The effect of ivermectin on the dynamic of egg shedding of gastrointestinal strongyles was studied in post-parturient ewes (n=12) from traditionally managed farm located in a north eastern province of Algeria. Fecal samples were collected during the postpartum period to monitor the worm egg count (WEC) before and 3, 22 and 34 days after subcutaneous administration of 0.2 mg/kg of ivermectin. Furthermore, the pretreatment level of ewe’s fat reserve was assessed by the scoring of body condition (BCS) and weighing of body weight (BW) to investigate its effect on ivermectin efficacy. The injection of ivermectin to low infected ewes caused a remove of egg shedding (0 eggs) and a perfect efficacy (100%) in some ewes during the first month post-treatment, mainly on day 3 corresponding to a theoretical time of ivermectin maximal plasmatic concentration. However, on day 34, ewes had markedly shed worm eggs in feces (P<0.05) expressing the diminution of ivermectin efficacy about 87%. This resumption of eggs excretion was significantly and closely correlated to pre-treatment BCS (r=-0.91, p=0.01). The present study has revealed a possible presence of strongyle resistant against ivermectin during the first week post-treatment from low infected ewes which displaying a postpartum energy deficit (poor body fat reserve), and the necessity to take in account a probable inconspicuous development of this resistance following the arbitrary use of this avermectin in traditionally managed flocks.
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