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

Overview

• This research reports the first observed outbreak of acquired equine polyneuropathy (AEP) in Iceland, describing its clinical signs, affected horse population, and potential epidemiological factors involved in the outbreak on a single farm.

Background

• Acquired equine polyneuropathy (AEP) is a neuromuscular disorder primarily marked by dysfunction of the digital extensor muscles, especially affecting the pelvic limbs of horses.
• The clinical hallmark is repeated knuckling of the affected limbs, indicating neurological impairment leading to difficulty in limb extension.
• Prior to this study, AEP had only been reported in Norway, Sweden, and Finland since its emergence in 1995.
• The cause (aetiology) of AEP remains unclear despite ongoing research.

Study Setting and Population

• The outbreak occurred at a combined breeding and training farm in Iceland.
• Within the study period (May to August 2019), 30 out of 145 horses (approximately 21%) showed clinical signs consistent with AEP.
• Affected horses ranged in age from 2 to 9 years and came from four of six free-ranging groups on the farm.
• An additional 40 horses stabled and fed a different batch of wrapped forage showed no clinical signs, suggesting a potential link with specific feed.

Clinical Signs and Outcome

• The main clinical features included:
  • Acute pelvic weakness
  • Pelvic limb digital extensor dysfunction
  • Repeated knuckling of the affected limbs
  • Recumbency (inability to stand)
• Eleven horses had to be euthanized due to severe symptoms, indicating a case fatality rate of 37% within the affected group.

Diagnostic Findings

• Histopathology was conducted on peripheral nerve tissue from one euthanized horse.
• The examination revealed severe demyelination, which is loss or damage of the myelin sheath around nerves, a hallmark of neuropathic conditions.

Implications and Significance

• This report constitutes the first documentation of AEP in Iceland, expanding the known geographic distribution of the disease.
• The high number of affected horses makes this one of the largest documented outbreaks to date.
• The findings strongly suggest a possible link between the disease and consumption of a particular batch of wrapped forage, as horses fed other forage batches did not become ill.
• The study provides valuable epidemiological and clinical insight, particularly in the context of mainly free-ranging horses kept outdoors, which resembles natural grazing conditions.
• These insights can help guide future research into the cause of AEP and preventative strategies, especially regarding feeding practices and pasture management.