Are mosquitoes, ticks and fleas turning California into a disease hotspot?

Some days it feels similar we’re losing a conflict against mosquitoes, ticks, fleas, flies and worms.

Just this month, tularemia, besides known arsenic rabbit fever and caused by a bacterium successful ticks and cervid flies, was detected successful radical surviving connected Long Island. A parkland successful Marin County was unopen down successful July aft it was recovered to beryllium teeming with fleas, which tin transportation plague and typhus. And successful April, Rickettsia, a uncommon but deadly tick-borne bacterium, was recovered successful a idiosyncratic surviving successful Sonoma County.

Lyme illness and Alpha-gal syndrome are expanding their range. Dengue and typhus are increasing. There are inactive reports of West Nile, Zika and Chikungunya, positive the imaginable introductions of Chagas and rat lungworm disease. Some of these are contracted overseas, portion others are seemingly settling successful for the agelong haul.

Experts accidental determination is nary azygous crushed for this surge successful disease. Some of it whitethorn beryllium hyped news, header sensationalism. When everyone is penning astir an outbreak of a disease, nary substance however isolated, it tin consciousness much pervasive and wide than it really is.

Increased monitoring and much precocious illness tracking tin besides amplify the consciousness that thing caller is afoot, adjacent if it was ever there.

But successful immoderate cases, determination truly does look to beryllium an summation — and California is beforehand and center.

You person to ask, “what are we doing to the satellite and does that mean we volition brushwood much diseases?” said Janet Foley, a illness ecologist astatine the UC Davis School of Veterinary Medicine. “The reply is: yes.”

This year, conditions including a record-smashing March vigor question led to a surge successful California’s autochthonal Culex mosquitoes, the transmitters of West Nile virus. Most infections are asymptomatic, but astir 1 successful 100 impact neuroinvasive disease: meningitis, encephalitis oregon paralysis. There’s nary circumstantial treatment.

Matt Willis is simply a erstwhile Marin County nationalist wellness serviceman who utilized to enactment astatine the Centers for Disease Control and Prevention arsenic an epidemiologist and present helps exemplary the wellness impacts of clime change. In a July 23 Substack post, helium wrote that aboriginal signs constituent to a perchance terrible play for West Nile microorganism successful California. L.A. County’s affirmative mosquito samples were 10 times higher this twelvemonth than past and nationally, quality cases were astatine a 20-year precocious for that constituent successful the season. The pursuing week, helium wrote that accrued enactment “has maine adjacent much concerned.”

As of mid-August, 373 West Nile virus-positive dormant birds person been reported statewide, compared to 76 astatine the aforesaid constituent successful 2025. The CDC confirmed 181 quality cases nationwide — 129 of them neuroinvasive.

“As somesthesia rises globally, and arsenic much bluish latitudes acquisition hotter temperatures connected average, and much utmost temperatures, immoderate of those pathogens and/or insects that are much prevalent successful those parts of the satellite migrate northward,” Willis said. “That is happening.”

Another mosquito-borne illness of expanding interest successful California is dengue. Also known arsenic “breakbone fever” — the painful, debilitating disease, which tin beryllium fatal, is caused by a microorganism adapted to an invasive, African mosquito known arsenic Aedes aegypti.

A caller survey successful the Lancet Regional Health Americas recovered that astir 18.2 cardinal Californians — chiefly successful the Central Valley, L.A. and San Diego metro areas — unrecorded wherever conditions are suitable for section dengue transmission astatine slightest 1 period a year, usually July oregon August.

“Under mean scenarios of clime warming and municipality expansion, an further 4.1 cardinal residents whitethorn beryllium astatine hazard by mid-century,” according to the survey led by UC Berkeley’s Lisa Couper.

The state’s archetypal 2 locally acquired cases (meaning the radical did not declaration the illness portion traveling) were logged successful 2023. The pursuing year, that fig ballooned to 18, with 14 successful Los Angeles County. The California Department of Public Health identified this section dispersed arsenic the astir notable caller vector-borne inclination it’s tracking.

There were 7 cases successful 2025, and truthful acold zero this year.

The black-and-white striped mosquitoes were archetypal detected successful 2013 successful California, apt arriving via quality question and cargo transport. The aforesaid mosquito carries the diseases Zika and Chikungunya.

And there’s flea-borne typhus, which is besides connected the emergence successful Southern California. In 2025, Los Angeles County wellness officials confirmed 220 cases — a crisp summation implicit the 187 cases reported successful 2024.

The reasons? More of the same. A warmer clime has expanded the flea season. And arsenic radical (and their pets) determination into areas person to wildlife, opportunities for the typhus bacteria to leap to radical grows.

But determination whitethorn beryllium different reason, too, said Jim Farrar, manager of the UC Statewide Integrated Pest Management Program: State laws and section ordinances that person banned definite rodenticides could beryllium inadvertently fueling typhoid outbreaks.

Last month, a parkland successful Fairfax successful Marin County was overrun with fleas. The metropolis had to unopen down the parkland for respective weeks. In 2001, Fairfax lawmakers enacted a prohibition connected each herbicides, insecticides and rodenticides connected nationalist land. The authorities besides has wide prohibitions connected respective rodenticide ingredients.

“I mean, determination was a bully crushed for this,” Farrar said. Wildlife officials had recovered that galore of these poisons worked their mode done the nutrient chain, sickening and sidesplitting owls, raptors, coyotes and upland lions.

But it has near an opening for immoderate of these vector reservoir animals — specified arsenic rats, mice and others — to instrumentality vantage of their changing clime and scenery to breed and grow their ranges; which leads to much accidental for wildlife-to-human illness transfer.

Foley, however, cautioned that trying to pinpoint immoderate 1 origin of illness maturation is astir apt shortsighted.

She said researchers trying to bash that tin often miss the complexity, scale, and earthy variability successful the systems they are investigating. Traditional approaches to studying cause-and-effect relationships are often not imaginable erstwhile investigating populations and ecosystems, which are large, analyzable and messy.

We are apt to spot much of these diseases. And caller ones. As radical determination deeper and deeper into ecosystems and territories wherever few, if any, had lived before, they hazard interacting with pathogens that were antecedently confined.

“We are dysregulating ecosystems and creating much opportunities for contact,” said Foley, the UC Davis veterinarian. She pointed to quality of roadworthy operation crews digging deeper and deeper into Brazil’s Amazon forest, which she said volition inevitably “bring america into interaction with birds and bugs” and the disease-baggage they carry.

Times unit writer Mack Baysinger contributed to this report.