Effect of Ascaris suum infection on performance of fattening pigs.

Scientific investigations of production losses in pigs due to roundworms are rarely conducted in commercial farms, despite the fact that this information is needed to decide whether anthelmintic treatment is cost-effective or not. Therefore, the aim of our study was to compare performance in fattening pigs treated or not treated for Ascaris suum infections. Two Danish pig farms producing fatteners and showing A. suum-induced liver white spot prevalences of 10-33% were selected for the study. In each farm, pens with fattening pigs were randomly assigned to either treatment with an anthelmintic (Flubenol, Janssen Animal Health), or a placebo. Pigs were treated by administering anthelmintic or placebo mixed in the feed for five consecutive days (5mg/kg body weight) on day 2-6 and day 36-40 after introduction to the finishing unit. Fecal egg excretion before first shipment, liver lesion scores (white spots), lean meat percentage at slaughter, average daily gain, and feed conversion were recorded weekly per pen and evaluated for the entire fattening period (30-100kg). A. suum egg excretion was detected in none of the 57 pens where pigs were treated with anthelmintic compared to 18.2% of 57 pens in the placebo group. Pen floor fecal sampling underestimated the prevalence of A. suum in the fattening units compared to individual rectal sampling; only 22% of pens where A. suum was present were diagnosed correctly by both sampling methods. The prevalence of white spots did not differ significantly between pigs treated with anthelmintic (26.7%) and pigs treated with placebo (23.7%), but considerable variation between batches and over time was observed. Liver lesion scores (none, few, moderate, many) were not significantly influenced by de-worming twice during the fattening period. There were no significant differences in average daily gain, feed conversion and lean meat percentage between pigs treated with anthelmintic or placebo. This lack of effect may be explained in part by a rather low infection intensity, as measured by fecal egg counts, but in contrast, white spot numbers were moderate to high. A possible negative influence of other disease, mainly diarrhea due to Lawsonia intracellularis, on performance could not be excluded. Treatment with flubendazole twice during fattening prevented A. suum egg excretion but did not reduce the occurrence of liver white spots. To improve performance significantly, repeated treatments over several consecutive rounds might be necessary.