Mosquito season in the United States is no longer just a summer nuisance. A cluster of new data on an obscure virus called Oropouche, alongside surging dengue and eastern equine encephalitis, suggests the country is entering a far more dangerous era for mosquito-borne disease. The pathogens are not yet everywhere, but the ecological and travel patterns that helped them spread across the Americas are now firmly in place on U.S. soil.
The headline threat is a little known infection sometimes nicknamed “sloth fever,” which has quietly arrived via travelers and is now being tracked by federal health officials. Taken together with invasive mosquitoes moving into new states and a warming climate that favors their survival, the picture that emerges is one in which millions of Americans could be exposed in the coming years if the country stumbles on surveillance and prevention.
What the new Oropouche data actually shows
Oropouche virus, or OROV, is not a household name, but it is already on the radar of federal epidemiologists. The Centers for Disease Control and Prevention has created a dedicated dashboard for Oropouche in the United States, with maps that track infections by state and travel status. A companion page notes that the underlying Data are current to mid January and distinguish between Travel associated and Locally acquired cases across each Year. That level of granularity is a sign that officials are treating Oropouche as more than a curiosity.
Clinically, Oropouche behaves like a tropical flu, with high fevers and severe headaches, but it belongs to a family of orthobunyaviruses that can cause neurological complications. A recent scientific review describes in its Abstract how Oropouche outbreaks have repeatedly swept through Amazonian cities, often with predominantly mild clinical courses but occasionally with meningitis-like symptoms. Another section labeled Introduction underscores that OROV is an emerging pathogen, not a historical footnote, and that its ecology overlaps with urban mosquitoes that already thrive in the United States.
From “sloth fever” to U.S. travelers
The virus’s leap toward American patients has come through airports rather than swamps. According to one clinical summary, Key Takeaways More than 20 people returning to the United States after traveling in Cuba have been infected with Oropouche, a pattern that immediately raised concerns about onward spread. The same report notes that the nickname “sloth fever” emerged because sloths are susceptible to the virus, a reminder that OROV is deeply rooted in forest ecosystems that are now colliding with human settlements.
Regional health agencies across the Americas are also sounding the alarm. In Washington, PAHO, formally The Pan American Health Organization, has issued an epidemiological update on Oropouche fever that details how infections have surged in some countries even as others such as Colombia have reported fewer cases. That hemispheric backdrop explains why U.S. clinicians are being briefed through pediatric references like Red Book Online, which notes that As of January a total of 110 Oropouche Virus infections had been documented in the Americas and flags Oropouche Virus and Possible Sexual Transmission as an area of active investigation.
Why climate and land use are supercharging mosquito risk
Even if Oropouche has not yet established local transmission in U.S. cities, the ecological table is being set. As Forests Shrink, Mosquitoes Are Turning to Humans for Blood, with researchers documenting how insects that once fed on a mammal from the dog family are now biting people more often as their natural habitats disappear. A complementary study notes that as forests disappear, mosquitoes are not vanishing with them but are adapting in ways that bring them closer to people and that this shift could be detected long before outbreaks become visible, a pattern summarized in the single word Instead.
Climate change is widening the geographic reach of the most dangerous mosquito species. One analysis of invasive vectors reports that An Invasive Disease Carrying Mosquito Has Spread into the Rocky Mountains The Aedes aegypti mosquito, historically unable to survive at those elevations, now has a population that is thriving in Western Colorado. Another report notes that More than 2,500 dengue-related deaths have been reported globally in 2025, with outbreaks in Brazil, India, Australia and other countries, underscoring how quickly Aedes-borne viruses can turn lethal when conditions are favorable.
Dengue, EEE and West Nile show what is already possible
To understand what Oropouche might do if it gains a foothold, it helps to look at the mosquito diseases that are already here. Federal health advisories note that in 2024 more than 13 million cases of dengue were reported in North, Central, South America and the Carib, a staggering regional burden that makes spillover into the United States almost inevitable. Public radio coverage has already warned that the U.S. is seeing increased risk of dengue infections, with NPR highlighting that more than 9 million infections had already been logged in the Americas at that point.
Other mosquito-borne threats are moving north as well. A detailed feature on a rare but deadly brain infection notes that eastern equine encephalitis is gaining ground in North America, after Once being confined to a few select regions. State agriculture officials in the Midwest have echoed that concern, with MDARD reporting that MDARD Confirms First Case of Eastern Equine Encephalitis for 2025 in a Benzie County Horse, an incident that prompted warnings from Agriculture and Rural Developmen leaders about vaccination for animals.
The southern frontline: South Carolina and beyond
Along the Atlantic coast, the risk is no longer theoretical. In South Carolina, university extension specialists announced the First 2025 Cases of Eastern in South Carolina, warning from COLUMBIA that mortality can reach 90 percent in unvaccinated horses. Later in the season, public health officials confirmed the first human EEE death in over 20 years in the state, with EEE detected in mosquito pools and Beaufort County responding by setting traps in affected neighborhoods, a step local leaders in Beaufort County described as essential for early detection.
These outbreaks are unfolding against a backdrop of broader mosquito expansion. Infectious disease specialists warn that mosquito-borne diseases are expanding and posing new threats, with Jan C. Semenza, a PhD and MPH at Ume University in Sweden and former head of a European disease program, arguing that the range of Aedes mosquitoes is growing and that “we cannot be sound asleep at the switch.” That warning resonates in coastal states and in inland hubs like New York and Michigan, where summer mosquito seasons are lengthening and public health departments are already stretched.
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