Dengue fever is a viral disease – an arbovirosis – caused by an RNA virus (flavivirus) transmitted by mosquito bites: mosquitos tiger and Aegypti. The number of cases worldwide continues to increase. It is estimated at 400 million per year among 4 billion people exposed mainly in tropical regions. There are serious forms, with hemorrhagic fever requiring hospitalization. They affect approximately 5 million patients each year, sometimes leading to death (20,000 per year). Cases tend to spread to countries in the northern hemisphere, linked to the migration of vector mosquitoes due to global warming. A few cases of indigenous contamination have already been reported in France.
There is no treatment for dengue fever. Prevention is difficult. It is based on individual protection against mosquito bites or collective measures (drying up water points, insecticides but the use of which is restricted by the risks of resistance and toxicity). The release of male mosquitoes rendered sterile could reduce mosquito populations but the effectiveness of this measure remains to be evaluated while it is also the subject of dispute. There are vaccines, but their use requires vaccination screening which complicates their use on a large scale.
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Reduce viral transmission
This is where the bacteriological warfare comes in. Normally, the virus ingested by the mosquito passes from its intestine to its salivary glands and is then inoculated by the bite (which provides the mosquito with the food bowl of blood). But within the cells of many insects, a class of bacteria called Wolbachia. We are talking about endosymbiosis. It was found that the presence of this bacteria reduced the mosquito’s life expectancy and, above all, reduced viral transmission. This observation gave rise to the idea of releasing female mosquitoes (the only ones that bite) infected with this bacteria into the wild. The experiments were conclusive. While mosquitoes carrying the bacteria are still found a year later, this approach has led to a 40% reduction in cases of dengue fever in the tested areas. A convincing result because people can also become infected by moving outside the treated areas!
After these experiments carried out in Malaysia more than fifteen years ago, the application was made in a dozen countries: Australia, Brazil, Indonesia, as well as in New Caledonia, treated on a small scale. In Indonesia, these releases led to a reduction of more than 70% of dengue cases and hospitalizations. Note that this approach can also benefit the prevention of infections by the Zika or Chikungunya viruses, also transmitted by these mosquitoes. We can only hope for the very large-scale dissemination of this clever strategy!
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But recently another discovery has come to broaden this strategy. A Chinese team has identified a new bacteria, “Rosenbergiella”present among others in the intestine of certain tiger mosquitoes. This bacterium also reduces the risk of transmitting the dengue virus. It acts by inducing an acidity in the insect’s intestine that is unfavorable to the virus. Interestingly, this team showed that, in regions of Hunan (in China), where the tiger mosquito carried this bacterium in its intestinal microbiota, cases of dengue were less frequent. Correlation does not equal causation, but tests in nature have also shown that infection of mosquitoes by this bacterium made them resistant to the replication of the dengue virus.
These promising results encourage the carrying out of tests of releasing tiger mosquitoes carrying this bacteria. This is all the more interesting since it can also effectively infect the mosquito. Aegyptithe other provider of arboviruses. This is a research avenue that should enrich the possibilities of using commensal bacteria of insect vectors of viral or parasitic diseases to limit infections. A great prospect for ecological, clean and elegant intervention!
Alain Fischer is president of the Academy of Sciences and co-founder of the Institute of Genetic Diseases
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