In an effort to eat compassionately, many people choose free-range chickens and eggs, assuming that the birds lived happy, high-quality lives before they became dinner.

But researchers at the National Veterinary Institute in Uppsala, Sweden have discovered that, if farmers aren't extremely careful, bacterial infections like E. coli can run rampant through free-range chicken flocks.

The finding, which appears in BioMed Central's open access journal Acta Veterinaria Scandinavica raises questions about what's best for both animals and people, reports Australia's news agency, ABC.

The study began when scientists noticed a sudden spike in dead laying hens submitted for necropsies.

The researchers guessed the surge in deaths was linked to the way farmers were housing the birds.

In Sweden, a new law bans the use of small laying cages and instead requires that chickens live more naturally - with access to nests, perches, and piles of dust they can roll around in.

This type of living is called a litter-based system. In a free-range litter-based system, the birds are also allowed to go outside.

Most Swedish farmers made the switch from cages to litter-based housing between 2001 and 2004. Around the same time, the number of dead hens presented to the Institute increased.

To further investigate, veterinary pathologist Dr Oddvar Fossum and colleagues analysed the necropsies of 914 hens from 172 flocks.

The researchers found that as many as 10 times more hens were submitted from litter-based and free-range setups than from caged systems during those years.

Compared to caged birds, free-ranging hens had more bacterial infections (the most common cause of death), more parasites, and more viruses.

They were also more likely to become victims of violent pecking and cannibalistic attacks.

Flock size was part of the problem, Fossum says. Cages held a maximum of 10 birds, but free-range flocks sometimes contained as many as 35,000 chickens.

Even though these chickens had the freedom to wander outside and roll in the dirt, they were more likely to bump into each other, fight and share diseases.

The findings add to a growing body of evidence that free-range chickens are particularly prone to disease and violent behaviour, says veterinarian and poultry pathologist Dr Rob Porter, of the University of Minnesota, Twin Cities.

It's hard to evaluate whether a chicken is or can be "happy," Porter adds. But other studies have failed to find a difference in stress hormone levels between chickens that are raised one way or the other.

"One of the largest attractions of free-range chickens is that it makes people happy to think about chickens outside pecking at the soil," says Porter.

"Although the perception of the general public is that these outdoor chickens must be healthier than others, time and time again this is shown not to be true."

Nutritionally, Porter adds, free-range eggs and meat are virtually identical to the same products from caged chickens.

Still, Fossum says, there are ways to keep a free-range flock healthy with a combination of breeding, vaccines, and behaviour control. Dimming the lights, for example, can calm a chicken down.

Sweden's experience, he says, offers lessons to chicken farmers elsewhere.