Release of Bacteria-Laden Mosquitos Reduces Dengue Incidence by 77%

NEW YORK (Reuters Health) – As cases of dengue fever make their way into the southern United States, with sometimes-fatal results, a new study in Indonesia offers evidence of an effective way to block the spread of the mosquito-borne infection.

The technique infects mosquitoes with a bacterium known as the wMel strain of Wolbachia pipientis. Those mosquitos are less likely to carry and pass on the dengue virus.

In the new test, done in Yogyakarta on the island of Java, the regions where wMel mosquitos were released saw the rate of dengue decline by 77%.

Hospitalizations for dengue were 86% lower.

“This represents ten years of work, which builds on other observations, and says this technique brings about a profound reduction that can produce a significant public health benefit,” coauthor Dr. Cameron Simmons of Monash University in Melbourne, Australia, told Reuters Health in a teleconference call.

“The community and government there are embracing this as a public-health measure, and doing it very quickly,” he added. “We’re looking forward to turning it into an area that may be dengue free, or close to it.”

The study appears in the New England Journal of Medicine, which also carries a report from Miami, Florida, documenting the first case of a person to die of the disease after acquiring it locally. The disease wasn’t detected until after her death, 29 days and five surgeries after her hospitalization.

An analysis suggests that the disease was imported from Cuba and spread to Miami mosquitoes. In 2019, 18 people contracted the disease in Florida but none died. The other 395 cases that year were probably imported.

The report puts out “a statement that dengue is a risk in the southern United States,” including states such as Florida and Texas, said epidemiologist and coauthor Dr. Tyler Sharp of the dengue branch of the U.S. Centers for Disease Control and Prevention, located in San Juan, Puerto Rico. “It is a burden that is worsening over time. We are seeing trends that are not going in the direction we would like.”

Globally, dengue affects 50 million to 100 million people each year. In about 5% of cases, the infection becomes severe, which can lead to shock, plasma leakage, hemorrhage and acute cholecystitis. The World Health Organization considers dengue one of the top global health threats.

The wMel bacterium is inherited maternally, so it is automatically passed on to any mosquito offspring. It does not naturally infect the breed of mosquito, Aedes aegypti, that carries dengue. Scientists decided to change that.

After getting community approval, they periodically released wMel-infected mosquitos – about two to five per person per week – into 12 randomly selected urban regions, each about a square kilometer, over 10 months. A dozen adjacent areas served as control regions.

The research team tried to establish areas where the geography might prevent the migration of treated or untreated mosquitos, using canals and large roads as natural barriers.

“A. aegypti are fairly lazy,” explained Dr. Simmons, Oceana director for Monash’s World Mosquito Program. “They usually don’t travel more than 50 to 100 meters in their lifetime, so large roads represent a significant barrier. They’re not impenetrable barriers, but they will slow them down.”

Among people living in the neighborhoods where the treated mosquitos were released, the rate of virologically confirmed dengue was 2.3%. In control neighborhoods, the rate was 9.4% (P=0.004).

The hospitalization rates for dengue were 0.4% where the treated mosquitos lived and 3.0% in geographical regions where they had not been released, also a significant difference.

The technique lowered the risk in all four serotypes of dengue virus.

The new results, combined with past research, “suggest that when wMel is established at high prevalence in local A. aegypti populations, reductions in the incidence of dengue follow,” the researcher team said.

In a Journal editorial, Dr. Davidson Hamer of Boston University calls the results “impressive” and says “the use of Wolbachia-infected mosquitoes has exciting potential to address the harms associated with dengue.”

“Future trials should explore the multivalency of the intervention, since laboratory studies suggest wMel could also attenuate transmission of Zika, chikungunya, yellow fever, and Mayaro viruses by A. aegypti,” Dr. Simmons and his colleagues write.

Climate change is expected to encourage the spread of dengue, Zika and other mosquito-borne diseases in the United States.

“The overall burden is worsening in the United States and elsewhere,” Dr. Sharp told Reuters Health by phone. “We need tools that are sustainable, effective and scalable to combat dengue. We don’t have that at the moment, unfortunately.”

In the case of the Florida woman, doctors were misdirected from considering dengue because, although the patient had returned from a two-week trip to Honduras, the return had been five weeks before she felt ill, and dengue makes people sick within two weeks of exposure.

“Timely diagnosis of this patient’s condition was complicated by a lack of available dengue rapid diagnostic tests, because none are yet cleared by the Food and Drug Administration for use in the United States,” Dr. Sharp and his colleagues write. “Had a rapid test been performed, surgery might have been avoided.”

“Diagnostics in the United States are not where we would like them to be,” said Dr. Sharp.

SOURCE:, and The New England Journal of Medicine, online June 9, 2021

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