Sheep blowfly strike imposes significant economic costs of $260 million annually from disruptions to farm management operations associated with high labour cost of controlling flystrike, sheep morbidity and mortality, and loss of wool productivity. Mulesing, the surgical removal of skin from the breech which minimises soiling and reduces susceptibility to flystrike, is performed without anaesthetic and is therefore perceived as cruel and painful as reflected by physiological and behavioural alterations in affected lambs.5, 6 Intra-dermal sodium lauryl sulfate (SLS) was trialled in this study as a potential chemical alternative to mulesing. A sound mulesing alternative would ideally provide efficient wound healing and hairless scar formation which increases the bare area of the breech while causing minimal discomfort to the animal.
In a sighting study, 3 Merino wethers were given intra-dermal injections of SLS at specific sites on the flank which were biopsied at different time points over 28 days. Assessments were made on gross and histological changes of wound healing as well as early behavioural indicators of pain. Based on the results of the sighting study, the following protocol refinements were made for the main study which used 6 Merino wethers, to:
1) reduce biopsy frequency with concentration around specimens of histopathologic interest;
2) expand lesion parameters and sharpen the grading process;
3) improve SLS delivery using a pressurized needleless applicator;
4) incorporate analysis of SLS’s mode of action at the ultrastructural level;
5) improve the approach for pain assessment.
Treated sheep showed minimal behavioural or postural indicators of pain or discomfort, their appetite was normal and there was a net gain in body weight, when compared with their untreated cohorts. Gross appearance of the treatment site consisted of initial swelling which subsided by day 14 leaving a firm, slightly raised, crust. At day 21, the treated area was depressed and covered by a scab (eschar) which partially sloughed off by day 28.
Histological examination revealed necrosis of soft tissue structures (including hair bulbs, collagen, vessels and nerves) in the subcutis and deep dermis at 2 minutes after treatment, followed by subsequent additional ischaemic necrosis of the superficial dermis and epidermis with progressive inflammation, proliferation and remodelling of the wound. Fibroplasia and angiogenesis were accompanied with reepithelialisation starting at day 7, subjacent to areas of full dermal-epidermal necrosis which subsequently formed into eschars and partially sloughed off. Wound healing with complete reepithelialisation took approximately 21 days. The reepithelialised scar tissue lacked adnexal units (hair follicles and their associated glands).
Ultrastructural examination revealed necrosis manifesting as severely damaged vascular endothelium and follicular basal lamina, which resulted in loss of structural integrity and accumulation of extracellular fluid. Connective tissue ground substance in the extracellular matrix adjacent to the degenerated basal lamina was disorganized which inferred disruption of protein fibres embedded in the mixture of proteoglycan molecules and their glycosaminoglycan units, and supported the presence of interstitial oedema. Degeneration of remaining soft tissue structures was inferred from the observed widespread histopathologic necrosis.
Histologic and ultrastructural findings confirmed the fundamental loss of structural integrity of cellular membranes of tissue components which is consistent with the mode of action of SLS which involves binding strongly to proteins of cellular membranes, resulting in protein unfolding and inactivation. Consequently, intra-dermal SLS caused rapid local necrosis followed by eschar formation and scarring of treated skin. From its effects on the destruction of sensory nerves and adnexal units, intra-dermal SLS has been shown in this study to be potentially useful as a less painful chemical alternative to mulesing. The resulting scar formation would tighten skin and increase the bare area of the breech, thus minimising faecal and urine contamination which predisposes to flystrike.