Sterile Insect Technique: Mosquito Birth Control
In 2019, on the island nation of Mauritius, a team of researchers flooded the village of Panchvati with swarms of Ades albopictus mosquitos, the primary vector of Chikungunya and Dengue viruses. By doing so, they hoped to prevent the very infections these mosquitos are known to spread. These were no ordinary mosquitos; they were all infertile males, sterilized in a laboratory using methodology standard to the Sterile Insect Technique (SIT).
What is Sterile Insect Technique?
SIT came into scientific interest in the early-mid 20th century when across the world from A.S. Serebrovskii in Moscow to E.F. Knipling in the United States to F.L Vanderplank working in rural Tanzania each independently began researching innovative ways to control insect populations. In the 1950’s, SIT became a staple method for “area-wide integrated pest management” (AW-IPM) after it was used to completely eliminate the New World screwworm, Cochliomyia hominivorax, a lethal livestock parasite, from the south-eastern United States.
Photo: Mosquito stinging through fabric.
Female screwworms mate only once in their lifetime. Scientists exposed male screwworms to a certain kind of radiation, rendering them sterile, and then released them into the general population. As females used their only mating opportunities on these sterile males, the screwworm population declined successively over generations.
SIT is a vital aspect of global agricultural pest control, but it also holds great promise in reducing global infectious disease burden by controlling the populations of disease vectors like mosquitos. As vectors of dengue to chikagunya, yellow fever to West Nile virus, zika to malaria, mosquitos pose a unique and distinct danger to susceptible human populations worldwide. Climate change, urbanization, and poor past control strategies have resulted in an ever changing range which mosquitos can inhabit and a global increase in incidence of mosquito-borne diseases. According to WHO, half of the world’s population are at risk of dengue infection.
Photo: The screwworm was the first pest to be eliminated using Sterile Insect Technique.
The efficacy of mosquito and mosquito-borne disease control efforts has begun to stall in recent years. In an absence of effective vaccines and safe, inexpensive medications, vector control is the most sustainable option. Mosquito nets fail to hit at the source of the issue and insecticides have only lead to a rise in insecticide-resistant mosquitos. Further, insecticides may be detrimental to the environment, unintentionally affecting and killing off-target species. SIT is an environmentally friendly, species-specific, long term, vector-focused solution for mosquito control.
SIT and Mosquitos
SIT begins by rearing mosquito populations in the lab. Mass-rearing methodology has already been established for the species Anopheles arabiensis, the primary malaria vector, and Ae. aegypti and Ae. albopictus which are known spread diseases such as dengue, zika, and yellow fever. The next step is to induce sterility in the males. Depending on the technique, sterility may be induced by gamma rays, x-rays, or chemical exposure.
SIT is only interested in releasing male mosquitos because males do not bite. Every mosquito bite you’ve ever had has been the result of a hungry female. By exclusively releasing males, the human population is not put at risk.
Induced sterility in SIT may take one of four forms:
- Males do not produce sperm or produce inactive sperm
- Males cannot mate
- Reduced fecundity (egg production) in females
- Dominant lethal mutations in reproductive cells that disrupt embryonic development
Sterile males are released in swarms into the general mosquito population. While there is still a chance a female will come across a healthy, fertile male and produce viable offspring, there is also a chance she will come across one of these sterile males, our public health sleeper agents. Over generations, which for insects can mean mere weeks or months, the mosquito population declines.
Incompatible Insect Technique
An alternative method to SIT is the Incompatible Insect Technique (IIT), which takes advantage of a naturally occurring form of mosquito birth and infection control, Wolbachia bacteria.
Wolbachia, discovered in 1924, is a bacterium that intra-cellularly infects many arthropods, including some mosquitos. Vectors that carry wolbachia are less likely to be infected by malaria parasites or arboviruses like chikagunya, yellow fever, West Nile, dengue, or zika. Wolbachia infection also results in changes in mosquito gametes, inducing cytoplasmic incompatibility, or a failure for the combination sperm and egg to result in fertilization. Some species such as Aedes aegypti and Anopheles do not naturally carry Wolbachia, but this can be fixed with a microinjection of bacteria at either the embryonic or adult stage.
In some Wolbachia population control methodologies, all Wolbachia-infected mosquitos (w-) male and female are released. When a w-female is released, she will produce more offspring than a wild-type female. Wolbachia bacteria is passed down maternally, so over time the w-mosquitos will replace the wild-type population.
IIT methods are similar to traditional SIT in that, in order to prevent any unintentional spread of disease, only male mosquitos are released. When a w-male mates with a wild-type female, her eggs to not hatch.
Fig: Sterile Insect Technique and the Incompatible Insect Technique act as “mosquito birth control” to reduce vector populations.
Implications for Public Health
One common critique of SIT is the possibility that reducing the vector population will create an open environmental niche for another harmful vector or that SIT will leave more resources available for the remaining mosquitos, unintentionally increasing their chance of survival.
Fortunately, there is little evidence at the present time to suggest that reducing one mosquito population makes room for a different one. Further as SIT techniques develop, it may be possible to alter bio-dynamics of sterility so that mosquitos continue to produce eggs that will hatch, enter the pupal stage, and actively use up resources before dying.
Others have expressed a fear that fewer mosquitos will mean less human herd immunity and a resulting increase in disease incidence. This phenomenon has indeed been noted in Singapore where there was a spike in dengue cases following a large-scale vector control program. However, proponents of SIT argue that if timed and placed strategically, SIT may be able to shrink mosquito populations well below the threshold of transmission for outbreaks to occur, even in populations with little-to-no herd immunity.
The true limiting factor for SIT implementation is money. Low- and middle-income nations are disproportionately impacted by the disease burden of mosquito-borne infections, so it is vital that mosquito-control methods like SIT can be implemented in the places that need them most. It takes significant time and fiscal resources to mass-rear generations of mosquitos, sterilize them, and then transport them in a manner that will not compromise their health and ability to fly and mate. Some mosquitos with especially short flight ranges like Ae. aegypti have required the use of elaborate strategies involving drones to ensure they were released in the most appropriate locations for maximized public health benefit.
Although widespread use of SIT for mosquito control may be out of range for some LMIC today, as research advances and technologies improve, SIT methods will become cheaper and more accessible. There are already many alternative methods being researched involving the more feasible release of eggs containing sterile males rather than figuring out how to cheaply transport tanks full of full-grown mosquitos.
SIT offers an environmentally friendly, sustainable, versatile, and exciting method for mosquito population control. In the past 70 years we have seen it change the field of agriculture on a global scale, increasing crop yield while eradicating pests. Looking to the future, with continued research and intervention, SIT now has the potential to change the landscape of infectious disease public health.
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