HONOLULU (AP) — Frustrated Hawaii health officials say they have faced a number of challenges trying to identify the source of a hepatitis A outbreak that has affected at least 74 people on the island of Oahu.

“It’s not as feasible as a lot of people might think,” state epidemiologist Dr. Sarah Park said of finding the source.

A major challenge has been the long incubation period of the disease, Park said. Symptoms, if there are any, can show up two to six weeks after exposure.

Hepatitis A is spread by contaminated food or water or close personal contact. Symptoms can include fever, fatigue, appetite loss, abdominal discomfort and diarrhea.

Health department staff members have scrambled to interview all those who have been sickened. But they say it can be difficult for them to remember who they came into contact with or what they consumed.

“We have to sit down with each person and say look at the calendar, go back almost two months and list every single thing that passed through your mouth – food or drink or what-not – in the past 52 days,” Park said.

Officials have been looking for commonalities such as shopping histories among the patients, but “there’s nothing really tying them together, unfortunately,” Park said.

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Seattle lawyer William Marler, who handles food safety litigation, said he is representing two of the Hawaii cases. Health officials have asked his clients for their purchase histories at two specific stores, he said.

“The health department is starting to dig deeper to look at things that might not at the beginning look like they are promising,” he said. “I’m pretty confident they’ll figure this out.”

People with confirmed cases range from 18 to 70 and are scattered across the island. Two now live on the Big Island and Maui but were on Oahu during their exposure period, the health department said.

The U.S. Centers for Disease Control and Prevention said Thursday officials aren’t aware of any other outbreaks in the nation or cases that may be connected to Hawaii’s outbreak.

In 2014, there were an estimated 2,500 acute hepatitis A infections in the U.S., according to the CDC.

Fortunately, there’s an effective vaccine, said Dr. Monique Foster of the CDC’s division of viral hepatitis.

All the Oahu cases involve adults – a good sign that children are being vaccinated, Park said.

Hepatitis A can range in severity from a mild illness lasting a few weeks to a severe illness lasting months. It’s usually spread when a person ingests contaminated fecal matter.

Investigating the outbreak has been taxing on health department personnel, and it’s possible they may never be able to track down the source, Park said.

“We understand the public’s frustration because we feel it, too,” she said.