by Rupinder Kaur, Sarah R. Bordenstein, and Seth R. Bordenstein
Amidst a global surge and focus on COVID-19 infections, other diseases that have had and will continue to have impact on human health cannot be forgotten. Dengue, for example, continues to be a major mosquito-borne viral infection, impacting more than 100 million people per year. Dengue can inflict joint pain, rash, and fever leading to severe illness with no specific treatment. In the absence of an effective vaccine and mosquitoes developing resistance to insecticides, the disease calls for an alternative solution with a long-lasting impact. A non-profit research consortium called World Mosquito Program (WMP) last month reported a dramatic drop in Dengue cases and related hospitalizations in Yogyakarta, Indonesia via the deployment of mosquitoes carrying a bacterial symbiont called Wolbachia, proving the efficacy of this biocontrol system to improve human health.
Wolbachia are Gram-negative, maternally-transmitted symbiotic bacteria that inhabit a wide variety of hosts including insects, spiders, mites, and terrestrial isopods, as well as filarial and plant nematodes. It was in the early 1920s that graduate student Marshall Hertig, along with his PhD mentor Dr. Simeon Burt Wolbach (Harvard University, Cambridge, Massachusetts), first described a bacterium, later named Wolbachia, in the gonads of various insects (Figure 1). Thirty years later, Hannes Laven (Johannes Gutenberg University, Mainz, Germany) discovered that some geographically isolated C. pipiens mosquitoes failed to breed, producing few or no progeny due to a phenomenon named cytoplasmic incompatibility (CI). In 1967, he established that an incompatibility factor transmitted from mothers to offspring is responsible for the failure. Janice Yen and Ralph Barr (University of California in Los Angeles, California) in 1971 proposed that Wolbachia are the ultimate incompatibility factor causing CI, and it was later discovered that Wolbachia-carrying (symbiotic) male sperms in fact get modified. This sperm modification prevents the normal development of fertilized embryos not carrying maternal Wolbachia (aposymbiotic), resulting in embryonic lethality. CI‑causing Wolbachia strains selfishly use this reduction in aposymbiotic host fitness to their favor, as symbiotic females are compatible and can successfully reproduce with both symbiotic and aposymbiotic males, providing a relative fitness benefit to symbiotic females that rapidly drive Wolbachia through the population.
In 2008, two independent research groups from Australia and the United Kingdom made a groundbreaking discovery that wMel Wolbachia from Drosophila melanogaster flies block the proliferation of natural viral pathogens to provide resistance to the host. Researchers all over the world then expanded the investigation to mosquitoes and showed that Wolbachia are also refractory to several other medically important mosquito-borne viruses (arboviruses) such as dengue, chikungunya, West Nile, and Zika. These findings led to an idea that the application of a CI-based drive could spread Wolbachia into wild mosquito populations to make them refractory to arboviruses and limit the virus spread to humans. Dengue‑spreading Aedes aegypti mosquitoes, however, do not naturally carry Wolbachia, posing a challenge to drive Wolbachia into wild mosquito populations. Leveraging microinjection techniques, wMel Wolbachia was injected into A. aegypti eggs and established with successful transmission to the next generations. Further, to implement the idea into the field, a non-profit Eliminate Dengue Program was established that is now called WMP, supported by the Bill and Melinda Gates Foundation and subsequently others. Theoretical modeling predicts the establishment of Wolbachia in A. aegypti populations under most epidemiological settings and thus the spread of the pathogen-blocking trait should markedly curb dengue transmission. Using population replacement strategy (PRS), symbiotic males and females are released to propagate and establish Wolbachia in the target population that eventually reduces the vector competence and thus disease burden in humans (Figure 2). The deployment of mosquitoes infected with the wMel strain of Wolbachia has been a resounding success. In Northern Australia, for example, the release of ten symbiotic mosquitoes per house per week for 10 weeks stably integrated Wolbachia into wild A. aegypti population at frequencies above 80–90%. Importantly, the bacteria have not been lost from the mosquito population in any of the areas after stopping the releases, highlighting success of the method. WMP started their field trials primarily in Australia in 2011 and, with the advancement of PRS efforts, reported no local dengue transmission in the area of Townsville, Australia. This led them to establish field sites and collaborations in many tropical countries with frequent cases of dengue infection including Indonesia, Brazil, Colombia, Vietnam, Sri Lanka, Mexico, and some Pacific Islands. Data confirmed that rise in Wolbachia-carrying mosquitoes associates with protection of the residents from dengue transmission.
An alternative strategy that has been successfully used for Dengue control is Incompatible Insect Technique (IIT) also known as Population Suppression, in which symbiotic males that are incapable of producing viable offspring after mating with aposymbiotic females are released, causing mosquito population breakdown. Along with WMP, multiple organizations are now deploying IIT including MosquitoMate (Kentucky, USA), Verily (California, USA), Commonwealth Scientific and Industrial Research Organization (Australia), and Singapore's National Environment Agency (Singapore). By releasing symbiotic male mosquitoes, their efforts have also proven successful, causing 95% population reduction in California, more than 80% reduction in Australia, and 90% reduction in Singapore.
In the most recent study, WMP provided a "gold standard" trial proving the efficacy of PRS in Wolbachia control measures. They released symbiotic mosquitoes in central Yogyakarta, Indonesia over a number of years. By selecting 12 wMel-infected mosquito release and non-release sites, they surveyed and compared the proportions of patients with dengue infection and dengue-related hospitalizations from release and control sites. This comparative and controlled method yielded a 77% significant reduction in the incidence of dengue cases and 86% reduction in hospitalizations among a community that endures frequent dengue outbreaks. Researchers have also reported a similar reduction in cases of dengue as well as chikungunya in an urban area near Rio de Janeiro, Brazil.
Every year, evidence for Wolbachia-based efficacy to control dengue and related mosquito-borne viruses is rapidly accumulating, highlighting the enormous potential of this approach to fight arboviral diseases at a global scale. That day is not far when governments and the World Health Organization will approve this microbial ally for even broader and more significant applications.
Drs. Rupinder Kaur, Sarah R. Bordenstein, and Seth R. Bordenstein are biologists in the Department of Biological Sciences, Vanderbilt University, Vanderbilt Microbiome Initiative in Nashville, Tennessee, USA. The laboratory endeavors to understand and disseminate the principles that shape interactions between animals, microbes, and viruses and the basic and translational outcomes of these interactions. The lab team has studied Wolbachia for more than 25 years and directs the worldwide science education series Discover the Microbes Within! The Wolbachia Project that engages students worldwide in biodiversity, biotechnology, and bioinformatics on the Wolbachia symbiosis.