Hawaii has declared a state of emergency on its Big Island as it battles the local spread of dengue fever, a disease outbreak that experts think may foreshadow how Zika virus transmission might occur in parts of the United States.

The declaration of emergency could help Hawaii authorities tap new funds to control mosquitoes that transmit the illness, which in turn could help the state lower the risk of the local spread of Zika. Both viruses are transmitted by the same type of mosquitoes and health officials have said places that have recorded spread of dengue are at risk of Zika transmission too.

The dengue virus does not typically circulate in Hawaii, but since last fall the state has reported about 250 cases of dengue fever. Island residents have made up the bulk of the cases, but some tourists have also been infected.

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This scenario — some local spread following introduction of the virus — is what experts at the Centers for Disease Control and Prevention have cautioned could  happen in parts of the United States with the Zika virus, now sweeping rapidly through South and Central America and the Caribbean.

Zika virus, which causes mild flu-like symptoms in people who contract it, is suspected to be linked to a surge in births of babies with abnormally small heads — a condition called microcephaly. Brazil, which has been experiencing an explosive Zika virus outbreak, has reported a substantial rise in microcephaly births in recent months.

Dengue and Zika are both flaviviruses. Dengue causes more severe illness than Zika virus does, though it has not been reported to trigger birth defects in the offspring of women infected during pregnancy.

Both are spread by Aedes mosquitoes. Southern sections of the United States have Aedes aegypti mosquitoes, which are known to transmit both viruses, while big parts of the central and eastern US have Aedes albopictus mosquitoes, which can transmit dengue virus and may be able to spread Zika too.

Hawaii’s Big Island has both types. And last October, health officials there learned that a woman who had not traveled off the island had tested positive for dengue infection. Several members of her family reported the same symptoms: fever, a rash, headaches, and muscle and joint pain.

An investigation started in late October by the state’s department of health — with assistance from the CDC — revealed there were many more cases.

The outbreak report, in the CDC’s online journal Morbidity and Mortality Weekly Report, details the findings to late November, by which point 107 cases had come to light.

Are you at risk for contracting Zika virus? Your level of risk depends in part on your living conditions. Alex Hogan/STAT

The number has more than doubled since then. The most recent update, from earlier this week, put the tally at 224 cases. Most were Hawaiian residents, but some were tourists.

Dengue used to be common in Hawaii before World War II, but since then there have only been two other outbreaks, in 2001 and 2011.

The report does not pinpoint how the virus made its way to Hawaii, though it suggests the route would be that a person infected elsewhere still had the virus in his or her blood upon traveling to, or returning home to, Hawaii. From there, the scenario works like this: Mosquito bites person. Mosquito becomes infected. Mosquito bites other people. And so on.

“It’s how this outbreak began although we’ve not been able to identify the index case,” Dr. Sarah Park, Hawaii’s state epidemiologist, told STAT in an email. “None of the visitors we identified as cases were among the earliest cases, so even if they had had history of travel from an endemic region, they couldn’t have been the ones to introduce the virus — it had to be another as yet unidentified traveler.”

Park said Hawaii has been alerting doctors to the risks of local outbreaks of dengue, Zika virus, and chikungunya (another virus spread by mosquitoes), because it has the mosquitoes that spread these viruses, loads of tourists coming from all over the globe, and a population that travels extensively.

When experts describe how they expect Zika virus spread to play out in the United States, they describe situations like the Hawaiian dengue outbreak.

“I think this is pretty consistent with what I would expect,” said Scott Weaver, an expert on viruses spread by mosquitoes and director of the Institute for Human Infections and Immunity at the University of Texas Medical Branch in Galveston.

Dr. Lyle Petersen, director of the CDC’s division of vector-borne diseases, has said he expects outbreaks to be small — not the millions of cases being reported in Latin America and the Caribbean — because in parts of the United States where Aedes mosquitoes are found, many people live in air-conditioned dwellings, with screens on windows and doors.

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