The Hawaii Department of Health says an outbreak of hepatitis A has grown to 31 confirmed cases.

The department says staff members worked through the holiday weekend to interview infected patients in an effort to find the cause of the infection.

State Epidemiologist Sarah Park says identifying the cause is difficult. She says accurately recalling all of the food consumed and locations visited during the infection period is challenging, especially for those who are still feeling ill.

Health officials say the virus can be spread by eating contaminated food, drinking contaminated water, close personal contact or sex. Symptoms include fever, fatigue, appetite loss, abdominal discomfort and diarrhea.

The department said last week there were at least 12 cases of hepatitis A on Oahu.