Getting under the skin of mosquito control

The real hurdle standing in the way of reducing mosquito-borne diseases is not technology – it’s the lack of persistence from governments, Duncan R Smith writes.

As a result of my work in the field of mosquito-transmitted diseases, I was recently invited to take part in a survey organised by the Brazilian health research institution Fiocruz on the future of arboviral diseases (which are caused by arboviruses).

The term arbovirus denotes a virus spread by an arthropod. There are over 500 known arboviruses, many of which are serious public health concerns, and they are spread predominantly by mosquitoes, ticks, and sandflies.

The mosquito-transmitted arboviruses are distributed over much of the world’s tropical and semi-tropical regions, where they exert a significant toll on the human population. Well-known mosquito transmitted arboviruses include dengue virus, Zika virus, chikungunya virus, and yellow fever virus.

The second question in the survey was along the lines of whether I thought arboviral diseases would still be a significant problem in the next two decades. I am not sure if it was the realist or the pessimist in me that selected the option “likely”.

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Later in the short survey were questions based around the possibility of controlling some arboviruses through the release of modified insects. For mosquitoes, there are a number of potential approaches.

One involves releasing large numbers of sterile male mosquitoes who compete with non-sterile males in breeding, thus reducing the number of successful mating encounters and thereby reducing the mosquito population. This approach has been successfully used for other insect species such as fruit flies and tsetse flies.

Another promising method is to release male mosquitoes that are infected with a particular type of bacteria – Wolbachia – that renders them reproductively incompatible with wild female mosquitoes.

Other methods currently in development involve genetic manipulation of male mosquitoes so they pass on a gene that will kill any offspring after a mating with a wild female mosquito. Pilot projects using this approach have been undertaken in a number of countries with apparently good results in reducing mosquito numbers.

It is certain that the application of any of these technologies will be able to have a large and immediate impact on local mosquito population species. But that has never really been the problem.

Over 130 years ago, the French started construction on the Panama Canal. Progress was slow due to engineering difficulties, the large numbers of death, and hospitalisation of up to 85 per cent of the workers due to yellow fever and malaria. Malaria is a plasmodium parasite rather than an arbovirus, but it is also transmitted by mosquitoes.

Work was effectively stalled on the construction of the canal until the early 1900s, when the United States took over the construction and William Crawford Gorgas as chief sanitary officer implemented strict vector control measures in the Panama Canal Zone.

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These procedures included inspecting homes for mosquito larvae and permanently destroying breeding sites if found, fumigating homes, draining swamps and wetlands, replacing rainwater collection with a domestic water system, using oil to lay on top of stagnant water, and quarantining patients with yellow fever (or malaria) to prevent infected patients being bitten by mosquitoes to continue the cycle of infections.

While some of the methods would be considered not particularly eco-friendly today, all of the methods were simple, practicable and effective. Within two years yellow fever and malaria were almost completely eradicated from the areas in which these controls were placed, and the Panama Canal was finished some ten years later. And all this occurred more than 100 years ago.

We have any number of low-technology ways to reduce mosquitoes and the diseases they spread. Simple mosquito proof meshes on doors and windows have been shown to lower rates of vector-borne disease transmission. Destruction of breeding sites in and around the home, fogging during outbreaks – all of these have an impact on disease incidence.

The problem, however, is the lack of sustained application of these methods. On national levels, governments lose interest in vector control when disease rates start to decline, and people get slack in their vector control measures when there are no active disease cases around them. These allow the mosquito vectors to spring back with a vengeance.

The current high-technology methods are not likely to eliminate mosquitoes completely, which means their sustained application will be required for the foreseeable future. The question again will be whether governments will keep their interest in continually applying this technology, even when disease rates have dropped dramatically.

The lesson of the Panama Canal is that it does not take high technology to control mosquito vectors. Instead, political will and sustained application are what’s needed to have real policy bite.