July 24, 2006 — Despite worries over animal-borne illnesses from avian flu to "mad cow" disease, in many respects animals actually reduce the transmission of infectious diseases to people, argue the authors of a new study.
Sleeping in close quarters with cows and other livestock, for example, may reduce the rate at which disease-carrying mosquitos bite humans, since the insects will often bite the cows first.
"I think the fact that cows produce so much carbon dioxide is one of the reasons they are so effective in attracting mosquito bites," said Andy Dobson, lead author of the new study.
Dobson believes the health benefit associated with cows could be one reason Hindus began to revere the animals in ancient times.
Dobson, a professor of ecology and evolutionary biology at Princeton University, said many Tanzanian herders today try to stay close to their cattle to minimize disease risk. Soviet agricultural communities deliberately used cattle as a barrier between mosquito breeding sites and human settlements.
Dogs may also protect against mosquito bites, he said, for the same reason. But that isn't the only way dogs protect people from disease.
"I suspect one of the key reasons we first kept dogs as pets is that they make excellent 'watch-dogs,' but they are happy to feed on feces, which could significantly reduce the burden of hookworms and Ascaris (a parasitic worm that can infect humans) around the house," Dobson told Discovery News.
"These worms are probably our oldest parasites and they still have a huge impact on the growth and intellectual development of the one billion children infected with them," Dobson added.
In North America, it appears that gray squirrels may afford some protection against the tick-borne bacteria that causes Lyme disease. Squirrels seem to have a natural immunity to the illness, and they deflect tick bites away from white-footed mice, which can spread the bacteria.
According to the study, which was published recently in the journal Public Library of Science Medicine, areas in the eastern U.S. with many gray squirrels tend to have fewer outbreaks of Lyme disease.
The researchers explained that fewer than 15 percent of ticks feeding on gray squirrels become infected with Lyme disease-causing bacteria, acting as a buffer that slows the bacteria's spread.
The scientists think similar animal barriers can reduce instances of tick-borne encephalitis, West Nile virus and bovine spongiform encephalopathy, commonly known as mad cow disease.
Dobson and his team wrote that mad cow disease is usually transmitted from carcasses in the wild, when nutritionally stressed animals eat infected remains.
When scavengers such as buzzards and coyotes, which seem to be naturally resistant to the disease, pick the bones clean, the disease cycle is stopped cold.
