Los Angeles County's West Nile count passes 180, the highest in nearly a decade

Residents have taken to sleeping in long sleeves and stuffing towels into the gaps under their doors ^[17]. The mosquito they are most likely to notice is Aedes, the "ankle biters" that first showed up in the region in 2014 and target humans ^[11]. "They like human blood," said Anais Medina Diaz, communications director for the San Gabriel mosquito and vector control district. "They will bite you multiple times to get a full blood meal." ^[13] People here have had less exposure to the species, so its bites can leave larger welts, according to Medina Diaz ^[12].

West Nile takes another route ^[7]. The virus moves through Culex quinquefasciatus, the region's most common native mosquito, known locally as "quinx" ^[7]. Quinx prefer birds to people. They pick up the virus from birds such as crows and pass it on when they do bite humans ^[7].

Districts follow that cycle with traps ^[8]. Brian Brannon, public information officer for the Orange County mosquito and vector control district, said the season's tally of positive traps "doesn't mean 850 mosquitoes" but "means 850 traps full of mosquitoes with West Nile virus" ^[9]. "We've had a pretty busy season," he said ^[10]. The reporting does not include the district's total number of traps, Los Angeles County's case counts for earlier years, or any measure of control effort ^[1][8]. Without those figures, nobody can calculate the share of traps testing positive or the size of this year's rise.

On cause, the official quoted points to the weather. "This year in particular for our area has been higher mosquito activity and higher disease activity," Medina Diaz said ^[2]. "We can attribute that to some connection to the environment and how warm it's been this year." ^[3] The Guardian goes further and ties the season to heatwaves and an extra-powerful El Niño ^[19].

We think warmth is the better-supported explanation for the West Nile rise, because it has a physical basis. Mosquito activity drops sharply as overnight temperatures approach 50F ^[4], and an unusually humid summer made breeding easier ^[5]. It is still one warm season set beside one high case count. Medina Diaz herself described the link only as "some connection" ^[3]. If the region's decade of struggling control contributed to this year's cases, the officials quoted did not say so ^[16].

Dengue is a separate question. The Guardian frames it as a fear that travelers could bring the virus in ^[19]. Aedes, the mosquito that would spread it locally, has been proliferating in the region for 12 years ^[18], and this year's moisture has especially helped it thrive, according to the Guardian ^[15].