‘Extremely Rare’ Case of Locally Acquired Dengue Fever Detected in California

Officials say the risk of exposure to the virus, which is spread by mosquito bites, is very low for residents

Mosquitoes on an angled netted surface
Aedes aegypti mosquitoes can spread dengue fever to humans via bites. CHRISTOPHE SIMON / AFP via Getty Images

A person in Pasadena, California, has caught a case of dengue fever without traveling outside of the United States, reports the Pasadena Star-News John Orona. The diagnosis marks the first local transmission of the virus in California. Still, the risk to residents is low, the Pasadena Public Health Department (PPHD) said in a statement last week.

Based on its years of surveilling mosquito-borne diseases in Pasadena, the PPHD has “confidence that this was likely an isolated incident and that there is very low risk of additional dengue exposure in Pasadena,” Matthew Feaster, an epidemiologist with the PPHD, says in the statement.

Dengue fever is a viral infection spread to humans by the bites of infected mosquitoes, primarily Aedes aegypti. It is endemic in more than 100 countries and appears most commonly in tropical and sub-tropical climates. About half of the global population lives in areas at risk for dengue, and between 100 and 400 million people get infected each year.

In the continental U.S., however, local transmission of the virus is “extremely rare,” per the statement—most reported cases are acquired while traveling. Still, more than 500 locally acquired cases of dengue have been recorded by the Centers for Disease Control and Prevention (CDC) this year, with nearly 90 percent occurring in Puerto Rico. The others were largely in Florida, with one case confirmed in Texas, prior to the recent California infection.

Cases of dengue are increasing across the globe and spreading to places that previously didn’t have the virus, according to the New York Times’ Stephanie Nolen.

Record rainfall and recent humid conditions have contributed to a growing mosquito population, as Jason Farned, district manager for the San Gabriel Valley Mosquito and Vector Control District, tells the Pasadena Star-News. Numbers of Aedes aegypti are increasing in Southern California, and spurred by the rain from Hurricane Hilary in August, lots of mosquito eggs recently hatched in the region.

Aedes mosquitoes thrive in warm and humid environments, so definitely climate change and rising temperatures and also extreme weather events are helping extend their habitat range,” Gabriela Paz-Bailey, an epidemiologist and public health expert with the CDC’s division of vector-borne diseases, tells the New York Times.

In June, the CDC reported that malaria, another mosquito-borne illness, had spread in the U.S. for the first time since 2003.

Most people who get dengue don’t develop symptoms, and most people get better in one to two weeks. But in rare cases, dengue cases can be severe and fatal.

Symptoms may include high fevers, headaches, pain behind the eyes, muscle and joint pain, nausea, vomiting, swollen glands and rashes. After the fever subsides, severe cases could lead to strong abdominal pain, persistent vomiting, rapid breathing and bleeding gums or nose.

Following the detected case in Pasadena, the San Gabriel Valley Mosquito and Vector Control District has deployed additional traps to aid in testing mosquitoes for dengue. The district has not identified dengue infections in the insects.

To reduce mosquito populations and stop the spread of mosquito-borne illnesses, the PPHD recommends eliminating areas of standing water, properly maintaining swimming pools and changing water in small containers like pet dishes and bird baths. The department advises people to wear insect repellant, long pants and loose-fitting, light-colored, long-sleeved shirts to prevent mosquito bites.

Public health officials think the infected individual was bitten by a mosquito that had previously bitten someone infected with dengue, per according to the Los Angeles Times’ Jeremy Childs. The person who tested positive is now recovering.

“I don’t think that the case in Pasadena or anything else that I’ve seen lately is an indication of any looming crisis in the United States in the short term,” Alex Perkins, an infectious disease epidemiologist at the University of Notre Dame, tells the New York Times. “But I think the general expectation that this is going to be a growing problem in the United States is reasonable.”

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