Diseases transmitted from animals to humans are referred to as animal-borne, or zoonotic, diseases. These diseases can be carried by various animals and insects (e.g., rodents, mosquitoes or ticks) and can then be transmitted to humans. Animal-borne diseases are more common during the summer when people tend to be outdoors more often and for longer periods of time and when insects and wild animals are more active.

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If you’d like more information, please contact our Environmental Health Manager:

Melvin Bustos
(970) 867-4918 ext. 2262
melb@nchd.org

Hantavirus and Safe Cleaning Practices

Hantavirus is carried by rodents, most commonly deer mice and rats, and can cause severe and potentially deadly disease in humans who have contact with them.

Exposure can include breathing in the virus when dust containing particles is stirred up, or from touching eyes, nose, or mouth after contact with an object or surface contaminated with rodent urine or droppings. People most at risk include those who handle or clean up after rodents, or who spend time in environments where rodents make nests. Early symptoms include fever, chills, muscle aches/weakness, abdominal pain, nausea, vomiting, and diarrhea. If you experience these symptoms and have had exposure to rodent droppings, seek medical care immediately.

For more information about the disease and risks, please visit the Hantavirus Pulmonary Syndrome webpage of the American Lung Association.

There are several simple cleaning recommendations to follow to reduce the chances of exposure to hantavirus from rodent droppings. Visit the CDC webpage, How to Clean Up After Rodents for details about how to safely clean up after rodents in a variety of spaces.

Two environmental health workers inspect mouse droppings at a location where hantavirus infected a human. They are wearing latex gloves, goggles, and respirator masks.

West Nile Virus: Protection is Prevention

West Nile virus disease, “West Nile,” is the most common mosquito-transmitted virus in the U.S. Birds and mosquitoes carry West Nile virus and infected mosquitoes pass the virus to humans and other animals through bites. Colorado consistently has reportable cases of West Nile every year, which include hospitalizations and deaths. In 2023, Colorado saw the highest number of cases since recording began in 2003, in humans, animals, and tested mosquitoes, including in the NCHD health district. There were 634 reported human cases, including 51 deaths — the highest number of cases in the nation. Although 2024 was a milder year, the number of cases can vary widely from season to season, and human cases are a signal to stay alert.

In 2025, the first mosquitoes carrying West Nile virus were detected in Larimer County during June, followed shortly by cases in mosquitoes and humans throughout northern Colorado. Mosquitoes remain active until the first hard freeze, and are most active during the time around dawn and dusk, especially in wet areas. The highest risk of West Nile virus transmission is from July through September.

Symptoms typically appear three to 14 days after being bitten and may include fever, severe fatigue, headache, body aches, skin rash and swollen lymph nodes. While most people infected with West Nile virus don’t have symptoms, some can develop a serious, potentially deadly illness. People aged 60 years and older and those with certain medical conditions are at greater risk of serious illness. Talk with a health care provider if you develop a fever with severe headaches or confusion.

West Nile isn’t contagious between people, but there is no vaccine or treatment, and the only way to protect yourself from the risk of disease is to prevent mosquito bites.

Prevent West Nile by following 4 basic recommendations:

  • Use insect repellent with DEET, picaridin, IR3535, oil of lemon eucalyptus, or para-menthane-diol. Follow label instructions.

  • Limit time outside at dawn and dusk, when mosquitoes that spread the virus are most active.

  • Wear long sleeves, pants, and socks in areas with mosquito activity and apply repellent to clothing. Use head nets and cover baby carriers with netting.

  • Remove standing water from yards and gutters weekly. Treat water that can’t be drained with Bti larvicide – learn about Bti for mosquito control.

  • Install or repair window and door screens.

West Nile Virus. How do you get it? Mosquitoes bite infected birds, then bite humans. Do you think you have it? Only about 20% of people have symptoms, which are most often mild, flu-like complaints. Symptoms usually show up 2-14 days after being bitten by an infected mosquito. Fewer than 1% of people get a serious illness that can be fatal. If you develop severe headaches or confusion, contact your health care provider. How do you fight it? Protect yourself: wear long sleeves, pants and socks in areas where mosquitoes are active. Use an effective insect repellent containing deet, picaridin, IR3535, and some oil of lemon eucalyptus and para-menthane-diol products provide the best protection. Stay inside, or take steps to avoid mosquito bites at dusk and dawn, when mosquitoes that carry West Nile virus are most active. Mosquito-proof your home: Drain standing water around your house often. Empty tires, cans, flowerpots, clogged gutters, rain barrels, toys and puddles. Install or repair screens on windows and doors.

More information about West Nile virus, prevention, and Colorado West Nile virus data can be found in the following links: