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Another case of dengue fever in California

A few days ago, I wrote about a case of dengue fever that was found in a non-traveler in Pasadena, California. But it was only one case, and despite the dangers of dengue fever, it was hard to be concerned about it.

But there has been a second case of dengue fever found in a non-traveler in Southern California, this time in Long Beach. One case may be easily dismissed, but two cases start to make me wonder if dengue fever is now more prevalent in the Los Angeles area than we originally thought.

This article will review the facts about dengue fever and what we know about the case in southern California.

close up shot of a mosquito on human skin
Photo by Jimmy Chan on

What is dengue fever?

Dengue fever is one of four viruses transmitted by the Aedes aegypti mosquitoes to humans. It infects over 100 million people a year worldwide. A. aegypti is endemic to areas around Los Angeles, including Pasadena and Long Beach.

The principal symptoms of dengue fever are high fever, severe headache, severe pain behind the eyes, joint pain, muscle and bone pain, rash, and mild bleeding (e.g., nose or gums bleed, easy bruising). Generally, younger children and those with their first dengue infection have a milder illness than older children and adults. 

Dengue hemorrhagic fever (DHF) is a more severe form of dengue infection. It can be fatal if unrecognized and not properly treated in a timely manner. DHF is caused by infection with the same viruses that cause dengue fever. With good medical management, mortality due to DHF is still significant but can be reduced to less than 1%.

Dengue fever is not transmitted from one person to another with direct contact, only from the bite of the A. aegypti mosquito.

Although dengue fever is relatively rare in the USA, there have been outbreaks in 2009 and 2010 in the Florida Keys along with one in 2005 in Texas. Dengue fever is endemic in Mexico, so carrier mosquitos could hypothetically cross the border into Southern California without much trouble, especially if the weather is wet (like it has been in 2023).

There are several outbreaks of dengue fever observed across the world in 2023. And with higher temperatures and sea level rises, the mosquito vectors for Dengue will move further and further north putting more of the world population at risk.

Florida has been trying to eradicate the dengue-carrying mosquitos by using genetically modified mosquitos to breed with the native population of mosquitos. The initial studies have shown a lot of promise, and without any foreknowledge on my part, they could be used in the Los Angeles area if more dengue fever cases appear.

There is a vaccine to prevent dengue. It is approved for children ages 9 – 16 who have previously had dengue and live in areas where dengue is common. Clearly, your local Los Angeles-based pediatrician is not going to have the vaccine available. And it probably won’t become widely available in Southern California unless there are many more cases of dengue.

Dengvaxia is the USFDA-approved vaccine for dengue fever and is indicated for preventing dengue virus serotypes 1, 2, 3, and 4. The Dengvaxia® vaccine is available for certain people following a pre-delivery diagnostic test review.

One of the best ways to prevent dengue is by avoiding mosquito bites:

  • Wear insect repellent with DEET or another U.S. Environmental Protection Agency (EPA)-registered insect repellent. Make sure to follow the instructions for using the repellant.
  • Wear clothes that cover your arms, legs, and feet.
  • Close unscreened doors and windows.

Dengue fever in Southern California

On November 1, 2023, the City of Long Beach Department of Health and Human Services (Health Department) reported one case of dengue in a resident who has not traveled outside the U.S. This is the first non-travel-related case of dengue in Long Beach, which has reported five travel-related cases in the past.

As in the previous case discovered in Pasadena in October 2023, the City of Long Beach health department and mosquito control teams are monitoring local populations of A. aegypti mosquitos to determine if dengue fever is becoming prevalent.

According to the Long Beach Health Department:

The Health Department monitors and investigates mosquito-borne illnesses and have enhanced these efforts in response to this case. The Department’s Vector Control team regularly sets traps and tests samples of the mosquitoes found in Long Beach. To date, no mosquitoes collected by the Department have tested positive for dengue. The Department is also coordinating with the California Department of Public Health, Pasadena Health Department and the Los Angeles County Department of Public Health and is conducting education and outreach near the area where dengue was identified. 

Again, this second case is not a reason to panic that the area is having a localized outbreak of dengue fever. That being said, there is cause for concern because both cases appear to have been caused by local mosquitos not ones from endemic areas of the world.

I would not panic about these cases if I lived in the area, but there I’d be really cautious and avoid mosquito bites. However, if more cases arise over the next few weeks, then it might be time to sound the alarm.


  • Centers for Disease Control and Prevention (CDC). Dengue hemorrhagic fever–U.S.-Mexico border, 2005. MMWR Morb Mortal Wkly Rep. 2007 Aug 10;56(31):785-9. Erratum in: MMWR Morb Mortal Wkly Rep. 2007 Aug 17;56(32):822. PMID: 17687243.
  • Graham AS, Pruszynski CA, Hribar LJ, DeMay DJ, Tambasco AN, Hartley AE, Fussell EM, Michael SF, Isern S. Mosquito-associated dengue virus, Key West, Florida, USA, 2010. Emerg Infect Dis. 2011 Nov;17(11):2074-5. doi: 10.3201/eid1711.110419. PMID: 22099104; PMCID: PMC3310564.
  • Radke EG, Gregory CJ, Kintziger KW, Sauber-Schatz EK, Hunsperger EA, Gallagher GR, Barber JM, Biggerstaff BJ, Stanek DR, Tomashek KM, Blackmore CG. Dengue outbreak in Key West, Florida, USA, 2009. Emerg Infect Dis. 2012 Jan;18(1):135-7. doi: 10.3201/eid1801.110130. PMID: 22257471; PMCID: PMC3310087.
Michael Simpson
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