Do not blame the mosquito


Vector-borne diseases are complex and it doesn’t help to address just one aspect of the equation

Head (back view) of Aedes vittatus. Photo: Walter Reed Biosytematics Unit

A mosquito was caught at the United States Naval Base in Guantanamo Bay, Cuba. This mosquito, Aedes vittatus, was first spotted in June 2019. Since then, more mosquitos have been caught around the base and one was even spotted in a bird bath in a residential area.

The mosquito was isolated for the first time in the United States. While genetic analysis suggests that this particular mosquito originated in the Indian subcontinent (as explained in an article published in the January 2021 issue of Acta Tropica magazine), the vector is found in tropical regions of Asia, Africa and the Mediterranean in Europe widespread. In fact, it was first isolated and described from Corsica in Europe in 1861.

The US authorities are concerned about this vector because it has the potential to transmit pathogens that cause dengue, yellow fever, zika, and chikungunya. As a result, they have taken steps to control the population of this mosquito.

But is finding a vector in a new area a cause for concern?

Not really, since the very presence of the vector does not lead to disease. The mosquito has not yet spread these diseases – neither in the United States nor in other parts of the world. For example, researchers at the Center for Research in Medical Entomology, Madurai, were unable to isolate the dengue virus from this species of mosquito, even in dengue endemic villages in Tamil Nadu.

This benign behavior has also been observed with other vectors and disease combinations. While India has Aedes aegypti, which is one of the main vectors for yellow fever, we don’t have the disease.

It is easier to target the macroscopic mosquito than the microscopic virus, however, and a large part of our disease control strategy has focused on controlling the vector. In addition to insecticides, genetically modified mosquitoes and biological control agents are used for this purpose.

These methods have proven to be mostly ineffective in fighting the disease. Instead, they harm the environment and are also expensive.

Perhaps it is time to focus our energies on the other components of this equation.

For one, we need to make sure that vectors travel around the world. Mosquito eggs piggyback on inanimate objects such as containers and tires. Using existing international conventions such as the Convention on the Application of Sanitary and Phytosanitary Measures and the International Convention for the Control and Management of Ballast Water and Sediments from Ships would help.

We also need to make sure that sick people don’t travel around. The novel coronavirus disease (COVID-19) pandemic has shown that it is possible to test people when they arrive in a new location. It would help simply to follow the International Health Regulations of 2005 recommended by the World Health Organization.

A clean environment would also help many vectors thrive in degraded areas. For example, A vittatus is a rock hole breeder – it lays eggs in small, natural bodies of water – and such rock holes are commonly found in natural environments in forests, near springs and lakes.

The degradation of the environment from mining has increased the number of places where water can collect. The urbanization process has also offered the mosquito alternative breeding grounds in the form of plastic waste such as cups and containers.

Managing these three factors is not that simple as our knowledge of the vectors, the pathogens and the environment is very poor.

For example, we do not have clear estimates of the microbes with pathogenic potential. One estimate assumes that only the Indian flying fox (Pteropus giganteus or the larger Indian fruit bat) harbors 58 different viruses.

If this is extrapolated to estimate the number of viruses in the 5,486 known mammals, we would have to deal with roughly 320,000 viruses in case they decide to cross the species barrier and infect humans. Protecting the environment is even more difficult as economic interests stand in the way.

While vector control seems the simplest, it has been shown to be largely ineffective. We need to get it right quickly, however, as with climate change, the environment encourages the growth of vectors that thrive in warm, humid conditions.

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