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Catching the mosquitoes that spread lymphatic filariasis

Dr Rogers Nditanchou, September 2019

Lymphatic filariasis (LF), a neglected tropical disease, is spread by mosquito bite and causes painful disability and disfigurement. But our research into mosquitoes is helping us to tackle it.

Lymphatic filariasis is usually seen as a rural disease but, as we move towards eliminating it, it’s important to check that the disease is not being spread in urban areas as people migrate from rural villages to towns and cities.

Finding out whether the disease is being transmitted in urban areas can be difficult, as current methods are based on rural areas and involve checking people to see if they have the disease. In cities, where there is increased migration, understanding transmission and prevalence is harder. The parasitic worms that cause LF can live in humans for years, so if people in urban areas test positive it is not necessarily an indication that LF transmission is happening in that area.

One potential course of action is to find out whether mosquitoes, rather than people, are carrying the parasite. Mosquitoes have short lifespans (less than three months) and breed locally, tending to remain in the area where they hatch. This means that if urban mosquitoes have the disease, people are potentially at risk.

Testing mosquito traps

Between May and September 2018 we conducted a research study to determine if LF is being transmitted in Kaduna and Minna, two cities in Nigeria, to understand if treatment is needed for people at risk. We tested various methods for trapping the two main types of mosquitoes known for spreading LF: Culex and Anopheles.

These two types of mosquitoes are very different. Culex mosquitoes are common and easy to catch in urban areas, but they are poor at transmitting LF, meaning that we needed to catch a lot of them to establish whether transmission is happening. Conversely, Anopheles are good transmitters, so we needed fewer of them to establish whether they were spreading LF. However, they are much rarer in urban environments and much harder to catch.

We used our transmission study to trial various ways of catching the mosquitoes, trying to find the best and most cost-effective combination. In both cities, we identified three communities where LF had previously been recorded, where poor sanitation and building structures meant mosquitoes were common. In each community, we set three different types of traps.

A close-up of a researcher's hands as they write data onto a sheet of paper.

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The traps we use to collect mosquitoes

A man empties a mosquito exit trap

Exit traps

These are nets on a cubic frame attached to indoor window ledges, which catch mosquitoes as they try to escape. They take advantage of night biting patterns and are most effective in catching Anopheles mosquitoes.

A mosquito PSC trap

PSCs

Pyrethrum spray catches (PSCs) target mosquitoes after they have bitten. Mosquitoes rest after biting, so by covering everything in white sheets and spraying an insecticide, the mosquitoes fall on the sheet and can be collected.

A mosquito gravid trap

Gravid traps

These traps are laid in outdoor areas where mosquitoes breed. They are designed to mimic a good breeding site for Culex mosquitoes, and trap pregnant (gravid) mosquitoes as they come in to lay their eggs.

We recorded the number and type of mosquitoes in each trap, collected daily. This meant we were able to calculate the cost of each mosquito collected to determine the most cost-effective way to trap them.

Our study shows that, in urban contexts, using a combination of gravid and exit traps is the optimum method for catching Culex and Anopheles mosquitoes in the most cost-effective way.

The findings from this phase are already being used to improve a similar exercise in Monrovia, Liberia, and will help the country as it moves towards eliminating LF. I will also be presenting these results at the 11th European Congress on Tropical Medicine and International Health (ECTMIH 2019) on 17 September 2019.

The importance of community researchers

Recruiting local people to work as community researchers is crucial for research like this to succeed. For the study to be successful, we needed people to give us regular access to their homes during unsociable hours. Our team of community researchers proved invaluable: recruiting people who were well known and trusted helped households to understand what we were doing and why, meaning people were more likely to agree to take part.

Our community recruits were also invaluable field researchers, which we had discovered during a similar research project in Ghana. After basic training, the researchers were able to help with all aspects of mosquito collection, laying traps then safely emptying them and transferring the caught mosquitoes to our field entomologists for lab processing. Using a mix of local women and men was also important: in some instances it was more acceptable for female researchers to enter people’s homes.

Their days involved early starts, often beginning around 6am to empty the gravid traps, before conducting an early morning visit to people’s homes, emptying exit traps and spraying the insecticides for the PSCs. Around sunset they would go around setting gravid traps, then do it all again the next day.

Unexpected benefits

During their rounds the community researchers chatted to people from the households taking part, answering questions or listening to their concerns. I lost track of the number of times they resolved issues or smoothed over misunderstandings about why some houses were taking part and others weren’t. Without them, our research would have faced many more, potentially insurmountable, obstacles.

As well as being paid for their time, the researchers were also involved in the study in other ways. Umar, a young community researcher I met in Kaduna who also worked as a florist, had taught his friends, family and colleagues how best to protect themselves from mosquito bites so they could avoid mosquito borne-diseases.

Results in action

Our findings on whether the mosquitoes show if LF is being transmitted in these urban areas will be released early next year. What we do know from the data we already have is the optimum methods for catching the mosquitoes that spread the disease.

I hope these findings will help others working to determine whether LF is present in urban communities. This will ensure treatment is more effective and can be redirected to where it is needed most, meaning more people can be freed from the risk of this debilitating disease.

Hassana from Nigeria sits in her home. SHe has lymphatic filariasis and her left leg is visibly swollen.

What is lymphatic filariasis?

This disease is transmitted via mosquito bite, usually during childhood, and affects the lymphatic system in later life, causing painful disability and disfigurement.

About the disease

Author


A head and shoulders picture of Dr Rogers NditanchouDr Rogers Nditanchou

Dr Rogers is a regional research advisor for NTDs, based in Sightsavers’ Cameroon country office. He will present this research at the ECTMIH on 17 September 2019.
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