MADISON, Wis. — A multistate outbreak of salmonella bacteria that is resistant to several drugs has been linked to infected dairy bull calves purchased in Wisconsin, according to federal and state health officials.

According to the Centers for Disease Control and Prevention, 21 people were infected in eight states from Jan. 11 through Oct. 24. Eight of the 21 people sickened were hospitalized, but no deaths have been reported.

Those who were sickened ranged in age from 1 year to 72. Wisconsin has the most cases with 12 people infected in eight counties. Among the Wisconsin patients, more than 90 percent had direct or indirect contact with dairy bull calves and more than half the illnesses occurred in children under 10 years of age, according to the Wisconsin Department of Health Services.

In Minnesota, health officials said a 16-year-old boy was sickened with the Salmonella Heidelberg strain in July shortly after purchasing calves from a Wisconsin dealer for a 4-H project. The boy recovered from his flu-like illness without hospitalization.

The CDC says its investigation identified dairy bull calves from livestock markets in Wisconsin as the likely source of infections. Federal and state health and agricultural officials are investigating where calves associated with the bacteria were sold or originated.

Minnesota and South Dakota each have two cases and California, Iowa, Idaho, Missouri and Oklahoma each have one.

Health officials say people should always wash their hands thoroughly with soap and water after touching livestock or anything in the area where the animals live. The agency has not issued any warning against eating beef or dairy products in relation to the outbreak.