Consumer Concerns

Mycobacterium avium subspecies paratuberculosis and Crohn's Disease

Mycobacterium avium subspecies paratuberculosis (Map) is one of a group of bacteria known as Mycobacteria, which occur naturally in the environment. Map has been known for many years to be the cause of a gut infection in cattle, sheep, goats and other ruminants, called Johne's disease, which leads to diarrhoea and weight loss. It is a worldwide problem and there is no effective treatment. Animals with Johne’s disease are known to shed the bacteria in their milk and their faeces.

Johne's disease resembles a chronic inflammatory bowel disease of humans, called Crohn’s disease, and MAP is one of several factors that have been suggested as a possible cause. Crohn's disease may be severe, prolonged and debilitating and although a number of treatments are available for the symptoms, there is currently no cure. The disease appears to be most common in developed countries but national figures on the incidence are not collected. Some studies suggest that the incidence has been increasing whilst others have found no clear-cut trend in incidence, or even a decrease.

Worldwide, there is no consensus as to whether there is a link between Map and Crohn’s disease. The joint Department of Health/Health and Safety Executive Advisory Committee on Dangerous Pathogens concluded in 1998 that a causal link could not be established on current evidence. A Working Group of the EU Scientific Committee on Animal Health and Animal Welfare has recently examined the available evidence and have called for further research on this topic.

Milk from cattle with Johne's disease has been suggested as a possible route of transmission of Map from cattle to humans. Surveillance by the Ministry of Agriculture, Fisheries and Food has shown Map to be present in raw and pasteurised cows' milk. Department of Health advice is that, on the basis of current evidence, there is no need for anyone to change their dietary habits.

Environmental waters can become contaminated with cattle faeces and it has been suggested that treated drinking water can contain Map and therefore be a possible cause of Crohn’s disease. This appears unlikely for a number of reasons. Firstly, because significant pollution of a water source would be immediately obvious and the water company would close the abstraction. Secondly, surface water sources receive treatment by coagulation and filtration and disinfection processes that remove bacteria. Finally, if there has been an increased incidence of Crohn’s disease in the last twenty years this would not be consistent with the significant improvements in the microbiological quality of drinking water that have taken place during the same period.

However, the Inspectorate has decided that it would be prudent to investigate whether drinking water might be a source of exposure to Map. Research projects have been commissioned on the geographical distribution of cases of Crohn’s disease and on the susceptibility of Mycobacteria and Map to removal or inactivation in drinking water treatment processes.

The Public Health Laboratory Service (PHLS) was awarded a contract to investigate the fate of Mycobacteria and Map in water treatment and distribution systems. Previous studies had been hampered by the lack of a suitable technique for culturing Map in water. PHLS developed a culture method and studied susceptibility to disinfection processes and removal of the organism in filtration processes. No Map were detected in water supplies although occasionally other Mycobacteria were found. A second study carried out by PHLS investigated whether biofilms in water supply systems, including household systems, might provide a source of exposure to MAC organisms Map. Again Mycobacteria were detected on occasion but not Map.

Drinking water derived from underground sources is unlikely to be affected by pollution from cattle. On the other hand, diffuse run-off in catchments with livestock farming could contribute low level contamination to some surface waters. The London School of Hygiene and Tropical Medicine (LSHTM) carried out a preliminary assessment of published studies on the relationship between incidence Crohn’s disease and exposure to water which revealed no evidence for any association with source of drinking water.

In addition, the Inspectorate commissioned the University of East Anglia carried out a case-control epidemiological investigation. The study was based around newly diagnosed cases of Crohn’s disease from fives centres in England to examine whether there was any demonstrable link with sources of drinking water risk factors.  The study  found no significant association between incidence of Crohn’s disease and measures of potential contamination of water sources with MAP (ie surface versus ground water sources), water intake  or water treatment.  This study did not provide any evidence of an increased risk of CD in association with exposure to drinking water for transmission pathways of MAP. Consequently the study did not support a role for MAP or drinking water in the aetiology of CD.

Based on this and the previous findings related above, the Drinking Water Inspectorate are not planning any further research in this area.