Region-specific modeling of refugia and anthelmintic resistance dynamics in gastrointestinal nematodes of Nigerian small ruminants.

Abstract

Gastrointestinal nematodes (GIN) are a major constraint to small ruminant productivity in Nigeria, causing economic losses through reduced growth, morbidity, and mortality. The widespread use of broad-spectrum anthelmintics has led to the emergence of anthelmintic resistance (AR), threatening the sustainability of parasite control programmes. Refugia-based strategies, such as targeted selective treatment (TST), have been recommended to slow resistance development, but no predictive framework currently guides their implementation in Nigeria's diverse livestock systems. Region-specific mathematical models integrating differential equations and discrete-time dynamics to simulate GIN population and resistance dynamics in two Nigerian systems were developed. They are commercial ranching farms in the humid south and mobile pastoral systems in the arid north. Parameters included seasonal reproduction/mortality rates, treatment coverage, efficacy, and resistant allele emergence. Treatment regimens were simulated at early, mid, and late wet season intervals. Model validation used epidemiological data and expert inputs. Southern farms, with 90% treatment coverage, showed low refugia (5-20%) and rapid AR expansion (up to 30% annually). In contrast, northern pastoral systems maintained high refugia (60-80%) and minimal resistance growth (≤10%). Late-season treatments consistently preserved higher refugia and limited AR in both regions. Model predictions were sensitive to treatment coverage, efficacy, and initial resistance frequency. This study provides the first predictive framework for optimizing refugia-based GIN control in Nigeria. Strategic, context-specific treatment timing, particularly in the late wet season can enhance drug sustainability. These models offer practical tools for integrating parasite control into Nigeria's livestock transformation agenda.