Dr John Hermon-Taylor, the IBD researcher, noted that there are seasonal differences in the prevalence of Mycobacterium paratuberculosis which may explain why some sufferers of Crohn's experience variations in the severity of their symptoms depending on the time of year (Scand J Gastroenterol, 1996; 31: 79-82).

In an animal study, he found that at peak times of the year for stress, up to 25 per cent of cows tested showed positive for M paratuberculosis.Other research has shown that cows can be infected with the bacterium without being symptomatic so clearly the culling of symptomatic cows is not the answer. In one study of asymptomatic cows, 27 per cent were infected (J Clin Microbiol, 1992; 30: 166-7).

It is not only dairy products which are affected. M paratuberculosis can also get into the water supply via infected animal feces. The survival of M paratuberculosis and other infectious organisms in farm slurry is well documented (Nord Vet Med, 1977; 29: 67-70; J Dairy Sci, 1994; 77: 1999-2007; Commun Dis Rep CDR, 1994; 4: R50-1).

It is believed that the majority of the population can tolerate Mycobacterium, but those with Crohn's or their close relatives who may be similarly genetically disposed may wish to switch to UHT milk which is less likely to be contaminated. Another theory is that the rise in Crohn's seems to have paralleled the growing use of antibiotics in human and animals. Our overuse of antibiotics may be encouraging the growth of the bacteria in cattle and greater susceptibility in some individuals (Hepatogastroenterol, 1994; 112: 549-51).