Efforts to control the spread of dengue, a tropical scourge so painful and debilitating that it is sometimes called “break-bone fever,” have long centered on its vector, the Aedes aegypti mosquito, but the latest approach goes well beyond such population control methods as fogging or spraying or even neutering – and it appears to work.
A two-year test of the method showed it was 77 percent effective against dengue, according to a report published this month in the New England Journal of Medicine.
“It’s really exciting news,” said Brett Ellis, director of the Virgin Islands Territorial Public Health Laboratories, who is familiar with the effort.
The test came after years of research by scientists who introduced a bacterium, Wolbachia, into mosquito populations in Yogyakarta, Indonesia, a region with a high incidence of dengue over the last 30 years. Once the bacterium is established in a mosquito, it essentially blocks the dengue virus from taking hold in the insect’s system. (Many other insects naturally have Wolbachia in their systems.)
Without that blockage, the normal transmission of dengue involves a mosquito obtaining the virus from an infected human, harboring the virus within its body and then passing it along to another human via a mosquito bite.
That is the same path taken by viruses that cause Zika and chikungunya, two potentially serious diseases that emerged in the Virgin Islands and other tropical locales in the early 2000s.
The Yogyakarta trial followed a series of other tests, according to a summary of the published report provided online by the World Mosquito Programme. Researchers have maintained a Wolbachia-infected colony of mosquitoes in the province since 2013.
The most recent trial followed consenting patients for 27 months and compared results of infections from a total of 24 areas – half with Wolbachia present, and half without.
The World Mosquito Programme suggested the bacteria approach may also prove effective against other diseases besides dengue.
“In general, we expect that in other locations where Wolbachia is established at a high level, we will see similarly significant reductions in local arboviral (insect-transmitted) disease incidence.
“The main caveat in replicating these results elsewhere is that differences in ecology, climate, altitude and the complexity of the urban environment are likely to affect the trajectory of Wolbachia establishment, and consequently the timing of the impact on disease,” the report summary says.
Ellis called the report of the trial “very impressive,” adding that within the scientific community the project has had much support. “A lot of time and effort and money has gone into that work.”
The territory was set to conduct its own large-scale mosquito population control project until COVID-19 interfered, and the work was halted last year.
The St. Croix Mosquito Project differed from the Yogyakarta trial in that it involved the release of modified males only, not females. Only female mosquitoes are capable of biting, so the males would not have affected the human population. What they would have done, Ellis explained, is mate with females and render their eggs infertile, thus reducing the overall population.
The St. Croix project was funded through a federal grant from the Centers for Disease Control and Prevention and was still in the public education phase. A website for it can be found at stxmosquitoproject.com.
If the Health Department can get funding support again to restart the project, “we’ll definitely revisit it in the future,” Ellis said. But, he added, “We always take a cautious approach,” when it comes to vector control. The approach “has to be acceptable to the community.”
Although there are historical references that suggest dengue has existed for hundreds if not thousands of years, and was well known at least 200 years ago, it really became prevalent globally beginning in the mid-20th century, spreading perhaps because of urbanization, international travel, and other factors.
The CDC website says three billion people, about 40 percent of the world’s population, now live in areas that are at risk for the disease. Wikipedia says the number of known cases has increased 30 percent from 1960 to 2010.
The U.S. Virgin Islands has suffered periodic outbreaks of dengue at least since the 1970s. The last outbreak – defined by the CDC as “an occurrence of disease greater than expected at a particular time and place” – was in 2012, according to experts at the V.I. Health Department. There were a handful of cases reported in late 2019 into early 2020.
About one-fourth of people infected with the dengue virus don’t develop any symptoms, according to the CDC. Cases range from very mild to severe and, rarely, fatal. Common symptoms include rash, nausea, eye pain and headache, muscle, joint and bone pain.
Health experts recommend residents protect themselves by limiting their exposure to mosquitoes and their bites. Advice includes eliminating standing water that can breed the insects, wearing protective clothing to prevent bites, and using repellants.