Outbreak of digital extensor dysfunction compatible with acquired equine polyneuropathy observed for the first time in Iceland

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Background Acquired equine polyneuropathy is a neuromuscular syndrome characterized by digital extensor dysfunction, primarily affecting the pelvic limbs, with consistent, repeated knuckling. Despite being recognized as an emerging disease in Scandinavia since 1995, the aetiology remains unknown, and cases have been limited to Norway, Sweden, and Finland. Case presentation On a combined breeding and training farm in Iceland, 30 out of 145 horses (21%) presented with acute pelvic weakness, pelvic limb digital extensor dysfunction, knuckling and/or recumbency, from May to August 2019. The affected horses, aged 2–9 years, were from four out of six free-ranging groups on the farm. All affected horses had been fed a specific batch of wrapped forage for 11 days or more, while none of the 40 stabled horses fed a different wrapped forage were affected. Eleven case horses were euthanised due to severe pelvic limb weakness, and/or recumbency, yielding a case fatality rate of 37%. Histopathological examination of peripheral nerves from one case revealed severe demyelination. Conclusions This case report documents the first recognized outbreak of equine polyneuropathy in Iceland. Describing one of the largest documented outbreaks of the disease, this report provides crucial insights into the epidemiology and clinical manifestation in mainly untamed horses kept and fed outdoors.