Prevalence of levamisole and benzimidazole resistance in oesophagostomum populations of pig-breeding farms in North Rhine-Westphalia, Germany.

The objective of this cross-sectional study was to estimate the prevalence of anthelmintic resistance in porcine strongylid populations in northern North Rhine-Westphalia. A random, weighted sample of 144 pig-breeding farms, stratified by organisation and herd size, was selected from a total of 706 farms with at least ten sows associated with three breeding organisations. On 61 of all visited farms, no or insufficient strongylid eggs were found in the faecal samples. Eighty-three farms were surveyed for resistance by a larval development test (LDT) using a cut-off concentration of 0.58 microg ml(-1) for levamisole and of 0.0113 microg ml(-1) for thiabendazole. The results indicated resistance to levamisole and benzimidazole in Oesophagostomum spp on one and two farms, respectively. Quantitative LDT showed increased LD50 values of levamisole (0.24 microg ml(-1)) and thiabendazole (0.01-0.015 microg ml(-1)) for these field populations when compared with a drug susceptible laboratory strain of O. dentatum (0.084 microg ml(-1) levamisole; 0.0093 microg ml(-1) thiabendazole). Faecal egg count reduction tests using single standard doses of levamisole and flubendazole corroborated the LDT results. Thus, the prevalence of levamisole and benzimidazole resistance was estimated to be 2-3.5% in sow herds of northern North Rhine-Westphalia. The economic importance of these data is assessed.