Modelling the Effect of Indoor Residual Spraying on Malaria Transmission in Nassarawa State, North-central Nigeria.

Background: Indoor Residual Spraying (IRS) is one of the two major strategies recommended by the World Health Organization for malaria vector control. Between 2012 and 2013, IRS was piloted in two Local Government Areas (LGAs) of Nassarawa State, North-central Nigeria. Uncertainties remain as to whether the intervention led to a decrease in the rate of malaria transmission or not.

Methodology: A simple SIRS model was used to generate a system of ordinary differential equations. The solutions of the model, obtained through Euler's method, were adapted to malaria surveillance data obtained from one of the interventions LGAs to estimate model parameters. The rate of malaria transmission, obtained from the intervention LGA, was compared with the one obtained from a carefully selected control LGA to ascertain the effect of IRS on malaria transmission, assuming other model parameters remained constant.

Results: The results showed a good fit of surveillance data to the numerical solutions of the model. The estimated rate of malaria transmission in the intervention LGA was lower than the rate estimated in the non-intervention LGA, even though the difference was marginal (0.95 versus 1.05). Over two years, IRS activities reduced the rate of malaria transmission in the intervention LGA by 10%. The modest decrease was attributed to the way IRS was implemented and the uncertainties associated with using routine surveillance data in Nigeria.

Conclusion: Future IRS interventions should consider the effect of spray frequency on disease transmission and adopt a robust data collection strategy that will support proper monitoring and evaluation.