Feminine Aedes aegypti mosquito, a provider of the Dengue fever, feeding on the human pores and skin, 2005. Picture courtesy Facilities for Illness Management (CDC) / James Gathany. (Picture by Smith Assortment/Gado/Getty Photographs)
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For the previous century, most mosquito-borne ailments like dengue, malaria and yellow fever have been thought-about issues of the tropics, not the continental United States. That’s beginning to change. Florida started coping with domestically acquired dengue over a decade in the past. Malaria instances acquired in Texas and Florida made headlines in 2023. And now a new paper within the CDC journal Rising Infectious Ailments paperwork what often is the most telling instance in a seven-week chain of sustained dengue transmission in Los Angeles County.
Fourteen individuals contracted dengue in LA County within the fall of 2024. Eight of them clustered inside a mile of one another within the San Gabriel Valley. The opposite six had been scattered throughout 5 neighborhoods greater than 20 miles aside, every probably a separate introduction from a returning traveler.
A lot of the southern United States sits on the environmental margin for vector-borne illness transmission. The mosquitoes are current. The local weather is heat sufficient for a part of the yr. Contaminated vacationers arrive recurrently. What has saved dengue from establishing is that circumstances have been simply barely inadequate to maintain native chains. The LA outbreak exhibits what occurs when “simply barely inadequate” suggestions the opposite means, even briefly.
Timeline and geographic distribution of 14 domestically acquired dengue instances, Los Angeles County, August–November 2024. Supply: Vaughan et al., Rising Infectious Ailments 32(5), 2026.
CDC
What Seven Weeks Means
Dengue requires a relay. A mosquito bites an contaminated particular person, the virus replicates contained in the mosquito for eight to 12 days, and the mosquito bites somebody new. That particular person incubates the virus for 5 to seven days earlier than changing into infectious to the subsequent mosquito. One full cycle takes two to a few weeks. A seven-week chain means the relay accomplished at the least twice, in all probability thrice, with out breaking.
The fourteen confirmed instances are virtually definitely a fraction of the entire. About three-quarters of dengue infections produce no symptoms or signs too gentle to set off a medical go to. Many dozens extra individuals had been seemingly contaminated in LA County and by no means knew it.
The dengue virus transmission cycle. A mosquito bites an contaminated particular person, the virus replicates contained in the mosquito over eight to 12 days, and the mosquito passes it to somebody new.
CDC
A Linked World
Why did this occur in 2024? LA County recorded 222 travel-associated dengue instances that yr, up from 35 in 2022 and 75 in 2023. Greater than 13 million instances had been reported throughout the Americas in 2024, surpassing earlier data. Each traveler who returns to LA with energetic dengue viremia is a possible spark. With 222 sparks touchdown in a county the place the dominant mosquito vector, Ae. aegypti, has been around since 2013, it will have been stunning if none of them caught.
That is how what’s world turns into native. LA County didn’t develop a dengue drawback as a result of its mosquito management failed. It developed one as a result of different locations on this planet had been surging with dengue and sufficient contaminated individuals got here residence to a spot the place the vector was ready. The ongoing hantavirus outbreak that started on the cruise ship MV Hondius is one other reminder that pathogens transfer on the pace of world journey.
Reported dengue instances within the Americas, 1980–2024. The 2024 whole of almost 13 million instances was thrice the earlier report set in 2023. Information: OpenDengue V1.2 (Clarke et al. 2024); 2024 whole from PAHO.
John M. Drake
Climate change pushes both sides of the equation, increasing the vary and season size of Aedes mosquitoes in temperate areas whereas intensifying transmission within the tropics that feeds the move of contaminated vacationers.
The Threshold Drawback
Ailments propagate themselves. The distinction between no native dengue and somewhat native dengue isn’t a matter of diploma. It’s a distinction in type.
Under the transmission threshold, each introduction dies out by itself. Public well being can afford to be sluggish, as a result of the maths is on its aspect. Above the brink, every an infection generates extra infections, and the burden compounds. Surveillance, prognosis and vector management all must work rapidly and in coordination, and all of it prices greater than prevention did.
Florida has discovered this over the previous decade. Key West saw its first modern dengue outbreak in 2009. Miami-Dade and different South Florida counties started reporting sporadic domestically acquired instances shortly after. By 2022–2023, Florida was recording over 60 locally acquired cases in a single season, managed by way of sustained funding in surveillance, speedy diagnostic capability and aggressive vector management.
California is earlier on that curve. The LA County investigation confirmed that the median time from symptom onset to a dengue take a look at was 10 days. One case was initially recognized as West Nile virus. 4 of the fourteen sufferers had been discovered by public well being groups going door to door, not by the medical system. In a spot the place dengue has by no means been endemic, the prognosis requires somebody to think about it first.
What Ended the Outbreak?
The 2024 outbreak ended when the mosquitoes went dormant in late October. The timeline of the California outbreak exhibits instances accumulating by way of August, September and October, then stopping. The general public well being response was aggressive, 318 family surveys, 9 discipline operations, multilingual outreach. Whether or not it will have damaged the chain by itself is unknowable. The season ended first.
Local weather change is lengthening that season. The worldwide dengue burden is rising. The introduction strain will hold rising. Each considered one of these forces pushes a marginal system nearer to the brink. It’s virtually at all times cheaper to maintain a illness out than to handle it as soon as it arrives.
