Campylobacter, which live in the digestive tracts of poultry, can cause cramps, fever and bloody diarrhea when they get into humans, and can lead to complications, such as arthritis or life-threatening blood infections.

Baytril has been licensed since 1996 to treat respiratory disease in chickens. Since then, strains of antibiotic-resistant campylobacter have increased, now accounting for 21% of the 1 million campylobacter infections in people each year, says Rob Tauxe of the Centers for Disease Control and Prevention.

In 2000, the FDA became "concerned that the drug was no longer safe," said Stephen Sundlof, director of the FDA's Center for Veterinary Medicine. It moved to withdraw approval for the use in chickens of Baytril, made by Bayer, and a similar drug made by Abbott.

Five-Year Legal Process

Abbott complied, but Bayer appealed through a five-year legal process ending with FDA Commissioner Lester Crawford's decision to withdraw the drug's approval. Unless Bayer appeals further to an appellate court, or asks the commissioner to reconsider, the decision will go into effect Sept. 12.

The FDA ruling doesn't apply to the use of Baytril in other animals, which do not pass campylobacter to humans.

"We applaud the decision," Tauxe said. "People with campylobacter infections in the future will be easier to treat, and their infections will be shorter."

Campylobacter, which live in the digestive tracts of poultry, can cause cramps, fever and bloody diarrhea when they get into humans, and can lead to complications, such as arthritis or life-threatening blood infections.

Drug-Resistant Strains

Baytril, when given to chickens, kills some of the bacteria, but those that survive become resistant to antibiotics such as Cipro, the standard treatment for campylobacter. These drug-resistant strains can pass into people who eat undercooked chicken.

"We are disappointed by the action of the FDA because, in our view, Baytril is an important medication that is used sparingly and responsibly," said Richard Loeb of the National Chicken Council, a producers group in Washington DC.

It is a drug of last resort to fight respiratory diseases in poultry and is used on 1% or less of chickens and turkeys in the USA, says Bob Walker of Bayer Animal Health in Shawnee Mission, Kan. "The poultry industry will only use it when they 100% need it," he said.

Respiratory illnesses can infect entire flocks, some as big as 20,000 birds, very quickly. "Mortality can reach 100%, and this was the only medication that was really effective in some of these infections," Loeb said. Chickens represent a $50 billion market in the USA.

Bayer's share price closed at $36.01 Thursday, up 60 cents.