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Despite reports of malaria cases in Florida and Texas (summer 2023), local transmission of malaria in the U.S. is very rare.

Most malaria cases are acquired when people travel outside of the U.S. where malaria transmission occurs. Anopheles mosquitoes that can spread malaria are present in California, but the parasites that cause malaria are not currently found in these mosquitoes in California. The last cases of local transmission of malaria in California were reported in 1989.

About 100 cases of malaria are reported each year in California from people who were infected while traveling in other countries. Since malaria is a reportable disease, all diagnosed imported cases are tracked by the state and local public health departments. The patient’s history and potential for exposure to local malaria vectors are evaluated and if need be, actions are made to prevent possible transmission within the local community.


What is malaria?

Malaria is a parasite that is transmitted to humans through the bite of an infected mosquito. There are four species of malaria parasites: Plasmodium falciparumP. vivaxP. ovale, and P. malariae. These four species differ in their biology and in severity of disease.

Malaria is the leading cause of death and disease worldwide and was once endemic throughout much of the United States. During the civil war, there were over 1.2 million cases of malaria among soldiers of both sides. In 1933, 30% of the human population in the Tennessee Valley were infected.

Malaria was rampant in California during the gold rush and early 1900s. In fact, many of the mosquito abatement programs in California were initially formed to combat malaria.

Today, there are about 1,200 cases of malaria reported in the US each year, approximately 300 of these (25%) occur in California. In almost every case, the disease is acquired outside the US in countries where malaria is common. However, there have been several occasions in which local transmission has occurred. Mosquitoes acquire the infection from persons with active malaria and transmit the parasite to local residents. The most recent of these occurred in 1974 in San Diego.

What are the signs and symptoms of malaria?

  • Recurring bouts of high fever and flu-like illness with shaking, chills, and sweating with headache, muscle aches, and fatigue
  • Severe infections with Plasmodium falciparum malaria can result in coma, renal failure, convulsions, and death

How can I get malaria?

Malaria parasites are acquired through the bite of an infected mosquito. Only certain mosquito species of the genus Anopheles are capable of transmitting malaria. Because the parasite lives in the blood, it can also be transmitted through blood transfusions, organ transplants, contaminated needles, and breast milk. Use an EPA-registered insect repellent to reduce your risk of mosquito bites.

What is the life cycle of malaria?

Mosquitoes become infected with the malaria parasite by feeding on the blood of an infected human. The parasites develop in the mosquito’s gut for about a week before moving to the salivary glands. Once in the salivary glands, the parasite can move into a new human when the mosquito acquires its next blood meal.

Malaria parasites move from a person’s bloodstream to the liver where they multiply and develop. The parasites may remain in the liver for 1 week to several months. After this, the parasites enter the bloodstream where they invade red blood cells. This stage multiplies in the red blood cells, periodically causing the cells to burst and release more parasites into the bloodstream. For two of the forms of malaria (P. ovale and P. vivax), some parasites can remain in the liver causing relapses months or years after the person was bitten.

Can I get malaria in San Mateo County?

Malaria is not currently present among mosquitoes in San Mateo County. However, two of the species of mosquitoes in San Mateo County are capable of transmitting the disease: Anopheles hermsi and An. punctipennis. Local mosquitoes could become infected by feeding on someone with an active case of malaria. For this reason, the District works closely with the County Health Department on malaria cases. When malaria 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 that has malaria?

Medications are available to prevent people from becoming infected with malaria while traveling. The type of medicine and the best time to take it is best determined on an individual basis through consultation with a physician. Ask your doctor for advice. Use an EPA-registered insect repellent to reduce your risk of mosquito bites.


Page last reviewed: December 28, 2023

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