Cryptosporidium study leads to safer drinking water

What is the infectious dose of Cryptosporidium - a waterborne pathogen that causes diarrhea? Dr. Pablo Okhuysen, professor of internal medicine in the Division of Infectious Diseases, collaborated with Dr. Cynthia Chappell, professor and associate dean for academic affairs at the School of Public Health, to research that question for an Environmental Protection Agency-sponsored study on water safety. They recently found the answer, which will result in safer drinking water nationwide.

Specifically, the EPA study focused on improving drinking water standards and decreasing disease and illness from Cryptosporidium, which is typically found in lakes, streams, rivers, and drinking water. It can infect many animals, particularly hoofed creatures like deer, cows, and horses.

"There are at least seven different species of Cryptosporidium circulating," Okhuysen said. "Some infect animals, and some infect humans. The ones that cause disease in animals can be transmitted to humans, but at least one species only causes disease in humans.

"It typically infects very young wildlife, and that reaches the streams and lakes from which we draw our drinking water," he continued.

Different factors contribute to its infectivity level, including its resistance to the environment and chlorination. Because of this, Okhuysen said the parasite can live in water for many months and is not easily destroyed.

"Almost anybody can get Cryptosporidium infections," Okhuysen said. "In the otherwise healthy person, it means having diarrhea for a week. But, in people who have immune-compromised systems from HIV or cancer, disease can be really severe."

In order for the EPA to create rules to improve drinking water standards, it needed to know how many parasites contributed to disease in humans. To answer the EPA's question, Okhuysen and his team performed studies with healthy volunteers who were given set amounts of Cryptosporidium parasites.

From the studies, they had several findings. "The infectious dose of the parasite is very, very low; it's only about 10 to 100 parasites," Okhuysen said.

They also concluded that people can be infected again, and not all parasites are the same. There are differences within and between species, as well as differences in their disease manifestations.

Based on these results, the EPA recently developed the Long Term 2 Enhanced Surface Water Treatment Rule (LT2 rule) to reduce disease from Cryptosporidium in drinking water.

"The data we generated is going to be applicable to the way water is treated for the whole country," Okhuysen said.

The LT2 rule is nationwide and applies to 14,000 water treatment systems that serve about 180 million people. It requires systems to increase monitoring for Cryptosporidium levels in water sources.

"Monitoring will start in October 2006, and it will target the large water systems that serve at least 100,000 people," Okhuysen said. "The smaller systems will start operations in 2008."

The cost to implement the rule ranges from $92 to $133 million, and the results will be safer drinking water and decreased illness and disease from Cryptosporidium. "By enhancing the safety of the water, you'll prevent other diseases that are waterborne also," Okhuysen added.

Okhuysen's study was conducted at the University Clinical Research Center (UCRC). The UCRC is administered by the Medical School and located at Memorial Hermann Hospital. Okhuysen has directed the center since 2002.

--Camille Webb