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West Nile outbreaks may become common in southern California

In a new study from UC Berkeley, experts have found that outbreaks of West Nile virus and other mosquito-borne diseases could become more frequent in coastal areas of southern California as the climate gets warmer.

The West Nile virus has been a threat to the Los Angeles metropolitan area since it first appeared in this region in 2003. While birds can also carry the deadly virus, it is most commonly transmitted to humans through mosquito bites. 

For the current study, the team analyzed data on nearly two million mosquitoes that had been captured in Los Angeles and tested for West Nile between 2006 and 2016. The experts used machine learning to identify certain factors, such as climate conditions, that influenced mosquito infection throughout various neighborhoods.

The analysis revealed that West Nile infection among the captured mosquitoes was strongly tied to the average temperature in the neighborhood.

Study lead author Nicholas Skaff is a former postdoctoral scholar in Environmental Health Sciences at UC Berkeley’s School of Public Health. 

“Our data revealed a sharp transition, where – as temperatures shift between 70 to around 73 degrees Fahrenheit – the likelihood of capturing infected mosquitoes in L.A. neighborhoods increases dramatically,” said Skaff. “Above this range, conditions become consistently favorable for transmission, and below this range, conditions are consistently unfavorable.”

The findings help explain why coastal communities in Los Angeles seem to be protected from the threat of West Nile outbreaks in some years, yet vulnerable in others.

According to study co-author Professor Justin Remais,  a greater number of West Nile cases can be expected along the southern California coast with significant warming expected over the coming decades.

“Coastal L.A. appears to be vulnerable to the expected warming of California’s climate by mid-century, which will push coastal climates more consistently into the favorable zone,” said Professor Remais. 

“Inland L.A. may not be as susceptible to these shifts, as the climate is already favorable. Yet, as climate warming progresses towards the century’s end, it is possible that temperatures become too hot in these areas.”

The researchers emphasize that one or even a few hot days do not appear to increase transmission risk. The outbreaks should be expected after sustained warm temperatures over the course of weeks, after West Nile has had the opportunity to become established.

“Our research suggests that, rather than focusing on daily weather reports, it’s important to examine temperatures over the long haul,” said Skaff.

“If coastal Los Angeles experiences a month or two of warm temperatures during the summer or early fall, it’s probably a good time to be extra careful to avoid mosquito bites. Inland parts of L.A. are almost always sufficiently hot during the summer, so other factors end up determining whether intense transmission occurs there.”

While the study clearly indicates that temperature will play an important role in transmission of the virus, the researchers emphasize that many factors ultimately determine whether a West Nile outbreak will occur.

“You can think of the favorable temperature range we identified as a prerequisite – if other things don’t go right for the vector or the virus, transmission may still not occur, even when temperatures are favorable,” said Skaff.

“For example, if most of the susceptible birds in the region were infected during the previous year or two because a large outbreak occurred, herd immunity will be high, and the risk to people will be limited.”

“Predicting the transmission of infectious diseases carried by animal hosts and vectors represents a complex puzzle, and machine learning can pick up patterns in vast epidemiological and ecological datasets that help us understand why certain people and neighborhoods are at the highest risk, as well as what the future holds,” said Professor Remais.

The study is published in the journal Proceedings of the Royal Society B.

By Chrissy Sexton, Staff Writer


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