Resistance of Oesophagostomum spp. in pigs to pyrantel citrate.

This study was undertaken to determine whether anthelmintic-resistant Oesophagostomum spp. populations occur in Danish swine herds. A controlled field trial on selected sow herds suggested resistance to pyrantel citrate in a herd where faecal egg count depression in response to treatment was only 28.3%. This was confirmed by subsequent experimental infection of pigs where the suspected resistant Oesophagostomum isolate was compared with a susceptible worm isolate. After treatment with the recommended dose of the drug, worm burdens of the suspected isolate were only reduced by 42.6% (p greater than 0.05) in contrast to the susceptible isolate, which was reduced by 94.6% (p less than 0.01). Differential counts of the resistant isolate suggested that O. dentatum might be more resistant than O. quadrispinulatum. The resistant isolate originated from a herd, where the anthelmintic had been administered four times a year over a period of 7 years. The potential for development of anthelmintic resistance in Oesophagostomum spp. under management practices in our country is discussed in the light of the current views on predisposing factors.