Emerging Equine Neurologic Syndromes:What Nature Has Taught Us Recently About Neurophysiology and Neuroanatomy

Over the last 5 years or so we have learnt much about existing and new disorders in horses from documentation of careful clinical observations and interventions and from painstaking pathologic studies with especial emphasis on clinicopathologic correlates. Some of these disorders have been discussed elsewhere in this conference and this paper will highlight a few of these disorders through which we have added to our understanding of anatomy, physiology and clinicopathologic correlates – the buildingblocks for advancing equine neurology. 

Acquired cervical torticollis associated with Parelaphostrongylus tenuis

Several Metastrongyloid nematodes are important causes of neurologic disease in wild and domestic ruminants in Europe, Asia, New Zealand and North America and likely occur wherever native Eurasian and American cervid populations exist worldwide and where suitable terrestrial molluscs can act as intermediate hosts. There are 2 genera of these so-called extrapulmonary lungworms affecting cervids - Elaphostrongylus and Parelaphostrongylus - that pass through the host CNS as part of their life cycle. Although they can cause neurologic disease in their primary hosts they tend to produce prominent neurologic infections, usually non-patent, in abnormal and aberrant hosts. 

One of the more common of these neurologic diseases involves Parelaphostrongylus tenuis that produces traumatic and inflammatory myeloencephalomeningitis in domestic ruminants, and recently in horses, showing syndromes relating to asymmetric spinal cord and brain stem involvement. The amazingly selective meanderings of these migratory nematode parasites are exemplified by the plight of Parelaphostrongylus tenuis in some horses that have been elegantly documented. This worm has caused an acute onset of a lateral deviation of the cervical vertebrae, or scoliosis, which remained static, and was the result of an exquisitely selective, continuous, cervicothoracic, inflammatory dorsal grey column lesion. Cutaneous hypalgesia to analgesia of the neck and/or thorax and mild unilateral ataxia and paresis but with no evidence of denervation atrophy in the epaxial muscles was observed in all affected horses. The scoliosis was clearly argued to be due to loss of afferent cervical proprioceptive inputs because of the dorsal gray column lesions with some white matter involvement accounting for ataxia and weakness [Figure 1]. [...]