CHILLICOTHE, Ohio — A mosquito collected from a trap in Colerain Township has tested positive for La Crosse virus, the Ross County Health District announced. No human cases have been reported in the county in 2026, according to the health district.
The mosquito species most responsible for spreading the virus, the Eastern treehole mosquito, is an aggressive daytime biter commonly found in wooded areas, health officials said. The Ohio Department of Health reports that Ohio records more human La Crosse virus cases than any other state, averaging about 20 cases a year.
Most people infected with La Crosse virus do not develop symptoms, according to the Centers for Disease Control and Prevention. When symptoms do appear, they can include fever, headache, nausea, vomiting and fatigue. In rare cases, the virus causes encephalitis, or inflammation of the brain, a severe complication that occurs most often in children younger than 16.
A Ross County resident tested positive for La Crosse virus last October in what health officials said was the county’s first known human case. No additional human cases have since been confirmed, according to the health district.
Because some mosquito species, including the one that carries La Crosse virus, bite during daylight hours, the health district urged residents to guard against bites throughout the day, not just at dusk. Recommended precautions include:
Dumping standing water around the home weekly to eliminate breeding sites.
Wearing long pants and long sleeves when possible.
Using insect repellent containing picaridin, DEET, oil of lemon eucalyptus or IR3535, all recommended by the CDC.
The health district said mosquito larvicide tablets are available at its office on a first-come, first-served basis for residents to treat standing water that cannot be drained.
The information contained in this story was obtained from the Ross County Health District.
