Editor: This news story, originally released Nov. 7, is being re-released with a clarification concerning the link between Johne's disease and Crohn's disease. Preliminary studies performed by Srinand Sreevatsan on a very limited number of sheep isolates of paratuberculosis show a genetic proximity to human isolates. However, the data is not sufficient to claim that Johne's organisms isolated from sheep cause Crohn's disease in humans. In other words, genetic associations may not be sufficient to conclude a causal association.
WOOSTER, Ohio - An animal disease that has potential links to a human intestinal illness may be more closely tied to sheep rather than dairy cattle, where the disease is most commonly associated.
Ohio State University veterinary researcher Srinand Sreevatsan said preliminary research results indicate that the organism that causes Johne's disease (pronounced yo-nees) in sheep may be related to the organism that causes Crohn's disease in humans, although the data is not sufficient to
claim that Johne's organisms isolated from sheep specifically cause Crohn's disease in humans.
Research may take as long as six months to a year to determine whether or not the preliminary data holds up. If so, it would finally answer the question of which animal host is linked to the human disease and how humans may be coming into contact with the organism that has been associated with it.
"It would mean that the disease is possibly coming from sheep," said Sreevatsan. "But right now the results don't logically explain how the organism is traveling from sheep to humans."
Johne's disease, also known as paratuberculosis, is a highly contagious bacterial illness that affects dairy and beef cattle, but it has also been found in sheep, goats and a variety of wild animals. The disease, which can be fatal, causes chronic diarrhea, rapid weight loss and emaciation.
Researchers theorize the disease is connected to Crohn's disease, also known as inflammatory bowel disease, a chronic disorder that causes inflammation or ulceration in the small and large intestines. Research reports that 30 to 75 percent of patients with Crohn's disease test positive for the bacterium that also causes Johne's disease.
Debate has raged for years over how humans are coming into contact with the organism - whether by consuming milk, dairy or meat products, or through the environment as an animal sheds the organism. The dairy industry has been at the center of attention as some research has shown that pasteurization does not destroy the bacterium. The disease is most prevalent in dairy cattle. The dairy industry loses approximately $200-$250 million annually, and each case of Johne's disease costs the farmer about $245 per infected cow a year. About 45 percent of United States dairy herds are currently infected with paratuberculosis.
Sreevatsan's research involves studying the organism at the molecular level using DNA fingerprinting and sequencing to determine if an animal bacterium closely resembles the human bacterium. So far he has collected over 1,000 cattle isolates, 10-15 goat isolates, seven human isolates and less than 10 sheep isolates, but only a small portion of the isolates have been analyzed so far.
"Each isolate is different for each animal species, but the diversity of the organism appears to be very limited," said Sreevatsan. "Its genome seems monomorphic. So it'll be a challenge to determine if isolates are different state to state, if there is limited sharing of the organism between animal populations or if there is a close association in geographical locality of their origin. Such answers could define ancestry and natural history of the disease, and cue us in to why certain clones are more prevalent than others."
Sreevatsan said that once that information is determined, the next step would be to identify and select virulence attributes of successful bacterial clones, develop vaccines and initiate management practices to help control the disease in the animal population. Such biomedical and management interventions would likely aid in eliminating the animal and human problems associated with the organism. But such work, said Sreevatsan, would take at least four to five years to complete.
"The disease is very difficult to work with as there is no good experimental animal model to use and signs of the disease take a long time to emerge," he said, adding that a cow may become exposed to the disease as a calf, but won't show symptoms until its first or second lactation. "We have to understand the epidemiology of the disease first to devise preventive measures in domestic and wild animal populations and determine if and how it affects humans."
For more information about Johne's disease, log on to the Ohio Department of Agriculture's website at http://www.state.oh.us/agr/addl/dp.htm.