West Nile Virus in Colorado: Symptoms, Who's at Risk, and Avoiding Mosquito Bites
West Nile virus spreads through mosquito bites, and Colorado sees cases every summer. Most people have mild or no symptoms, but some get seriously ill. Here's what to know.
West Nile virus spreads through the bite of an infected mosquito, and Colorado reports cases every summer and early fall, often among the most in the country. Most infected people, about 8 in 10, have no symptoms. About 1 in 5 get a fever with headache, body aches, tiredness and sometimes a rash. Fewer than 1 in 100 develop a serious brain or spinal cord infection, with a stiff neck, confusion or weakness, which is an emergency. There's no specific treatment, so preventing bites matters most.
When and where
In Colorado, West Nile season usually runs from June through September, peaking in August. The mosquitoes that carry it are most active at dusk and dawn, and breed in standing water.
Symptoms
West Nile fever (about 1 in 5 people), usually 2 to 14 days after a bite:
- Fever
- Headache
- Body aches and joint pain
- Tiredness, which can last weeks
- Sometimes a rash, vomiting or diarrhea
Serious illness (fewer than 1 in 100):
- High fever and severe headache
- Stiff neck
- Confusion or disorientation
- Muscle weakness or paralysis
- Tremors or seizures
- Vision loss or numbness
Who's most at risk of serious illness
- Adults over 60
- People with cancer, diabetes, high blood pressure or kidney disease
- People with a weakened immune system, including organ transplant recipients
Treatment
There's no antiviral medicine for West Nile. Mild illness is treated with rest, fluids and pain relievers. Serious illness needs hospital care.
Preventing mosquito bites
- Use an EPA-registered insect repellent with DEET, picaridin, IR3535 or oil of lemon eucalyptus, following the label
- Wear long sleeves and pants at dusk and dawn
- Empty standing water around your home every week: buckets, birdbaths, flowerpots, gutters and toys
- Keep window and door screens in good repair
- Use extra care in late summer, when risk is highest
When to get seen
Come in for fever with headache and body aches in summer or early fall, especially if you've had mosquito bites, are over 60, or have other health conditions. A clinician can check you and decide whether testing or a referral is needed. At an AfterOurs clinic, a standard self-pay visit is $175, and a self-pay in-clinic visit never costs more than $330. We contract with all major commercial and government plans, including Colorado Medicaid and Medicare. See our fever page and the cost and insurance page. Our tick bite post covers the other summer bug bite risk in Colorado.
Go to the ER or call 911 if a fever comes with a stiff neck, severe headache, confusion, unusual sleepiness, muscle weakness, trouble walking, a seizure, or vision changes.
Where to find us
Four clinics across metro Denver, all ages welcome, se habla español:
- Hampden (Southeast Denver): 8 AM–11 PM daily
- Littleton: 8 AM–11 PM daily
- Thornton: 8 AM–11 PM daily
- Highlands: Mon–Fri 5–11 PM, Sat–Sun 8 AM–11 PM
Walk in, or book online.
Medically reviewed by Peter Martinuzzi, MD
This article is general health information, not medical advice for your situation. If you are worried, come in or book a telehealth visit — and if it is an emergency, call 911.