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VERO BEACH, Florida – In the United States, West Nile virus circulates among wild birds, transmitted by Culex mosquitoes, which transmit West Nile virus and other viruses in the southern United States. Humans can become infected when a person is bitten by a West Nile carrier mosquito.
Scientists previously thought that Culex mosquitoes feed primarily on birds and mammals. The researchers believed that the interaction facilitated transmission of WNV among birds and subsequent infections of humans and horses.
The new discovery helps scientists understand how diseases carried by Culex mosquitoes spread through ecosystems and how they might reach humans and pets.
Mosquitoes bite a wide range of animals, and each species of mosquito has distinct preferences for certain types of animals. Scientists call these animals “hosts”.
“The process of transmission of mosquito-borne diseases to humans and wildlife depends on which mosquitoes feed on which animals,” Reeves explained. “Only certain species of mosquitoes can transmit viruses, and only certain animal species can become infected and transmit virus to new mosquitoes.”
Whether lizards play a role in the transmission of these Culex mosquito-borne diseases is a key question emerging from the study. It offers researchers, like Reeves, a new direction to explore.
“If lizards are poor hosts for these viruses, mosquito bites on lizards could result in fewer flying West Nile virus-carrying mosquitoes,” Reeves said.
Culex mosquitoes, vectors of WNV and SLEV, were previously known to feed almost entirely on birds and mammals. This understanding is based on large-scale studies done in the 1960s and 1970s, before the brown anole and other invasive lizards were widespread in Florida, Reeves explained.
For Reeves’ study, the team collected blood-fed mosquitoes in Florida and Arizona and identified the animals they were feeding on. They assessed host associations for 10 Culex species from Florida and Arizona using DNA barcoding. This process includes extracting and sequencing DNA from each blood-fed mosquito as part of the process.
In Florida, scientists collected mosquitoes from five counties: Alachua, Levy, Indian River, Pinellas and Miami-Dade. In Arizona, mosquitoes were collected from wild areas in the Sky Islands region of Cochise, Pima, and Santa Cruz counties.
Here’s what they learned:
• Not all Culex species feed on lizards to the same degree.
• Only daytime active lizard species associated with trees were bitten by mosquitoes.
• In Florida, brown anoles — the small brown lizards often seen on sidewalks — were the most frequently fed hosts of Culex nigripalpus, one of the most important vectors of WNV and VSLE.
“This information is important because lizards could help reduce the transmission of WNV and SLEV – both viruses infect birds, and whenever a human is infected with one of these viruses, that virus would come from a vector mosquito that had previously fed on an infected bird,” Reeves said.
Culex mosquitoes, known to spread zoonotic flaviviruses like WNV and SLEV, pose a public health risk because they spread viruses among wildlife and, occasionally, wildlife to humans, Reeves said.
Culex mosquitoes catch these viruses when they feed on the blood of an infected bird. Then, when that mosquito feeds on new animals, the virus can be transmitted when that mosquito draws blood from a subsequent host. If a bird is infected, any mosquito that feeds on it while the infection is active can pick up the virus. On the other hand, some animals, such as humans, represent dead ends for these viruses: if a human is infected with WNV or VSLE, mosquitoes that feed on that human will not pick up the virus.
“The key question that arises from our study is whether lizards can serve as hosts for these viruses,” he said. “Otherwise, if they are dead ends, it is possible that the presence of lizards, including some of these invasive species, has a beneficial effect by reducing the transmission of these viruses and the risk of humans becoming infected”
On the other hand, if lizards are dead-end hosts for WNV and SLEV, each mosquito bite that goes to a lizard, rather than a bird or a human, is one less opportunity for WNV or the SLEV to be transmitted.
“If so, it may be possible to use lizards as a sort of biological control to help curb the transmission of West Nile virus and other similar viruses,” Reeves said.