If you ask a Coloradan what the ‘scariest animal’ is in the Centennial State, they’ll probably say a mountain lion, bear, or wolf – maybe even a moose (that’s what this writer fears the most, after all). That said, another animal in Colorado is far deadlier and it often goes unnoticed, weighing about 2 milligrams – the mosquito.

This year in Colorado, 17 people died from the mosquito-borne West Nile virus, and that’s far from the deadliest year on record.

Since the Colorado Department of Public Health and Environment started keeping records on West Nile virus in 2003, a total of 246 locals have succumbed to the disease – an average of 10.6 people per year. While this year did bring an above-average number of West Nile virus deaths, 2023 (51 deaths), 2022 (20 deaths), and 2003 (66 deaths) each resulted in more West Nile virus-related fatalities.

While death is obviously the most severe potential outcome of West Nile virus, it’s also worth noting that of the 285 people that were found to be affected by the disease this year, 154 were hospitalized (54%) and 143 ended up with neurologic symptoms (50%). A death rate of six percent is scary in itself, but it’s important to remember that these other symptoms can have a lasting impact on an individual’s life, too. It may also be worth noting that this year’s death rate was higher than the long term norm of about 3.5 percent.

In 2025, northern Colorado counties of Larimer, Boulder, Weld, and Adams combined accounted for 152 of the 285 total cases (about 53%). This falls in line with historical data as these four counties have accounted for about half of all known cases since 2003 – 3,505 of 7,075 cases, or about 49.5 percent.

It’s also extremely likely that many more cases of West Nile virus occurred throughout the year, going unnoticed as symptoms may not have been deemed severe enough for an individual to seek formal medical care.

While some outliers may exist, West Nile virus cases tend to pop up from May through October in Colorado, with August typically being when the most cases are recorded.

When it comes to protecting oneself from West Nile virus, preventing mosquito bites is key. The CDPHE recommends using insect repellents (see their list of essential ingredients here) and limiting time outside when mosquitos are most active (dusk and dawn). Protective clothing and limiting skin exposure can also help, as can working to ‘mosquito-proof’ your home by removing items that may collect standing water and by installing or repairing screens on doors and windows.

As far as the symptoms of West Nile virus go, these tend to appear about two to 14 days after a bite. Fever and a headache can progress into a stiff neck, disorientation, tremors, and coma, with more severe cases resulting in brain infections like meningitis or encephalitis. Permanent brain damage and death can occur. It’s also worth noting that most deaths involve people 50 years of age or older. Unfortunately, there’s no treatment or vaccine available for the disease.

Read more about mosquito safety and the West Nile virus in Colorado here.

Editor’s Note: West Nile virus is most commonly spread to humans by bites from infected mosquitos. In an extremely small number of cases, it has also spread through blood transfusions, organ transplants, and from a mother to a baby during pregnancy or at time of birth.

STAY INFORMED: Get free Colorado news with our daily newsletter (Click here)