THIRD WORLD NETWORK BIOSAFETY INFORMATION SERVICE
Dear Friends and colleagues,
RE: Survival of GM mosquitoes in the presence of tetracycline contamination
This TWN Biosafety Briefing addresses two interrelated issues that are of importance for the forecasting of potential environmental and health effects of GM mosquitoes using Oxitec’s RIDL technology: that of survival in the presence of the antibiotic tetracycline and evidence of mosquitoes breeding in sewage aquatic environments.
Laboratory experiments inadverdently contaminated by tetracycline in animal feed increased the survival rate of the GM mosquitoes to 15%. The presence of tetracycline in the environment is therefore a risk factor that may lead to contamination of Aedes aegypti breeding sites and subsequently lead to successful emergence of adult mosquitoes and affect the efficacy of the RIDL technology. Survival may also pose unknown risks due to the presence of increased numbers of biting females expressing the transgenic trait.
Aedes aegypti is generally believed to only breed in clean water, and this information has been used during the risk assessment for environmental release of the GM mosquitoes. However, a growing number of publications are showing evidence that sewage-contaminated breeding may be significant. The scenario of breeding and development in potentially tetracycline-contaminated aquatic environments, with the risk of suppressing the lethal system, should be considered as tetracycline is one of the major antibiotics used for humans, and can be found in sewage.
With best wishes,
Third World Network
131 Jalan Macalister,
TWN Biosafety Briefing
GM mosquitoes: Survival in the presence of tetracycline contamination
By Camilo Rodriguez-Beltran
New scientific findings and information should be considered in the assessment of genetically modified (GM) Aedes aegypti mosquitoes releasedusing RIDL technology. This technology, developed by biotech company Oxitec, releases mainly male GM mosquitoes carrying a genetic regulation that, in the absence of the antibiotic tetracycline, causes death at the larval stage of the offspring. This application, intended to result in mosquito population suppression, aims to reduce the incidence of dengue fever, which is spread by Ae. aegypti.
This TWN Biosafety Briefing addresses two interrelated issues that are of importance for the forecasting of potential environmental and health effects of this genetic engineering application, for which new and updated information has been made available.
1. Survival in natural presence of tetracycline
It has been acknowledged by Oxitec that in the absence of tetracycline, the survival rate of the GM mosquito larvae is about 3% under laboratory conditions. This level of survival, while in itself raising questions as to the implications on ecosystems and health, has apparently been accepted by risk assessors, who have given approvals for the field release of the GM mosquitoes in the Cayman Islands, Malaysia and Brazil, with further approvals pending in the United States. It has been also described that the tetracycline concentration used for larval survival during the experiments is 30 µg/ml (Alphey, 2005), a concentration at which the tetracycline toxicity does not affect larvae.
A confidential report by the company (Nimmo et al.) that has been recently made publicly available describes issues regarding tetracycline contamination in the laboratory experiments. It was brought to attention that in an independent experiment, the survival rate of the transgenic line rose to 15%. This result was explained because the food used in this experiment contained chicken that was probably raised with antibiotics such as tetracycline. The company acknowledged that tetracycline contamination in this experiment was the source of larvae survival (Oxitec, 2012).
While the concentration of tetracycline contamination in this experiment has not been described, it should be of interest for biosafety regulators in order to assess the fate of released GM mosquitoes that are likely to encounter antibiotics-exposed animals and soil/water containing manure from these animals. Dose-response testing should be made available for assessment, in particular based on specific environmental conditions where the testing will be performed. The assertion that when “tetracycline can be found in the environment in isolated areas it is not present in sufficient quantity to ensure survival of the mosquitoes” (Oxitec, 2012) should be confirmed with independently derived and peer-reviewed, scientifically sound data.
The presence of tetracycline in the environment, particularly in commercially available animal feeds, has been acknowledged as a risk factor that may lead to contamination of Ae. aegypti breeding sites and subsequently lead to successful emergence of adult mosquitoes and affect the efficacy of the RIDL technology (Patil et al., 2010). Survival of transgenic mosquito larvae to adulthood could limit the technology’s potential for effective population suppression and may pose unknown risks due to the presence of increased numbers of biting females expressing the transgenic trait.
In light of this information, questions that regulators should ask in assessing the GM mosquitoes include:
- Have the concentrations of the tetracycline family of antibiotics been measured in farm animals and in agricultural soil and water in the specific environments where the GM mosquitoes have been released?
- What are other possible sources of tetracycline in the environment?
- What is the minimal concentration that triggers the switching off of the lethal system in these specific environments?
- Would all members of the tetracycline family of antibiotics and their breakdown products behave in a similar manner and hence have the same minimal concentration?
- What will be the behaviour of the mosquitoes in these situations?
- What will be the risk management strategies in case of environmental tetracycline contamination?
2. Mosquitoes breeding in sewage aquatic environments
Aedes aegypti is generally believed to only breed in clean water (Christophers, 1960), and this information has been used during the risk assessment for environmental release of the GM mosquitoes. However, a growing number of publications are showing observational and experimental evidence that sewage-contaminated breeding may be significant.
A group of Puerto Rican researchers have found large numbers of Ae. aegypti in septic tanks in southern Puerto Rico (Barrera et al., 2008). Similar observations have also occurred in Nigeria (Irving-Bell et al., 1987), but it has been highlighted that the “presence of the mosquitoes is not sufficient proof that their immature development has occurred in the polluted water” (Burke et al., 2010).
However, two recent studies have confirmed the presence of Ae. aegypti larvae in septic tanks in Florida (Hribar et al., 2004) and in Puerto Rico (Burke et al., 2010). The study by Burke and colleagues showed a significant correlation between the number of Ae. aegypti immatures collected in the water and the number of adults emerging from septic tanks in the study area, strongly suggesting that this species is using raw sewage as an aquatic habitat and not merely as a resting place (Burke et al., 2010). Furthermore, the national health authorities in El Salvador have confirmed the findings of scientists at the Centre for Health Research and Development who discovered that Ae. aegypti was able to reproduce in dirty water and not exclusively in clean water (Martinez, 2011).
A Brazilian study confirms the development of Ae. aegypti in environmental sewage (Beserra et al., 2010). The scientists conclude that Ae. aegypti does not demonstrate any particular preference for laying eggs in clean water. In this study, raw sewage from the urban area of Campina Grande was used in a laboratory test, with five other different sources of water, to research the oviposition behaviour and development of Ae. aegypti in two conditions (with or without food). Despite having a relatively low rate of larval survival (37% with food, 55% without food), complete larval development occurred in raw sewage. The longevity of males and females raised in raw sewage was not different from those raised in other environments. The scientists also researched oviposition attraction from females to the diverse water sources, and they found no statistical difference in the preference for raw sewage compared to the other aquatic environments. They finally conclude that females were not attracted by clean water sources, which represents a flexible characteristic for adaptation of Ae. aegypti to diverse environmental situations, even in highly polluted sources of water. These findings are important to redirect actions for control and inspection, especially in areas where basic infrastructure for sewage systems is located relatively close to housing areas (Beserra et al., 2010).
This body of recent literature supports the need for further studies before dismissing sewage and human disposal aquatic environments as habitats for Ae. aegypti, as has been done so far. This information is of prime interest for risk assessors of Oxitec’s GM mosquitoes, as the scenario of breeding and development in potentially tetracycline-contaminated aquatic environments, with the risk of suppressing the lethal system, should now be considered. It has been well documented that tetracycline is one of the major antibiotics used for humans, and can be found in the sewage system due to its presence in urine after treatment or consumption of meat treated with tetracycline or from direct disposal of drugs.
In the case of releases of GM mosquitoes in inhabited areas, the following are pertinent questions to ask:
- Have the concentrations of the tetracycline family of antibiotics been measured in local sewage systems?
- What will be the exposure of the GM mosquitoes to these tetracycline-contaminated aquatic systems? And what will be their fate?
- Has survival in these aquatic systems been forecasted in risk management strategies?
Camilo Rodriguez-Beltranis a researcher and lecturer in interdisciplinary innovation at the Universidad del Desarrollo, Chile. He is also a biosafety consultant and member of the Centre for Integrated Research in Biosafety (INBI), University of Canterbury, New Zealand.
The author thanks two reviewers for their comments on this briefing paper.
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Martinez L. (2011). Aedes ya se reproduce en agua sucia. La Prensa Gráfica, 29 October.
Nimmo D., Gray P. and Labbé G. (undated). Eliminating tetracycline contamination. Internal report from Oxitec, http://libcloud.s3.amazonaws.com/93/de/e/986/MosquitoDocOriginal.pdf.
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