SEATTLE (AP) — State health officials are warning that cases of mosquito-borne illnesses are on the rise in travelers returning to Washington after vacations in warmer weather.

The Seattle Times reports that combined cases of dengue fever and chikungunya have risen this year, with 37 cases as compared to 23 cases in 2014 and even fewer in the years before.

Six travelers were hospitalized and two developed neuroinvasive disease, a serious complication.

In Hawaii, an outbreak of dengue fever has sickened more than 150 people this year. Chikungunya has been on the rise in Mexico for more than a year and other popular travel destinations such as the Caribbean and central and south America have also seen an increase in cases.

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