On the aggregated nature of chronic Sarcoptes scabiei infection in adult pigs

The prevalence and body distribution of Sarcoptes scabiei and associated dermatitis was investigated in sows and boars from four herds with long standing mange. Macroscopic hyperkeratotic dermatitis (crusted mange) was present in 1-6% of herd sows. Mite estimated prevalence (95% CI) in ear scrapings was 11% (6-17%) including 100% (13/13) and 2% (3/134) in sows with and without crusted mange, respectively, and the later had very few mites compared to the former. S. scabiei body distribution and dermatitis were further investigated in 59-64 skin scrapings/sow taken post-mortem from four culled sows including two (sows 1 and 2) with and two (sows 3 and 4) without crusted mange. The proportion of skin samples with eggs, instars or adults was 59% in sow 1, 84% in sow 2, 0% in sow 3 and 3% in sow 4. S. scabiei distribution in sows 1 and 2 ranged from being present in all skin ear and head samples to absent in those from the inner side of the limbs and mammary glands. Crusted lesions were observed in the skin of the ears, neck and lower limbs and contained the largest mite populations. Histopathological analysis of skin samples identified mites, inflammatory cellular infiltrate (mainly lymphocytes, neutrophils and eosinophils) and hyperkeratosis, acanthosis and spongiosis in 78%, 54%, 20% and 25% of samples from sows 1, 2, 3 and 4, respectively, being lesion severity positively associated to mite presence. The study provides further evidence that in herds with long-standing exposure to S. scabiei, infection becomes highly overdispersed with large mite populations present only in a few pigs and in specific body areas. Although the reasons for mite aggregation have not been identified, it is important controlwise because treating or eliminating a few and easy to identify heavily infected adult pigs, should markedly decrease the herd's parasite load and reduce the use of acaridal drugs.