What facts address the concern that Wolbachia-infected mosquitoes released into the environment will have a negative impact?

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Skip to question Many arthropods are infected with a microbe called Wolbachia, a genus of Gram-negative parasitic bacteria found in more than 40% of all insect species. These bacteia have enormous potential for use as a biological control agent for diseases such as Dengue fever. Five different but related dengue viruses (DENV-1–5) can cause Dengue fever, a flu-like illness that causes severe joint pain and sometimes life-threatening disease. These viruses, along with others such as Zika virus, are transmitted by mosquitoes of the Aedes family. Currently, preventing them relies on controlling the populations of Aedes mosquitoes, a challenging problem. However, given that the World Health Organization (WHO) estimates that half of the world's population is at risk of contracting dengue fever, controlling the transmission of the causative viruses is essential. Now researchers are looking to Wolbachia as a possible way of controlling viruses such as Dengue virus.

 

One approach to control aims to reduce the number of DENV-carrying mosquitoes in an environment —a process called population suppression. It relies on the effect that Wolbachia has on the reproduction of the mosquito. Wolbachia species can infect the ovaries and testes of mosquitoes. If a female mosquito is infected with Wolbachia, her eggs also contain the bacterium, but they hatch normally whether she mates with an infected or a non-infected male. However, when an uninfected female mosquito mates with a Wolbachia-infected male, the female's eggs don't hatch, because of something called cytoplasmic incompatibility (CI)—the Wolbachia-infected sperm are lethal to the non-infected egg. Thus, if Wolbachia-infected male mosquitoes are introduced into a population, any uninfected females that mate with them will not produce offspring, and the population of mosquitoes will decline, protecting humans. This approach would require ongoing release of the male mosquitoes.

 

A different approach, called population replacement, aims to increase the number of Wolbachia-infected mosquitoes of both sexes into a habitat. Research has shown that certain viruses cannot replicate effectively in Wolbachia-infected mosquitoes, and transmission of these viruses from the mosquito to human hosts is blocked. If a mosquito population is mostly composed of Wolbachia-infected mosquitoes, there will be less dengue virus in that mosquito population, and less chance of transmission of dengue viruses to humans. This approach would only require one release of infected mosquitoes.

 

There are some challenges with using Wolbachia. First, Wolbachia does not naturally occur in Aedes mosquitoes. However, it can be introduced into mosquitoes in the laboratory. Second, although Wolbachia itself is not transmitted to humans, people are concerned that the bacteria might spread to other insects and/or animals. However, as other researchers in the field point out, Wolbachia occurs naturally in the environment and infects significant numbers of insects and it has not been found in any animals. In addition, human volunteers have allowed themselves to be bitten by multiple Wolbachia-infected mosquitoes, and have never contracted the virus. Using Wolbachia in this way offers an alternative to insecticides that may have an environmental impact. 

 

Sources: Johnson, K. N. (2015). The Impact of Wolbachia on Virus Infection in Mosquitoes. Viruses, 7(11), 5705–5717. http://doi.org/10.3390/v7112903; Ritchie, Scott A. et al. “Application of wMelPop Wolbachia Strain to Crash Local Populations of Aedes Aegypti .” Ed. Jason L. Rasgon. PLoS Neglected Tropical Diseases 9.7 (2015): e0003930. PMC. Web. 31 Dec. 2017.

What facts address the concern that Wolbachia-infected mosquitoes released into the environment will have a negative impact? 

(NOTE: Please change all question marks to checkmarks for correct answers or empty boxes for incorrect answers.)

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1. Wolbachia already occurs naturally in a significant number of insect species. Wolbachia already occurs naturally in a significant number of insect species.
2. Wolbachia kills any insect that it infects. Wolbachia kills any insect that it infects.
3. Wolbachia is not transmitted to animals. Wolbachia is not transmitted to animals.
4. Wolbachia is extremely rare in nature, so will hardly impact it. Wolbachia is extremely rare in nature, so will hardly impact it.
5. Volunteers who have been bitten by Wolbachia-infected bacteria do not become infected.

Answer 1

This is more of a reading comprehension test than a knowledge test. You don't have to know about Wolbachia going in. First let's read the material to find out which of the points are supported by the text.

1.  Wolbachia already occurs naturally in a significant number of insect species.

The authors mention in paragraph 1 that Wolbachia occurs in 40% of insect species. This claim is supported.

1.  Wolbachia kills any insect that it infects.

Not supported by the text. They mention that the infected sperm are lethal to the eggs, but the infected insects themselves still live to mate. The fact that the mosquitos don't die right away means they can waste time mating with mosquitos that can't give them offspring and producing useless sperm and eggs.

1. Wolbachia is not transmitted to animals.

This claim isn't supported as strongly as other points the article makes, but they do mention that the naturally-occuring Wolbachia infection hasn't been seen in humans or non-insect animals, and that bites from lab-infected mosquitos don't cause infection in humans.

1. Wolbachia is extremely rare in nature, so will hardly impact it.

No, the article goes directly against this claim. The authors say about 40% of all insect species have it.

1.  Volunteers who have been bitten by Wolbachia-infected bacteria do not become infected.

I think they mean mosquitos, since bacteria can't bite. If they do, this is mentioned in the last paragraph.

So the text-supported points we have are:

1. Wolbachia already occurs naturally in many insect species
2. Wolbachia is not transmitted to (non-insect) animals
3. Volunteers who have been bitten by lab-infected mosquitos don't become infected

The question to answer is which of those points address the concern that the wolbachia-infected mosquitos will have a negative impact. All three of the true points address this concern:

1. Wolbachia is naturally-occuring in many insect species <- so we aren't going to release some weird new pathogen into the environment. It's already there in relatively high amounts.
2. Wolbachia is not transmitted to (non-insect) animals <- it would be bad if the bacteria infected animals that previously weren't exposed to it, since this could cause negative environmental impacts. I wish the text had more evidence to support this claim, but at least some is presented.
3. Humans bit by lab-infected mosquitos don't become infected -> We don't want to cause a new pandemic. Having some evidence this won't jump to humans helps.

And that answers this question! Other things I'd like to see addressed though:

1. Why is this approach better than existing approaches, like insecticides?
2. What is the cost of this approach? Can areas the most affected by Dengue fever afford to use it?
3. Is transmission of the infection from lab-grown mosquitos limited to the same species of mosquito? Genus? Other insect species? Are we confident it won't spread to other species that don't naturally have the infection?
4. Is there evidence that female mosquitos will still mate with infected males? If there is sexual selection with infection as a factor, the population control might be less effective (since the mosquitos would be wasting less time with unproductive mating)

Are there any other concerns you'd like the article to address?