History

A sexually intact male horse was adopted from the US Department of Interior Wild Horse population when it was approximately 1 year old. In November 1998, the horse was castrated and a tetanus toxoid injection was administered. In December 2007, the horse was evaluated because of a hoof abscess and was given a tetanus toxoid booster injection. The horse did not receive any other vaccinations and lived in a pasture by itself. On April 1, 2009, the 12-year-old gelding was found laterally recumbent in its pasture. An employee at the location where the horse was being housed reported that the horse was running and eating apparently normally during the preceding day. Another observer reported that the horse appeared to have had pelvic limb lameness of several days' duration.

Clinical and Gross Findings

Upon physical examination by the veterinarian, the horse's rectal temperature was 39.5°C (103.1°F) and auscultation of the heart and lungs revealed no abnormalities. The veterinarian reported that the horse had paresis of its pelvic limbs, a flaccid tail, and a large urinary bladder and that the rectum contained a large amount of feces. The horse had no signs of cranial nerve deficits, and mentation was reported to be normal. The veterinarian treated the horse with dexamethasone (30 mg/kg [13.6 mg/lb], IV), but the horse did not respond. Because of a poor prognosis related to the clinical findings and the lack of response to treatment, the horse was euthanatized 2 days after being found in lateral recumbency. A necropsy was performed at the Athens Diagnostic Laboratory.

On gross examination, the carcass had minimal autolysis and was in overall good postmortem condition; the horse's body condition score was 3 of 5. The mucous membranes were moderately congested and reddened. The scleras of both eyes were icteric. The right lung was diffusely congested, and the left lung had emphysematous areas along the cranioventral border of both lobes. Approximately 30 bot fly larvae (Gastrophilus intestinalis) were attached to the nonglandular portion of the stomach along the margo plicatus. The rectum was filled with feces. The urinary bladder was distended by a large amount of urine. From the region of the C1 vertebral body to the distal end of the spinal cord, the gray matter was diffusely reddened (Figure 1). In the lumbar region, the gray matter had tracts of parenchymal softening in addition to the reddening.

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Photograph of transverse spinal cord sections (A) and a longitudinal section of the lumbar portion of the spinal cord (B) of a horse that had apparently normal mentation, a flaccid tail, pelvic limb paresis, and urinary bladder atonia. In panel A, notice that the gray matter is diffusely hemorrhagic in the cervical (left), thoracic (center), and lumbar (right) portions of the spinal cord. In panel B, congestion of the gray matter and leptomeninges is characterized by prominent blood vessels. A focally extensive area of the gray matter is granular and softened. Bar in both panels = 3 cm.

Citation: Journal of the American Veterinary Medical Association 237, 3; 10.2460/javma.237.3.277

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Formulate differential diagnoses from the history, clinical findings, and Figure 1—then turn the page →

Histopathologic Findings

Both the spinal cord and brain were removed during necropsy and further processed for histologic examination. The vessels of the gray matter of multiple sections of the spinal cord (cervical, thoracic, and lumbar regions) were diffusely congested and lined by plump endothelial cells. Scattered hemorrhages were present throughout the gray matter. Multifocally, the Virchow-Robin spaces, mainly of the gray matter, were distended by perivascular cuffs composed of 1 to 3 layers of lymphocytes, macrophages, and rare neutrophils (Figure 2). Scattered neurons in the dorsal horns of the cervical portion of the spinal cord and in the ventral horns of more distal spinal cord regions had dispersed Nissl substance, central pallor of the cytoplasm, and eccentric nuclei with loss of nuclear detail (neuronal degeneration). There were occasional axonal spheroids within the ventral horns of the gray matter and the lateral and ventral funiculi of the white matter. Viable neurons contained multiple, variably sized, eosinophilic, intracytoplasmatic inclusion bodies (Negri bodies; Figure 3). All of the aforementioned microscopic changes were more severe in sections of the lumbar portion of the spinal cord. The brain had mild perivascular cuffing mainly throughout the neuraxis (brainstem, thalamus, and basal ganglia), which extended multifocally to the leptomeninges. In these areas, the neuropil and neuroparenchyma contained multiple foci of gliosis. Rare Negri bodies were seen within neurons of the thalamus and basal ganglia. Scattered neurons within the hippocampus were shrunken and eosinophilic.

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Photomicrograph of a section from the lumbar portion of the spinal cord in the horse in Figure 1. Perivascular hemorrhages and lymphocytic cuffing expand the Virchow-Robin space. H&E stain; bar = 100 μm.

Citation: Journal of the American Veterinary Medical Association 237, 3; 10.2460/javma.237.3.277

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Photomicrograph of a section from the lumbar portion of the spinal cord in the horse in Figure 1. An oval eosinophilic intracytoplasmic Negri body is present within the perikaryon of a neuron in the dorsal horn of the spinal cord. H&E stain; bar = 20 μm.

Citation: Journal of the American Veterinary Medical Association 237, 3; 10.2460/javma.237.3.277

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Morphologic Diagnosis

Extensive, acute, hemorrhagic, lymphocytic poliomyelitis with intraneuronal Negri bodies and mild multifocal lymphocytic meningoencephalitis with gliosis and rare Negri bodies.

Comments

The gross and microscopic lesions in the CNS of the horse of this report were characteristic of rabies virus infection, which was confirmed on the basis of results of a fluorescent antibody test for rabies. Rabies virus is an enveloped, negative-sense, nonsegmented RNA virus (genus, Lyssavirus; family, Rhabdoviridae).^1,2 This virus is responsible for causing a deadly meningoencephalomyelitis and ganglionitis and is capable of affecting any mammal because lyssaviruses are genetically analogous and have modified themselves to reproduce in the CNS of mammals.^2,3

Three forms of rabies are described—the dumb, furious, and paralytic forms¹—each with characteristic clinical signs. Although the clinical signs observed in mammals infected by the virus are typically the same, there are minor differences among species.¹ The 2 most common clinical signs in mammals affected with rabies are progressive paralysis and alteration in behavior.¹ In addition to the common clinical signs, horses in particular can have clinical signs associated with spinal cord injury.^3–5 The clinical signs begin as slight pelvic limb lameness that advances to loss of proprioception in 1 or both metatarsophalangeal joints, followed by ataxia and eventually pelvic limb paralysis.⁵ In addition, colic and genitourinary tract signs frequently develop in affected horses.⁵ There are other species-specific differences in the clinical progression of rabies.³ For example, cattle are more likely to vocalize and drool.³ In swine, often, there are no clinical signs and the animals are found dead.³ Sheep display passive behavior, whereas dogs become agitated or anxious.³

Clinical signs are usually associated with the location of the most severe microscopic lesions, but gross lesions in rabies-affected animals are usually absent. There are few documented cases^4,6,7 with gross lesions, and these mostly have involved horses. Gross lesions in those reports included congestion of the meninges to central areas of hemorrhage,⁴ disseminated brain edema,⁴ diffuse hemorrhagic poliomyelomacia,⁶ hemorrhagic myelitis,⁷ and ulcerative cystitis.⁶ Hemorrhage in the spinal cord seems to be a consistent lesion in rabid horses, as seen in the horse of this report. Unlike other species, horses appear to have a high concentration of the virus within the spinal cord, which is consistent with the clinical signs as well as the 2 predominant forms of rabies in horses.⁴

On the basis of the gross findings in the spinal cord of affected horses, the differential diagnoses in addition to rabies are eastern equine encephalitis (caused by a virus of the genus Alphavirus), equine herpesvirus myeloencephalopathy (caused by equine herpesvirus-1), equine protozoal myeloencephalitis (caused by Sarcocystis neurona and Neospora hughesi), and, less commonly, fibrocartilaginous embolism.³ To rule out each disease, it is important to submit sections of brain or spinal cord for diagnostic assessment. Virus isolation and PCR assays are required to confirm eastern equine encephalitis or equine herpesvirus myeloencephalopathy.³ Sarcocystis neurona and N hughesi can be observed microscopically in H&E-stained sections; however, results of PCR assay or immunohistochemical staining are necessary for definitive diagnosis.³ To establish a postmortem diagnosis of fibrocartilaginous embolism, it is important to submit spinal cord sections that contain gross lesions for histologic examination; staining of sections with alcian blue stain may be necessary to detect the emboli.⁸

Specific histologic lesions are important to make a definitive diagnosis of rabies, and these include nonsuppurative meningoencephalomyelitis with intracytoplasmic neuronal inclusion bodies (ie, Negri bodies).⁹ However, these changes can be mild or even absent within the CNS.⁹ Historically, the Negri bodies are often located in the Purkinje cells of the cerebellum in herbivores and in neurons of the hippocampus in carnivores.⁹ In most domestic species, Negri bodies are identified in approximately 70% to 85% of animals infected with the rabies virus.⁹ However, in horses, only 30% to 50% of rabies-affected animals have detectable Negri bodies.^4,5 It is thought that the number of Negri bodies within the CNS is related to the duration of the clinical signs before death.⁹ There is evidence of irregular distribution of rabies virus antigen and Negri bodies within different segments of the CNS.¹⁰ In rabid horses, viral antigen also appears to be more prominent along the neuraxis⁵ and, in our experience, rare inclusion bodies are present in the cerebellum, hippocampus, and cerebral cortex. Therefore, in horses suspected of being infected with rabies virus, samples of the brain as well as a portion of the spinal cord should be collected for fluorescent antibody testing and histologic examination to maximize the likelihood of identifying the virus and Negri bodies, respectively.

As deadly as rabies is, animals can be protected from effects of the virus via administration of an inactivated vaccine prior to exposure.⁴ It is also important to know that following infection with the virus, herbivores, unlike carnivores, are more likely a dead-end host.¹¹

The history and clinical signs in the horse of this report were helpful in prioritizing rabies as the most likely differential diagnosis. Gross lesions were detectable in this case, which is rare in animals affected with rabies, and it should be remembered that gross changes do not preclude the possible diagnosis of rabies. The histologic findings and fluorescent antibody test result confirmed the diagnosis of rabies in this horse.