t in causing Crohn's disease.
"Crohn's disease has a very spotty distribution in the world," notes Dr. Walter Thayer, an expert on the disease at Rhode Island Hospital who worked with Chiodini to culture Mp from Crohn's patients. "But it's seen only in milk-drinking areas -- Australia, southern Africa, Europe, the United States, Canada, New Zealand. Interestingly, it's not seen in India, where they do drink milk, but they boil it first.
"The other thing, Crohn's disease wasn't really known until the '20s. And there were only a few cases in the literature. Now it's a very common disease. What has happened to dairying in that time? Do you get milk from your local dairy? No. You get it from big conglomerates that buy from local dairies and pool all the milk. I think this is possibly the reason the disease has spread so quickly."
Just as Crohn's disease is increasing in the human population, Johne's disease is spreading among dairy cattle. According to a National Animal Health Monitoring System study conducted in 1996 for the United States Department of Agriculture (USDA), an estimated 22 percent of U.S. dairies are infected with the Johne's organism. Larger herds are more likely to be infected -- about 40 percent of the herds with more than 300 cows had at least a 10 percent infection rate.
According to the same study -- despite both its having been identified more than a hundred years ago and the increasing incidence of the disease in dairy herds, which is costing the industry an estimated $1.5 billion a year -- 45 percent of dairy producers are either unaware of Johne's disease or know little about it. This is perhaps due to the fact that between 1992 and 1996, less than 1 percent of the USDA's National Research Initiative Competitive Grants funding was allocated to Johne's research and/or education. Regardless of any human health consequences this organism may cause, clearly a problem has been allowed to fester.
"It's one of the most economically significant infectious diseases," says Collins about the prevalence of Johne's in the nation's dairy herds. "It has appeared among our dairy cattle and is spreading, and we don't yet have the means, or possibly the will, to stop it.
"Experts agree that [Mp]...is excreted directly into the milk of infected cows...and that it happens before the animal shows signs of [Johne's disease]," says Collins. "The question is whether Mp in raw milk is sufficiently heat-resistant to withstand pasteurization. Four studies indicate that Mp is capable of surviving pasteurization, and two studies say it is not. It was found in pasteurized retail milk in England in 1996," results that were reproduced by Dr. Irene Grant, a respected researcher from Ireland.
Dusting for fingerprints
Shortly after Chiodini reported his successful efforts to cultivate Mp from Crohn's patients, he sent some of these cultures to Dr. John Hermon-Taylor, chairman of the department of surgery at St. Georges Hospital Medical School in London. An internationally recognized expert on Crohn's disease, Hermon-Taylor used these cultures to identify a unique genetic sequence in Mp, known as IS900.
This genetic "fingerprint" was then used by Hermon-Taylor to investigate whether Crohn's patients are more likely to be infected with Mp than other populations. In 1991, Hermon-Taylor reported that 65 percent of Crohn's patients tested positive for the presence IS900 (and thus the presence of Mp), whereas only 4.3 percent of ulcerative colitis patients and 12.5 percent of people without Crohn's disease tested positive.
Hermon-Taylor also received a small amount of funding to test retail milk in Great Britain for the presence of IS900. Between 1990 and 1994, his laboratory reported that 7 percent of the pasteurized milk they tested was positive for the DNA fingerprint of Mp. After culturing the milk for up to three and a half years, Hermon-Taylor says, "16 percent of the retail milk samples that originally tested negative came up with long-term cultures which tested strongly positive." Significantly, the same pasteurization process used