Date:3 February 2016
In recent months the mosquito-transmitted Zika virus has been a subject of much concern in the Americas.
The virus was originally isolated in Uganda in 1947 and is similar to the dengue, yellow fever, West Nile viruses and Japanese encephalitus – all these viruses stem from the Flaviviridae family.
Although the symptoms of the virus are mild in infected adults, Zika could hold serious prenatal consequences. During late November 2015 the Brazillian Ministry of Health established a relationship between the Zika and microcephaly in newborn babies.
Microcephaly is a rare congenital condition associated with incomplete brain development in the womb. Although the condition can be detected at birth, many children suffering from microcephaly show signs of abnormal post-natal development. Research by the World Health Organisation (WHO) shows that pregnant women have the same likelihood as the general population of being infected by the virus and, like some, might not show any symptoms of the infection. Research around perinatal transmission of the virus is currently limited, but the WHO has advised pregnant women travelling to South America to consult a doctor before doing so.
The WHO and the Pan American Health Organization (PAHO) are working on detecting and responding to the spread of Zika, that has been reported in 26 countries and territories across the Americas. The aim is to better understand the relationship between Zika, microcephaly and other complications, reports the PAHO.
Meanwhile researchers at the Pest & Environmental Adaptation Research Group are using Wolbachia bacteria to infect mosquitoes and block the transmission of viruses like Zika. Certain strains of the bacterium Wolbachia can spread among mosquitoes, preventing viruses from surviving in its body and shortening the insect’s lifespan. The method looks promising in field tests with dengue fever, and it could work for Zika as well. Ecological geneticist Gordana Rašić explains.
ResearchGate: What are Wolbachia and how do they reduce the spread of mosquito-borne diseases?
Gordana Rašić: 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?
Rašić: 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?
Rašić: 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?
Rašić: 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.
The interview with Gordana Rašić was originally published by ResearchGate.