Experimentally induced monensin-resistant Eimeria tenella and membrane fluidity of sporozoites.

Two resistant lines of Eimeria tenella (H) to monensin were developed after 35 passages in chickens medicated with 100-125 ppm or 125-200 ppm monensin in the diet. Drug sensitivity of the induced lines to different level drugs were estimated with mean lesion scores (LS), mean oocyst productions (OP), percentage optimum anticoccidial activity (POAA), reduction of lesion scores (RLS), relative oocyst production (ROP), anticoccidial index (ACI) and global index (GI), respectively. Membrane fluidity of sporozoites of the sensitive line (i.e. the parent line, coded as MON-S((S))) and two resistant lines (coded as MON-R((S))-1 and MON-R((S))-2) with and without in vitro exposure to monensin were determined. Membrane fluidity of MON-R((S))-1 and MON-R((S))-2 were significantly lower than that of MON-S((S)). In vitro exposure to monensin significantly increased membrane fluidity of MON-S((S)), but had a much less effect on those of MON-R((S))-1 and MON-R((S))-2. Sporozoits of the MON-S((S))and MON-R((S))-2 with or without in vitro exposure to monensin were examined by SEM, and the sensitive sporozoites (MON-S((S))) appeared swollen and bulgy after treatment with monensin, while there was no obvious morphological deformation in the resistant sporozoites (MON-R((S))-2). The results suggest that the altered membrane fluidity in the membranes of E. tenella may be related to the decreased sensitivity to monensin.