I share it here in the dual interests of education and curiosity. If you can interpret any piece of it in a way that sheds light, let me know! If you are just deeply curious about what a highly technical pathology report looks like, wonder no more.
If you just want to read my vet's layman's explanation for this, it's what I wrote up on Friday.
The histologic findings show a severe eosinophilic dermatitis. The top differentials for this lesion are habronemiosis, eosinophilic granuloma, or cutaneous mast cell tumor. Some horses have an atypical eosinophilic response to bacterial infection and some fungal infections such as, oomycetes. No bacteria or fungi are noted. No habronema parasites are seen in the examined sections but additional deeper sections to further examine for residual parasites are pending and additional results will follow in an addendum. Cutaneous mast cell tumor in the horse can be a highly eosinophilic condition with very few mast cells present. The clusters of mast cells required for the diagnosis of cutaneous mast cell tumor are not appreciated in these biopsy sections but histochemical staining to better highlight mast cells are also pending and results will follow in an addendum. Excluding these differentials, the remaining differential is equine eosinophilic granuloma, which is a common skin lesion of uncertain etiology. A hypersensitivity response to insect bites is one speculated cause. Lesions of equine eosinophilic granuloma occur most commonly on the neck, withers, back, and girth region and are often alopecic without ulceration. Early eosinophilic granulomas generally respond to treatment with corticosteroids while chronic lesions may require surgical excision. Strict insect control may diminish recurrences in cases due to insect bites.I googled "habronemiosis" because what the heck, did a lot of digging, and came up with this slightly more readable explanation:
So...we wait. Some more.