Seasonal patterns, climatic forcing, and cross-state synchrony of malaria incidence in Northwest Nigeria: a wavelet analysis

Malaria transmission in Nigeria's Northwest geopolitical zone, comprising Jigawa, Kaduna, Kano, Katsina, Kebbi, Sokoto, and Zamfara States, is strongly seasonal and collectively accounted for over 80 million confirmed cases between 2015 and 2024, with all seven states recording prevalence levels well above the National Malaria Elimination Programme's 2025 target. Despite this burden, the temporal structure of malaria seasonality, the lead-lag relationships between climatic drivers and malaria incidence, and the cross-state synchrony of malaria transmission across the zone have not been characterised. This gap limits the operational precision with which seasonal malaria control interventions can be timed and coordinated. Monthly confirmed uncomplicated malaria incidence rates and concurrent records of monthly temperature, rainfall, and relative humidity were analysed for all seven states covering January 2015 to December 2024. Wavelet-based models were applied to decompose these data into their constituent time-frequency components, identify dominant periodicities and their temporal evolution, quantify the climate-malaria lead-lag relationships, and assess the cross-state synchrony of annual malaria cycles across all 21 pairwise state combinations. The 12-month annual cycle was confirmed as the dominant periodicity in malaria incidence across all seven states throughout most of the study period, with Jigawa and Kebbi showing the most temporally stable annual signal and Kaduna and Zamfara showing periods of weakened power around 2019 to 2021. Secondary semi-annual signals were identified in Kano and Kaduna but were weaker and less persistent than the annual cycle. A consistent climatic ordering was identified across the zone: temperature led malaria by between three months five days in Kaduna and four months twenty days in Kebbi; rainfall led malaria by between one month seventeen days in Kaduna and two months ten days in Kebbi, with all states recording leads close to two months; and relative humidity led malaria by between twenty days in Kaduna and one month fifteen days in Kebbi. Temperature peaked before rainfall by between one month thirteen days and two months nine days, and rainfall preceded humidity by between eighteen days and one month across all states, an ordering consistent with the West African monsoon progression. Cross-state wavelet coherence analysis revealed uniformly high annual-band synchrony across all 21 state pairs (range: 0.774 to 0.988), with the strongest broadband synchrony cluster comprising Jigawa, Kano, Katsina, and Kebbi (all pairwise correlations exceeding 0.642), while Kaduna, Sokoto, and Zamfara showed weaker broadband synchrony despite their high annual-band coherence. Malaria transmission in Northwest Nigeria is governed by a consistent and biologically coherent climatic cascade in which temperature, rainfall, and humidity lead malaria incidence with state-specific but reproducible lag structures. The approximately two-month rainfall lead provides a reliable advance warning of malaria case peaks sufficient for pre-emptive seasonal malaria chemoprevention deployment and long-lasting insecticidal net distribution across the entire zone. The strong synchrony among Jigawa, Kano, Katsina, and Kebbi supports regionally coordinated intervention scheduling for this four-state cluster, while Kaduna, Sokoto, and Zamfara require state-specific calendars that account for their divergent inter-annual transmission dynamics. These findings provide an evidence base for optimising the timing and coordination of malaria control interventions across a zone that carries a large share of Nigeria's malaria burden.