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Drought may have caused record-breaking year for West Nile virus activity in California



April 10, 2015
Area(s) of Interest: Infectious Diseases Public Health 

The California drought may be responsible for a record-breaking year for West Nile virus (WNV) activity in 2014, according to the California Department of Public Health (CDPH).


In 2014, California had the second-highest number of human cases of WNV since the virus first invaded California in 2003. In 2014, California recorded 801 cases of the potentially fatal disease. In 2005, CDPH detected 880 cases of WNV.


The highest number of cases was in Orange County (263 cases) and the highest incidence occurred in Glenn County (35.3 cases per 100,000 population).


It is possible that the ongoing drought contributed to West Nile virus activity by creating more limited sources of water for birds and mosquitoes, according to CDPH Director and State Health Officer Karen Smith, M.D.


“As birds and mosquitoes sought water, they came into closer contact and amplified the virus, particularly in urban areas. The lack of water could have caused some sources of water to stagnate, making the water sources more attractive for mosquitoes to lay eggs,” said Dr. Smith.


The level of WNV activity last year broke several records, including: 



  • 561 cases of West Nile neuroinvasive disease, the more serious neurological form of the disease that often results in encephalitis or meningitis, were detected.

  • The number of fatal WNV cases, 31, exceeded all previous years.

  • The proportion of mosquitoes infected with WNV was the highest level ever detected in California (mosquito infection rate = 6.0; epidemic conditions equate with 5.0).

  • The prevalence of WNV infection in tested dead birds, 60 percent, was the highest ever detected in California.


Physicians should consider WNV in any person with a febrile or acute neurologic illness who has had recent exposure to mosquitoes, a blood transfusion or organ transplantation, especially during the summer months in areas where virus activity has been reported.


The diagnosis should also be considered in any infant born to a mother infected with WNV during pregnancy or while breastfeeding. More information on WNV in pregnancy and breastfeeding is available here.


All cases of WNV should be reported to local public health authorities in a timely manner. The incubation period for WNV is typically two to six days, but ranges from two to 14 days and can be several weeks in immunocompromised people.


An estimated 70-80 percent of human WNV infections are subclinical or asymptomatic. Most symptomatic persons experience an acute systemic febrile illness that often includes headache, weakness, myalgia or arthralgia; gastrointestinal symptoms and a transient maculopapular rash also are commonly reported. Less than 1 percent of infected persons develop neuroinvasive disease, which typically manifests as meningitis, encephalitis or acute flaccid paralysis.


Click here for the Centers for Disease Control and Prevention’s information on clinical evaluation and treatment summary.

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