Efficacy of an anthelmintic combination in sheep infected with Fasciola hepatica resistant to albendazole and clorsulon.

In the current study, we identified five sheep flocks with fasciolosis in the province of León (northwestern Spain) in order to determine the anthelmintic resistance status to three commonly used anthelmintics, namely albendazole (ABZ), triclabendazole (TCBZ) and clorsulon (CLOR). The identification of one flock resistant to ABZ and CLOR was shown after the faecal egg count reduction test (FECRT). The reductions in eggs per gram values were -17.6% and -68% against immature and adult flukes, respectively, after ABZ treatment; 85.15% and 44.91% against immature and adult flukes, respectively, after CLOR treatment; and 97.06% against both stages, after the administration of TCBZ. As an alternative to control the infection, two combinations of ABZ and CLOR were tested. In the first, both drugs were administered at the recommended dose of each; in this case, the efficiency reached values above 95% against both immature and adult flukes. However, when the combined drugs were administered at half the recommended dose of each, the efficiency of the combination was very low, i.e. 16.67% and -11.11% against mature and immature flukes, respectively. In conclusion, this preliminary report suggests a possible interaction between ABZ and CLOR after their joint administration. However, these results should be confirmed in other flocks.