The rains that have fallen across Colorado this spring and summer have been a blessing for a drought-stricken state. For the first time in several years, agriculture is irrigating with a degree of comfort rather than caution. Reservoir levels are up, and all the benefits of a water-rich season are abundant. But—and there is always a but—all this water is creating a near-record year for West Nile virus, a mosquito-borne virus that has so far been fatal to 11 people in Colorado in 2023.

The bounty of moisture after consecutive years of drought has meant a boom in the mosquito population, and specifically the Culex Mosquito, the insect that carries West Nile virus.

Colorado state health epidemiologist Dr. Rachel Herlihy calls the mosquito explosion this year “unprecedented” and the “highest we’ve seen in years.” Over the last seven years, the highest number of documented West Nile virus cases in the state occurred in 2015 when four cases were recorded. State health is worried that things could get worse since August and September are usually the months when cases spike.

West Nile virus, first discovered in 1937 in the West Nile region of Uganda, can affect humans, birds, horses and other mammals. Its symptoms include fever, muscle weakness, vision loss, convulsions, paralysis, coma and death. There have also been cases in which victims have suffered meningitis or encephalitis.

As of August 22, state health has documented 74 cases of the virus with Larimer and Weld counties recording the highest incidents with 14 and 13 cases respectively. Denver and El Paso counties have each recorded seven cases. The virus has also accounted for 40 hospitalizations.

The current West Nile outbreak has been predominantly clustered along the Front Range with Boulder and Larimer counties recording higher levels than other areas. But it has moved even farther, says the state health department. Pueblo and Weld counties have also been hit and Pueblo is one of the Front Range counties where a single death was attributed to the virus.

But anywhere there is standing water is a potential West Nile breeding ground. An area the size of a bottle cap, state health warns, “can hold enough water for mosquito eggs to survive.”

The virus spreads in an almost circular fashion. The cycle begins when a bird, often hawks or owls or scavengers like crows feeding on dead birds that have been infected after eating sick or dead birds already infected with West Nile contract the virus. Other times a bird can become a host by eating an infected mosquito. A mosquito can also contract West Nile by biting an infected bird.

If a host mosquito bites a human, it doesn’t always follow that West Nile will automatically set in. Health experts say about one in five people are affected. The first sign of the virus is usually a high fever but things can go seriously downhill from there to include—though in rare cases—even paralysis.

Fortunately, only about 1 in 150 people bitten by a carrier mosquito develop a serious or fatal illness. Those most at risk are people aged 60 and older or people with certain medical conditions.

There is currently no vaccine or medications for treating West Nile virus in humans, but there is a vaccine for horses that has worked remarkably well. The equine vaccine, said CDC, has all but eliminated the virus in horses and reduced mortality by 40 percent.

Among humans, it is still a challenging conundrum. Still, there are plenty of things one can do to minimize the risk of contracting it, says the health department.

Because any standing water—even the tiniest accumulation—is a potential mosquito breeding ground, do whatever you need to do to eliminate these collection areas. Mosquitos lay their eggs in old tires, rain barrels, birdbaths, puddles, and just about anywhere water can pool.

There is no need to isolate, says the health department. When outside use a good repellent, ones that contain DEET, picaridin, IR3535, oil of lemon eucalyptus or para-menthanediol products. (Some repellents are clearly marked ‘DEET.’)

Health officials also suggest that you consider wearing long-sleeved shirts and long pants. Also consider spraying the repellent not just on your skin but also on your clothing. Finally, if you have even tiny holes in your screens, repair them and consider limiting time outdoors at dusk and dawn, the period when mosquitos are most active.

When West Nile virus arrived in the United States in 1999, it had only regional impact, only New York and Connecticut. But by 2004, the virus had spread to the West Coast and by 2012, said the CDC, it had found homes in all 48 contiguous states and the District of Columbia. A similar upward trend of West Nile virus has also been recorded in Canada.

Scientists are still puzzled about how West Nile virus, a disease that had been isolated to Africa for more than sixty years came to the United States. One theory is that it got here via airplanes. There is also an argument that migrating birds impacted by climate change may be the cause.

But there are incontrovertible truths about West Nile virus that cannot be ignored, say scientists. Among them are that since West Nile virus first arrived in 1999, there have been more than 51,000 clinical cases reported and more than 2,300 deaths with an estimated 7 million people actually having been infected. Finding answers, however, may be some time in coming as research funds have been cut back over the last several years.

For more information on West Nile virus, visit www.cdc.gov/westnile.