The growing number of dengue fever cases in Hawaii County has prompted the county’s mayor to declare a state of emergency, a measure that aims to reduce the spread of the disease.

William P. Kenoi issued the declaration on Monday as the state Department of Health reported that there have been confirmed 251 cases of the mosquito-borne illness since last September.

All of the confirmed cases have been in Hawaii County, which is the state’s largest island.

Kenoi’s declaration allows people to dump tires at county landfills, which had been outlawed. The Centers for Disease Control and Prevention (CDC) says used tires can collect rain, making them a good place for mosquitos to lay eggs, and mosquito control is key to combating dengue.

Hawaii Governor David Y. Ige issued a statement saying he supported Kenoi’s emergency proclamation, but he would only declare a statewide emergency if certain conditions were met.

According to the World Health Organization (WHO), dengue fever usually triggers a flu-like illness that lasts up to a week. Symptoms include headaches, vomiting, muscle pains and rashes. In some cases, however, dengue can become severe and lead to potentially fatal complications.

The WHO says severe dengue causes about 500,000 hospitalizations and 12,500 deaths every year, though access to proper medical care significantly lowers the disease’s mortality rate.

Hawaii health officials have not reported any deaths as a result of this outbreak.

The Hawaii Department of Health says dengue isn’t usually found in Hawaii, but there have been some cases of infected travelers coming to Hawaii from areas where the disease is spread. But it says this outbreak is of locally-acquired dengue, the state’s first such event since 2011.

Dengue is different than the Zika virus, another mosquito-borne illness. However, both dengue and Zika can be transmitted by the Aedes aegypti mosquito and both have spread to new areas.

The WHO says dengue has reached more than 100 countries over the past 50 years, recording a 30-fold increase in its incidence. About half the world’s population is now at risk of infection.

The CDC has issued numerous travel warnings about the Zika virus, and the WHO recently deemed Zika an international public health concern as scientists investigate its potential connection to a rare birth defect called microcephaly that affects head size.

The Hawaii Department of Health has said that one Oahu child who was born with microcephaly had been infected with Zika, though his mother likely got infected when she was living in Brazil. The country saw a substantial rise in microcephaly last year.

Only about 20 percent of those infected with Zika show any symptoms, according to the CDC. Those symptoms include fever, rash and joint pain and most people fully recover within a week.