Rare Virus Outbreak Means Maricopa County Mosquitoes Doubly Dangerous

Arizona is calling in U.S. health officials to investigate an outbreak of the rare — and sometimes fatal — mosquito-borne illness St. Louis Encephalitis.

Nationally, there are typically only about 20 cases of St. Louis Encephalitis annually, says Dr. Rebecca Sunenshine, medical director for the Disease Control Division at the Maricopa County Department of Public Health. But just in Maricopa County since July, the disease has killed one and infected 11 others.

At the same time, Sunenshine says, the county has kicked off what’s looking to be a “pretty heavy” season for the West Vile virus. Forty-two people have contracted the disease, which also is transmitted through mosquito bites, and two people have died.

This is the first time officials have seen the two viruses at the same time and in the same place, she said.

Both West Nile and St. Louis Encephalitis are transmitted through birds. Previously, scientists thought the viruses did not spread concurrently because a bird that gets West Nile is immune to St. Louis Encephalitis.

“We can’t say much more because we don’t know," Sunenshine said.

A team from the U.S. Centers for Disease Control and Prevention visited Arizona last week to investigate the situation and train healthcare providers how to test for St. Louis Encephalitis.

The symptoms for the two diseases are similar. About 80 percent of infected people feel fine. Twenty-percent of people experience flu-like headaches and muscle aches. In less than 1 percent of cases, typically those involving the chronically ill or people over the age of 50, the viruses invade the brain or spinal cord, causing paralysis or death.

"Prevention is vital." — Dr. Rebecca Sunenshine

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“They are so similar that no physician is going to be able to tell the difference by looking,” Sunenshine said. “It’s important for healthcare providers to know if they are testing for a disease that looks just like West Nile and the test comes back negative, there’s another disease that looks just like it.”

There is no vaccine and no treatment for either West Nile or St. Louis Encephalitis, so “prevention is vital,” Sunenshine says. 

To lower your risk of infection, the Maricopa County Department of Public Health recommends using insect repellent that has DEET, picardin, or oil of lemon eucalyptus and wearing long sleeves and pants.

Mosquitos breed in standing water, so the county suggests changing the water in bird baths and pet bowls twice weekly, properly maintaining decorative ponds and pools, and checking for puddles after rain storms.

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Elizabeth Stuart
Contact: Elizabeth Stuart