Amanda Mansz
Animal Health Laboratory, University of Guelph, Guelph, ON.
AHL Newsletter 2024;28(4):21.
Following the euthanasia of a 15-year-old Dutch Warmblood mare with a long history of progressively worsening loss of tail tone, urinary incontinence and passive defecation/fecal retention, the animal was submitted to the AHL for postmortem.
On gross examination, there was significant hind end muscle atrophy most visible at the base of the tail, as well as sabulous urolithiasis, most likely associated with the inability to fully express urine (Fig. 1). The sacrocaudalis dorsalis muscle and ventralis muscle of the tail were markedly atrophied and mottled pale brown/white to completely white (Fig. 1a). Coccygeal muscles and regions of the gluteal muscles were similarly affected. The urinary bladder was filled with a 15 cm x 10 cm x 7 cm foul smelling, putty-like mass of sabulous light yellow crystalline sediment (Fig. 1b).
Microscopic lesions of the nervous system were largely restricted to the sacral spinal nerves (cauda equina), and included a severe chronic granulomatous and lymphocytic polyradiculoneuritis with marked axonal degeneration and epineurial fibrosis (Figs. 2b, 2c). Additional changes in other organs included marked muscle atrophy of the tail muscles (Fig. 2a), and evidence of a chronic cystitis.
Polyneuritis equi (cauda equina neuritis) is an uncommon sporadic disease in horses that can result in chronic clinical manifestations including sensory and motor deficits involving the tail and perineum, and urinary and fecal incontinence. This syndrome is of unknown etiology, but is considered to be an autoimmune or immune-mediated disorder following a viral infection. Clinically, animals exhibit a slowly progressive peripheral neurological disorder localized to the sacrococcygeal nerves (cauda equina), and it is most often observed in females. Formerly known as cauda equina neuritis in horses, the name of the disease was updated to polyneuritis equi based on descriptions in which the involvement of spinal nerves at various levels of the spine and cranial nerves were also reported. Clinical signs attributable to cranial nerve root involvement include facial paralysis, head tilt, and wasting of the masticatory muscles. In this case, no lesions were found in nerves distant to the sacral nerves.
Figure 1. Gross lesions of polyneuritis equi. 1a. A cross section of the proximal tail comparing an affected (left) and an unaffected (right) horse. There is marked muscle atrophy and pallor of the sacrocaudalis dorsalis muscle (SCD) and ventralis (SCV) muscles of the tail in the affected horse (left). 1b. Excessive accumulation of calcium carbonate sludge within the urinary bladder is characteristic of sabulous cystitis/urolithiasis which is often associated with urinary bladder paralysis or other physical or neurologic disorders hindering complete emptying.
Figure 2. Histologic lesions of polyneuritis equi. H&E stain. 2a. Marked skeletal muscle atrophy of the SCV with small, individualized skeletal myocytes surrounded by fibrous and adipose tissues. 4x. 2b. The epineurium of sacral spinal nerve bundles is markedly expanded and fused with adjacent nerve bundles. Sacral spinal nerves are variably effaced by a cellular infiltrate. 4x. 2c. Marked infiltration and replacement of degenerate nervous tissue by lymphocytes (black arrow) and macrophages (blue arrow). Streams of expansile epineurial fibrous tissue are also present (green arrow). 20x.
References
1. Cantile C, Youssef S. Nervous system. In: Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. 6th ed. Maxie MG, ed. Elsevier, 2016;vol 1:374-375.
2. Van Galen G, et al. The composition of the inflammatory infiltrate in three cases of polyneuritis equi. Equine Vet J 2008; 40(2):185-188.