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By Trina Wood on August 3, 2016

It’s hard to read the news — in print or online — without coming across at least one article on Zika virus and anxiety about increased transmission rates, especially with the Olympics starting this week in Brazil. Of primary concern are women of childbearing age whose pregnancies could be affected by the mosquito-borne virus linked with severe birth defects such as microcephaly.

Aedes aegypti mosquito
The Aedes aegypti, a known carrier of Zika, is found in California but is not a common as the Culex  mosquito. (James Gathany/photo)

Living in Yolo County, I’ve been more concerned about members of my family contracting West Nile virus than Zika. But just last week, researchers in Brazil announced that the southern house mosquito Culex quinquefasciatus, common to Central and Southern California, has been identified as a new carrier for Zika.

This mosquito is more abundant in many areas  than the two types of Aedes (Aedes aegypti and Aedes albopictus) mosquitoes that previously were known to transmit Zika.

Epidemiologist is not sure Culex is a carrier

“We’re not 100 percent sure that the house mosquito is a carrier,” says epidemiologist Chris Barker with the UC Davis School of Veterinary Medicine. Barker says he is awaiting confirmation in peer-reviewed scientific literature.

The bigger question is whether Culex mosquitos can transmit Zika efficiently enough to be important in passing the virus from one human to another. Barker points out that the southern house mosquito primarily bites birds and other mammals. Human-biting Aedes species still are likely to be more efficient transmitters of Zika, he says, even if scientists in the lab show that Culex species have the ability to transmit the virus.

Predicting new outbreaks is more difficult

Aedes albopictus mosquito
Another Zika carrier is the  Aedes albopictus mosquito. (marcouliana/GettyImages photo)

The invasive Aedes mosquitoes that can transmit Zika virus are now spreading in many cities in California, aided by human trade and travel, and infected travelers are being detected after returning with Zika virus infections.

“The potential for spread of Zika virus is increased by the fact that both mosquitoes and people know how to get around,” Barker says.

That’s the reason for concern about so many people traveling to Rio during a Zika outbreak.

This interactive surveillance map shows where the Aedes aegypti and Aedes albopictus, both known carriers of Zika, are concentrated in California.

Since the Brazilian outbreak began last fall, approximately 1,600 Zika cases in the U.S. have come to light, all associated with travel to Zika-affected countries — until last week. Florida health officials announced that four individuals in Miami-Dade and Broward counties have been infected with Zika by local mosquitoes, the first known cases of the virus being transmitted by mosquitoes in the U.S.

“We have a lot of pieces of the epidemiological puzzle worked out with regard to mosquito-borne viruses,” Barker says.

“We’re reasonably good at starting with an outbreak and working backward to explain likely causes, but predicting when and where a new outbreak will occur is more difficult. The biggest risk associated with the Olympics is a collective one. If even a few people return from the Olympics and start transmission in new areas, that will present big challenges for public health and vector control programs.”

West Nile remains the great concern here

Barker says West Nile virus is still of greater concern for people in this area than Zika. In fact, Yolo County officials announced that two more residents had tested positive for the virus last week.

Although many of those infected with West Nile are symptom-free, the virus still poses a serious health risk. And this year, thanks to improved water conditions and hot weather, the mosquito levels are particularly high — five to six times the normal levels in some places, which means that more of those mosquitoes carry West Nile.

4 tips to protect yourself (and others)

No matter where you live and what mosquito-borne illness you may be more at risk of contracting, the same rules of avoiding infection apply, Barker says. 

Contact your local vector control agency if you have questions or concerns about mosquitoes or the viruses they carry.

  1. Wear protective clothing such as long-sleeve shirts and pants.
  2. Remove sources of mosquito breeding grounds by draining standing water around the house.
  3. Wear repellent; those containing DEET are most effective.
  4. If you travel to the Olympics or another country with high rates of Zika transmission, protect yourself from bites while traveling and after returning home to limit transmission risk. (Asymptomatic people may be able to infect mosquitoes.) If you have symptoms of a viral illness after returning home, tell your physician where you’ve been.

Communications and marketing officer Trina Wood is the UC Davis School of Veterinary Medicine’s communications “Jill of All Trades.”