After the first human West Nile virus (WNV) case was reported in May in Harris County, Texas and cases trickled in for a couple of months, it appears that WNV season is in full swing with cases being reported from multiple states on a daily basis.Just in the past day, WNV cases have been reported from Florida, Ohio, Idaho, Pennsylvania, and Colorado among others.

As of Tuesday, a total of 36 states have reported West Nile virus infections in people, birds, or mosquitoes in 2015. Overall, 38 cases of West Nile virus disease in people have been reported to Centers for Disease Control and Prevention (CDC). However, the cases reported to the CDC are typically well behind what the states and localities are reporting, hence, the total case count is likely quite higher.

West Nile virus has been a scourge on the United States since the first 62 cases were reported in New York in 1999. Since that time, nearly 42,000 human WNV cases have been reported, with only Alaska never reporting a case.

West Nile virus activity is picking up in recent weeks, so how bad will this season be?

Any public health professional would say it is impossible to predict how bad the WNV season might be. Colorado Public Health Veterinarian, Jennifer House said, “Although we can’t predict how much West Nile virus activity will occur this summer, we know the virus is present, and that means people are at risk.” However, there are some things to look at.

In California, where more than 800 human cases were reported in 2014, the most in the US, California Department of Public Health (CDPH) Director and State Health Officer Dr. Karen Smith has noted recently, “West Nile virus activity is more widespread in 2015 than in years past. Californians need to be vigilant in protecting themselves.”

I bring up California for a specific reason: drought. The Golden State has experienced a serious drought this year, which may affect the mosquito burden. How does that happen?

The Contra Costa Mosquito and Vector Control District offers a good explanation:

In a typical non-drought year, healthy creeks and rivers flow freely and typically don’t produce mosquitoes. Mosquito eggs are fragile and can’t survive in these flowing or agitated waters. Standing, or still water is the culprit. Female mosquitoes only lay eggs in still waters where they won’t be harmed. During drought conditions, creeks dry up and leave scattered puddles that hold still and standing water perfect for immature mosquitoes–water with no agitation nor predators. In Contra Costa County, mosquitoes that can transmit West Nile virus can lay up to 400 eggs in as little as two tablespoons of water.

When water is scarce in drought years, the remaining puddles become critical water sources for insects, birds and animals that all gather at the same locations. This high concentration of birds, which can carry West Nile virus, and mosquitoes which can transmit the virus from birds to people, creates a greater health risk.

The risk of mosquito-borne illness also increases in a drought year when areas of water that once held mosquito-eating fish dry up, because without enough water, the fish cannot survive to eat the mosquitoes. In addition, when rain does eventually fall, the water deposits in areas that now no longer hold fish. Without these predators, mosquito activity can flourish. More mosquitoes may mean more virus.

In addition, rainy seasons are also implicated in increases in mosquito activity. On Tuesday, experts from Colorado State University report the high number of Culex mosquitoes, the mosquito vector of WNV, in northern Colorado may translate to an increase in WNV cases. Why? A wet spring and summer in Northern Colorado has led to the increases of the mosquito vector.

Colorado has reported more human WNV cases since 1999 than any other state in the union with more than 5,000.

Related: 13 Diseases You Can Get From Mosquitoes

West Nile Virus (WNV) was first isolated in a woman in the West Nile district of Uganda in 1937, according to the World Health Organization. In 1999 a WNV circulating in Israel and Tunisia was imported in New York producing a large and dramatic outbreak that spread throughout the continental United States of America (USA) in the following years.

West Nile virus is usually spread by the bite of a mosquito that has fed on an infected bird.  In very rare cases, West Nile virus can be spread through: blood transfusion, organ or tissue transplants, pregnancy (from mothers to unborn babies), breast milk and exposure of laboratory workers to infected medical specimens.

Symptoms can range from mild to severe. Most people (70 percent to 80 percent) who are infected with West Nile virus have no symptoms. Some people have mild symptoms that can include: fever, headache, body aches, mild rash and swollen lymph glands. The first symptoms usually appear within 2 to 15 days after infection.

Anyone infected with West Nile virus can be at risk of developing more severe symptoms and health effects. Adults 50 years or older and those with underlying conditions or weaker immune systems, however, are at greater risk.
Very few people (fewer than 1 percent of people infected with the virus) will develop severe symptoms and health effects. In many of these cases, the infection can affect the central nervous system. This is the nervous system tissues in the brain and spinal cord).

Prevention of West Nile virus and other mosquito borne disease include avoiding mosquito bites, using repellent that contains DEET, wearing clothes that cover skin if possible, mosquito-proof your home and eliminate mosquito breeding sites.

There is no preventive vaccine for WNV although there is one currently in Phase I clinical trials. There is no specific treatment for WNV, just treat the symptoms.

Time will tell how bad this WNV season ends up; however, certain conditions are in place for a possible heavy season.