When mosquitoes change their habits
Could changes in when and where mosquitoes bite keep malaria going in Lagos?
The background
Bed nets and indoor spraying work best when mosquitoes bite indoors, late at night. But studies show some mosquitoes are changing. They bite earlier in the evening, bite outdoors, and sometimes feed on animals instead of people.
Lagos already has low malaria. In 2021, fewer than 5% of young children tested positive. That makes it a good place to ask a hard question: could these new habits quietly keep malaria alive?
What I did
I built a model with two halves. One follows people through four stages: susceptible, exposed, infectious and recovered. The other follows mosquitoes through three: susceptible, exposed and infectious.
I added the new biting habits (early evening, outdoors and on animals) together with bed net and spraying coverage. Then I ran it over one year and three years under different scenarios.
I calculated the reproduction number, and ran a sensitivity analysis to see which factors pushed transmission up the most.
How the SEIR-SEI model is built
People and mosquitoes each move through their own set of boxes. Mosquito bites link the two.
The numbers that matter
baseline R₀: transmission is low
R with more nets and spraying: only slightly lower
sensitivity index for biting rate and feeding on animals
malaria in under-fives in Lagos, 2021
Transmission stays below 1, so malaria isn't spreading fast in Lagos. But scaling up nets and spraying barely moved the numbers.
The biggest drivers of spread were a high human biting rate and mosquitoes switching to animals. The number of mosquitoes able to carry malaria stayed high, and the number of people with no immunity kept growing.
Low numbers can hide a risk. If nets and spraying are pulled back too early, malaria could bounce back.
My advice: keep coverage up, replace worn nets on time, and add protection for early evening and outdoor bites, such as screening homes.
Slides and figures
These come straight from my presentations and model outputs. Tap any one to see it bigger.
