or para- tuberculosis), a chronic, infectious enteritis that can affect a variety of animals, most commonly domestic ruminants such as cattle, sheep and goats (Cocito et al. 1994; Collins 2003a,b; 2004). Other animals such as red deer (Godfroid et al. 2000), rabbits (Greig et al. 1999) and other nonruminant wildlife species (Beard et al. 2001) can also be infected by MAP. The Office International des Epizooties (OIE) considers Johne’s disease a disease of major global importance and categorizes it as a List B transmis- sible disease, considered to be of socioeconomic and /or public health importance within countries and significant in the trade of animals and animal products (Office International des Epizooties 2001). Johne’s disease has been reported on every continent and its prevalence is increasing in many countries, particularly in dairy cattle. When an animal is exhibiting clinical symptoms of Johne’s disease (persistent diarrhoea, weight loss, progressive ema- ciation), and comes to the attention of the farmer, it has reached the clinical stage of the disease. It has been estimated that for every clinical case of Johne’s disease existing in an infected herd, as many as four to eight other animals may have subclinical disease and be asymptomatic carriers of infection (Council for Agricultural Science and Technology 2001). These animals are infected by MAP but are not yet exhibiting the characteristic signs of Johne’s disease. They are, however, shedding the organism periodically in both faeces and milk (Sweeney et al. 1992; Streeter et al. 1995). The main problem with Johne’s disease in many respects is that it rep- resents a hidden threat for farmers. Although most animals are infected at an early age, the onset of clinical symptoms is usually delayed by several years (typically 2–5 years). In the intervening years between infection and clinical manifestation of Johne’s disease, asymptomatic carrier animals will have been contaminating the farm environment, potentially infecting other animals in the herd, or will have been sold on to another farmer thereby spreading the disease to other herds. PUBLIC HEALTH SIGNIFICANCE OF MAP A possible link between MAP and Crohn’s disease in humans has long been suspected but is currently Paper of presentation given at Yakult conference, Dublin, March 2005