Even after several rounds of insecticide sprays a year, not to mention use of drug-treated nets and other measures, mosquitoes continue to spread the dreaded dengue across India. In 2016 alone, over 100,000 tested positive for dengue in the country, with hundreds dying from infection. Now, there are plans to pit bacteria against the virus.
A team of Indian researchers from the Vector Control Research Center (VCRC) is importing Australian larvae that are infected with the bacterium Wolbachia, a genus that has the ability to block the growth of viruses like the ones that spread dengue. It will be used to contain the spread of dengue, chikungunya, zika and other viruses spread by mosquitoes.
Wolbachia effectively sterilizes mosquitoes’ gonads, or reproductive organs. When male mosquitoes infected with Wolbachia mate with uninfected females, the latter will lay eggs that don’t hatch. When Wolbachia-infected females mate with infected, or even uninfected, males, all their offspring will have Wolbachia.
An expert in the area of vector control, Professor A. P. Dash avers that indoor residual spray (IRS) of insecticides and use of long-lasting insecticide impregnated nets (LLINs) remain the mainstay of vector control, as such methods have a proven impact on mosquito populations and disease transmission. “However, LLINs are not used for control of dengue, since Aedes mosquitoes have vast breeding sites in and around human habitations where nets may not be effective,” says the former Regional Adviser for the World Health Organisation (SEARO) and present Vice-Chancellor of Central University of Tamil Nadu, in Thiruvarur. “Moreover, dengue vectors bite during the day, with peak biting in morning and at dusk.”
Wolbachia, a natural bacterium first identified in 1924 in Culex mosquitoes, could be used as a novel form of biological control by getting it into the Aedes aegypti mosquitoes that spread dengue, Prof. Dash tells Asia Times.
Another expert, John R. Timmer – who is the Science Editor of Ars Technica – says: “Wolbachia can do some amazing things to their insect hosts, such as changing their sex, killing their offspring and perhaps even creating new species to ensure their [own] spread. Wolbachia is a bacterial genus that may include a variety of species and infects over one million known insect species.
“Lots of bacteria find homes on the surface of organisms, and even more inside their orifices and guts. Wolbachia is a bit more aggressive. It actually enters individual cells and finds a home for itself there. In the process, it’s able to interact with and manipulate the cells’ components.”
Although Wolbachia can grow in a variety of tissues, it has a particular attachment to gonads and reproduces itself through infecting female eggs.
“To ensure its spread, Wolbachia uses several methods, including ensuring that all the male offspring die, causing infected males to develop as females or even allowing females to produce fertile offspring without ever mating,” continues Dr. Timmer. “Once in cells, Wolbachia seems to interfere with many (but not all) other pathogens, including dengue virus. We’re just not sure how this happens. There’s some signs that Wolbachia competes with other parasites, that it activates the insect version of an immune system, and that it activates other defenses in the mosquitoes. But which (if any) of these is involved in limiting dengue virus isn’t clear. But it is clear that mosquitoes with Wolbachia growing in them produce far less dengue virus, and thus pose less of a risk of passing the virus on to new victims.”
Elaborating on the dengue threat, Prof. Dash says: “It is a disease that is re-emerging and fast spreading to newer niches in the world. South East Asia is the epicenter of this spread. It is one of the most important mosquito-borne viral diseases in South East Asia Region (SEAR). It causes more human morbidity and mortality than any other arboviral disease. Recent estimates are that approximately 390 million people are infected each year and 96 million manifest with clinically apparent disease. Asia contributes to 70% of these infections, 34% being from India. As many as 3.9 billion people in 128 countries are at risk of infection. Dengue is spreading from urban to rural areas [and] is still an acute public health problem in spite of tremendous progress.”
But will Wolbachia work in India? “It will work in certain urban areas of India, but in areas where there are scattered houses with huge numbers of small breeding containers in domestic and peridomestic localities, Wolbachia-infected males may not be able to infect the total Aedes population.”
Trials to begin
The Indian Council of Medical Research, in New Delhi, is stepping up field trials. “An expert group meeting was held recently,” says Prof. Dash. “India is a large country with diverse ecosystems. However, the study in India will be very relevant to other Asian countries.”
He proposes a combination of strategies in controlling dengue with Wolbachia: “One is suppression of mosquito populations by large-scale releases of males incompatible [in reproductive terms] with native females and this intervention requires ongoing releases. Another intervention transforms wild mosquito populations [with] Wolbachia-infected females; [this] potentially requires just a single, local release for area-wide disease control.” These strategies, together, will shorten mosquito life and block viral transmission.
Prof. Dash adds: “Clinically, it has been proven that all Wolbachia strains are capable of antiviral protection if a density threshold is reached – but this is yet to be tested experimentally.”
Dr. P. Jambulingam, Director of the Vector Control Research Center (VCRC) in Puducherry, tells Asia Times that lab preparations are underway for controlled trials of Wolbachia under an Indian Council of Medical Research-funded project, and that “we are awaiting permission to import the larvae.”