Abstract
Two ruminant acute phase proteins (APPs), haptoglobin (Hp) and serum amyloid A (SAA), were evaluated as serum biomarkers (BMs) for sheep scab–a highly contagious ectoparasitic disease caused by the mite Psoroptes ovis, which is a major welfare and production threat worldwide. The levels of both APPs increased in serum following experimental infestation of sheep with P. ovis, becoming statistically significantly elevated from pre-infestation levels at 4 weeks post-infestation. Following successful treatment of infested sheep with an endectocide, Hp and SAA serum levels declined rapidly, with half lives of less than 3 days. In contrast, serum IgG levels which specifically bound the P. ovis-derived diagnostic antigen Pso o 2 had a half-life of 56 days. Taking into account pre-infestation serum levels, rapidity of response to infestation and test sensitivity at the estimated optimum cut-off values, SAA was the more discriminatory marker. These studies illustrated the potential of SAA and Hp to indicate current sheep scab infestation status and to augment the existing Pso o 2 serological assay to give disease-specific indications of both infestation and successful treatment.
Highlights
Sheep scab, caused by the infestation of sheep skin with the highly contagious mite Psoroptes ovis, affects the productivity and welfare of sheep as it produces intensely pruritic lesions and wool loss
The qualitative results for serum amyloid A (SAA) in the lesion resolving sheep (Figure 4a) where the band densities increased decreased around 2 weeks before the lesion size changes, suggested that SAA levels in the serum of infested sheep may increase before obvious clinical signs were apparent, which would be an important factor in the pre-clinical diagnosis of sheep scab
These levels were similar to those of Hp and SAA measured in Alpine ibex (Capra ibex) infested with clinical sarcoptic mange (Sarcoptes scabiei infestation) where mean Hp levels during naturally-acquired infestation increased to 3.72 ± 0.65 mg/mL compared with healthy, uninfested controls (0.58 ± 0.09 mg/mL) and SAA levels were 130.7 ± 0.16 μg/mL and 8.7 ± 0.13 μg/mL in infested and uninfested animals respectively [25]
Summary
Sheep scab, caused by the infestation of sheep skin with the highly contagious mite Psoroptes ovis, affects the productivity and welfare of sheep as it produces intensely pruritic lesions and wool loss These factors, along with the high cost of treatment, mean this disease has significant economic implications for the sheep industries of affected countries [1]. Accurate diagnosis can be challenging - clinical signs in P. ovis infested sheep may be minimal during early infestation (up to several months in the field) and missed but these animals represent a source of infestation [5] In such a Several immunoassays have been developed to detect serum antibodies with specific reactivity to P. ovis antigens from infested sheep [9,10,11]. An improved diagnostic test for sheep scab should combine the pathogen specificity and early indication of infestation (as provided by the Pso o 2 ELISA [12]) with a biomarker (BM) element to differentiate between successfully-treated animals and those with active disease
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