Two types of mosquito known to carry dengue and yellow fever - tropical illnesses rarely seen in California - have been spotted around the state, including San Mateo County, and now vector control experts are scrambling to kill them off before they become seasonal threats.

Specialists in mosquito control are going door-to-door in several Bay Area counties to hunt for signs of Aedes aegypti, also called the yellow fever mosquito, which was found in San Mateo County in August. The same mosquito type was seen in at least two other California counties over the summer.

There also have been numerous reports of another invasive pest called Aedes albopictus, or Asian tiger mosquito, in Los Angeles County over the past three years, raising concerns that it too could spread to other parts of the state.

All of the reports have vector control experts worried that these mosquitoes could become established in the state. Unlike native species, these mosquitoes are able to spread viruses like dengue to humans.

For now, the risk of being bitten and developing dengue or yellow fever is small, since the viruses themselves aren't often present in California. But the state does see occasional cases of both illnesses exported from other parts of the world, and if the mosquitoes become prevalent, those viruses will have a relatively easy vehicle for spreading among humans.

"We don't really have yellow fever here at all, and the only dengue fever cases are imported," said Deborah Bass, a spokeswoman for the Contra Costa Mosquito and Vector Control District. "What we have now, with these mosquitoes, is a mechanism for transmission."

Disease returning

Dengue fever, which for decades was pretty much wiped out in the United States due to aggressive vector control efforts, has been creeping back into communities in southern Florida in recent years along with the appearance of carrier mosquitoes.

Both dengue and yellow fever are spread only by mosquito bites - the viruses cannot pass from human to human. So as long as there are no mosquitoes to carry the virus, there is no risk to the larger population if a single person is infected.

Symptoms of dengue fever are elevated temperature, headaches and intense pain in the muscles, joints and bones; the illness is sometimes called break-bone fever. Severe cases can lead to dengue hemorrhagic fever, in which the capillaries can become leaky and patients can die from circulatory failure or shock.

There is no specific treatment for dengue fever.

Symptoms of yellow fever include fever, headaches and body aches, and like dengue, more severe cases can cause bleeding and shock that lead to death. There is no specific treatment for yellow fever, although there is a vaccine.

History may fuel the concern of vector control and public health over the newly found mosquitoes.

West Nile virus - which is also spread primarily through mosquitoes - was nonexistent in the United States just 15 years ago. Now it's a regular seasonal threat, with 275 cases reported in California so far this year. Bass said there were five cases in Contra Costa County this year, which is normal.

No clear answer

No one can say for sure why the mosquitoes that carry dengue and yellow fevers are starting to spread into California, said David Brown, manager of the Sacramento-Yolo Mosquito and Vector Control District and a member of the Mosquito and Vector Control Association of California.

Climate change is one possibility - these mosquitoes prefer areas with relatively warm winters - along with increased travel to and from tropical areas where the mosquitoes are normally present, he said.

Both types of mosquitoes are known to lay eggs that can stay dormant for months or even years in dry environments. Brown and other mosquito experts say a "queen" could lay eggs in a flower pot in Central America that is then carried into the United States. When that pot is filled with water - from winter rains or a watering can - that can refresh the eggs and allow them to hatch.

"All it would take is for someone to take that pot into San Francisco and fill it with water, and you could have a new mosquito population," Brown said. "The challenge is to keep it from getting established in an area."

It's critical to identify new populations early and kill them before they can expand, Brown said.

In the Bay Area, vector control experts are advising residents to look out for unusual mosquito activity, especially insects that bite during the day, instead of at dusk or dawn, and stick around after the typical mosquito season has ended in mid- to late October.

Distinctive look

The invasive mosquitoes look strikingly different too, said Bass. They're both about a quarter of an inch long, with distinctive white stripes along their black bodies.

Anyone who sees such a mosquito, or who finds mosquitoes biting in the middle of the day, should report the pests to the county vector control department.

"We have 23 different kinds of mosquito in Contra Costa alone. Aedes aegypti could be here already and we just haven't found it yet," Bass said. "But we're looking for it. We're going to have to rely on the public more than ever to help us."