11/9/15 Update from the Miami Herald
Monroe County voters and those in the Lower Keys neighborhood of Key Haven on Tuesday were split on releasing genetically modified mosquitoes in Key Haven as a test to see whether the measure can reduce the population of Aedes aegypti mosquitoes, which carry Zika and other viruses.
In a nonbinding referendum, about 58 percent of voters countywide favored the mosquito release. In Key Haven, where the same question was asked, about 65 percent of voters opposed the release.
The final decision on whether millions of GMO mosquitoes will be released in Key Haven in spring 2017 to battle the Zika virus falls on the Florida Keys Mosquito Control Board. The agenda has not yet been prepared for the board’s Nov. 19 meeting, and Beth Ranson, public information officer, could not confirm whether it will be discussed.
11/2/16 Update: Highly recommended Zika-related reading, PLOS Science Wednesday redditscience AMA with researchers Colin Carlson and Eric Dougherty (UC Berkeley) discussing an ecological model they developed using historical data to map Zika transmission patterns. Read the study: “An Ecological Assessment of the Pandemic Threat of Zika Virus” in PLOS NTDs.
By Gaétan Burgio
Zika virus infection is a devastating disease for its association with neurological disorders (Guillain-Barré syndrome and microcephaly). For the last few months Zika virus has spread at a rapid pace over the continent of South America and has now reached the US and Asia (Taiwan and Singapore). The World Health Organization declared Zika a public emergency and special funds from Brazil, Singapore and the US (after months of political battle in the US congress) have been committed to combat the infection and to contain the outbreak.
Despite this strong effort to eradicate Zika virus infection there is still no available cure against the virus and a vaccine has a long way to go before being widely implemented. Meanwhile, we have learnt a lot about this virus.
- Importantly we know now that the virus transmits only to few mosquito species, such as Aedes aegypti and Aedes albopticus or eventually Aedes polynesiensis.
- We also know there is, to date, no evidence of a transmission of Zika through Culex or other endemic species in the US or elsewhere [1, 2].
These two factors certainly restrict the reservoir available to the virus and could better facilitate control of transmission through the currently available remedies of insecticide spraying and preventing contact with the vector. However, given their limitations, discussed next, we must also assess newer, experimental methods for eradicating and controlling Zika, including the release of GMO mosquitoes, the CRISPR/Cas9 gene drive, and Wolbachia infection.
Insecticide sprays only work temporarily and are largely ineffective
The immediate response to this Zika virus outbreak adopted by authorities in Florida, Puerto Rico, Taiwan and Singapore has consisted of a combination of aerial sprays of the organophosphate insecticide Naled and/or ground sprays the larvicide Bacillus Thuringiensis israelensis or biti. As a result, a reduction in Aedes aegypti prevalence in Miami Beach after a month of intense aerial and ground sprays was observed according to the CDC in late September 2016.
However, a rebound in the Aedes aegypti prevalence has been recently observed following the insecticide spray campaign at Miami Beach according to an October 3, 2016 CIDRAP report.
Aedes aegypti is an indoor mosquito resident and it is highly challenging to eradicate since the larvae have the ability to survive for months in standing water in every possible indoor or outdoor surface (plastic bottles, tree holes….) as long as the water is available. To eradicate the larvae and mosquitoes, it would require absolute control of the indoor and outdoor sites and eliminate all standing water.
Past experience with the malaria vector Anopheles has demonstrated that an aerial spray is largely ineffective for indoor mosquitoes and indoor sprays are effective but expensive, and required to be conducted on a large scale, at regular intervals and importantly with the full commitment of the communities.
Is the release of GMO mosquitoes a solution to control the outbreak?
The control of vectors using genetically modified insects has been the focus of many researchers and entomologists since the 1950s. Several strategies have been tested to control the transmission of mosquito borne diseases, including:
- Sterile insect techniques (SIT) using irradiated sterilized mosquitoes.
- Engineered (GM) mosquitoes carrying a dominant lethal mutation (RIDL) affecting the offspring at pup stage to larval development.
For Zika virus infection, the FDA has approved field trials by Oxitec (a Oxford based company) using the RIDL strain OX513A Aedes aegypti — to be performed in the small community of Haven in the Florida Keys. Florida remains the only state in the US with locally-acquired cases of Zika, making the need for Zika control measures particularly acute. The Center for Disease Control (CDC) reports that, as of October 12 2016, there has been 128 locally acquired Zika cases on record, and experts predict the aftermath of Hurricane Matthew may lead to greater risk of a homegrown Zika outbreak in Florida.
Despite the threat of Zika in Florida and approval from the FDA to begin a trial using GM mosquitoes, no such trial has yet taken place due to growing public resistance, culminating in a petition to stop the trial based on fears about the potential impact of the GM mosquitoes on the local people and environment. Before proceeding with any trial, Florida officials have placed a non-binding voter referendum on the affected county’s November 8 election ballot.
So, what is known about GM mosquito effectiveness and environmental impact?
From previously published field trials on Dengue virus infection using OX513A, we now have an idea of the effectiveness of this strategy on the mosquito prevalence and its epidemiological impact. Several field trials were conducted in Brazil or in Malaysia [3, 4] using sustained release of OX513A over a year to control or suppress Aedes aegypti population. The results of these trials were very encouraging leading to up to 90% suppression of Aedes mosquitoes and demonstrated that mosquito prevalence could be controlled for over a year. No safety concerns, environmental effects or the emergence of resistant mosquitoes were reported during or after these trials.
Despite these encouraging results, it is still too early to come to a conclusion about the longer term efficacy of this strategy. It would certainly require several years of field trials before envisaging a large-scale implementation.
Meanwhile, the outcome of a showdown between community opponents and the Florida Keys Mosquito Control Board — which is charged with determining whether the available science is sufficient to safely proceed with a local trial — will be known at some point after November 8th.
Is there any future for a CRISPR/Cas9 gene drive strategy as an alternative to insecticide sprays or the release of GMO mosquitoes to eradicate Zika virus?
Gene drive strategy using CRISPR/Cas9 genome editing technology has sparked much interest in the scientific community and the general public, to a point where it has been widely hyped as a solution for the eradication of mosquito-borne diseases such as Zika and malaria. While the hype and the excitement are tangible, there are also many fears over the use of CRISPR/Cas9 genome editing technology to control mosquito-borne infection.
Meiotic drive, simplistically described as a natural process that favors the over-representation of an allele in a gamete during meiosis (e.g. could be a transposable element in the genome) which occurs commonly in Drosophila or in mosquitoes such as Aedes aegypti or the malaria vector Anopheles, is seen as a powerful, potential mechanism for natural population replacement. Recently, researchers took advantage of meiotic drive and combined this with CRISPR/Cas9 genome editing technology to accelerate the population replacement of the gene of interest to disrupt genes required for fertility or transmission of the pathogens. The experiments in a laboratory setting have worked remarkably well, with a high success of up to 99% in generating infertile mosquitoes in the laboratory population, raising many hopes of the eradication of wild mosquito populations .
However, some major limitations to this approach have become apparent to researchers in just the last few months. In the field, the mosquito population displays considerable genetic variation. As such, targeting a gene with a CRISPR/Cas9 gene drive approach might have a very limited effect, or be immune, as the genetic variation in the population at the Cas9 target site could considerably affect the efficiency of the drive .
One solution to avoid this limited effect could be the introduction of a second drive. Recent studies have shown that resistance to gene drive occurs fairly rapidly in natural populations and would definitively impede the efficiency of the drive . Thirdly, the Cas9 enzyme creates a double stranded break in an error prone repair pathway leading to indels and off targets effects that are not negligible. Finally besides environmental and ethical concerns such as the lack of biocontainement and escape from edited mosquitoes leading to a ‘chain reaction’ in wild population or eventually the development of the gene drive technology as a bio-weapon, there is no remedy to date on how to stop the chain reaction generated from gene drive.
Given these issues, I would predict a very limited effect of this strategy to the control and the eradication of mosquitoes-borne diseases.
Wolbachia: a potential alternative to GMO mosquitoes to contain Zika outbreak?
The endosymbiotic bacteria, which could be simply defined as a bacteria living within the body of another living organism, Wolbachia pipentis, is an intracellular bacteria which infects the Aedes or Anopheles host and has been shown to reduce or block the transmission of Zika infection [8, 9]. Wolbachia is present and specific in many insect species and has been shown to be safe for the environment, as the bacterium doesn’t transmit to animals or humans. Field trials performed in Australia and Vietnam for Dengue virus showed great success but also a lack of persistence in the establishment of the infection into wild populations of Aedes [10, 11]. Further field trials on Zika and other viruses will determine the sustainability of this strategy and will certainly be very interesting to follow.
How to efficiently control this Zika virus outbreak?
To control this Zika outbreak, sadly there are not many immediate options available. The use of GMO, CRISPR/Cas9 or Wolbachia infected mosquitoes are still at the developmental stage and would require years of field trials before attempting a larger scale release.
The Zika vaccine is still under development and again, will require years of clinical trial before its implementation.
Barring the use of GM mosquitoes in Florida, the only option currently available to contain this outbreak and combat Zika virus infection remains the application of indoor and outdoor sprays, along with the implementation of preventive measures delineated by the CDC.
One of several available CDC Zika-prevention public information brochures is displayed below.
- Fernandes, R.S., et al., Culex quinquefasciatus from Rio de Janeiro Is Not Competent to Transmit the Local Zika Virus. PLoS Negl Trop Dis, 2016. 10(9): p. e0004993.
- Hall-Mendelin, S., et al., Assessment of Local Mosquito Species Incriminates Aedes aegypti as the Potential Vector of Zika Virus in Australia. PLoS Negl Trop Dis, 2016. 10(9): p. e0004959.
- 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, 2015. 9(7): p. e0003864.
- Lacroix, R., et al., Open field release of genetically engineered sterile male Aedes aegypti in Malaysia. PLoS One, 2012. 7(8): p. e42771.
- Hammond, A., et al., A CRISPR-Cas9 gene drive system targeting female reproduction in the malaria mosquito vector Anopheles gambiae. Nat Biotechnol, 2016. 34(1): p. 78-83.
- Drury, D.W.S., D.J.; Zentner, G.E.; Wade, M.J. , CRISPR/Cas9 gene drives in genetically variable and non-randomly mating wild populations. BioRxiV, 2016.
- Unckless, R.L.C., A.G.; Messer, P.W., Evolution of resistance against CRISPR/Cas9 gene drive. BioRxiV, 2016.
- Aliota, M.T., et al., The wMel strain of Wolbachia Reduces Transmission of Zika virus by Aedes aegypti. Sci Rep, 2016. 6: p. 28792.
- Dutra, H.L., et al., Wolbachia Blocks Currently Circulating Zika Virus Isolates in Brazilian Aedes aegypti Mosquitoes. Cell Host Microbe, 2016. 19(6): p. 771-4.
- Hoffmann, A.A., et al., Successful establishment of Wolbachia in Aedes populations to suppress dengue transmission. Nature, 2011. 476(7361): p. 454-7.
- Nguyen, T.H., et al., Field evaluation of the establishment potential of wMelPop Wolbachia in Australia and Vietnam for dengue control. Parasit Vectors, 2015. 8: p. 563.
Gaétan Burgio @GaetanBurgio completed his PhD in 2008 from Pasteur Institute in France and migrated to Australia to join Professor Simon Foote as a postdoctoral researcher at the Menzies Research Institute (University of Tasmania) from 2008 to 2012, and the Australian School of Advanced Medicine at Macquarie University from 2012 to 2015 investigating the host response to the malaria parasite using large scale ENU mutagenesis in mice. In 2015 he was appointed as a Group Leader at The John Curtin of Medical Research, and Head of the transgenesis core facility at the Australian Phenomics Facility to establish a new research program of the host pathogens interaction on multidrug resistant bacteria and the malaria parasite, using cutting edge technologies such as CRISPR/Cas9 genome editing. His research is supported by the National Health and Medical Research Council (NHMRC) and the National Collaborative Research Infrastructure Strategy (NCRIS) in Australia.
Sara Kassabian and Victoria Costello contributed reporting for this post.
The PLOS Zika Collection featuring the latest PLOS-published research on the Zika virus and outbreak, including the following papers of particular relevance to this post:
- Genetically Modified (GM) Mosquito Use to Reduce Mosquito-Transmitted Disease in the US: A Community Opinion Survey Amesh Adalja, Tara Kirk Sell, Meghan McGinty, Crystal Boddie PLOS Currents Outbreaks 25 May 2016
- A Cost-Effectiveness Tool for Informing Policies on Zika Virus Control Jorge Alfaro-Murillo, Alyssa Parpia, Meagan Fitzpatrick, Jules Tamagnan, Jan Medlock, Martial Ndeffo-Mbah, Durland Fis…PLOS Neglected Tropical Diseases 20 May 2016
- Preventing Zika Virus Infection during Pregnancy Using a Seasonal Window of Opportunity for Conception Micaela Elvira Martinez PLOS Biology 28 Jul 2016
- An Ecological Assessment of the Pandemic Threat of Zika Virus Colin J. Carlson, Eric R. Dougherty, Wayne Getz PLOS Neglected Tropical Diseases 26 Aug 2016