GM Mosquitoes to The Rescue?

Mosquitoes are the deadliest animals on earth. Often disregarded as a small nuisance, mosquitoes are responsible for the deaths of millions of people every year due to the diseases they carry. The pending release of genetically engineered mosquitoes in Florida could pave the way for new technologies in preventing the spread of mosquito-borne diseases.

Voters in Florida Keys on election day were asked an unusual question:

“Are you in favour of the Florida Keys Mosquito Control District [FKMCD] conducting an effectiveness trial in Key Haven using genetically modified mosquitoes to suppress an invasive mosquito that carries mosquito-borne diseases?”

The FKMCD, along with a UK-based commercial company, Oxitec, are planning a trial release of genetically modified mosquitoes in Florida (fig.1). After significant public opposition to the trial after the project was given the green light by the Food and Drug Administration, the decision was put to vote. Although non-binding, three of the five members of the Mosquito Control Board have said they will abide by the public’s decision.

As the results came in on the 8th November, there were mixed responses. The original location, Key Haven, voted against the trial. Monroe County however, voted in favour, mobilising plans to release GM Aedes aegypti in spring 2017. This could be the first time a genetically modified animal is released into the wild in the US.

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Figure 1. A scientist releases genetically engineered mosquitoes. Source: Getty Images

The Science

Zika, dengue, chikungunya, and yellow fever are all transmitted to humans by the A.aegypti mosquito, an aggressive daytime biting species known to target humans with “sneak attacks”. Only females bite, they need protein from blood to produce eggs. A.aegypti was responsible for outbreaks of dengue in Florida in 2010, a virus that causes high fever, vomiting, and server pain. The spread of Zika across South America and cases reported in South Florida have increased pressure to find a solution to the spread of mosquito-borne diseases. Current methods of control, such as fogging, are ineffective, expensive and involve extensive use of harmful pesticides. The mosquito is developing resistance to many pesticides and living around human habitation can make it very difficult to reach. Early control of vector populations has been recommended by The World Health Organisation as an effective method to prevent disease outbreaks.

The trial proposed is a variant of the Sterile Insect Technique, whereby large numbers of male insects are bred and subsequently made sterile before release into the wild. Wild females will mate with the infertile, lab-bred insects and fail to produce offspring. This causes a rapid decline in the population and has been used successfully in managing populations of agricultural pests since the 1950s1. However, this technique has been criticised, as the method of sterilization can decrease the health of insects. This puts the lab-bred males at a disadvantage to their wild counterparts, making them less attractive to females and thereby less effective at reducing the population. This is a proven problem in the control of the Olive Fly2.

Oxitec will be using another, controversial technique to sterilise male mosquitoes, genetic modification. OX513A is a self-limiting strain of the A.aegypti mosquito. The tTAV gene is inserted into A.aegypti, which prevents it from reaching maturity. When expressed, this gene prevents cells from functioning properly, causing the insect to die. Males of this strain will be released to mate with wild females. The offspring of these matings will carry this gene and die before reaching adulthood, reducing the number of individuals in the population. To breed OX513A mosquitoes in the lab, Oxitec administer an antidote that disables the tTAV gene (fig.2).

Release of OX513A mosquitoes has already been trialled and proven successful in Brazil, The Cayman Islands and Panama, each showing reductions in the wild A.aegypti population by over 90%3,4. There have been no noted adverse effects yet there is still substantial opposition in the US.

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Figure 2. An infographic explaining how the self-limiting gene works to reduce Aedes aegypti numbers. Source: Oxitec

The People

A handful of Key Haven residents lobbied in opposition of the trial (fig.3). The petition “Say No to Genetically Modified Mosquitoes” garnered 170,000 signatures. Opponents worry about the resulting impact on the environment. Oxitec’s previous field trails have been criticised for lacking transparency and were deemed scientifically deficient in a review by scientists in 20125. The company have financial interests intrinsically linked to the success of these trials and with an Oxitec employee as named author on the studies, it is easy to see the potential for bias.

There is also a worry among opponents about the small percentage of female OX513A mosquitoes that are released and the consequences if they were to bite a human. Only female mosquitoes bite, therefore they need to be removed before release into the wild. Oxitec sorts mosquitoes by sex according to larval size, a method which isn’t guaranteed to be 100% accurate. The company counters that the engineered genes produce non-toxic proteins which don’t show up in the saliva of the mosquito that’s transferred to mammals during a bite. Therefore, the bite from a genetically modified mosquito would be the same as from a wild mosquito.

Another concern is that suppression of A.aegypti could cause the populations of other mosquito species to increase due to lack of competition. Notably, the tiger mosquito, Aedes albopictus, which is another vector of human diseases including Zika and dengue. However, trials in Panama found no evidence for this over short periods suggesting there is little competition between the two species.4

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Figure 3. A satellite image of Key Haven, Florida, the proposed trial site for the release of genetically modified mosquitoes. Source: Google Earth

The Monroe County vote has proven that there is support for the trial in the resident community, but have they made the right choice? From what evidence is available, there is little to suggest any detrimental impacts to the environment or the humans that inhabit it. Keeping people safe is an ethical obligation, but with great potential benefit to humans in the future, do the risks outweigh the costs?

 

References

  1. Klassen, W. & Curtis, C. F. in Sterile Insect Technique (eds. Dyck, V. A., Hendrichs, J. & Robinson, A. S.) 3–36 (Springer Netherlands, 2005). doi:10.1007/1-4020-4051-2_1
  2. Zervas, G. A. & Economopoulos, A. P. Mating Frequency in Caged Populations of Wild and Artificially Reared (Normal or γ-Sterilized) Olive Fruit Flies. Environ. Entomol. 11, 17–20 (1982).
  3. Carvalho, D. O. et al. Suppression of a Field Population of Aedes aegypti in Brazil by Sustained Release of Transgenic Male Mosquitoes. PLoS Negl. Trop. Dis. 9, 3864 (2015).
  4. Gorman, K. et al. Short-term suppression of Aedes aegypti using genetic control does not facilitate Aedes albopictus. Pest Manag. Sci. 72, 618–628 (2016).
  5. Reeves, R. G., Denton, J. A., Santucci, F., Bryk, J. & Reed, F. A. Scientific Standards and the Regulation of Genetically Modified Insects. PLoS Negl. Trop. Dis. 6, 1502 (2012).

 

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