02019nas a2200193   4500008004100000020001400041245011300055210006900168260000800237300001400245490000800259520143300267100001701700700001301717700001801730700001801748700001401766856004501780       2012                        eng d  a0031-182000aAnthelmintic tolerance in free-living and facultative parasitic isolates of Halicephalobus (Panagrolaimidae)0 aAnthelmintic tolerance in freeliving and facultative parasitic i  cSep  a1301-13080 v1393 aStudies on anthelmintic resistance in equine parasites do not include facultative parasites. Halicephalobus gingivalis is a free-living bacterivorous nematode and a known facultative parasite of horses with a strong indication of some form of tolerance to common anthelmintic drugs. This research presents the results of an in vitro study on the anthelmintic tolerance of several isolates of Halicephalobus to thiabendazole and ivermectin using an adaptation of the Micro-Agar Larval Development Test hereby focusing on egg hatching and larval development. Panagrellus redivivus and Panagrolaimus superbus were included as a positive control. The results generally show that the anthelmintic tolerance of Halicephalobus to both thiabendazole and ivermectin was considerably higher than that of the closely related Panagrolaimidae and, compared to other studies, than that of obligatory equine parasites. Our results further reveal a remarkable trend of increasing tolerance from fully free-living isolates towards horse-associated isolates. In vitro anthelmintic testing with free-living and facultative parasitic nematodes offers the advantage of observing drug effect on the complete life cycle as opposed to obligatory parasites that can only be followed until the third larval stage. We therefore propose Halicephalobus gingivalis as an experimental tool to deepen our understanding of the biology of anthelmintic tolerance.1 aFonderie, P.1 aBert, W.1 aHendrickx, F.1 aHouthoofd, W.1 aMoens, T.  uhttps://nematodes.myspecies.info/node/67