Anthelmintic resistance in gastrointestinal nematodes of small ruminants in Southern Africa: a review and comparative analysis

Gastrointestinal nematodes (GINs) significantly limit small ruminant production in Southern Africa, causing substantial economic losses and reduced livestock productivity. Control largely depends on anthelmintic drugs, but intensive and improper use has accelerated the development of anthelmintic resistance (AHR). This review synthesizes published evidence on AHR in GINs of sheep and goats across Southern Africa, drawing on studies from six countries, and summarizes resistant parasite species, drug classes, diagnostic methods, and associated risk factors. Comparative analyses showed a significant association between anthelmintic class and the proportion of farms exhibiting resistance (P < 0.05). Macrocyclic lactone (ML) resistance, particularly to ivermectin, was most prevalent, with significantly more farms harbouring resistant GIN populations than susceptible ones (P < 0.05). In contrast, the proportions of farms showing resistance to benzimidazoles (BZ) and imidazothiazoles (IMD) did not differ significantly from those with susceptible populations, reflecting variability in resistance patterns at farm level. Salicylanilide compounds exhibited significantly lower resistance prevalence (P < 0.05). Haemonchus spp. was the dominant resistant genus, with 62.2% of farms reporting resistant populations, significantly exceeding those retaining susceptibility (P < 0.05). In contrast, lower resistance levels were observed in Teladorsagia, Trichostrongylus, Oesophagostomum, and Cooperia species. These findings indicate growing evidence of resistance to BZ, ML, and IMD compounds in GINs of small ruminants in Southern Africa. The underlying reasons are unclear but are likely driven by frequent whole-flock treatments, improper dosing, limited drug rotation, reduced refugia, cross-border livestock trade, communal grazing, and non-prescription use of anthelmintics.