MADISON, Wis. (AP) -

Animal health officials say a strain of salmonella bacteria resistant to several drugs has been linked to infected dairy bull calves purchased in Wisconsin.

State and federal officials are investigating 12 human cases of infection in eight Wisconsin counties.

People can become infected by salmonella through contact with animal manure and by person-to-person contact.

State Veterinarian Paul McGraw is urging dairy farmers to take precautions to protect people who work closely with animals.

Symptoms of salmonella infection include diarrhea, fever and abdominal cramps.