The bacteria that could fight Zika virus

Researchers are using Wolbachia bacteria to infect mosquitoes and block the transmission of viruses like Zika.

GRCertain strains of the bacterium Wolbachia can spread among mosquitoes, preventing viruses from surviving in their bodies and shortening the insects’ lifespans. The method looks promising in field tests with dengue fever, and it could work for Zika as well. Ecological geneticist Gordana Rasic explains.

ResearchGate: What are Wolbachia and how do they reduce the spread of mosquito-borne diseases?

Gordana Rasic: Wolbachia are bacteria common in mites, spiders, nematodes and insects, including mosquitoes. The mosquito Aedes aegypti, the main transmitter of Zika, doesn’t naturally carry Wolbachia infections, but scientists have been able to transfer different Wolbachia strains from other insects to this species. Initially, this was done to reduce the mosquito lifespan. Later, it was discovered that certain Wolbachia strains also block replication of viruses inside mosquito tissues. Mechanisms behind this blocking effect are complex and not entirely resolved. We know that Wolbachia manipulate the host defense system to ensure their own persistent infection, and they outcompete viruses over nutritional resources needed for replication. This means Wolbachia can effectively immunize mosquitoes against viruses that are ingested during a blood meal. Because viruses like dengue and Zika spread through mosquito bites, immunized mosquitoes break the viral transmission cycle.

RG: Might Wolbachia be effective against the Zika virus?

Rasic: Most Wolbachia research has concentrated on dengue viruses, to which Zika is related, so it is possible. However, our knowledge of the Zika virus is currently very limited, and we need to test this in the lab. The good thing about the Wolbachia-based strategies is that they can be used in combination with other methods like mosquito sterilization, providing an extra layer of protection.

RG: What is the state of research on Wolbachia?

Rasic: Currently there is great interest in using Wolbachia to control mosquito-borne human diseases. Many research groups around the world are investigating the effect of different Wolbachia infections in major disease-transmitting mosquito populations. Wolbachia and the viral pathogens are very complex, so research questions and approaches are numerous. What we see emerging from these studies is that Wolbachia effects can be different across mosquito hosts and pathogens, so the research community needs to test as many Wolbachia strains as possible to find the ones that are optimal for a given mosquito and/or disease. There are many Wolbachia strains in nature, and the techniques for their transfer to mosquitoes are becoming more accessible. So, I suspect we will see more exciting results in the near future.

RG: Have approaches using Wolbachia been tested in the field?

Rasic: One strategy that has been implemented in the field is releasing Wolbachia-infected mosquitoes and allowing Wolbachia to spread to local mosquitoes, immunizing them against dengue. For example, Aedes aegypti populations in northern parts of Queensland (Australia) and Yogyakarta (Indonesia) still have a high frequency of Wolbachia infections several years after the introduction of lab-infected insects. It will take a few more years of monitoring to show that this is an effective way to suppress dengue in those areas, but the results so far have been encouraging.

Wolbachia within an insect cell (Scott O'Neill / CC BY)

Wolbachia can also be used to reduce or even eliminate local mosquito populations. For example, some Wolbachia strains make mosquito eggs extremely sensitive to drought. Such strains can invade a mosquito population during a rainy season and then kill off resting eggs during the following dry season. Field cage experiments have recently shown that this can be a viable strategy for a region like central Queensland in Australia.

Furthermore, mosquito populations can be crippled by releasing males infected with Wolbachia. When these infected males mate, their offspring die. This way you are effectively sterilizing females indirectly, driving a population to crash. Such an approach was initiated in southern China last year to fight Aedes albopictus, the mosquito responsible for dengue epidemics in that region. We are looking forward to updates on whether that project was successful.

RG: Are there risks or drawbacks to using Wolbachia to combat mosquito-borne diseases?

Rasic: It’s possible, though unlikely, that some Wolbachia strains could actually enhance, rather than suppress, the proliferation of the Zika virus within a mosquito. Of course, researchers would be able to determine if these undesirable effects exist before a strain is released into the field. Another potential drawback could be Wolbachia’s blocking effect diminishing over time as a result of evolutionary changes in the genomes of the mosquitoes, the Wolbachia, or the virus. From what we can gather so far, such changes are unlikely to be rapid. It is worth noting that other strategies like the use of insecticides or genetic modifications face the same problem: all populations evolve and can become resistant. Therefore, it is not wise to depend only on one solution. A favorable feature of the Wolbachia-based strategies is that they are compatible with other control programs, and they should be pursued as such.

ResearchGate has put together a comprehensive collection of emerging Zika virus research, including scientific papers, interviews with researchers, and discussions among them. 

Featured image courtesy of Ian Tunbridge.