Wolbachia-based Biocontrol Methods Limit the Spread of Mosquito-borne Illness

By Lina Sorg

Aedes aegypti mosquitoes spread dengue fever, chikungunya, Zika, and other vector-borne illnesses (see Figure 1). The species originated in Africa but now inhabits tropical, subtropical, and temperate regions around the world. Traditional mosquito control efforts involve insecticides, but recent years have seen a rise in Wolbachia-based biocontrol methods to limit mosquito populations and the subsequent spread of disease.

Wolbachia is a symbiotic bacterium that exists in more than 50 percent of insect species. Though it does not naturally occur in Ae. aegypti, injection of this bacterium into female mosquitoes ensures that they transmit it to their eggs. The presence of Wolbachia in Ae. aegypti eventually causes cytoplasmic incompatibility (CI): a phenomenon wherein sperm and eggs are unable to form viable offspring. For instance, when Wolbachia-infected female mosquitoes mate with males that do not have Wolbachia, all of their offspring will possess the bacterium. When both male and female mosquitoes have Wolbachia, their offspring will have it as well. And when male mosquitoes with Wolbachia mate with Wolbachia-free female mosquitoes, the females lay eggs that will not hatch. Once an infected population is established, Wolbachia then suppresses the replication of different viruses in Ae. aegypti and prevents their spread to humans.

During the 10th International Congress on Industrial and Applied Mathematics, which is currently taking place in Tokyo, Japan, Olga Vasilieva of Universidad del Valle in Colombia modeled Wolbachia’s ability to control wild mosquito populations. Vasilieva considered three strains of Wolbachia for the prevention of Aedes-borne diseases: wMel, wMelPop, and wAu. She began by addressing four key ways in which thermal stress impacts Wolbachia mosquitoes in the context of these three strains.

• Under high daily temperatures of 30° Celsius or more, CI can be partially lost in adult insects with wMel or wMelPop.
• Temperatures above 30° Celsius may disrupt the vertical transmission of Wolbachia and induce imperfect maternal transmission.
• Exposure of Wolbachia-carrying larvae to water temperatures that cycle between 27° and 37° Celsius may reduce bacterium density and completely eliminate Wolbachia infection from wMel and wMelPop strains in adult insects.
• The wAu strain resists thermal stress but does not induce CI, meaning that some of the resulting eggs might be viable.

Though the wAu strain retains Wolbachia infection under thermal stress (unlike wMel and wMelPop, which have a much lower tolerance), the absence of CI is significant. When planning field releases of mosquitoes with Wolbachia, scientists must therefore decide which strain is most effective.

To do so, Vasilieva developed a simple model of adult mosquitoes that accounts for parameters such as fecundity and mortality. She then displayed phase portraits for the wMel, wMelPop, and wAu strains. Vasilieva determined that wMelPop requires a higher number of mosquitoes as input (when compared to wMel) in order to establish a sustained Wolbachia infection within the population. In terms of the wAu strain, Wolbachia infection is unsustainable without the constant input and release of additional infected mosquitoes.

During the second half of her presentation, Vasilieva compared the use of Wolbachia bacteria to traditional insecticide-based chemical treatments. When Wolbachia is established in a population via the wMel strain, the persistent wild population of mosquitoes falls to slightly under six percent of the original level. The wMelPop strain performs even better and reduces the wild population to just 1.5 percent of the original level. Vasilieva uses these thresholds as points of comparison for two types of insecticide interventions—larvicide fogging and adulticide spraying—to see whether these chemical techniques attain the same impressive level of wild population control. 

To answer this question, she utilized a model that relied on the same dynamics as before — but under chemical control intervention rather than Wolbachia injection. Vasilieva ran the simulation at 80 percent effectivity for the larvicide and 100 percent effectivity for the adulticide, then compared the results to the wMel and wMelPop strains. Both the weekly and biweekly application of insecticide at the aforementioned effectivity percentages meets the wild population level of the wMel strain, while monthly application wavers somewhere in between. However, a lower effectivity percentage for the larvicide or adulticide will not reach the wild population level of wMel at either a weekly or monthly application rate. The effectivity standards to meet the wild population rate of the wMelPop strain are even more stringent.

After reviewing the monetary cost of treating one hectare (100 acres) of land with insecticide versus the wMel strain of Wolbachia, Vasilieva concluded that Wolbachia-based biocontrol is more cost effective and favorable than chemical sprays. “The goal of both of them is to keep the same level of wild mosquito population,” she said, adding that the wMel strain does so more effectually.

Lina Sorg is the managing editor of SIAM News.