Health & Medicine

Drought, heat suggest West Nile virus danger in Sacramento region

A mosquito draws blood from a horse.
A mosquito draws blood from a horse. Sacramento Bee file

Rising temperatures and a historic drought suggest that the Sacramento region and Central Valley will likely see high West Nile virus activity this summer, researchers say.

The prediction comes after 31 people in California died last year from the virus, including two in Sacramento County.

New research by the National Center for Atmospheric Research and the Centers for Disease Control and Prevention has found a link between higher-than-average annual temperatures and the intensity of West Nile virus outbreaks. The research published last month in the American Journal of Tropical Medicine may prove useful in predicting high rates of West Nile virus activity.

In Sacramento, the average annual temperature rose from 60 degrees in 2011 to just above 64 degrees in 2014, according to data from the National Oceanic and Atmospheric Administration.

Rising temperatures, especially at night, extend the daily period that mosquitoes can feed and transmit the virus, said Micah Hahn, lead author of the new study.

“It’s possible that warmer nighttime temps may extend mosquito blood-feeding activity earlier and later in the day,” said Hahn, of the National Center for Atmospheric Research (NCAR). “This would amplify the bird-mosquito transmission cycle and increase the number of infected mosquitoes, and increase human risk.”

Warmer temperatures also allow West Nile virus to replicate faster within mosquitoes, allowing them to transmit the virus sooner after they become infected, said Chris Barker, professor of pathology, microbiology and immunology at UC Davis.

“This makes it more likely that adult mosquitoes will be able to transmit the virus before they die and therefore increases risk for infection,” he said.

No human cases have been reported this year so far in California. But last year, 801 individuals tested positive for the virus in the state, and the 31 fatalities were the second-most recorded in California since 2003, when the virus made its first appearance in the state.

31 Number of West Nile virus-related deaths in California in 2014

Health experts are particularly concerned that 561 cases last year involved the more troublesome West Nile neuroinvasive disease, which often results in encephalitis or meningitis.

As of May 29, the virus had been found in 12 California counties this year, according to the California Department of Public Health. The closest county so far is Solano.

This year, 49 birds have tested positive for West Nile virus in California, compared to 34 by the same time last year. Meanwhile, mosquito samples testing positive for the virus are running four times higher this year compared with last year.

In the Central Valley, the virus will have the strongest foothold where agricultural areas abut cities, such as Davis and Woodland, Barker said.

The California Department of Public Health concurs with the NCAR research, said State Epidemiologist Gil Chavez.

The department is collaborating with the UC Davis Center for Vector-borne Diseases to study the relationship between climatic conditions and temperatures on West Nile virus activity to form a response plan, Chavez said. The drought is chief among the conditions whose effects are being reviewed.

“It’s possible that the ongoing drought contributed to West Nile virus activity by creating more limited sources of water for birds and mosquitoes,” Chavez said. “As birds and mosquitoes seek water, they come into closer contact and amplify the virus, particularly in urban areas.”

The drought has resulted in farmers, notably rice farmers, leaving acreage fallow. Typically, rice fields are flooded during cultivation, creating surrogate wetlands for birds along the Pacific Flyway. When such wetlands disappear, birds must congregate in larger numbers in remaining wetlands. It is there that birds are likely exposed to mosquitoes at a greater rate, Barker said.

Meanwhile, Chavez said, the lack of precipitation can cause water sources to stagnate, making them more attractive for mosquitoes to lay eggs.

Despite the recent findings, a lack of data remains a challenge with West Nile virus research in California, Barker said. On the East Coast, more data exists because the virus made its first appearance there in 1991.

“There is no single factor that perfectly explains or predicts West Nile virus outbreaks, and our region’s 11-year history with West Nile virus is still short by climate scientists’ standards,” Barker said.

Edward Ortiz: (916) 321-1071,

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