Puerto Rico has declared a public health emergency amid an ongoing outbreak of dengue infections, a mosquito-spread viral infection that can cause fever, aches, rash, vomiting, and, in about 5 percent of cases, a severe disease marked by internal bleeding and shock.

The US territory has tallied 549 cases since the start of the year, representing a 140 percent increase compared with cases tallied at this point last year, according to the territory's health department. The Associated Press reported that more than 340 of the 549 cases have been hospitalized.

In 2023, the island nation of more than 3.2 million people had over 1,000 cases of dengue throughout the year.

But this year's cases are rising rapidly, leading health officials to warn residents to be vigilant and take prevention measures. That includes wearing insect repellent and draining or treating any standing water where mosquitoes can breed. The mosquitoes that transmit dengue—Aedes aegypti—seem to prefer breeding in artificial containers, such as flower pots, buckets, water drums, vases, plastic containers of any kind, the inner rings of discarded automobile tires, and trash in general. They can breed in tiny amounts of water, such as in bottle caps.

Puerto Rico's outbreak of dengue is part of a larger regional trend, the island's health department noted. Last year, the Americas saw the highest number of dengue cases ever recorded, totaling 4,565,911, according to the Pan American Health Organization (PAHO), a division of the World Health Organization. So far this year, the region has reported 3,578,414 cases, including 1,039 deaths. The bulk of the cases are in Brazil, which has reported over 2.9 million cases so far. Paraguay has reported over 191,000 cases, and Argentina has reported over 134,000 cases.

In a December 2023 risk assessment, the PAHO deemed the risk to human health from dengue to be "high" in the region. The report noted that the effects of climate change are driving higher rainfall, higher temperatures, and more humidity, allowing A. aegypti to expand into new areas and increase breeding. A. aegypti are already found in much of South America, Central America, and the southeastern US.

This post was updated to include additional case tallies.