By Brenda Goodman, MA
WebMD Health News
Reviewed by Brunilda Nazario, MD
March 17, 2016 -- San Juan, P.R. -- In Puerto Rico, they're trying to figure out once again how to kill the mosquitoes.
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The island U.S. territory has been fighting Aedes aegypti mosquitoes and the diseases they spread -- including dengue fever, chikungunya, yellow fever, and now Zika -- for over a century. But Aedes are hardy and adaptable. They've thrown public health experts some curveballs that are forcing them to rewrite the mosquito-fighting playbook here.
And they're looking halfway around the world for help.
"Nobody, except for Singapore, has been able to give Aedes aegypti the good fight," says Brenda Rivera Garcia, DVM, state epidemiologist for the Puerto Rico Department of Health.
Singapore's natural features and population are a lot like Puerto Rico's. Both islands have tropical climates, which means mosquitoes are a year-round problem. Both have higher-than-average numbers of people per square mile. Both have been battling epidemics of dengue fever for decades.
In the 1960s, Singapore carried out an extensive public health campaign using an army of inspectors, who scoured homes and residential areas for potential breeding sites, Rivera says.
The campaign was effective. By the early 1970s, the country had tamped down dengue so well that experts think most people in Singapore lost their immunity to the virus. That has allowed dengue to return there full force in recent years.
Puerto Rico doesn't have the money to roll out a full Singapore-style campaign, but health department officials are hoping to borrow some of the main strategies used there to head off Zika.
The first step in fighting Aedes mosquitoes is always getting rid of even tiny amounts of water that may collect in and around a house.
In Singapore, an ad campaign prompts residents to "Do the 5-step Mozzie Wipeout." The five steps target the most common mosquito breeding sites around homes -- urging people to empty vases and flower pot plates every other day, cover bamboo pole holders, turn over water-holding containers like buckets, and regularly treat roof gutters with insecticides.
Puerto Rico is working on a similar public awareness campaign. They don't exactly know how it will look yet, but Rivera says they're honing in on the idea of "herd immunity" to protect the most vulnerable residents on the island: pregnant women. Zika virus has been strongly linked to serious birth defects of the brain and nervous system in babies of infected women.
Herd immunity is a term borrowed from vaccine campaigns. It means everybody needs to take action to protect people who can't protect themselves -- in this case, unborn children. Puerto Rican officials hope it's a message that will resonate in this family-oriented culture.
But the regular steps people in Puerto Rico should take are probably going to look a little different, starting with septic tanks.
In 2010, scientists here discovered that Aedes mosquitoes were able to continue to breed during the dry season by adapting to live in the sewage that collects in septic tanks -- something these "clean water" mosquitoes don't normally do.
An important part of the mosquito-control strategy here will involve repairing cracked tanks and covering their access tubes with screens, a huge job in a place where about half the homes rely on septic tanks to treat their wastewater. Septic tanks will also need disks of a chemical that kills the mosquitoes in their early, larval stages, before they turn into flying adults.
Another top priority is to find a chemical that will kill the flying pests.
"We're literally working around the clock on this," says Audrey Lenhart, PhD, an entomologist with the CDC, who is leading the insecticide experiments.
In Puerto Rico, Aedes aegypti have become resistant to permethrin, the insecticide that's sprayed around the island to keep them at bay. The chemical isn't killing mosquitoes, but it is killing bees, which many farmers rely on to pollinate their crops and make honey.
Working out of the CDC's dengue branch office, scientists have gathered mosquito eggs from 17 different sites around the island. In a room called an insectary, they're hatching the eggs and rearing the young mosquitoes into flying adults.
When the adults are between 2 and 5 days of age, they put them in glass bottles that have been coated with different EPA-approved insecticides. Every 15 minutes, scientists check the bottles to see how many mosquitoes have died. After 2 hours, if the mosquitoes are still alive, they are considered to be resistant to the chemical.
"We're checking for resistance, and we're finding a lot of it," says CDC Director Tom Frieden, MD, who recently traveled to Puerto Rico and observed the agency's insecticide resistance experiments.
The race is on to find a chemical that can be sprayed on surfaces in and around homes, particularly the homes of pregnant women, before the rainy season hits Puerto Rico and causes the mosquito population on the island to explode.
But Lenhart says they know from studies done in Mexico that insecticide resistance is a tough and highly local problem. They'll probably need an arsenal of different insecticides to make a dent.
"In one site the mosquitoes might be completely susceptible to an insecticide, but 50 miles down the road it could be a different story," she says. "A one-size-fits-all, island-wide strategy may not even be feasible at this stage."
SOURCES: Brenda Rivera Garcia, DVM, state epidemiologist, Puerto Rico Department of Health, San Juan, PR. Audrey Lenhart, PhD, research entomologist, U.S. Centers for Disease Control and Prevention, Atlanta, GA. Tom Frieden, MD, Director, U.S. Centers for Disease Control and Prevention, Atlanta, GA. Medical and Veterinary Entomology, June 2010. American Journal of Tropical Medicine and Hygiene, 2012.
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