An unusual outbreak of dengue virus fever has now made 139 people sick in Hawaii and authorities are debating what to do about it.

"Of the confirmed cases, 122 are Hawaii Island residents and 17 are visitors," the state department of health said in an update released Monday.

But there's not much they can do, an expert at the Centers for Disease Control and Prevention said.

"Dengue control is extremely difficult. We don't have a very good way of stopping dengue outbreaks," said Dr. Lyle Petersen, an expert on mosquito-borne infections at CDC who's just back from Hawaii.

"Maybe you can slow it down somewhat but there's no way to stop it."

That's in part because dengue usually causes a very mild illness, so people are still apt to go outside, where they are likely to be bitten by mosquitoes — which in turn will spread the virus to others.

Petersen says there are "clearly" more than 139 cases now on Hawaii. "You can't count them all. You are counting people going to the doctor and seeking medical help and getting tested. But people with more mild illness may not go to the doctor," he said.

Dengue is carried by mosquitoes — especially the Aedes aegypti mosquito that thrives in tropical zones. There's no treatment for dengue and no vaccine.

It can cause a very severe form called dengue hemorrhagic fever, which kills 20 percent of victims, although good medical care can lower death rates dramatically.