The New England Journal of Medicine (NEJM) published online the results of an interesting study on 10th June 2021. It demonstrated the efficacy of Wolbachia-infected mosquito deployments to control dengue, which according to the WHO is one of the top ten global health threats placing half the world population at risk.

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Introduction

Wolbachia is a bacterium. Researchers have been developing biological technology for the past many years. The NEJM paper highlights the first randomised controlled trial — the gold standard in clinical research — of a completely new approach to controlling dengue. The journal NATURE observed that epidemiologists are typically cautious in the language they use. However, some of the comments they made on the successful trial to control dengue were, “staggering” and “epochal”, the journal disclosed on August 27, 2020. The notable appreciation is very encouraging.

Dengue is a mosquito-borne, acute viral syndrome caused by any of the four serotypes of the dengue virus (DENV). When dengue is mild, the symptoms include fever, lethargy, headache, eye pain, nausea and rash. When severe, dengue may cause severe abdominal pain, persistent vomiting, rapid breathing, signs of bleeding and vascular leakage. Life-threatening complications include dengue shock syndrome and/or severe organ dysfunction. According to the WHO, there is no specific treatment for dengue/severe dengue; early detection of disease progression associated with severe dengue, and access to proper medical care lowers fatality rates of severe dengue to below 1%.

The global incidence of dengue presently occurs at an estimated 100-400 million infections each year. Dengue epidemics occur annually or at multiyear intervals, and the surge in case numbers places considerable pressure on health services.

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Dengue control methods

One can control dengue infection effectively by avoiding Aedes aegypti mosquito’s bites. These mosquitoes are the primary vectors for the disease. One way to prevent the spread of this disease in any region is by controlling the population of Aedes aegypti mosquitoes. Using insecticides or implementing environmental management methods have not been effective in controlling dengue. The mosquito develops resistance to insecticides.

The World Mosquito Program (WMP) pioneered another method. According to WMP, Wolbachia is a very common bacteria that occur in 60 per cent of insect species that include some mosquitoes, fruit flies, moths dragonflies and butterflies. These bacteria live inside insect cells and are passed from one generation to another through the insect’s eggs. Aedes aegypti mosquitoes do not normally carry Wolbachia.

A paper published in the journal NATURE on 24 August 2011, by researchers from Australia and the USA revealed that infection with Wolbachia makes the mosquitoes that transmit dengue virus resistant to viral infection and the resistant population can rapidly replace the natural, susceptible mosquito population. “They are safe for humans and the environment. Independent risk analysis indicates that release of Wolbachia-infected mosquitoes poses negligible risk to humans and the environment,” the WMP clarified.

Aedes aegypti mosquitoes loaded with Wolbachia, prevent them from being infected by viruses. Wolbachia spreads very quickly. By releasing a small number of carrier mosquitoes into a neighbourhood, we can make virtually all of the local insects dengue-free within a few months. The “bites” by virus-free mosquitoes will have only some nuisance value!

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Wolbachia- virus competition

The WMP discovered that when Aedes aegypti mosquitoes carry Wolbachia, the bacteria compete with viruses such as dengue, Zika, chikungunya and yellow fever. When the bacteria compete against the viruses it is harder for viruses to reproduce inside the mosquitoes. And the mosquitoes are much less likely to spread viruses from person to person. This means that when Aedes aegypti mosquitoes carry natural Wolbachia bacteria, the transmission of viruses like dengue, Zika, chikungunya and yellow fever is reduced.

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The study completed

In August 2020, the World Mosquito programme of Monash University, and its Indonesian partners the Tahija Foundation and Universitas Gadjah Mada have announced in a press release the first results of the study showing a 77% reduction in the incidence of virologically-confirmed dengue (VCD) in Wolbachia-treated areas of Yogyakarta, Indonesia, compared to untreated areas.

However, the researchers published the peer-reviewed paper on it in NEJM on 10th June 2021. `

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The study details

The researchers regularly released Wolbachia-infected mosquitoes into a wild mosquito population in the Yogyakarta region over several months. Wolbachia facilitates its own population introgression (the transfer of genetic information from one species to another as a result of hybridisation between them and repeated backcrossing) by manipulating reproductive outcomes between wild-type and Wolbachia-infected mosquitoes. The only viable mating outcomes are those in which the progeny are infected with Wolbachia. More than two years after the completion of mosquito releases, Wolbachia has persisted at a very high level in the local mosquito population.

The trial site was a contiguous urban area of 26 km² with a population of approximately 311,700. The researchers subdivided the area into 24 clusters, each approximately 1 km² in size, and where possible, having geographic borders that would slow the dispersal of mosquitoes between clusters. Of the 24 clusters, researchers randomly assigned 12 to receive deployments of open-label Wolbachia-infected mosquitoes (intervention clusters); they assigned 12 clusters to receive no deployments (control clusters), termed “untreated clusters” in the protocol.

In intervention clusters, most community members were unaware of the cluster assignment because release containers were placed discretely in a minority of residential properties for a limited time.

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Wolbachia deployment and entomologic monitoring

Researchers sourced A. aegypti infected with the wMel strain of Wolbachia from an out-crossed colony. They placed mosquito eggs in intervention clusters from March through December 2017. Each cluster received between 9 and 14 rounds of deployments. They monitored the progress with the use of a network of 348 adult mosquito traps.

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Enrollment of participants

The researchers recruited the participants from a network of 18 government-run primary care clinics in Yogyakarta and the adjacent Bantul District. Eligible participants were 3 to 45 years of age, had fever (either reported by the participant or measured in the clinic and defined as a forehead or axillary temperature of >37.5°C) with onset 1 to 4 days before presentation, and had resided in the trial area every night for the 10 days preceding the onset of illness. Participants were not eligible if they had localising symptoms suggestive of a specific diagnosis other than an arboviral infection (e.g., severe diarrhoea, otitis, and pneumonia) or were enrolled in the trial within the previous 4 weeks.

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Procedures

Participants provided demographic information, a geo-located residential address, and detailed travel history for the 3 to 10 days before the onset of illness. Researchers obtained a 3-ml venous blood sample for arbovirus diagnostic testing. Researchers contacted the participants 14 to 21 days after enrollment to obtain vital status and to determine whether they had been hospitalised since enrollment

The NATURE journal summed up the procedure thus:

“The project was the first randomised controlled trial — the gold standard in clinical research — of a completely new approach to controlling dengue. The technique breeds Aedes aegypti mosquitoes, which transmit the dengue, Zika and chikungunya viruses, such that they carry a bacterium called Wolbachia. The bacterium subdues the viruses and prevents the mosquitoes from passing them on to humans. Eggs from the modified mosquitoes are then placed around the city, often in people’s homes. Small trials in Australia and Vietnam had produced tantalising results. But Yogyakarta, a dense city of nearly 400,000 people with high rates of dengue transmission, provided a much bigger stage for a trial.”

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Results and conclusion

After successful introgression of wMel into the intervention clusters, researchers enrolled 8144 participants; 3721 lived in intervention clusters, and 4423 lived in control clusters. In the intention-to-treat analysis, virologically confirmed dengue (VCD) occurred in 67 of 2905 participants (2.3%) in the intervention clusters and in 318 of 3401 (9.4%) in the control clusters The estimated protective efficacy of the intervention was 77.1% and was similar against the four dengue virus serotypes. This translates to people being four times less likely to develop the disease.

The incidence of hospitalisation for VCD was lower among participants who lived in intervention clusters (13 of 2905 participants [0.4%]) than among those who lived in control clusters (102 of 3401 [3.0%]). The protective efficacy was 86.2%. Researchers un-blinded the data in June 2020 — a few months earlier than scheduled, because of rising numbers of coronavirus cases in Indonesia.

They concluded that introgression of wMel into A. aegypti populations were effective in reducing the incidence of symptomatic dengue and resulted in fewer hospitalisations for dengue among the participants.

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Limitations and benefits

In his eminently readable article in The Atlantic (10th June 2021) Ed Young who received the Pulitzer prize recently, stated that the bacterium takes months to establish itself, so it can’t be “deployed to contain an outbreak today,” quoting Gonzalo Vazquez-Prokopec of Emory University, who studies vector-borne diseases.

Young clarified and cautioned:

“As the Yogyakarta trial showed, it works only when Wolbachia reaches a prevalence of at least 80 percent, which requires a lot of work and strong community support. And the dengue viruses could eventually evolve some way of resisting Wolbachia. It out-competes the viruses for the nutrients they need to reproduce and also boosts the mosquito’s immune system. That should make it harder for the viruses to get around the Wolbachia blockade.

According to Young, the method is self-amplifying and self-perpetuating: If enough Wolbachia-infected mosquitoes are released initially, the bacterium should naturally come to dominate the local population and stay that way.

“Unlike insecticides, Wolbachia isn’t toxic, it doesn’t kill beneficial insects (or even mosquitoes), and it doesn’t need to be reapplied, which makes it very cost-effective,” he described the other benefits.

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Seminal contribution from Dr. Adi Utarini

Dr Adi Utarini, Public Health Professor at Gadjah Mada University, who co-leads the World Mosquito Program, Yogyakarta, made seminal contributions to the project. Mr Ed Young disclosed that Dr Utarini herself had her first of two bouts of dengue fever in 1986 when she was still a medical student. F“Her home city of Yogyakarta, Indonesia has one of the highest rates of dengue in the country, which itself has one of the highest rates of dengue in the world,” he added.

According to Wikipedia, after graduating in 1989 she completed two master's degrees, one at the UCL Great Ormond Street Institute of Child Health, United Kingdom (1994) and one at Umeå University, Sweden (1997). She remained at Umeå for her doctoral research, where she focused on a malaria control programme in Central Java. She completed her doctorate in 2002. Her academic credentials to co-lead the team are unparalleled.

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One among NATURE’s 10

The Journal NATURE (15th December 2020) chose Utarini as one of the 10 people who helped shape science in 2020. The journal acknowledged her role in the project and aptly called her “Adi Utarini: Mosquito Commander”. Utarini’s team could get support from the community that would host the mosquitoes. “That was what was so successful about the team that Adi led,” the journal quoted Oliver Brady, a virus modeller who studies dengue at the London School of Hygiene & Tropical Medicine.

The task of the team was un-enviably tortuous; they were to successfully persuade the folks to allow them to release mosquitoes in their dwellings!

NATURE recalled:

“The team used media announcements, wall paintings, face-to-face meetings and even a short-film competition to inform the community about the technology and to answer people’s questions about the trial. More often than not, community members were keen to be in the treatment areas.”

Though the ream faced some problems initially, Dr Utarini negotiated with several government ministries, winning regulatory approval soon after.

NATURE noted that according to her colleagues Utarini — whom they describe as quiet but persuasive — has been a key part of the study’s success.

Almost 10,000 volunteers helped distribute egg-filled containers to local backyards. Within a year, about 95 per cent of the Aedes mosquitoes in the 12 release zones harboured Wolbachia. Overall, it was a huge project.

NATURE recalls that Utarini’s triumphant year has also been touched by tragedy. Sadly, her husband, a pharmacologist, died of COVID-19 in March. In difficult times, she has turned to her other passions — piano playing and cycling. “Whenever I have unsolved problems, I try to get ideas through that.” She said.

 

Disclaimer- The views and opinions expressed in this article are those of the author's and do not necessarily reflect the official policy or position of M3 India.

Dr K S Parthasarathy is a former Secretary of the Atomic Energy Regulatory Board and a former Raja Ramanna Fellow, Department of Atomic Energy. A Ph. D. from the University of Leeds, UK, he is a medical physicist with a specialisation in radiation safety and regulatory matters. He was a Research Associate at the University of Virginia Medical Centre, Charlottesville, USA. He served the International Atomic Energy Agency as an expert and member in its Technical and Advisory Committees.