Amplification of ionophore resistance in a field strain of Eimeria tenella by treating free sporozoites with monensin in vitro.

Monensin resistance in a field strain of Eimeria tenella (FS139) was successfully amplified by an in vitro method. Sporozoites of parent FS139, designated as FS139(0), were treated with monensin at 5 micrograms ml-1 before inoculation into chickens. Oocysts developing from these drug-treated sporozoites were designated FS139(5). Sporozoites of FS139(5) were then treated with a higher level of monensin (25 micrograms ml-1), which produced another line called FS139(25). Sporozoites of FS139(25) received a monensin treatment of 100 micrograms ml-1 which yielded line FS139(100). After one passage in chickens, sporozoites of FS139(0), FS139(25) and FS139(100) were treated with either 0, 1, 25, or 100 micrograms ml-1 of monensin and inoculated into chicken primary kidney cell cultures to observe sporozoite invasion rates and development of first generation schizonts. Data on invasion rates and development of schizonts showed that FS139(100) was the most drug-resistant line while FS139(25) was the second most resistant line compared with the parent line FS139(0). These in vitro treatment techniques could be used to develop resistant coccidia for laboratory study of the physiological mechanisms of resistance.