Insects may spread foodborne microbe to chickens.

Flies sucked through ventilation shafts into industrial chicken coops may be the primary carriers of a major cause of food poisoning in people, a Danish study suggests.

Campylobacter bacteria, such as the common Campylobacter jejuni, plague even the cleanest of chicken farms. Nearly half of Danish broiler chickens acquire the bacteria during their 6-week lives. In U.S. flocks, the infection rate is greater than 80 percent.

"Once a chicken gets infected, [campylobacter] spreads very rapidly throughout the whole broiler house," says Karl Pedersen of the Danish Institute for Food and Veterinary Research in Arhus. The bacteria don't harm the chickens but can cause diarrhea in consumers who eat undercooked chicken. Other raw meats, unpasteurized milk, and contaminated water can also transmit the bacteria to people.

Chicken flocks raised during the summer face the greatest risk of campylobacter infection--a seasonal pattern that had lacked explanation.

Coops are thoroughly disinfected after each flock goes to slaughter, so campylobacter must repeatedly reenter facilities. Preventing infections is crucial in Europe because regulations there prohibit treating livestock and carcasses with chemicals used in the United States.

"Strong evidence suggests that farmers moving in and out of chicken houses are a source of campylobacter transmission," says Diane Newell, a London microbiologist who manages the European veterinary network Med-Vet-Net. Therefore, many Scandinavian farmers don fresh clothing and dip their shoes in mild acid upon entering poultry houses. Such measures have dramatically cut the prevalence of salmonella, another troublesome foodborne pathogen, but their effect on campylobacter has been minimal, Newell says.

To test whether flies could be smuggling campylobacter into coops, Pedersen, his colleague Birthe Hald, and five other scientists collected and counted flies that entered a large Danish coop in July, which is peak fly season. From their tally, they estimate that 30,000 flies entered the coop every 6 weeks at that time of year.

The researchers also captured 96 flies outdoors near the facility. In the August Emerging Infectious Diseases, they report finding viable C. jejuni on 8.2 percent of the flies.

Sheep outside the facility had the same C. jejuni strain as the flies and nearly all of the chickens had, and the Danish team conjectures that the flies acquired the bacteria from sheep manure.

"The new study fits very nicely with the epidemiology that shows elevated infection rates in flocks at warmer times, when the flies are more active," says Norman Stern of the U.S. Department of Agriculture in Athens, Ga. Chickens presumably ingest the bacteria by eating infected flies, he says.

"Controlling flies is going to be difficult," Stern notes. Screens added to cover air ducts in U.S. coops could block flies, but they'd cut the efficiency of ventilation fans, he says.

The Danish researchers are now testing whether loosely surrounding the air intake ducts with mesh can keep flies out. In the Netherlands, some poultry farms already have gauze fly screens in coop ventilation ducts, says Jaap Wagenaar of Wageningen University in the Netherlands.

Stern suggests that it might also be possible to control campylobacter just before chicken slaughter. For example, he has identified a natural bacterial-killing protein that, when added to chicken feed 3 days before slaughter, substantially reduces campylobacter numbers in the birds' intestines. He'll present those data at this week's International Association of Food Protection meeting in Phoenix.