Efficacy of <i>Beauveria bassiana</i> fungal formulations against immature stages of gastrointestinal nematodes from horses

Strongylids are among the most prevalent helminths affecting equines and are responsible for significant health and economic losses. This study evaluated the efficacy of aqueous suspensions and oil-based formulations of the entomopathogenic fungus Beauveria bassiana against equine gastrointestinal nematodes under laboratory and semi-natural conditions during the rainy and dry seasons in the Recôncavo region of Bahia, Brazil. Faecal samples were collected from naturally infected equines. Four treatment groups were established: water control, oil control, aqueous suspension of B. bassiana, and oil-based formulation of B. bassiana. Semi-natural assays were conducted using pots containing Urochloa brizantha MG-5, while in vitro assays consisted of quantitative stool cultures. The fungal product used was Boveril® WP (B. bassiana), and nematode larvae were recovered and quantified using the Baermann technique. Under laboratory conditions, efficacy reached 80.4% for the aqueous suspension and 92% for the oil-based formulation. In semi-natural assays, the aqueous suspension showed low efficacy during both the rainy 10.51% and dry 14.28% seasons. In contrast, the oil-based formulation achieved high efficacy during the rainy season 92.59% and moderate efficacy during the dry season 46.9%. Soil analysis confirmed fungal persistence under both climatic conditions, with B. bassiana remaining detectable up to 52 days after application. The most frequently identified larvae were Strongyloides westeri, cyathostomins, Strongylus equinus, and Strongylus vulgaris. These results demonstrate that oil-based formulations of B. bassiana are effective against immature stages of equine gastrointestinal nematodes, particularly under favourable environmental conditions.