Secondary granulomatous meningoencephalomyelitis has also been recorded due to primary fungal and protozoan pathogens.
Primary GME is a central nervous system disorder is characterized by focal or multifocal non-suppurative granulomatous lesions within the white matter of the brain and cervical spinal cord, characterised by dense aggregates of inflammatory cells arranged in whorling patterns around blood vessels.
Like Greyhound meningoencephalitis and necrotizing meningoencephalitis (Pugs, Maltese Terriers, Pekingese, Yorkshire Terriers), this disease is thought to be either an immune-mediated disease or caused by infectious pathogens.
Mycoplasma canis has been identified with PCR analysis of postmortem brain samples from affected dogs and immunohistochemical studies have shown a preponderance of anti-T cell receptor complex antibodies.
Clinically affected dogs usually present with a brief history (3 - 5 days) of progressive neurological symptoms such as behavior abnormalities, seizures, compulsive pacing, circling, or cranial nerve abnormalities may be observed. Choroiditis, retinal detachment, and secondary glaucoma may lead to blindness. Severely affected dogs usually progress to tetraparesis and absent postural reflexes.
The antemortem diagnosis of these disorders is clinically challenging. Blood tests, CSF analysis and imaging are often inconclusive and histopathology is often required for a definitive diagnosis. When there is no definitive histopathological diagnosis, the terminology meningoencephalitis of unknown etiology (MUE) may be used.
Histologically, lesions consist of perivascular cuffs distributed within the CNS but they occur most commonly within the white matter of the cerebrum, cerebellum, caudal brainstem or cervical spinal cord. Comparable lesions may also be observed in grey matter and there may be lesions involving the vasculature of leptomeninges or choroid plexus. These perivascular cuffs comprise principally macrophages and occasional lymphocytes, monocytes, plasma cells, neutrophils and multinucleate giant cells.
A differential diagnosis would include meningoencephalitis caused by Cryptococcus spp, neospora caninum, canine distemper virus, Histoplasma capsulatum, Coccidioides immitis, Cladophialophora bantiana, Aspergillus spp, Fusarium spp, Sporobolomyces roseus and Balamuthia mandrillaris.
Although radiation therapy and pharmacotherapy with azathioprine, cytosine arabinoside, cyclosporine, procarbazine and prednisolone has been shown to alleviate clinical symptoms in the short-term, this disease is rapidly progressive with poor survival times after clinical onset.
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