New vector control methods
We have already looked at the various methods being used to manage mosquitoes as a way of preventing the spread of Zika. Unfortunately, the occurrence of insecticide resistance and the impracticalities of destroying their breeding sites has limited use of most standard approaches. It is therefore crucial that other new, or novel, options are considered that can work synergistically with traditional methods of mosquito vector control. In this step, we consider genetic and microbial mosquito control as described by: Yakob, L. & Walker, T. Zika virus outbreak in the Americas: the need for novel mosquito control methods. The Lancet. Volume 4, March 2016.
What options are left for control of the Zika virus vectors?
The significant burden of mosquito-transmitted diseases such as dengue and malaria has driven development of a range of novel approaches to insect control. Scientists across the world are looking outside of traditional strategies, and many of the methods they develop could be of benefit in the fight against Zika.1
Two particularly promising new approaches that have gained considerable attention are genetic modification of Aedes aegypti and the introduction of bacteria into mosquito populations. The intended effect of these methods is to reduce transmission, and this may be achieved by directly reducing the numbers of female mosquitoes in an area or reducing their ability to support development of the virus.
One genetic control strategy currently being pursued involves creating a strain of mosquitoes with a lethal gene. This gene produces a toxin that kills the mosquito. The team behind this strain are able to produce large numbers of healthy mosquitoes in an insectary by giving them a dietary supplement that suppresses the activity of the lethal gene, but in the absence of this supplement these same mosquitoes will eventually die and their offspring won’t survive to adulthood.
Males of the genetically modified mosquitoes can be released into the wild to mate with females, and because their offspring won’t live to adulthood the mosquito population is expected to decline. A recent field trial near Juazeiro in Brazil involved releasing the mosquitoes in a sustained effort over several weeks, and the population of mosquitoes in the area was indeed suppressed.2 In fact, trapping of adult mosquitoes indicated that the population had declined by as much as 95%. This implies that the release of genetically modified mosquitoes with a lethal gene could be a highly effective method of preventing disease epidemics.
Endosymbiotic bacteria are bacteria that live inside other organisms. A group of bacteria commonly found living in insects are Wolbachia. Studies have shown that Wolbachia taken from fruit flies and introduced into Aedes mosquitoes can directly inhibit the replication of dengue virus within Aedes.3 Wolbachia also reduces the capacity of these mosquitoes to harbour and transmit additional medically important viruses, including Yellow fever virus and Chikungunya virus.
A trial has been performed in Queensland, Australia in which Wolbachia-infected Ae. aegypti were released over a series of weeks into the wild.4 Collection of larvae showed that the mosquitoes carrying Wolbachia had successfully invaded the local population, and within a few months the Wolbachia were found in 90% or more of the larvae sampled. What effect this approach will have on the transmission and epidemiology of viruses is currently unknown, but further releases of Wolbachia-infected mosquitoes are ongoing in countries endemic for dengue, including Indonesia, Vietnam, and Brazil.
Could Wolbachia be used to fight Zika? In a recently published study Wolbachia-carrying Ae. aegypti were experimentally infected with Zika virus isolated from Brazil. These mosquitoes were found to be highly resistant to the virus. Indeed, saliva collected from the Wolbachia-carrying mosquitoes did not contain infectious Zika virus.5 The authors of the study therefore concluded that Wolbachia could greatly reduce transmission in field populations of Ae. aegypti, and become an effective option for combatting the Zika burden in Brazil and other countries.
The biggest concern for the Wolbachia approach will be any long term evolution of the virus: like any control method, evolutionary responses are expected and over time it is possible that Zika or other viruses may overcome the inhibitory effect that the Wolbachia have, enabling transmission by the mosquito once again. While this can’t be readily tested, scientists have generated a ‘superinfected’ line of mosquitoes, containing two different strains of Wolbachia, which could offer an effective strategy to help manage potential resistance by viruses.6 What effect either Wolbachia or genetically modified mosquitoes will ultimately have on Zika transmission and epidemiology in the field remains uncertain, but this is applicable to any novel strategies that are undergoing preliminary trials.
The two new methods outlined above are species-specific, and because they don’t rely on the use of chemical insecticides they are considered environmentally friendly options for insect control. Furthermore, as Ae. aegypti is the vector for several related viruses, suppressing the population of this species or rending it unable to transmit viruses offers the potential to simultaneously tackle Zika, dengue, chikungunya, and yellow fever.
What do you think?
Which of these methods looks the most promising to you? Perhaps there are other approaches you have heard of that aren’t covered here and other learners may be interested in?
© London School of Hygiene & Tropical Medicine