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Don't blame the cows

Research rubs out the perception livestock contribute to beaver fever-type diseases in humans.

Livestock grazing within community watersheds may put cattle at the scene of the crime, but they are innocent bystanders rather than culprits in cases where water-borne bacteria are suspected of making humans sick, says a University of Calgary researcher.

Dr. Merle Olson, director of the Medicine Animal Resource Centre.

Cattle do shed bacteria, says Dr. Merle Olson, director of the Medicine Animal Resource Centre, but it's not the type that affects humans. Certain strains of commonly known Cryptosporidium spp. (crypto) and Giardia spp. are intestinal parasites that can infect mammals, including humans, causing watery diarrhea, fever and nausea.

But genetic testing showed Olson, a veterinarian and U  of  C researcher who is both a microbiologist and a gastrointestinal specialist, that the strain of bacteria shed by cattle are not infectious to humans.

"They are very similar, they look identical under the microscope, but genetically they are different," says Olson. "The bacteria shed by cattle are not human infective."

That finding shatters the long-standing speculation that because cattle shed bacteria and parasites in feces, it poses a threat to human health.

Outbreaks make news

Outbreaks of crypto within the past decade have been well publicized. Water-borne bacteria were cited in 1996 for making as many as 17,000 people ill in separate outbreaks in Cranbrook and Kelowna, B.C. A few years later, several thousand residents of North Battleford, Sask. became ill due to contaminated water. And in the United States in 1993, as many as 100 people died and another 400,000 became ill in Milwaukee, Wisconsin from drinking contaminated water. The parasites survive in the digestive tracts of warm-blooded animals and are shed in feces. The bacteria can be found in cow manure and in the feces of many wild animals such as deer, elk, muskrat and beaver. Giardia infections, often known as beaver fever, can be treated with drugs, while Cryptosporidium cannot. Both infections can be particularly severe, even life threatening, for the elderly, infants or those with reduced immunity. In some of the disease outbreaks cattle grazing in watersheds or near reservoirs were blamed as the source.

Humans to blame

But, not so, says Olson. "In most cases, the bacteria that makes people ill is due to the inappropriate disposal of human sewage," he says. "We are doing it to ourselves. Somewhere along the line, effluent from sewage systems is entering a community water system, or human activity within a watershed is contaminating a water source."

There is one limited exception to the livestock connection to illness in humans, he points out. Young calves will shed a form of Giardia in their feces,which can cause beaver fever in humans. "But it's a narrow window," says Olson. "These calves are infectious only during the first two or three weeks of their lives. So in most cases these calves are still on the farm or ranch in a confined environment and not out in range areas that usually make up watersheds. It needs to be noted, but it is important to keep risk in perspective."

Olson's findings on the specific bacteria genetics follows a broader three-year study in Alberta that looked at the impact of grazing cattle and related livestock on water quality issues in the North Saskatchewan River Basin west of Edmonton.

The $364,000 research project was led by Sandra Cooke, a biologist with Alberta Agriculture, Food and Rural Development (AAFRD). The research was one of about 60 beef-related projects funded by the $16.4 million Canada Alberta Beef Industry Development Fund (CABIDF).

The study found that grazing cattle and related livestock activities can contribute to the occurrence of parasites and bacteria in streams, rivers and other water sources, but they are not the only sources of contamination. The researchers monitored water quality within about half the area of the North Saskatchewan River Basin, 14,000 kilometres² of forest and farmland lying between Edmonton and Rocky Mountain House.

Key findings

• Concentrations of Cryptosporidium and Giardia were significantly higher in streams draining from watersheds with more intensive agricultural (livestock) production, compared with non-agricultural watersheds.
• In spring, levels of Cryptosporidium and Giardia in streams correlated with total livestock density and percentage of cattle yards along watershed streams. As well, Giardia levels correlated with density of beef cattle in spring. In summer, levels of both organisms were correlated with total livestock density. Giardia also correlated with beef cattle density and grazing factors, whereas Cryptosporidium correlated with density of dairy cattle and hogs. In fall, Giardia concentrations correlated with dairy cattle and hogs.

"That research, suggests that cattle contribute to overall bacteria loading," says Olson. "But, on closer examination, we found they don't carry the strain of bacteria that makes humans sick.

"Proper management of grazing livestock is still important for a variety of reasons, but it is really human-related activity that poses the greatest risk to human health," he says.

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