How Zika spreads (and who’s to blame)
A mosquito that’s common in California has become the latest identified carrier of the Zika virus, potentially multiplying the population of vectors capable of spreading the disease.
Until last week, researchers believed the virus could only be spread by two types of aedes mosquito much rarer in California and the U.S. But new research by a UC Davis ecologist and a top Brazilian science institution pins the southern variety of the culex mosquito – known as the southern house mosquito – as a potential vector of the disease, which can only be transmitted when a mosquito bites an infected person and then bites someone else.
Walter Leal, a chemical ecologist at UC Davis and a collaborator on the research, helped make the finding while testing mosquitoes in the northeastern Brazilian city of Recife. Zika, an often symptomless illness, has been blamed for a neurological defect called microcephaly in hundreds of babies around the world and has reached epidemic levels in Brazil.
“Everyone thought the yellow fever mosquito was the only vector, but if you look at the data from Recife, there are places where the population of the yellow (fever) mosquito didn’t justify the high number of (Zika) cases,” Leal said “Now that we have culex also in the picture, we have to be careful about that as well.”
There still isn’t sufficient evidence, however, to show that the culex mosquito can pass the virus on to humans, said Chris Barker, an epidemiologist with the School of Veterinary Medicine at UC Davis who has been studying the mosquito’s transmitting capabilities in a lab.
There are two types of culex mosquitoes – the southern house mosquito studied in Brazil and the closely related northern house mosquito, which sticks to cooler climates. Southern California is a hot spot for the southern variety. The Sacramento area is home to the northern variety as well as northern-southern hybrids, both of which transmit West Nile virus, Barker said.
Luz Maria Robles, spokeswoman for the Sacramento-Yolo Mosquito and Vector Control District, called the northern culex variety “very, very common” in the area, noting that they dwell in agricultural fields as well as in stagnant water around people’s homes.
This week, following two human cases of West Nile virus in Yolo County, the control district began aerial spraying in the city of Woodland to keep culex and other mosquitoes at bay. Mosquito numbers across the board have been especially high this year due to the hot weather, she said.
In vector mosquitoes, the Zika virus moves beyond the insect’s gut into other tissues, eventually infecting its salivary glands, and the virus moves to a new host at the next blood meal. In carrier mosquitoes that can’t transmit Zika, the virus goes no farther than the stomach.
“When you test mosquitoes in nature with a virus like Zika circulating, a mosquito biting people will get infected – it just doesn’t prove it’s a vector,” Barker said. “Unless it’s been proven a vector, infection doesn’t mean it’s a transmitter.”
Even if further research confirms the culex mosquito as a vector, it’s unlikely to be a major Zika threat because that particular breed is much more inclined to bite birds than humans, Barker said.
“It would be a secondary vector at most, and I’d be surprised to find an area where this would be the primary vector and continue to spread Zika in the absence of other vectors,” he said.
The news of a potential new vector comes amid growing concerns about Zika spread in the United States. There have been roughly 1,400 Zika cases in the U.S. and 98 cases in California since the Brazilian outbreak began, all of which were associated with travel to Zika-affected countries. Officials are investigating two cases in Florida that do not appear to be tied to travel, which if confirmed would be the first instances of local transmission in the United States.
Sacramento County Public Health Officer Dr. Olivia Kasirye said staff members have heightened their awareness of the Zika threat in the area and are informing people about what Zika is and what types of mosquitoes carry it.
If the culex were confirmed as a vector for the virus, the office would set traps and put out notices warning the public about the possible danger – the same process they follow with mosquitoes infected with West Nile virus.
“The difference is we already know we have West Nile virus in the area and we’ve had some cases of it that we’re investigating now,” Kasirye said. “But the message would remain the same: prevention is the best protection.”
Leal said the study authors announced their findings early to help people prepare for another possible vector.
“The problem with Zika is that it’s being brought in by many people and we don’t want these people to be close to the culex mosquito because we now know it is a vector,” Leal said. “Zika is a very serious problem. It’s very relevant information for people.”
Culex mosquito: the next Zika vector?
- There are two culex mosquito types: the northern and southern house mosquitoes.
- Both types are vectors for West Nile virus, but only the southern version has been shown to carry Zika virus.
- The southern culex mosquito is common in central and southern California and is not found in Northern California.
- The female mosquitoes bite at dawn or dusk and prefer birds, but will eat human blood as well.
- The two types of culex mosquitoes can interbreed; there are hybrid culex mosquitoes in the Sacramento area.