Some days it feels like we\u2019re losing a battle against mosquitoes, ticks, fleas, flies and worms. <\/p>\n
Just this month, tularemia<\/a>, also known as rabbit fever and caused by a bacterium in ticks and deer flies, was detected in people living on Long Island. A park in Marin County<\/a> was shut down in July after it was found to be teeming with fleas, which can carry plague and typhus. And in April, Rickettsia,<\/a> a rare but deadly tick-borne bacterium, was found in a person living in Sonoma County.<\/p>\n Lyme disease and Alpha-gal syndrome are expanding their range. Dengue and typhus are increasing. There are still reports of West Nile, Zika and Chikungunya, plus the potential introductions of Chagas<\/a> and rat lungworm disease<\/a>. Some of these are contracted overseas, while others are seemingly settling in for the long haul. <\/p>\n Experts say there is no single reason for this surge in disease. Some of it may be hyped news, headline sensationalism. When everyone is writing about an outbreak of a disease, no matter how isolated, it can feel more pervasive and widespread than it actually is. <\/p>\n Increased monitoring and more advanced disease tracking can also amplify the sense that something new is afoot, even if it was always there.<\/p>\n But in some cases, there really does appear to be an increase \u2014 and California is front and center.<\/p>\n You have to ask, \u201cwhat are we doing to the planet and does that mean we will encounter more diseases?\u201d said Janet Foley, a disease ecologist at the UC Davis School of Veterinary Medicine. \u201cThe answer is: yes.\u201d<\/p>\n This year, conditions including a record-smashing March heat wave led to a surge in California\u2019s native Culex mosquitoes, the transmitters of West Nile virus.<\/a> Most infections are asymptomatic, but about one in 100 involve neuroinvasive disease: meningitis, encephalitis or paralysis. There\u2019s no specific treatment.<\/p>\n Matt Willis is a former Marin County public health officer who used to work at the Centers for Disease Control and Prevention as an epidemiologist and now helps model the health impacts of climate change. In a July 23 Substack post,<\/a> he wrote that early signs point to a potentially severe season for West Nile virus in California. L.A. County\u2019s positive mosquito samples were 10 times higher this year than last and nationally, human cases were at a 20-year high for that point in the season. The following week, he wrote<\/a> that increased activity \u201chas me even more concerned.\u201d <\/p>\n As of mid-August, 373 West Nile virus-positive dead birds have been reported statewide<\/a>, compared to 76 at the same point in 2025. The CDC confirmed 181 human cases<\/a> nationwide \u2014 129 of them neuroinvasive. <\/p>\n \u201cAs temperature rises globally, and as more northern latitudes experience hotter temperatures on average, and more extreme temperatures, some of those pathogens and\/or insects that are more prevalent in those parts of the world migrate northward,\u201d Willis said. \u201cThat is happening.\u201d<\/p>\n Another mosquito-borne disease of increasing concern in California is dengue.<\/a> Also known as \u201cbreakbone fever\u201d \u2014 the painful, debilitating disease, which can be fatal, is caused by a virus adapted to an invasive, African mosquito known as Aedes aegypti.<\/p>\n A recent study<\/a> in the Lancet Regional Health Americas found that approximately 18.2 million Californians \u2014 primarily in the Central Valley, L.A. and San Diego metro areas \u2014 live where conditions are suitable for local dengue transmission at least one month a year, usually July or August.<\/p>\n \u201cUnder moderate scenarios of climate warming and urban expansion, an additional 4.1 million residents may be at risk by mid-century,\u201d according to the study led by UC Berkeley\u2019s Lisa Couper.<\/p>\n The state\u2019s first two locally acquired cases (meaning the people did not contract the disease while traveling) were logged in 2023. The following year, that number ballooned to 18<\/a>, with 14 in Los Angeles County. The California Department of Public Health identified this local spread as the most notable recent vector-borne trend it\u2019s tracking.<\/p>\n There were seven cases in 2025, and so far zero this year.<\/p>\n The black-and-white striped mosquitoes were first detected in 2013 in California, likely arriving via human travel and cargo transport. The same mosquito carries the diseases Zika and Chikungunya.<\/p>\n