MRCU: Release on modified mosquitoes ‘misleading’

Officials: Claims of mossies reproducing are inaccurate

Genetically-engineered mosquitoes released in Grand Cayman in 2009 to help fight the potential outbreak of dengue fever are not reproducing as some organisations have claimed recently, said the head of the Mosquito Research and Control Unit in the Cayman Islands. 

A release published 12 January by several organisations opposed to genetic modification claiming that Oxitec – the company that provided genetically-modified mosquitoes released during a six-month trial period in East End – concealed important information is “inaccurate and makes no sense,” said Dr. Bill Petrie, director of the Mosquito Research and Control Unit in Grand Cayman. 

In its report, the Friends of the Earth US, GeneWatch UK and Third World Network had claimed that the, “mosquitoes described by their manufacturer, UK company Oxitec, as ‘sterile’ are in fact not sterile and their offspring have a 15 per cent survival rate in the presence of the common antibiotic tetracycline.” 

The groups claim tetracycline, an antibiotic frequently used in agriculture, may be found in sewage as well as in industrially farmed meat. They assert that mosquitoes capable of carrying dengue fever “are known to breed in environments contaminated with sewage where they are likely to encounter widespread tetracycline contamination.” 

Dr. Petrie acknowledged the impact the antibiotic may have on the insects but took exception to the veracity of those assertions by saying, “tetracycline is the antidote for the genetically-modified state the insects are in and is required to be able to breed the mosquitoes in the lab. However, it would need to be 150 times higher in concentration that is usually found in sewage for it to be effective. Furthermore, they claim that dengue carrying mosquitoes breed in sewage, which is not true here and has never been found to be so.” 

Dr. Petrie said the only mosquito in the Cayman Islands which breeds in sewage is the Souther House Mosquito, which does not carry dengue. 

“They don’t care what lies they tell,” he said. “Only male mosquitoes were released and they cannot bite and do not live long.” 

Dr. Petrie said the exercise, conducted by MRCU and Oxitec of Oxford University two years ago in an isolated area of the District of East End, was simply a form of insect control using what is known as the sterile insect technique. The technique isn’t new and the difference now is the insects are made sterile by genetics as opposed to chemicals or radiation.  

The trial was carried out in three blocks; one block was where the method was introduced, while nothing was done in another block. A third block was used to monitor the natural population of the Aedes Aegypti mosquito. 

Other countries have expressed interest in fighting Dengue Fever using genetically modified mosquitoes.  

According to the report released by Oxitec to countries interested in the genetic modification technology, the significance of releasing just males is that only females contaminated with tetracycline may pass on the tetracycline contamination to the next generation through larvae and compromise the modified gene.  

“These are people with an agenda and it’s scare mongering,” Dr. Petrie said. “The report is targeting Oxitec rather than us here in the Cayman Islands, but they have even said that we have no regulations here which is not true.”  

The director said MRCU was required to get a permit from the Cayman Islands Department of Agriculture and had to demonstrate feasibility and visibility to get the permit for the eggs and ability to release. 

“We also went to the Department of Environment and told them what we wanted to do even though we did not have to,” he said. “Furthermore, the information the groups are touting as being withheld by Oxitec was not Oxitec’s in the first place and was not theirs to withhold or otherwise.” 

The Cayman Islands saw an 80 per cent reduction in the Aedes Aegypti mosquito, which is a known carrier of dengue fever in the test area, according to the MRCU officials. 

“We have heard nothing for the advantages of the study,” Dr. Petrie said. “Their view is biased.”  



The release issued by the organisations may be found at 


The Aedes Aegypti is a known carrier of dengue fever. – PHOTO: SUBMITTED

If you value our service, if you have turned to us in the past few days or weeks for verified, factual updates, if you have watched our live streaming of press conferences or sent an article to a friend... please consider a donation. Quality local journalism was at risk before the coronavirus crisis. It is now deeply threatened. Even a small amount can go a long way to sustaining our mission of informing the public. We need our readers’ financial support now more than ever.



  1. Oh please… they will tell Dr. Petrie anything, and give him their findings! I dont think he is qualified enough to detect what truth is, coming from a private company like Oxitec! Maybe we should arrive back at the first question: Why was genetically modified mosquitoes released on civilians in the first place – and not in the UK where the said company is based???!!!

  2. Born Caymanian, I’ve had Dengue twice in my life and I can assure you, you don’t want it. People who are in less the great physical condition die from this disease.

    Releasing modified mosquitoes who will breed but fail to reproduce is a well tested concept to reduce mosquito populations. The only thing anyone can argue about here is that these mosquitoes were genetically modified, but it’s not like any Frankenstein mosquitoes were made. They just failed to reproduce, as planned, and the proof of concept is the large reduction in the mosquito population.

    Frankly I think this should be done with other mosquito species as well.

  3. Our press release was not inaccurate, it is available on: It cites a confidential document which shows that levels of tetracyline present even in heat treated chicken (in cat food) were sufficient to cause a 15% survival rate of Oxitec’s GM mosquitoes in the lab. We also provide a link to a redacted version of the document, released to the UK parliament and in response to Freedom of Information requests, in which the 15% survival rate was blacked out. There is ample evidence in the scientific literature that tetracycline contamination can occur in sewage as well as in industrially-farmed meat and that Aedes aegypti mosquitoes can breed in potentially contaminated sites. For example, a 2004 study in Florida (on: ) found that: Sewage treatment plants, septic tanks, and cesspits were larval development sites for Cx. quin-quefasciatus, Cx. nigripalpus, and Ae. aegypti. We, unlike the company or the regulators, are placing this information in the public domain, as we believe the public, ultimately affected by the use of these technologies, have a right to know, and make informed decisions.

    Yours sincerely,
    Helen Wallace (GeneWatch UK), Eric Hoffman (Friends of the Earth US), Lim Li Ching (Third World Network)