Acquired Equine Polyneuropathy (Scandinavian Knuckling Syndrome)
Robert J. MacKay
Since 1995, a disease characterized by bilateral knuckling of the metatarsophalangeal joints has been observed in more than 400 horses in Sweden, Norway, and Finland.
Originally known as Scandinavian knuckling syndrome, the condition was renamed acquired equine polyneuropathy.1'1 Adult horses of any age, all breeds, and both sexes are affected. Large-fiber polyneuropathy with inflammatory demyelination-remyelination and conspicuous Schwann cell inclusions has been reported in affected horses.2,3 The cause of the disease is unknown, although an ingested neurotoxin is considered the probable cause, possibly acting to trigger autoimmune demyelinating neuropathy.3 Clinical signs appear mainly in late winter to spring, and almost all affected horses have been fed wrapped forage (haylage/ silage). In Sweden and Norway, the disease has clustered within farms, and all cases on a particular farm occurred within 100 days of the index case on that farm.1 The clinical signs are bilateral pelvic limb hyperflexion and collapse of the limb and knuckling of the metatarsophalangeal joints, so that the dorsum of the hooves contact the ground. Surprisingly, there is no or only slight atrophy of digital extensor muscles, and neurogenic atrophy is not found on histologic study.3 It has been suggested that the affected nerves are type 1a and 1b sensory nerves from muscle spindles and that clinical signs of acquired equine polyneuropathy may be related to impaired reflex modulation of α motor neurons to the digital extensors of the pelvic limbs.3 Mild and inconsistent thoracic limb weakness has been described in a few cases. The disease apparently has no cutaneous sensory or proprioceptive component, and cranial nerve abnormalities are not found clinically. In a longitudinal study of Norwegian horses with acquired equine polyneuropathy, 35% were euthanized within 6 months of diagnosis because of severe or persistent signs.4 Of 57 surviving horses with long-term followup, all but 3 returned to a previous or higher level of performance. Those 3 horses experienced a relapse or new attack during the follow-up period. Possible disease-related clinical signs (intermittent pelvic limb weakness, fetlock knuckling) were reported in 14 of the surviving 57 affected horses. An apparently similar syndrome, termed idiopathic peripheral polyneuropathy, has been reported in horses in Japan5; however, this syndrome severely affects all limbs, and limb muscles show marked neurogenic atrophy.■
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