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Ordering free-range chicken at a restaurant may make you feel good, but it turns out you may not be doing the birds any favour.

Uncaged chickens are exposed to higher levels of bacteria, parasites and viruses that put them at greater risk for disease and infection compared with their caged counterparts, new research from Sweden's national veterinary institute shows..

Chickens not kept in cages are often housed in shelters where the floor doubles as a giant litter box. As a result, hens have direct contact with bacteria and microorganisms that grow in the litter, which can greatly increase health risks, said the study, available on BioMed Central's journal Acta Veterinaria Scandinavica.

"We found that there was a higher occurrence of bacterial diseases, parasite disease in birds housed in the litter-based systems," said Oddvar Fossum, assistant state veterinarian at Sweden's National Veterinary Institute.

Exacerbating the problem is that hundreds of uncaged chickens are often kept in close quarters, allowing disease and infection to spread quickly.

Housing a large population of uncaged birds in close proximity also leads to increased incidences of pecking, which can result in disease or death.

While common wisdom suggests chickens are healthier and happier out of cages, that may not be accurate, said Trevor Smith, an animal and poultry science professor at the University of Guelph.

"It's just the opposite," he said. "It's actually more benign than one might think to have the birds in cages."

Although the term "free-range" is not legally defined in Canada, it generally means birds have some access to the outdoors. Uncaged chickens can include free-range and non-free-range.

Demand for free-range chicken has been growing in Canada in recent years as more consumers demanded more ethical treatment of animals.

But partly because of Canada's harsh winters, which make it difficult to let chickens outside, only a small percentage of chickens raised here are truly free-range. But nearly all chickens being raised for meat in Canada are uncaged and housed in a litter-based system, according to Lisa Bishop-Spencer, manager of communications for Chicken Farmers of Canada, which represents about 2,800 producers.

Sweden banned the use of cages for chickens in 1988. Now, 60 per cent of chickens in that country are raised in litter-based systems, while the remaining 40 per cent are in "furnished" cages, which are larger than conventional ones and provide room for activities such as nesting and perching. Similar systems have emerged throughout Europe and North America, though litter-based systems have become one of the most common ways to house flocks.

Dr. Fossum and his colleagues began the study after noticing increased demand for necropsy, or autopsies, on laying hens between 2001 and 2004, when many producers switched from conventional cages to litter-based or other more humane housing. They studied necropsy results of 914 hens from 172 different flocks.

Bacterial diseases, particularly those caused by E.coli, were the most common cause of death among all chickens included in the study. About 73 per cent of litter-based flocks and 74 per cent of free-range flocks were infected with bacterial diseases, compared with 65 per cent of caged flocks.

Parasitic diseases such as coccidiosis were found in 23 of 129 litter-based flocks, including five of the 23 free-range flocks included in the study. Only two of 20 caged flocks were infected with parasitic diseases.

Viral diseases infected 15 litter-based flocks, one free-range flock and six caged flocks, according to the study.

Meanwhile, cannibalism, or pecking, was noted in 24 litter-based flocks, six free-range flocks and one caged flock.

Canadian chicken farmers follow national safety guidelines and mitigate disease risk by changing the litter and sanitizing the facility after a flock leaves for slaughter, Ms. Bishop-Spencer said.

She also said government officials inspect chickens before they're processed to check for infection and disease.

The organization hasn't heard reports of bacterial infection or other disease among chickens housed in litter-based systems, she said.

Dr. Fossum said more research should be done to determine the risks of litter-based housing, and how to decrease the chance of bacterial infection or other disease. He said Swedish health officials have begun vaccinating and using some medicine to stop the spread of disease.