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Polyneuritis equi is an overlooked disease Part 1

Todays challenge is showing that the uncommon disease, polyneuritis equi, PE, is a recognizable condition that veterinarians should consider as a diagnosis in horses with neuromuscular disease.  There are only a few published papers and manuscripts report the cause is unknown.  The cause is probably due to an infection that stimulates immunity.  Once turned on the horses immune reaction attacks the insulation on nerves, myelin proteins, to cause disease. There are two presentations of PE, classical and atypical.  The atypical presentation involves the (cranial) nerves, these are nerves that exit the skull above the cervical vertebrae. The classical presentation involves the sacral and coccygeal nerves leading to paralysis of the tail. rectum, and bladder with loss of sensation in the area controlled by these nerves.

A horse can have both classical signs with atypical components that include behavior changes, ataxia, proprioceptive deficits and hemi-paresis (sidewinder gait). Horses of any age, breed, or sex can be affected, ages of reported cases range from 1 to 35 years old. The amazing thing is that there is a specific marker,  antibody, anti myelin P2  antibodies, that are found in the serum of affected horses.  Identification of the marker was first published in 1981.  The ELISA test for detecting antibodies was published in 1987. We published our test in 2015 after looking a  statistically significant number of horses that presented with unusual, unresponsive disease.  These horses were “diagnosed” as equine protozoal myeloencephalitis.  These were chronic, relapsing cases often receiving multiple treatments for EPM.

Signs of polyneuritis equi are diffuse because the inflammation affects multiple nerves in the body.  A similar rare condition is cauda equina neuritis, this is a chronic untreatable disease that has an apparent predisposition to involve the extradural nerve roots of the “cauda equina”. The cauda equina nerves exit the spine after the terminal part of the spinal cord (see picture). Cauda equina neuritis (CEN) has a specific microscopic lesion, granulomatous inflammation of the peripheral nerves.  It is most likely that polyneuritis equi is a spectrum of disease that terminates in CEN.

Polyneuritis equi may be the result of the bodies response to several different organisms, perhaps virus, parasites, or even some bacteria.  Most veterinarians at referral clinics that suspect CEN look for the classic histopathology or perhaps ultrasound/radiographic evidence to the classic lesions. I spoke to 7 veterinarians at 5 university clinics and one veterinarian from Kentucky.   They don’t see or look for early PE. They are biased to look for chronic disease because end stage cases are referred to them, not the early ones.  It is logical they wouldn’t look for a rare disease, especially early in the course of the process. They overlook these cases.  It is probable that the peripheral neuropathy is reversible in polyneuritis equi, if it is detected and controlled early in the disease process.

If you think your horse has signs of polyneuritis equi discuss it with your veterinarian.  Have your veterinarian read our paper on the proposed mechanism of disease, especially the part on why the disease is overlooked.  Send a serum sample using the ELISA submission form, mark the Sidewinder MP2/MPP and CRP box.  We can discuss the most appropriate plan for the horse.

cauda equina anatomy