GM mosquitoes cut rate of viral disease in Brazil | Science News for Students

GM mosquitoes cut rate of viral disease in Brazil

The genetically modified insects were unable to produce blood-sucking young
Aug 3, 2016 — 7:00 am EST
GMO mosquitoes

Immature GM mosquitoes being raised in the lab. Adults from such breeding programs were released into the wild. They helped cut the rate of dengue fever in one Brazilian community.


Tweaking the genes of mosquitoes that can carry disease can transform those blood suckers into weapons that fight disease. This is the finding of a new study in Brazil.

Dengue (DEN-gay) fever is a viral disease spread by Aedes aegypti mosquitoes. This potentially life-threatening infectious disease causes high fevers, headaches, pain and sometimes mild to severe bleeding. Because there is no cure, health officials are always on the lookout for ways to slow the spread of this virus. And one new approach is to slow the growth of mosquito populations.

By altering the genes in male mosquitoes, researchers have just impaired the ability of these insects to reproduce. Then they released the guys throughout several neighborhoods in the city of Piracicaba. And over the next year, the presence of those altered males put a dent in the town’s number of new dengue cases.

Health officials used regular methods to limiting new skeeter-breeding grounds throughout much of the city. Such measures included keeping stagnant water from collecting outside of homes in buckets, plant saucers or other spots.

mosquito egg
Here a scientist is about to inject a mosquito egg with a gene that will prevent the male insects from fathering young that can live to adulthood.

Where such control measures were taken, the number of new dengue cases dropped by 52 percent over a year, beginning in mid-2015. But in neighborhoods where the altered males were released as an extra control, the results were much, much better. Dengue cases there dropped 91 percent — from 133 to a mere 12.

Oxitec is the biotechnology company based in Abingdon, England, that created the altered male insects. On July 14 it reported that its new data linked the release of those those altered male insects with a sharp drop in disease.

It works by romantic deception

The gene changes in the male mosquitoes was small — but important. The guys can still mate. Their mates will still lay eggs that hatch and grow. But these young are not healthy. Their DNA has a built-in self-destruct switch. As a result, the young die off in the wild before they are mature enough to bite.

This is a modern biotech twist on a decades-old insect control technique. It relies on releasing sterile males, ones that cannot fertilize a females’ eggs. (If enough sterile guys are flying around in the wild, the females will waste their reproductive effort. Eventually, the population breeds itself out of existence.)

In earlier tests in other parts of the world, Oxitec showed that mosquito numbers will fall when the genetically altered males swarm through a neighborhood. But the company’s new report is the first claim that when mosquito numbers fall, so do the cases of disease.

mosquito scientist
Adult GM mosquitoes flying around in the screened boxes. Males from this program can mate with healthy females. But their offspring won’t live to adulthood — when they can transmit disease.

That information — though preliminary — could help to fill a data gap that has provoked a debate in the Florida Keys over a proposed release of such mosquitoes there. Many people oppose releasing genetically modified, or GM, organisms into the wild. The company says that almost all the releases will be males that will die and leave no adult offspring. (The adults bite animals, which can spread disease.)

Brazil, however, has suffered for years from dengue — and now the Zika virus. Conditions in some areas have been so dire that officials in that land are much more welcoming of tests with GM mosquitoes.

In the recent trial, Oxitec looked at the numbers of dengue cases reported mid-year to mid-year from Piracicaba’s dengue surveillance program. The GM mosquito test focused on just one area in the city. Called CECAP/Eldorado, some 5,000 people live here, where the city’s dengue rates were highest in 2014 to 2015. More than five out of every 200 people there became infected. Elsewhere in the city, the rate was less than two out of every 200.

But dengue rates fell dramatically after special mosquito-control measures went into effect, including the release of the GM mosquitoes. Now dengue rates are lower here than throughout the rest of the city — roughly half of the citywide rate.

Data on mosquito populations and diseases are rare and important, says Grayson Brown. He directs a lab on insects and public health at the University of Kentucky in Lexington. He wonders how far down the GM mosquitoes drove the wild mosquito population before dengue rates began to fall. (Oxitec reports that mosquito numbers dropped by 82 percent. But Brown asks: 82 percent of what?)

Plenty of programs monitor disease outbreaks as they treat mosquitoes, Brown notes. Still, it’s hard to justify choosing to leave some parts of a city untreated as part of a test, he adds. And that makes it hard to see how much a treatment has helped. Here that could be tried by adding the extra measure of the GM treatments.

Power Words

Aedes aegypti    A species of mosquito that can transmit the viruses responsible for several tropical diseases, including dengue fever, yellow fever and West Nile disease.

biotech      Short for biotechnology, which is the use of living cells to make useful things.

dengue fever    A potentially lethal infectious disease transmitted by mosquitoes. No vaccine yet exists to prevent infection with the virus responsible for the disease, which causes high fevers, severe headache, joint pain, pain behind the eyes, rash, bone pain and sometimes mild bleeding. A more severe form of the disease, known as dengue hemorrhagic fever can cause uncontrolled bleeding if not treated right away.

DNA    (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

entomology     The scientific study of insects. One who does this is an entomologist.

epidemiologist   Like health detectives, these researchers figure out what causes a particular illness and how to limit its spread.

fertilize   (in biology) The merging of a male and a female reproductive cell (egg and sperm) to set in create a new, independent organism.

gene   (adj. genetic) A segment of DNA that codes, or holds instructions, for producing a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.

genetic engineering   The direct manipulation of an organism’s genome. In this process, genes can be removed, disabled so that they no longer function, or added after being taken from other organisms. Genetic engineering can be used to create organisms that produce medicines, or crops that grow better under challenging conditions such as dry weather, hot temperatures or salty soils.

genetically modified    An adjective, also known as GM, used to describe organisms that have been altered through the use of genetic engineering.

incidence      The rate or number of times something occurs.

infection    (v. infect) A disease that can spread from one organism to another.

population    (in biology) A group of individuals from the same species that lives in the same area.

skeeter       Colloquial term for mosquitoes.

sterile     An adjective that means devoid of life — or at least of germs. (in biology) An organism that is physically unable to reproduce.

surveillance      A term for watching or keeping track of the behavior of others, usually in a stealthy manner or from a distance.

virus   Tiny infectious particle consisting of RNA or DNA surrounded by protein. Viruses can reproduce only by injecting their genetic material into the cells of living creatures. Although scientists frequently refer to viruses as live or dead, in fact no virus is truly alive. It doesn’t eat like animals do, or make its own food the way plants do. It must hijack the cellular machinery of a living cell in order to survive.

Zika virus     A virus that can be transmitted to humans via mosquitoes. About 20 percent of infected people get sick. Symptoms include a slight fever, rash and pinkeye and usually fade quickly. A growing body of evidence suggests that the virus could also cause a devastating birth defect — microcephaly. Evidence suggests it may also cause neurological conditions such as Guillain-Barré syndrome.


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S. Milius. "GM mosquitoes succeed at reducing dengue, company says" Science News. July 15, 2016. 


Oxitec.  Dengue fever cases drop 91% in neighbourhood of Piracicaba, Brazil, where Oxitec’s Friendly™ Aedes were released. Press release, July 14, 2016.