The first salmonella outbreak linked to MiceDirect began in Great Britain in August 2008. Since then, more than 400 people have fallen ill there, about two-thirds of them have been children under 10, according to Chris Lane, a senior epidemiologist of the Health Protection Agency’s Center for Infections in London. Although the shipments of tainted mice were halted last year, people continue to get sick there, Mr. Lane said.

The first case in the United States appeared in January 2010, according Dr. Barton Behravesh. The C.D.C. has identified more than 30 cases in 17 states with the same strain as the British outbreak. She said the cases were not concentrated in one region but spread across the country. Half the victims were under 12.

Accounts from both sides of the Atlantic suggest that American authorities were slow to react to indications of a problem.

British investigators looking into the outbreak found that many of the victims came from families where snakes were kept as pets. They eventually began looking at the frozen mice fed to the snakes and found shipments from MiceDirect that contained the same strain of salmonella as that isolated from the victims.

British officials contacted MiceDirect, Mr. Lane said, and the company promised to act to prevent further contamination.

Kristen Nordlund, a C.D.C. spokeswoman, said the British officials told the agency in May 2009 of the outbreak there and the connection to MiceDirect.

She said that the C.D.C. found no cases of infection in the United States at that time. But it did tell the Food and Drug Administration, which regulates pet food companies like MiceDirect, about the British investigation.