Climate change may accelerate spread of West Nile virus

Climate change may accelerate spread of West Nile virus
A new model suggests the warming climate will boost transmission of the West Nile virus and other mosquito-borne viruses in elements of america where temperatures are below the insects’ optimum range.

West Nile virus, which mosquitoes - as principal vectors - transmit to humans, first found its way to THE UNITED STATES in 1999. Since that time, it has become the most frequent mosquito-borne infection in the U.S., Canada, and Europe.

Scientific models at Stanford University, CA, now predict the virus will spread more easily in cooler parts of the country as average summer temperatures rise due to climate change.

Rising temperatures may also likely mean seasonal transmission of the virus starts earlier in spring and ends later in fall.

However, as well, the virus may spread less easily in hotter areas.

The model shows that the optimum temperature for transmission is 24-25°C (75.2-77°F). Around 70% of the U.S. population stay in regions where summer temperatures are below this range, whereas 30% are in areas above the number.

The authors write in the journal eLife, “we may expect a net increase in transmission of West Nile virus in response to the warming climate, even while hot temperatures suppress transmission in some places.”

Complex picture
“As the climate warms, it is advisable to know how temperature changes will affect the transmission of mosquito-borne diseases,” says lead author Marta Shocket, who was simply a postdoctoral fellow at Stanford when the models were developed and is currently a postdoctoral researcher at the University of California, LA.

Apart from temperature, many factors may donate to the transmission rates of mosquito-borne viruses, including land use, control measures, and the evolution of the viruses and their insect vectors.

“Climate change is poised to boost the transmission of West Nile and other mosquito-borne viruses in a lot of the U.S.,” says senior author Erin Mordecai, Assistant Professor of Biology at Stanford.

“But these diseases also rely upon human connection with mosquitoes that also contact wildlife. So factors like human land use, mosquito control, mosquito and virus adaptations, and the emergence of new viruses make predicting the continuing future of mosquito-borne disease a challenge.”

The main hosts of all the viruses they studied are wild birds - which become “reservoirs” of infection - so their shifting geographical range will also donate to changes in transmission rates, say the researchers.

Six viruses
Each mosquito species is a vector for a number of viruses, and each virus can be carried by a number of different species.

Therefore, for simplicity, the scientists centered on six viruses transmitted by a restricted number of extensively studied vectors. In total, they modeled 10 vector-virus pairs.

The viruses were:

  • West Nile
  • St. Louis Encephalitis
  • Eastern and Western Equine Encephalitis
  • Sindbis
  • Rift Valley fever
Their vectors are mosquito species from the genera Culex, Aedes, Coquillettidia, and Culiseta.

To build up their models, the scientists used data from previous research that measured how temperature affected these insects in the lab. The temperature-dependent factors driving transmission by the insects were:

  • survival
  • biting rate
  • number of offspring
  • development rate
  • ease of acquiring and transmitting the virus
  • virus incubation rate
The models predict that transmission of the viruses will peak at intermediate temperatures and decline in extremes of cold and heat.

The scientists validated their West Nile virus model by comparing its predictions with actual transmission patterns by county in the U.S. This confirmed that the virus spreads most rapidly at moderate temperatures, but is checked when temperatures are higher or lower than its insect vectors’ optimal range.

They write that their models are “the most comprehensive synthesis to date” of how temperature affects the transmission of the diseases.

However, they remember that some gaps stay in the data used to build the models. Filling these holes will increase the accuracy and precision of their predictions.

They conclude:
“As carbon emissions continue steadily to increase and severe climate change becomes increasingly inevitable, it is important that we know how temperature change will affect the transmission of mosquito-borne diseases in a warmer future world.” 
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