Acquired equine polyneuropathy (AEP), also known as scandinavian knuckling syndrome, is an emerging disease affecting horses living in Sweden, Finland, and Norway. Cases of AEP were first reported in horses in 1995 and since then, have occurred mainly during the late winter to early spring (January through May). Between 1995 and 2004, there was 334 documented cases of AEP in horses, residing at 136 different farms throughout Norway and Sweden. There was reported cases every year (1995-2012) except for 1997. 88% of the affected farms were located in Norway, and 12% were in Sweden. The associated fatality rate for AEP is between 29-53%.

AEP most commonly presents in affected horses as signs of bilateral pelvic limb knuckling. While assuming the knuckling stance, weight is supported by the dorsal surface of the foot, with severely affected horses becoming paraplegic and recumbent. The severity can range from mild to severe, in which a grading system is used to better rate the stage of the condition in horses:

• Grade I: Intermittent knuckling of one or both hindlimb fetlocks when eercised or stressed that was corrected immediately.
• Grade II: Knuckling of one or both hindlimbs when exercised or stressed that remained in the abnormal position for over 3 seconds.
• Grade III:: Knuckling of both hindlimbs when stressed, unable to run, or collapse of the pelvic limbs when attempting to run.
• Grade IV: Paraplegia and recumbency

Recent research suggests the condition is caused by exposure or ingestion of a toxin, possibly from the forage.