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.
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.