The number of cases of dengue fever in Hawaii has risen to 139, prompting health authorities this week to warn residents and travelers to the popular winter vacation destination to take precautions to avoid contracting the virus.

The Department of Health said in its latest statement on the outbreak that 122 residents and 17 visitors on Hawaii Island, the largest of the state’s eight main islands, have been confirmed to have dengue fever.

The first cases in the current outbreak were traced back to September, and over time they developed into what is now being treated as a cluster of 108 adults and 31 children. The cluster has now grown to rival the last major dengue outbreak, which took place in 2001 and lasted about 10 months with 92 cases on Maui, 26 on Oahu and four on Kauai.

The outbreak is taking place at the start of the island’s peak tourism season, which usually begins around mid-December and lasts until March or mid-April. The Hawaii Tourism Authority published an alert last month instructing tourists to take precautions against the illness.