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Black and white microscopic image of dengue virus
This transmission electron microscopic (TEM) image depicts a number of round, Dengue virus particles that were revealed in this tissue specimen. (CDC/ Frederick Murphy)

What is dengue?

Dengue (den-gee) is a disease caused by any one of four closely related dengue viruses that are spread by mosquitoes. The mosquitoes that spread dengue (specifically Aedes aegypti and Aedes albopictus, also called “Aedes mosquitoes”) live in many parts of the world. About 100 million people get sick with dengue in the world every year.


Where does dengue occur?

Dengue is common in many tropical and sub-tropical areas of the world, where Aedes mosquitoes thrive. These areas include parts of Africa, the Middle East, India, Southeast Asia, South America, Central America, and North America (specifically Mexico). Dengue is common in many U.S. territories, including Puerto Rico and American Samoa. According to the U.S. Centers for Disease Control and Prevention (CDC), almost half of the world’s population, about 4 billion people, live in areas with risk of dengue. In California, cases of dengue are reported each year in people who have traveled to areas of the world where dengue is common.

Dengue is not common in the United States, but local outbreaks have been reported in some states where Aedes mosquitoes are found, including parts of southern Texas, southern Florida, and Hawaii. In 2023, two cases of dengue from mosquitoes in California were reported - one in Pasadena and one in Long Beach.


How do people get dengue?

People get dengue from the bite of an infected Aedes mosquito. Dengue virus cannot spread directly from person to person. An Aedes mosquito becomes infected when it bites a person who has dengue virus in their blood. About a week after a mosquito bites an infected person, the mosquito is able to transmit the virus to other people when it bites.

The mosquitoes that spread dengue, Aedes aegypti (yellow fever mosquito) and Aedes albopictus (Asian tiger mosquito), are not native to (or naturally found in) California, but they have been introduced to the state and are now found in multiple counties in California.  It is possible for Aedes mosquitoes in California to spread dengue in the future under the right conditions.


What are the signs and symptoms of dengue?

Not everyone who is infected with dengue virus from a mosquito bite gets sick. About 3 out of every 4 people who are infected never develop any symptoms. If a person does get sick with dengue, their symptoms can be mild or severe.

Mild Dengue (or “Dengue Fever”) - Symptoms of dengue fever usually last 2-7 days. Most people get better in about a week. The most common symptoms of dengue fever include fever and any of the following: severe headache, pain behind the eyes, joint pain, muscle and bone pain, rash, bruising on the skin

Severe Dengue - Severe dengue usually starts with fever and other mild symptoms of dengue fever. However, once the fever starts to go away, signs and symptoms of severe dengue develop within a few hours. Severe dengue causes blood vessels in a person’s body to leak, causing swelling and fluid build-up in the stomach and around the lungs. If not treated, severe dengue can result in death. Severe dengue is a medical emergency. If you have had dengue once before, you are more likely to develop severe dengue. The warning signs of severe dengue include: bleeding from the nose or mouth (gums), severe pain in the stomach or abdomen, vomiting (at least 3 times in 24 hours), vomiting blood or blood in the stool

If you think you have severe dengue or any warning signs of severe dengue, go to the emergency room immediately. Be sure to tell the doctor or healthcare provider if you have traveled recently or if you have had dengue before. If you remember being bitten by mosquitoes recently, let your doctor know.


Can I get dengue in San Mateo County?

Dengue is not currently present among mosquitoes in San Mateo County. However, a species of mosquito capable of transmitting dengue, Aedes aegypti, has been found in San Mateo County as recently as 2015 (but not since). Local mosquitoes could become infected by feeding on someone with an active case of dengue. For this reason, the District works closely with San Mateo County Health Department on all mosquito-borne disease cases. When a mosquito-borne disease is detected in a patient in San Mateo County, the health department notifies the District. District staff will then conduct mosquito surveillance in the area to ensure that vector mosquitoes are kept at very low numbers.

What can I do if I am traveling to a country where dengue is present?

There is currently no vaccine or medication to prevent infection with dengue. Travelers are advised to avoid mosquito bites. Use an EPA-registered repellent to protect yourself from mosquito bites and the disease they carry.


For more information:

CDPH's Dengue Fact Sheet (including several questions/answers above)

CDC's Dengue webpage


Page last reviewed: March 5, 2024

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