Control of Sarcoptes scabiei var. suis with ivermectin: influence on scratching behaviour of fattening pigs and occurrence of dermatitis at slaughter.

The behaviour of fattening pigs, the occurrence of erythematous papular dermatitis in pigs at slaughter and the effects of treatment for mange with ivermectin at the start of the fattening period were evaluated at ten farms. At each farm, trial pigs were randomly allotted to a control or a treated group. At the start of the trial, the control group was injected intramuscularly with 5 mg kg-1 levamisole, and the treated group was injected subcutaneously with 300 micrograms kg-1 ivermectin. Skin scrapings, taken from each pig before medication at the start of the trial, and at slaughter, were examined for presence of sarcoptic mites. Pig behaviour was monitored at 2 week intervals from Week 5 to Week 13 of the trial. Carcasses of trial pigs were inspected for dermatitis at slaughter. Low to moderate Sarcoptes scabiei var. suis infestations were demonstrated at the start of the trial on five farms (S+ farms). At slaughter, the mean percentage of Sarcoptes-positive pigs per pen on S+ farms was 34.8% (range 11-60%) for the control group as compared with 0.5% (range 0-2.7%) for the ivermectin-treated group (P < 0.01). No S. scabiei was recovered from any pig at any time from the five other farms (mange-free (S-) farms). The low initial levels of mange in the controls at S+ farms resulted in a consistently high scratching index. Ivermectin treatment resulted in a significantly (P < 0.01) lower prevalence of scratching, comparable with the prevalence observed at S- farms. High prevalences of generalized dermatitis at slaughter were observed in control pigs from all S+ farms. Ivermectin treatment resulted in much lower prevalences, reflected in a significantly (P < 0.01) lower grand mean dermatitis index per pen for this group as compared with the controls. The majority of pigs from both treatment groups at all S- farms were free of papular lesions. These results indicate that low levels of sarcoptic mange at the start of fattening, left untreated, will result in a high prevalence of scratching during the fattening period and high prevalences of Sarcoptes and dermatitis at slaughter. Treatment with ivermectin at the start of fattening results in behaviour and dermatitis prevalences similar to those observed in mange-free herds.