Parasitology
Eosinophils and Sheep Scab
Dr Doug Jones and Dr John Huntley
Figure 1. Sheep Scab Mite .
Since its discovery in the late nineteenth century the eosinophil, a member of the white blood cell family, has been a focus of research and speculation regarding its biological role(s). A remarkable and unique feature of eosinophils is their intimate relationship with allergic and helminth parasite conditions. The extensive accumulation (eosinophilia) at sites of parasitic infestation has underpinned the view that eosinophils are host-protective cells responsible for killing and disposing of invading parasites.
At Moredun, recent studies on host-parasite relationships have centred on the eosinophil in the context of sheep scab. This debilitating condition, resulting from infection with the ectoparasitic mite, Psoroptes ovis (see Figure 1), is characterised by a dermal lesion composed almost entirely of eosinophils. Infesting mites appear to depend on lesion exudates for their nutrient supply, and hence their survival and propagation. Since the eosinophilic response in sheep scab confers no discernible benefit on the host, we have examined whether the mites themselves may be directly responsible for manipulating the host inflammatory response to their own advantage.
We have now accumulated compelling evidence that live mites actively secrete/excrete factors with potent chemotactic activity for ovine eosinophils in vitro. Moreover, we have recently demonstrated that, within 24 hours of topical application, live mite washings induce an intense dermal eosinophilic infiltration reminiscent of the mite-induced lesion. This is dramatically illustrated in Figure 2, which shows a section of skin after exposure to live mites in vivo for 24 hours and stained with anti-galectin 14, a highly specific eosinophil marker.
Whilst mite-derived chemoattractant(s) may not be exclusively responsible for scab formation, it is likely that they play an important early role in mite establishment. We are currently attempting to identify the factor(s) responsible and determine their importance in vivo. The possibility is raised that mite-derived eosinophil chemokines may provide novel targets for therapeutic intervention.
This research is funded by SEERAD