Maricopa County Investigating Mosquito-Borne Outbreaks, West Nile Virus, Encephalitis

The Maricopa County Department of Public Health (MCDPH) and Arizona Department of Health Services (ADHS) are investigating two simultaneous mosquito-borne outbreaks, West Nile virus (WNV) and St. Louis Encephalitis (SLE), and are reminding the community to do its part in preventing the spread of mosquitoes.

“Having outbreaks of these two diseases at the same time has never been reported, and we are working very closely with county Environmental Services, healthcare providers, laboratories, the State Health Department and the Centers for Disease Control and Prevention (CDC) to ensure we have a strong surveillance system and prevention strategies in place,” said Dr. Rebecca Sunenshine, medical director for the Disease Control Division at MCDPH. “We can’t stress enough the importance of avoiding mosquito bites. Apply insect repellent and cover up whenever you are outdoors, and do your part to rid your property of mosquito breeding sites. A little effort can go a long way in protecting the whole community from these mosquito-borne diseases.”

Statewide, there are 46 cases of West Nile (41 in Maricopa County) and 12 cases of SLE (10 in Maricopa County). There are three WNV deaths (two in Maricopa and one in Pima County) and one death in Maricopa County from SLE awaiting CDC confirmation.

Most years, there are only about 20 SLE cases in the U.S., typically in July through September but occasional large urban outbreaks happen every 10-20 years. Last year, no SLE cases were reported in Maricopa County; however, one was reported in Pinal County. The highest year of reported SLE cases was 2003, with five cases confirmed statewide, all from Maricopa County. Usually, there are around 100 WNV cases and zero SLE cases reported each year in Arizona.

MCDPH and ADHS are working with local healthcare providers to remain vigilant in looking for signs and symptoms of mosquito-borne illness and how to test for these diseases. West Nile and SLE share similar symptoms, although only about 20 percent of those infected will develop them. Those that do develop symptoms usually report fever, headache, body aches, and muscle weakness. In less than one percent of infections, both diseases can cause severe illness involving the brain and nervous system. These people, typically over 50 years old, experience severe headache, neck stiffness, and/or encephalitis or inflammation of brain, which can lead to paralysis or death.

“We are advising our health care community that in addition to West Nile virus, to consider SLE in their diagnosis of patients, especially those hospitalized patients who are presenting with a high fever and may be testing negative for West Nile virus,” said Dr. Cara Christ, director of ADHS. “If providers do suspect SLE, please contact your local health department for testing procedures.”

Because there is no commercially available test for SLE, and the rarity of the disease, testing performed by public health laboratories does not always give clear results. The CDC is assisting the Arizona State Public Health Laboratory with their testing strategy for both SLE and West Nile. The CDC is also supporting ADHS and MCDPH to develop an enhanced surveillance system, working with healthcare providers and laboratories around the state and to understand more about these two diseases.

West Nile and SLE are both transmitted through the bite of an infected mosquito. Maricopa County Health officials urge all people to “Fight the Bite” and follow simple precautions to avoid mosquitoes and the diseases they may carry:

  • Wear lightweight clothing that covers your arms and legs and use an insect repellent when outdoors when mosquitoes are active. Always follow the directions on the label.
  • Make sure doors and windows have tight fitting screens and remain closed.
  • Eliminate mosquito-breeding sites around the home by removing standing water in potted plants, tires, bird baths and other containers where water may collect.
  • Ensure that swimming pools and decorative water features are properly maintained.
  • Change water in flowerpots, birdbaths and pet watering bowls located outdoors at least twice per week.

Many local vector control programs around the state have been treating mosquito breeding habitats and some counties have been fogging to kill the specific mosquitoes that spread WNV and SLE.

Maricopa County Environmental Services Department (MCESD) began testing for SLE this past summer in addition to enhancing their surveillance efforts. The same type of mosquito that carries West Nile also carries SLE. So far this year, MCESD’s lab has confirmed 57 SLE and 84 West Nile virus positive mosquito samples. “These numbers are a reminder of how critical it is to dispose of any standing water, which mosquitoes need to breed,” said Johnny Diloné, MCESD Public Information Officer. “If you notice green pools or any mosquito problem in your community, talk to your neighbors or call us; we can follow-up if we are not already monitoring and treating the area.”

In Maricopa County, for more information on West Nile virus or SLE, go to or

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