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July 2023 Newsletter

Open House - August 12th

Photographs of people holding and standing near insects

We are excited to be hosting an Open House!  We will have kid-friendly activities, insect displays, information about rats and yellowjackets, displays of our equipment, and light snacks. The Open House will be held on Saturday, August 12th, from noon to 4pm.  Learn more HERE.  We look forward to seeing you!


2023 West Nile virus detections to-date. Green counties have had detections in non-human samples, such as mosquitoes or dead birds.

West Nile Virus 2023 Season

San Mateo County

To date, West Nile virus (WNV) has not been detected in any dead bird, mosquito, or sentinel chicken samples in San Mateo County in 2023.


Thus far, in 2023, WNV has been detected in 17 counties statewide, including Alameda County and Santa Clara County. These detections include 283 mosquito samples and 65 dead birds. No sentinel chickens have tested positive for WNV, and there have been no human cases reported in California. For additional information, visit


Water in Marshes - Preventing Mosquitoes

Photograph of a person in a uniform and hip-high rubber boots standing outside in a grassy-looking area. The person is looking down into a white cup on the end of a long metal stick.
Vector Control Technician Evan checks for mosquito larvae in a marsh after a high tide.

Three to four times a year, a “King Tide” event occurs in the Northern Hemisphere. Tides can also be higher than average during “full” or “new” moon events, which occur twice per lunar month. Both of these celestial events can flood our local marshes with brackish water. These flooded marshes are ideal for two species of day-biting, saltwater marsh mosquitoes. District staff inspect these areas accordingly: during the warmer months, inspection occurs 5-7 days after an above-average tide, and in the colder months, 3-4 weeks after the very high tide. District staff use several online platforms that predict tide levels, so appropriate planning for inspecting tidal marshes can be done.

Some areas of San Mateo County that District staff inspect include Pillar Point Marsh in Half Moon Bay, Faber Marsh in East Palo Alto, east of Shorebird Park in Foster City, and Outer Bair Island in Redwood City. Bair Island is approximately 92 acres and requires a boat to access.

Controlling these kinds of saltmarsh mosquitoes is one of the primary reasons San Mateo County Mosquito & Vector Control was formed. Work was started in the early 1900s in San Mateo, Burlingame, and Hillsborough when entomologists determined that the mosquitoes biting residents were from salt marshes along San Francisco Bay. Learn more about our District's early history HERE.

Over time, San Mateo County has had a significant amount of growth, development, and cooperation from stakeholders who manage these areas, reducing the amount of salt marsh we inspect for mosquitoes. Even though the inspection method has mostly stayed the same, it has become more efficient with the introduction of modern equipment and mosquito larvicides that can control mosquitoes for up to 40 days in these specific habitats. The knowledge we have gained over the years from the early innovators in mosquito and vector control still supports how we protect public health today.

In the News - Malaria

Photograph of a  brown mosquito with its abdomen full of red blood sitting on skin
Anopheles freeborni is a local mosquito capable of transmitting the malaria parasite. District staff regularly set traps to determine where in our county this mosquito can be found.

You may have seen the news that malaria cases were diagnosed in separate outbreaks in Florida and Texas recently. The Centers for Disease Control and Prevention (CDC) believe the two outbreaks were unrelated and are investigating the circumstances. Cases in both states were locally acquired, meaning the disease was not associated with travel to a country where malaria is endemic.

Malaria has not been a significant disease in the United States since the 1940’s and 50’s, when it was eradicated primarily through an aggressive mosquito control campaign. Today, malaria is most common in tropical and subtropical areas, including parts of Africa, Asia, and Latin America.

Historically, malaria was the cause of significant mortality throughout North America, including California. The disease was rampant in the state during the gold rush era, where it had a devastating impact on Native American tribes. The last known local transmission of malaria in California occurred in 1989. In recent years, about 100 travel-related cases (cases where someone traveled abroad and returned with the infection) have been reported in the state annually.

Malaria is a serious illness caused by small parasites of the genus Plasmodium. Symptoms of malaria include fever, shaking chills, headache, muscle aches, and tiredness; the disease can be fatal. The illness spreads from person to person via bites from infected Anopheles mosquitoes. While Plasmodium no longer regularly occurs in the United States, the potential vector mosquitoes are widespread.

San Mateo County has three mosquito vector species for malaria: Anopheles freeborni, Anopheles punctipennis, and Anopheles hermsi. The District traps Anopheles mosquitoes every summer to understand the distribution of these mosquitoes throughout the county. Staff also work with the San Mateo County Health Department to ensure that travel-acquired malaria cases do not lead to a local outbreak. Despite the recent cases in Florida and Texas, the chance of catching malaria in the United States remains extremely low.

Residents can minimize the risk of any mosquito-borne disease by controlling mosquito breeding and avoiding mosquito bites. This includes dumping and draining containers that hold standing water, using EPA-registered insect repellents, and reporting mosquito problems to the District.

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