Two disease-causing microorganisms, Cryptosporidium and Giardia, are the targets of intensive investigations by researchers at Cornell's colleges of Veterinary Medicine and Agriculture and Life Sciences. The scientists' goal is to maintain the safety of drinking water for millions of residents in the New York City metropolitan area, while preserving a way of life in the upstate farming communities where the water originates.

If the campaign succeeds in identifying sources of potentially lethal microbes and instituting farm-management practices that significantly reduce the risk to water quality, both upstate New Yorkers and Big Apple water drinkers will come out ahead: Dairy farms will continue to operate in the environmentally sensitive watershed, where rain trickles through barnyards on its way to New York City's reservoirs. And without New York City having to install a multi-billion-dollar filtration system, faucets from the Bronx to Brooklyn will continue to deliver high-quality water.

"We are looking for sources of contamination and for ways to manage the risk. Hopefully, we can provide the city with clean water and sustain farming in the watershed," said Hussni O. Mohammed, Cornell associate professor of veterinary clinical science and the epidemiologist in charge of the pathogenic parasite study.

This is one of a large number of projects conducted by Cornell researchers and staff members in several colleges to assist the New York City Department of Environmental Protection.

Both Giardia and Cryptosporidium are one-celled protozoa that live in the gastrointestinal tracts of mammals ≠ including cattle, wild animals and humans ≠ and find their way into water supplies by way of their hosts' fecal matter. People with healthy immune systems usually survive the infections, which can cause intestinal pain and severe diarrhea. But people with less robust immune systems ≠ such as AIDS patients, the elderly, infants and people receiving immunosuppressant drugs ≠ can die from giardiasis or cryptosporidiosis.

New York City, like most other major metropolitan areas, has a large population of residents with susceptible immune systems. Giardia and Cryptosporidium are resistant to the most common means of purifying municipal water supplies ≠ chlorine disinfection. Sophisticated filtration systems ≠ which in the case of New York City would cost several billion dollars to build and millions of dollars each year to operate ≠ is one way to trap the tiny microorganisms and keep them from reaching the water faucets.

Unless, that is, the parasites can be kept out of the water at its source. That is New York City's strategy: to comply with federal clean-water regulations and avoid in stalling costly filtration systems by reducing environmental sources of contamination in its watershed.

Although New York City owns and controls much of the land surrounding six reservoirs in its Catskill-Delaware Watershed (a five-county area west of the Hudson River), most land around streams and rivers feeding the reservoirs is in private hands. The water shed is dotted with towns and hamlets, some of whose sewage-treatment facilities are of questionable integrity, and with about 300 dairy farms.

Hoping to protect the watershed, the city has contracted with a team of scientists through the New York State Water Resources Institute (WRI) at Cornell to examine the problems and plan the solutions. One contamination source ≠ human populations ≠ is addressed increasingly through improved sewage-treatment facilities.

Farm animals as sources of contamination are addressed through the New York City Watershed Agricultural Program, which assists farm owners in controlling pollutants that may wash from their land into watershed streams. Among the farm-based pollutants are phosphorous and nitrogen from barns, barnyards and fields where manure is spread; pesticides; man made fertilizers and soil sediment eroding off fields; petroleum fuels; and water-borne pathogens.

Well-understood farm practices can handle most of the pollution problems ≠ and if financial assistance is available to farm owners, the practices are more likely to be followed ≠ according to WRI Director Keith Porter. But much less is known about the contaminants with the most dire human-health implications, the pathogenic parasites.

"We can't even tell, at this point, whether parasites found in a water sample are coming from cattle, humans or wild animals," said Susan Wade, the veterinary parasitologist in the watershed program's pathogen group. DNA tests now under development may eventually allow disease detectives to trace the parasites back to their hosts, she said. But for now, the focus is on the most -likely suspects ≠ farm animals.

"Our first task in risk assessment and risk management was to determine whether or not there is a pathogenic parasite problem on the farms," Mohammed said. A preliminary survey of 99 watershed farms in 1994 found Giardia in 20 percent of dairy cattle. Cryptosporidium was found in only 1 to 2 percent, "a much lower rate than we had expected," Wade commented. Cryptosporidium infect dairy cattle only during their first 30 days of life, whereas Giardia can be carried by cattle of all ages, she noted.

"At the same time we test for infection on the farms, we also look for risk factors that may be associated with higher levels of infection," Mohammed explained. Among the possible factors are crowding of young animals, inadequate sanitation and presence of rodents or wild animals around cattle, and nutrition and general health of the cattle. "Not all factors that are found in association with disease are necessarily significant causes," the epidemiologist observed. "Some associations are merely coincidences, and removing those coincidental factors probably won't have much impact on reducing the infection."

So the parasite study, now in its second phase, is scrutinizing 40 farms that were selected as a random, representational sample of those in the watershed. Twenty of the farms had detectable levels of Cryptosporidium and Giardia infection in preliminary tests, and 20 were free of infection. Fecal samples, taken every three months from cattle at the 40 farms, are analyzed at Cornell's Veterinary Diagnostic Laboratory. The scientists also document the presence or absence of known risk factors on the farms.

"By the end of this year, we should know which factors are consistently associated with infections," Mohammed said. "We will be able to make recommendations, based on a scientifically sound study, of which factors may be manipulated successfully to reduce infection."

For example, housing young animals in separate quarters may lessen the spread of infection among herds. Proper nutrition and veterinary care could make animals more resistant to disease. And fencing off streams certainly would keep animals from wading in water that is destined for someone's drinking glass. But which factors are truly associated with high levels of infection is not yet clear, Mohammed said. Rushing in to attack all factors at once would be a waste of everyone's time and money.

Through Watershed Planning Teams, cooperating farm owners will be encouraged to make changes in farm operation that are judged most likely to affect infection levels. (Participation in the study is voluntary, and confidentiality is assured to the farm owners, who receive a modest stipend and 100 percent financial assistance for implementing recommended changes.) Parasitologists also will continue quarterly testing of cattle in the 40-farm sample.

"At the end of the third year, we will have enough information about what causes these diseases and what can be done to control them," Mohammed said. "We will be able to recommend, with confidence, which best -management practices can be implemented on all watershed farms."

That knowledge is vital to the health of New York City water drinkers and to the continued diversity of a rural area that provides the water, WRI Director Porter said. Scores of farms and entire villages were eliminated during this century when New York City flooded rural valleys for its reservoirs, taking private land by power of eminent domain. Ill will toward the city continues to this day, and many watershed residents view with suspicion the city's attempts to manage its watershed under the stricter federal regulations. The farm program represents a new partnership between watershed residents and New York city.

"No one wants to see these farms shut down," Porter said. "They're a vital part of this state. But at the same time, no one wants to see illness and death from disease. This experiment will show whether we can manage the environment so that healthful water and food products come from the same place."