Invasive Mosquitoes a Concern as California Grapples With Zika Risk

A map from the California Department of Public Health shows where invasive mosquitoes have been found in California. These mosquitoes could theoretically transmit Zika virus.
A map from the California Department of Public Health shows where invasive mosquitoes have been found in California. These mosquitoes could theoretically transmit Zika.

There’s one topic San Diego obstetrician and gynecologist Dr. Anupam Garg gets asked about constantly these days by patients who are pregnant or looking to become pregnant.


The mosquito-borne virus linked to serious birth defects that is infesting large swaths of Latin America hasn’t reached the continental United States yet, although over 500 people have contracted the illness while traveling overseas. But with summer – and mosquito season – fast approaching, medical practitioners, some health officials and Dr. Garg’s patients are getting increasingly nervous.

“It really is one of the top concerns of women right now,” said Dr. Garg. “What’s particularly concerning about it is we don’t really have a lot of really powerful prevention tools, and once there is a case of Zika virus we don’t have any effective way of treating it.”

So far, no one has contracted Zika in California — but some travelers have returned to the state already infected. Epidemiologists say that it’s possible the virus could take hold in pockets of the state, if certain mosquitoes bite those with Zika infections, but the likelihood of a widespread epidemic is low. Zika typically results in a mild illness, but pregnant women who contract it are at increased risk of having babies with a serious birth defect called microcephaly, as well as other severe fetal brain defects.

San Diego and Los Angeles counties are at the top of the list when it comes to the number of travel-related Zika infections in California. They account for about half of the 49 cases detected in the state so far.

Southern California is also the area of the state where two types of mosquitoes capable of transmitting the Zika virus have been detected, although the mosquitoes have been found further north as well. If one of those mosquitoes were to bite an infected person here, Zika could start to spread locally and not just through travel-related infections, officials said. That would likely occur in small clusters, they said.

“I think it’s a matter of time,” said Kenn Fujioka, district manager of the San Gabriel Valley Mosquito and Vector Control District, one of five such districts in Los Angeles County. “The question for vector control folks is, how bad will the cluster be?”

Mosquito-transmitted viruses

It’s not the first time vector control and health officials in the state have had to worry about dangerous diseases spread by mosquitoes. Dengue, which can cause hemorrhagic fever and death, and Chikungunya, a source of fever and debilitating joint pain, have also appeared in the continental United States among people who had traveled to infected countries. Small clusters of locally contracted dengue cases have occurred in Texas, Florida and Hawaii over the past decade.

Both diseases are transmitted by aedes aegypti and aedes albopictus mosquitoes, which can also carry the Zika virus.

West Nile virus, which is spread by a different species of mosquitoes, already has a foothold in California. More than 5,000 human cases of the virus have been reported in the state since 2003, including 229 deaths.

Laurene Mascola, chief of the Acute Communicable Disease Control program for Los Angeles County, said although Zika is a concern for public health officials there, West Nile virus poses a greater threat. The county had over 300 reported cases of West Nile virus last year, she said.

Zika, on the other hand, is more likely to follow the trajectory of the other aedes-transmitted infections Dengue and Chikungunya, which are occasionally detected in travelers but never through mosquitoes present in California, she said. The few locally transmitted Dengue cases in the United States have occurred in tropical areas where mosquitoes proliferate, she noted. California has a much dryer climate.

“Anything’s possible, but it’s not happened before with a virus that’s also spread by the aedes mosquito,” Mascola said. “First of all, 13 (Zika) cases is not a whole lot of cases when we had over 100 chikungunya cases last year reported to us, we’ve had dengue cases reported to us, and we’ve never seen local transmission with either one of those.”

Epidemiologist Thomas Scott at the University of California, Davis, agreed that the risk of locally transmitted Zika infections in the state is low. He said small local outbreaks could occur, but the virus is unlikely to get permanently established here. That’s because living conditions in California reduce people’s exposure to mosquitoes, making it more difficult for the virus to spread.

“In places like Brazil or Central America and Columbia where we’re seeing huge numbers of people being infected with Zika, people don’t have screening on their houses, they don’t live the same way as we do with air conditioners. Mosquitoes essentially come and go and live in the house with them,” he said. “The way that we live in California …we essentially create a physical barrier that prevents that from happening.”

Working to stop the spread

Still, vector control officials are worried that the presence of the aedes mosquito species is growing in California, increasing the risk of local mosquito-borne infections such as Zika, said Fujioka. These invasive mosquitoes, which have been detected as far north as Alameda and San Mateo counties, are extremely difficult to eradicate, Fujioka said. They are small and fast, bite during the day, and can breed in tiny sources of water such as rainwater that pools in a plant leaf, inside the hole of a bowling ball, or in a crumpled up piece of paper, he said.

In San Gabriel Valley, vector control officials are asking the public to help with eradication efforts by removing standing water sources, and reporting any sighting of the black-and-white insects. So far, the San Gabriel Valley and Greater Los Angeles Vector Control districts are the two areas in the county where the aedes mosquitoes have been found, he said.

“We are enlisting the help of every organization that we can identify – community based organizations, neighborhood watches, city councils, code enforcement, public works people,” he said. “The challenge for California is to change human behavior, where these small sources of water are looked at seriously, wearing repellent is looked at seriously. And it’s going to be difficult. It hasn’t been accomplished yet.”

The California Department of Public Health is also working with local vector control agencies to stop the spread of aedes mosquitoes, and is keeping local health departments and health care providers updated with the latest news and recommendations from the federal Centers for Disease Control and Prevention about Zika, said public information officer Abram Arredondo. He said the department also plans to apply for federal funding made available to help fight Zika.

People traveling to Zika-infected regions of the world, or who live in areas where there are diseases spread by mosquitoes are advised to take steps to avoid mosquito bites. These include wearing insect repellent and clothing with long sleeves, staying in places with air conditioning and screened windows and doors, and removing standing water sources where mosquitoes could breed.

Physicians such as Dr. Garg are advising pregnant women to avoid traveling to regions where Zika is present if at all possible. Women who have travelled to such regions should wait at least eight weeks before getting pregnant. Because Zika can be sexually transmitted, men who have traveled to infected regions must also take precautions.

The medical community is watching Zika very closely, Garg said.

“We’re kind of hoping that we’ll stay safe for at least a little while, but we’ll definitely be watching for any signs or reported cases of it,” he said. “When that happens then we’re obviously going to have our patients take a much higher level of precaution for avoiding mosquitoes.”

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