Psoroptes ovis mange in sheep: Organ and tissue weights as indicators of the impact on organismal metabolism, lymphatic system and body composition

Although adverse effects on the welfare, skin pathology and host immune responses of ovine psoroptic mange are well described, there is only limited research on the impact on the performance of the animals and underlying pathogenicity. To provide objective data on effects of the disease at organismal level, weights of organs and tissues of sheep included in a parasiticide study with groups of mange-infested, mange-infested and treated and non-infested animals were evaluated. For the study, 48 young Merino sheep were formed randomly into three equal-sized groups on sex and body weight - to develop mange without intervention (ʽinfestedʼ), to develop mange and treated 10 weeks after initial infestation (ʽinfested-treatedʼ) or to be prevented from mange infestation – and infested twice with Psoroptes ovis mites. Mange developed in all ʽinfestedʼ and ʽinfested-treatedʼ animals and was maintained by the ʽinfestedʼ animals throughout the study while the ʽinfested-treatedʼ animals became mite-negative four weeks after treatment. No mites established on the pre-infestation-treated sheep which served as ʽnon-infestedʼ controls for the analyses. At slaughter 16 weeks after the initial mite infestation, the ʽinfestedʼ animals had significantly (p < 0.05) lower relative (organ/tissue weight divided by live weight) omental fat, perirenal fat and thymus weights and higher relative liver, adrenal glands, prescapular and precrural lymph nodes, gastrointestinal tract, metacarpus bone and skin weights than the ʽnon-infestedʼ animals. They also had significantly lower relative thymus and gastrointestinal tracts weights and higher relative adrenal glands, prescapular lymph nodes, precrural lymph nodes and skin weights than the ʽinfested-treatedʼ animals. The organ/tissue weights of the ʽinfested-treatedʼ animals indicated that many changes observed in the ʽinfestedʼ animals improved substantially within six weeks of treatment. Analysis of the organ weights demonstrates the impact of ovine psoroptic mange on protein and energy metabolism (lower adipose tissues and higher liver and adrenal glands weights as measures of increased demand for maintenance or chronic stress of the mite infestation) and defense mechanisms (lower thymus weight reflecting suppression from increased adrenal activity and higher weight of the peripheral lymph nodes as local reaction to the continuous exposure of antigens originating from the mite-induced exudative dermatitis). In addition, higher gastrointestinal tract plus skin weights and higher metacarpus bone weight leading to lower dressing percentage and higher carcass bone ratio, respectively, reflect the change in body composition and tissue disposition resulting in overall reduction in market value.