Arizona leads nation in West Nile infections

By Mitzi Brabb
Blog/Connection Correspondent

While cooler weather may mean fewer mosquitoes, the pesky insects still pose a threat to humans, especially in Arizona.

According to Dr. Anne Justice-Allen, wildlife health specialist for Arizona Game and Fish, the West Nile Virus (WNV) has taken the lead in human infection compared with other states during the past year.

WNV, which is a mosquito borne virus most commonly transmitted through mosquito bites, was first introduced to North America in 1999, when it was detected in New York City.

Birds are the natural hosts for the virus. They are not contagious, but when a mosquito infects a bird, a healthy mosquito may contract WNV when they bite the same bird.

Humans, however, are a dead-end host to the virus. The survival rate for humans is much higher than that for infected birds.

Justice-Allen said there were 19 human cases of WNV confirmed in 2009 in Arizona, 18 of which were in Maricopa county and one in Coconino County. That figure was significantly lower than 2008 when there were 108 reported cases. Unfortunately, the numbers for 2010 are much higher.

The Center for Disease Control (CDC) reported that Arizona leads the country, with nearly twice the number of confirmed cases compared to any other state.

As of Oct. 5, a total of 114 cases had been reported in Arizona. Most cases, 91, had been reported in Maricopa County, and only four came from Gila County.

Those numbers reflect both mild and severe cases that have occurred since Jan. 1. Of these 114 cases, seven deaths have resulted.

WNV shows up in different ways for different people, affecting individuals with mild to severe symptoms. Neuroinvasive cases, such as West Nile encephalitis and West Nile meningitis are the most severe forms of the disease, affecting the nervous system.

Encephalitis refers to an inflammation of the brain, meningitis is an inflammation of the membrane around the brain and the spinal cord.  In cases resulting in meningitis, a person experiences a sudden onset of weakness in the breathing muscles and or limbs.

There have been 67 neuroinvasive cases this year in Arizona, and 47 nonneuroinvasive disease cases. Typically the cases classified as nonneuroinvasive are less severe, and are more commonly recognized as West Nile Fever.

“The number of cases reported (as with all diseases) may be limited by whether a person affected seeks care, whether laboratory diagnosis is ordered, and the extent to which cases are reported to health authorities by the diagnosing physician,” the CDC reports.

The CDC also considers presumptive viremic blood donors (PVDs) in their statistics, of which 20 cases have been reported this year.

“PVDs are people who had no symptoms at the time of donating blood (people with symptoms are excluded from donating) through a blood collection agency, but whose blood tested positive in preliminary tests when screened for the presence of West Nile virus.”

United Blood Services, a national blood collection agency, reports that WNV infection may be transmitted by blood transfusion or organ transplantation.

The incubation period for the virus ranges from two days to two weeks, but an individual will generally develop symptoms within two to six days.

“The biggest culprit for the West Nile Virus are mosquito pools,” said Justice-Allen. “In October 2008, Arizona had 208 mosquito pools. Last year there were 88 mosquito pools reported.”

The “mosquito pools” referred to are large standing water holes -- a big attraction for mosquitoes and a popular breeding ground for their eggs.

For WNV prevention, Justice-Allen recommends using mosquito repellent with DEET when outside near dusk and dawn, and to wear long sleeve clothing at those times.

Another precaution to minimize exposure and prevention to the virus is to get rid of still water outlets. These include buckets, drums, old tires, and poorly maintained swimming poles. Also, it is a good idea to repair leaky pipes and outside faucets, and to move air conditioner drain hoses. It is also recommended that a person change water in birdbaths, and animal watering pans at least twice a week.

The upcoming winter will not kill off those obnoxious mosquitoes either. Typically mosquito eggs hatch into worm-like larvae in water. They feed on plant and animal matter before they emerge as flying adults. During the winter the eggs don’t hatch, but will stay in the cold or frozen water until the following spring.