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West Nile Virus On The Rise Due To U.S. Drought Conditions

West Nile_CA Drought 4

As drought continues to plague much of the western United States, public health officials are warning of a lesser-known but serious side effect of the dry conditions: a greater likelihood of contracting West Nile virus.

This week, officials from the California Department of Public Health (CDPH) confirmed that a 65-year-old woman in Nevada County, California, died from an infection with the mosquito-borne disease. She was the first person in the state to succumb to West Nile virus this year. Earlier this month, two people died from West Nile virus infections in Maricopa County, Arizona — an area that is also experiencing severe drought conditions.

Of the 58 counties that make up the state of California, 33 have reported West Nile virus activity already this year. At this time in 2014, only 29 California counties had reported the disease, according to the CDPH. There were more than 800 confirmed cases of West Nile virus in California in 2014, resulting in 31 deaths — that’s twice as many cases as in 2013 and twice as many associated fatalities.

West Nile was first detected in California in 2003, and in the following two years, there were hundreds of cases. By 2005, health officials had controlled its spread with spraying and other measures. The recent spike in West Nile activity is the first influx since then, and it could be even worse. The rate of infection among birds in California was the highest ever in 2014—60 percent of tested dead birds had it, up from just over 30 percent five years earlier. The proportion of mosquitoes infected with West Nile also set a record as the highest level ever detected in California, exceeding the infection rate equated with epidemic conditions.

Too close for comfort

The state’s soaring temperatures and lack of rainfall are related to those growing numbers, according to public health officials. As natural freshwater sources dry up, certain mosquito species that need this water must rely more heavily on artificial water sources, like swimming pools and other small bodies of standing water, which are located in residential areas. Mosquitoes — as well as the birds that mosquitoes feed on — are two of the species drawn to the backyards of Californians and Arizonians in greater numbers during a drought, according to the CDPH.

Photo Credit: San Jose Mercury News

Photo Credit: San Jose Mercury News

“As birds and mosquitoes [seek] water, they come into closer contact and amplify the virus, particularly in urban areas,” Karen Smith, director and state health officer of the CDPH, told the Los Angeles Times. “The lack of water could have caused some sources of water to stagnate, making the water sources more attractive for mosquitoes to lay eggs.”

Wild birds are the major amplifying host for West Nile virus, meaning that they help hasten the spread of the disease. They quickly become infected with West Nile virus after being bitten by a mosquito carrying the disease (the incubation for the virus is just a few days in birds). Most birds that are infected don’t die from the virus, but there is enough of the virus circulating in their blood that, when another mosquito feeds on them, the infected bird spreads the virus to that mosquito, carrying on the vicious cycle of the disease. Humans can get the virus when bitten by an infected mosquito.

Drought also dramatically alters the reproductive behavior of the Culex tarsalis, the particular species of mosquito that carries West Nile virus in the Western United States. Under normal environmental conditions, Culex females take one final meal — consisting of the blood of a bird or other small animal — and then hunker down to lay their eggs and let them mature. This process, called a gonotrophic cycle, lasts about five days. The typical Culex female goes through three consecutive gonotrophic cycles and then dies around the 15th day. That’s lucky for humans, because the average incubation period for West Nile virus is 15 to 21 days. So even if a mosquito’s final meal happens to be the blood of a wild bird infected with West Nile virus, the mosquito will never get a chance to become a vector for the virus because she will die before the incubation period has ended.

But under drought conditions, this cycle gets interrupted. Since Culex mosquitoes must lay their eggs in a flooded environment, drought forces them to wait — sometimes for weeks — for the right conditions to arise. As the female mosquitoes wait, they may be incubating West Nile virus. If so, then the next animal they bite after laying their eggs could end up with West Nile virus, too — thereby increasing the number of infected birds (or other animals) who can then spread the virus to feeding mosquitoes.

This results in what is called “drought-induced amplification,” a cycle that involves increased and prolonged contact between mosquitoes and West Nile-carrying birds, and ultimately leads to abnormally large populations of both infected mosquitoes and potential vectors (mostly birds). Experts say this drought-induced amplification plays a key role in West Nile epidemics in human populations, and studies have confirmed that outbreaks of the virus are often preceded by drought conditions.

Playing it safe

The end result of these drought-induced changes, experts say, is that more infected mosquitoes are coming into closer contact with humans.

West Nile_CA Drought 6The effects that infected mosquitoes have on the humans who share their drought-reduced habitats are sobering. While only 31 of the 801 cases of West Nile virus reported last year resulted in deaths, 561 cases involved neuroinvasive illness, according to the Mosquito and Vector Control Association of California (MVCAC). The neuroinvasive form of West Nile virus affects the nervous system and can cause inflammation of neural tissues, leading to meningitis, in which the membranes around the brain and spinal cord become inflamed, and encephalitis (inflammation of the brain).

It’s very likely that many more people were actually infected with the virus and simply did not exhibit symptoms last year, health officials say. According to the Centers for Disease Control and Prevention, only about 1 in 5 people infected with West Nile virus experience severe symptoms, including fever and rash, and less than 1 percent of those infected experience severe neurological illness.

To lower you chances of contracting the mosquito-borne disease, the MVCAC recommends practicing what it calls the 3 “Ds”:

  • DEET: Apply insect repellent containing DEET, picaridin, oil of lemon eucalyptus or IR3535, according to label instructions.
  • Dawn and dusk: Wear protective clothing and repellent if you are outside during these times. Make sure that your doors and windows have tight-fitting screens to keep out mosquitoes. Repair or replace screens with tears or holes.
  • Drain: Mosquitoes lay their eggs on standing water. Eliminate all sources of standing water on your property, including in flower pots, old car tires and buckets.

California’s West Nile virus website includes the latest information on West Nile virus activity in the state. Californians are encouraged to report all dead birds on the website or by calling toll-free 1-877-WNV-BIRD (968-2473).



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