Jamestown Canyon Virus Turned Up in Five Mosquito Species Across Northeastern Illinois
Jamestown Canyon virus (JCV) has now been detected directly in mosquito populations from suburban forest preserves in northeastern Illinois, a finding that pushes the virus from abstract regional concern into a tangible local exposure risk. In 2024, investigators working in Cook, DuPage, and Lake counties screened mosquito pools from wetland habitats bordering residential development and found viral RNA in five species, with the strongest signal in Anopheles punctipennis and the Aedes stimulans group.
The result matters because JCV is not a rare biological curiosity. It is a mosquitoborne orthobunyavirus capable of causing febrile illness and neuroinvasive disease, including encephalitis and meningitis, yet human case counts almost certainly understate its true burden. Many infections are asymptomatic or mild, and routine testing has historically been limited, so case-based surveillance misses much of the transmission that is happening in the background.
Wetland Mosquitoes at the Edge of Suburban Development
The sampling strategy targeted an ecological seam where human contact with vector habitats becomes more likely. Eight suburban forest preserves were chosen because they contained vernal woodland pools and other seasonal wetlands that support univoltine Aedes species and other suspected JCV vectors. Those habitats sit beside residential areas, creating repeated opportunities for mosquitoes, deer, and people to share the same landscape even when direct contact is invisible to residents.
Field collections began in early May 2024 and continued through October. At each site, investigators used CDC light traps baited with dry ice and, at one location, added BG-Sentinel traps with octenol and UV-LED or skin lure combinations to better capture mammalophilic mosquitoes. The design was built around the timing of enzootic amplification, when infection in vector populations is most likely to rise and when human exposure risk can climb quickly.
One sentence captures the main public health concern plainly: infected mosquitoes were present in the same season when people spend time in the preserves.
Infection Concentrated Early in the Season
Across the season, 213,996 female mosquitoes and 16,374 male mosquitoes were identified to species, species group, or genus level. From that haul, 73,917 female mosquitoes from 11 taxa were placed into 2,029 pools for JCV testing. The average pool contained 36.4 female specimens, allowing large-scale screening while preserving the ability to estimate infection prevalence.
JCV RNA appeared in 33 pools spanning five taxa. The highest estimated infection rates were found in An. punctipennis and the Ae. stimulans group, followed by Ae. canadensis, Ae. trivittatus, and Coquillettidia perturbans. Detection was not evenly distributed across the calendar. The earliest positive pool came from an Ae. stimulans group collection on May 24, and infections in that group rose through June and July.
No JCV was detected in mosquitoes collected after July 23.
That abrupt seasonal drop suggests a short but intense transmission window, the kind of pattern that can be missed when surveillance begins too late or relies too heavily on human case reports. For a virus maintained in wildlife-mosquito cycles, the timing of infected vectors is not just a field detail; it is the biological clock of human risk.
Why Anopheles punctipennis and Aedes stimulans Stand Out
The mosquito findings fit established patterns from other endemic regions. Ae. stimulans has been identified elsewhere as an early season host and vector, while Ae. canadensis and related species have also been implicated in JCV maintenance in parts of the northeastern and upper Midwestern United States. In this Illinois cohort, the infection profile points most strongly to An. punctipennis and the Ae. stimulans group as the likely primary enzootic vectors, with Ae. canadensis and Cq. perturbans possibly contributing secondarily.
The biology makes sense. Mammalophilic mosquitoes that feed on deer and other vertebrates can sustain arboviral amplification when their seasonal emergence overlaps with infected reservoir hosts. White-tailed deer are a primary JCV reservoir, and prior serologic work in Illinois has already suggested active transmission in suburban forest preserve habitats around Chicago. The current mosquito data close an important gap by connecting that reservoir exposure to direct entomologic evidence.
Within the Ae. stimulans group, female mosquitoes identified specifically as Ae. stimulans had a higher estimated infection rate than other members of the group that could not be assigned to species level. Infection in that group also differed by month, with July showing a significantly higher estimated infection rate than May. Those patterns matter because they suggest species-level ecology is shaping transmission intensity, not just broad habitat suitability.
Human Exposure Risk Is Likely Underrecognized
The cumulative vector index values ranged from 0.05 to 0.25 across sampling locations during May through July, indicating low to moderate seasonal risk but also clear geographic heterogeneity. Even at the lower end of that range, a virus that can cause encephalitis is not trivial when the affected sites are adjacent to neighborhoods, trails, and other recreational spaces.
The surveillance gap in Illinois is easy to see. West Nile virus dominates arboviral attention in the state, while JCV has been reported only rarely in humans. Yet the absence of frequent human diagnoses does not mean the virus is absent from the environment. It more likely reflects a combination of under-testing, mild or unrecognized infection, and the challenge of distinguishing JCV from other California serogroup viruses in clinical laboratories.
For clinicians, the implication is direct: summer and early-season neurologic or febrile illness in people with mosquito exposure in northeastern Illinois should keep JCV on the differential diagnosis, especially when routine testing has not been informative. For public health agencies, the ecological signal argues for continued mosquito surveillance in suburban wetlands where human development presses against natural habitat.
Entomologic Surveillance Now Carries the Main Warning
The study shows how local mosquito ecology can reveal risk that case counts alone miss. Five mosquito taxa carried JCV RNA, infected pools clustered early in the warm season, and the pattern fit known JCV transmission biology in North America. The strongest evidence points to An. punctipennis and the Ae. stimulans group as the most important species to watch in northeastern Illinois, with other competent vectors likely adding smaller contributions.
Continued surveillance will be essential because mosquito abundance, wetland conditions, and human land use all shift from year to year. The current data define a seasonal exposure window, but they do not settle how often spillover reaches people, how local land management affects risk, or how much transmission occurs beyond the sampled preserves. Those questions remain open, and the next field season may show whether the July peak seen here is consistent or merely one year’s expression of a larger ecological cycle.