After the hurricane came more mosquitoes

Super-charged hurricanes not only wreak havoc in the areas they are passing through, but also give rise to one of the deadliest animals on earth: mosquitoes.

As the world is experiencing a warming climate, we can expect more intense hurricanes, which bring about perfect breeding conditions for mosquitoes, namely increased instances of standing water.

Rising temperatures and precipitation patterns will also continue to affect the spread of mosquitoes into more regions. Certain species of mosquito have already adapted to environmental changes and have learned to reproduce in water that is slightly saltier than the freshwater they traditionally prefer.

Mosquitoes are a nasty disease vector, responsible for spreading pathogens that cause diseases including dengue, malaria, West Nile virus and Eastern Equine Encephalitis (EEE). Each can be deadly for humans.

Many studies have shown why hurricane-prone states of the United States should worry about the rise of mosquitoes and mosquito-borne diseases — especially Florida. Of the 292 hurricanes that occurred in the continental United States between 1851 and 2018, 123 (41%) of them struck Florida.

In a recently published study, scientists quantified the risk of mosquito-borne illnesses after hurricanes by counting mosquito populations before and after the devastating Hurricane Irma that struck Florida in 2017.

The study took place in Miami-Dade and involved mosquito surveillance data provided by the Miami-Dade County Mosquito Control and Habitat Management Division from Aug. 15, 2016 to Oct. 15, 2018.

“There’s a lot of studies that have been done on mosquitoes, but usually they only look at one-time point. They’ll look before or after,” explains Imelda K. Moise, the lead author of the study and the director of Global Health Studies at the University of Miami.

“This was the first study to kind of look at pre- and post-analysis to see if there’s an association in mosquito composition, and also speciation and abundance,” she added.

The result reveals that the mosquito abundance rose after the hurricane — to 5.3 times what it was before — and then slowly declined over the year after the storm.

As for the population composition, Cx. nigripalpus dominates the species of captured mosquitoes (70.4%). This mosquito species is also known as the Florida SLE mosquito and is a primary vector for several pathogens, including the Saint Louis Encephalitis virus as well as West Nile virus.

The next most common mosquito species in the inventory were Aedes taeniorhynchus (10.1%) and Aedes albopictus (7.8%), which carry the Eastern Equine Encephalitis and West Nile pathogens respectively.

Other captured mosquito species included Aedes albopictus (7.8%), which carries Dengue, Chikungunya and Zika, and some smaller percentages of Aedes aegypti (3.6%) and Cx.quinquefasciatus (3.5%), both of which carry Dengue fever.

This change in the mosquito abundance is likely to have public health consequences in Florida and beyond. In fact, cases of Dengue fever have already begun to pop up in Florida’s Tampa Bay region weeks after Hurricane Milton hit the area on Oct. 9, 2024. As of Jan. 8, the Hillsborough County Department of Health has confirmed seven cases of locally-acquired Dengue fever.

“Traditionally, [mosquito-borne disease] cases originated from travelers coming from outside the country who came back with a fever,” says Matthew Philips, a physician scientist who specializes in infectious diseases at Harvard Medical School. “But now we’re seeing normal endemic spread in Florida and Southern Texas. They’re just there now.”

Another notable concern is that several mosquito species remained high in number in Florida, even after Hurricane Irma in 2017.

“What is interesting is that some important vectors elevated in number in 2017 and remain elevated in 2018, which is the Aedes taeniorhynchus,” says Ranjan Ramasamy, a researcher focusing on vector-born disease at the University of Jaffna, who was not involved in the study.

The common name for the A. taeniorhynchus is the Black Salt Marsh mosquito, which is one of the mosquito species to have developed a tolerance for water with a small amount of salt, as opposed to freshwater mosquitoes. The Black Salt Marsh mosquito carries the Eastern Equine Encephalitis virus and is commonly found in North America, including the United States.

Some mosquito species, like the notorious Aedes aegypti, are known to reside in stagnant freshwater, which can form in everyday things like buckets, flowerpots and discarded tires. But they, too — according to a study by Ramasamy — have recently been found to develop in brackish water (a mixture of freshwater and saltwater) in coastal areas.

Not only have they been shown to be less picky about where they breed, but the Ae. aegypti larvae that emerged from brackish water habitat have also been observed to possess thicker cuticles and greater resistance towards adulticides (insecticides used against adult mosquitoes). The researchers discovered the latter finding by directly testing the mosquitoes’ susceptibility to different substances of adulticides.

This greater resistance to insecticides among brackish water mosquitoes might then pose a new challenge in mosquito control efforts. This is a telltale sign indicating how mosquitoes can adapt to the surrounding environment, which will ultimately affect the mosquitoes’ ecological and geographical distribution.

Experts suggest that public health officials should adopt mitigation efforts accordingly to respond to the development of pathogen-carrying mosquitoes and curb disease transmission effectively, especially during the times when super-charged storms will occur more often than they did 20 years ago.