Objectives: To determine reduction of Ascaris suum egg shedding and ovicidal effects in naturally infected commercial female breeding swine treated with fenbendazole.

Materials and methods: Five shedding and three embryonation experiments across three commercial sow farms were conducted. Ascaris suum-infected sows were allocated to four treatments: untreated controls; 545.5 mg fenbendazole, 1 day (Treatment 1); 545.5 mg fenbendazole, 3 consecutive days (1636.5 mg total) (Treatment 2); and 1636.5 mg fenbendazole, 1 day (Treatment 3). Fecal samples were collected and evaluated by a standard flotation method (shedding study) or eggs were isolated and incubated (embryonation study) to determine embryonation rates. Groups were compared for time-to-negative (Kaplan-Meier survival analysis); percent negative (chi-square analysis); environmental burden (analysis of variance); and embryonation rates (analysis of variance).

Results: Time-to-negative ranges were 9.3-13.1, 8.9-13.1, and 9.8 days post treatment (DPT) for treatments 1, 2, and 3, respectively; control ranges were 13.4-28.2 DPT. Treatment sows were 90%-100% negative, compared to 0.0%-28.6% of controls. Environmental burden ranges were 7.0%-60.9%, 13.9%-60.8%, and 29.3% (treatments 1, 2, and 3, respectively) and 60.4%-219.0% for controls. All treatment values differed from controls (P < .05), but not from each other. Embryonation rates were lower for treatments than controls at 6 and 8 DPT (P < .001).

Implications: Fenbendazole at various dosages is effective against A suum infections in sows. Treatment should begin 14 days prior to movement into clean farrowing facilities. Under the conditions of this study, fenbendazole demonstrates ovicidal activity against A suum at 4-8 DPT.