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Nematophagous fungi are a biological control tool used against gastrointestinal nematodes in livestock. These fungi prey on free-living larvae in faeces and could be affected by active drugs excreted post-treatment. This study aimed to determine in vitro and under environmental conditions the effect of the following anthelmintics on the fungus Duddingtonia flagrans: ivermectin, levamisole, albendazole, fenbendazole and ricobendazole. The in vitro effect of anthelmintics on fungal growth and predatory capacity was assessed in corn meal agar and coprocultures, respectively. Ivermectin (1, 2 and 10 ppm), fenbendazole (0.027, 0.054 and 1 ppm) and albendazole (1 ppm) significantly affected fungal development. The fungal efficacy against L3 was high in the control and levamisole coprocultures but decreased significantly in the presence of albendazole, fenbendazole, ricobendazole and ivermectin. The impact of levamisole on D. flagrans was further assessed under environmental conditions in autumn and winter; the fungal efficacy measured in faecal pats and the surrounding herbage was not affected by levamisole at any time. This study shows that using albendazole, fenbendazole, ricobendazole or ivermectin may compromise fungal activity, as these drugs affect the free-living stages of nematodes in faeces, but levamisole can be safely considered in parasite control strategies involving D. flagrans and anthelmintic treatments.

The purpose of using nematophagous fungi as biological control agents of gastrointestinal nematodes of livestock is to reduce the build-up of infective larvae on pasture and thus avoid clinical and subclinical disease. As the interaction of fungus-larval stages takes place in the environment, it is crucial to know how useful the fungal agents are throughout the seasons in areas where livestock graze all year-round. This study was designed to determine the predatory ability of the nematophagous fungus Duddingtonia flagrans against gastrointestinal nematodes of cattle during four experiments set up in different seasons. In each experiment, faeces containing eggs of gastrointestinal nematodes were mixed with 11,000 chlamydospores/g and deposited on pasture plots. A comparison between fungal-added faeces and control faeces without fungus were made with regard to pasture infectivity, larval presence in faecal pats, faecal cultures, faecal pat weight, and temperature inside the faecal mass. In three of the four experiments, Duddingtonia flagrans significantly reduced the population of infective larvae in cultures (68 to 97%), on herbage (80 to 100%), and inside the faecal pats (70 to 95%). The study demonstrated the possibility of counting on a biological control tool throughout most of the year in cattle regions with extensive grazing seasons.