Abstract: Narcolepsy is a neurological sleep disorder characterized by excessive daytime sleepiness, cataplexy (loss of muscle tone), sleep paralysis and hypnagogic hallucinations, also called the „tetrad of narcolepsy“. Although the pathogenesis is not completely understood, the disorder is well described in humans and it has been shown that a lack of the hormone hypocretin (orexin) synthesized in the hypothalamus is crucial. Narcolepsy with cataplectic attacks has also been reported in dogs, horses, cattle (STRAIN et al., 1984) and a lamb (WHITE und DE LAHUNTA, 2001).
In dogs up to 17 breeds have been shown to be affected sporadically, while familial forms occur in dobermans, labrador retrievers and dachshounds (TONOKURA et al., 2007). In horses there appear to be two syndroms (HINES, 2005), the first in which animals are affected within a few days after birth (possibly a familial form, reported in Suffolk, Shetland ponies, Fell ponies, Warmbloods, Miniature Horse foals (MAYHEW, 2011), Lipizzaner (LUDVIKOVA et al., 2012) and Icelandic horses (BATHEN‐NÖTHEN et al., 2009)) and the second in which animals are affected as adults (adult-onset narcolepsy).
It has been shown that both forms of canine narcolepsy are associated with a deficit in hypocretin/orexin neurotransmission (LIN et al., 1999). In the horse a similar etiology is suspected, but so far there are no studies to support this hypothesis.
The cataplectic attacks in humans and dogs occur during excitement or emotional stimulation such as laughing in humans or eating and playing in dogs. In contrast, the cataplectic or sleep attacks in adult horses happen almost exclusively while resting. The collapses observed in equines vary from drowsiness with hanging of the head, swaying, buckling at the knees or total collapse (see fig.1). Affected horses often show injuries and scars at the dorsal fetlocks, dorsal knees or at the face and the lips. ALEMAN et al. (2008) describe some of the suspected adult-onset narcolepsy cases as possible examples of sporadic idiopathic hypersomnia instead of true narcolepsy.