5-month-old, female, Labrador retriever, canine (Canis familiars). Approximate 3-month history of progressively abnormal gait and ataxia. Neurological examination revealed mild to moderate tetraparesis and mild proprioceptive ataxia in the pelvic limbs. Evaluation of the spinal reflexes showed reduced flexion in all four limbs and also reduced muscle tone in the pelvic limbs. The patellar reflexes were reduced bilaterally. MRI findings indicated cerebellar atrophy.
Examination of the brain reveals mildly increased amounts of clear cerebrospinal fluid surrounding the brain. The cerebellum is diffusely small relative to the cerebrum and medulla with the caudal aspect of the fourth ventricle extending beyond the caudal margin of the vermis. The folia appear diffusely thin with widening of the sulci and moderate congestion of the meninges. The remaining carcass is grossly unremarkable.
Brain (cere-bellum and brainstem): The cerebellar folia are partially to extensively disrupted by multifocal to coalescing foci of neuropil rarefaction, loss and collapse with marked Gitter cell infiltration and fibrous astrocyte proliferation, gliosis, and proliferation of small caliber vessels lined by reactive endothelium Low to moderate numbers of plasma cells, lymphocytes, fewer eosinophils and neutrophils are interspersed amongst the collapsed folia, expand the overlying meninges, and distend perivascular spaces within the subjacent cerebellum and brainstem. Smaller, randomly scattered foci of gliosis, nonsuppurative inflammation, and mild rarefaction are scattered randomly within the cerebellar peduncles and brainstem. Within the foci of inflammation, and randomly distributed within the neuropil, are variably frequent protozoal tissue cysts that measure up to 113 μm in diameter. These cysts have a 2 - 3 μm thick cyst wall and contain numerous 8 x 2 μm basophilic bradyzoites (Figure 4). Sporadic foci of axonal swelling, digestion chambers containing few debris-filled Gitter cells, and occasional spheroids are noted throughout the white matter.
In tissues not included for conference material, also contain similar lesions and protozoal tissue cysts. These lesions were noted in the cerebrum, midbrain, thalamus, and spinal cord with mild infiltration of the spinal nerve roots.
Brain (cerebellum and brainstem): Meningoence-phalitis, lymphoplasmacytic, histiocytic, necrotizing, subacute to chronic, multifocal, moderate to marked with protozoal tissue cysts (consistent with Neospora caninum)
Cytological examination of the cerebrospinal fluid shows moderate PCR positive for Neospora caninum and PCR negative of canine herpesvirus, canine parvovirus, Toxoplasma gondii, and Sarcocystis sp.
Additional findings that were not present in all slides include rare clusters of free 2 x 6 μm, spindle-shaped tachyzoites within the neuropil and fibrinoid degeneration scarcely noted within the walls of arterioles. Overall, the microscopic findings and PCR results confirm the etiologic diagnosis of Neospora encephalitis as the underlying cause for the progressive ataxia in this 5-month-old Labrador retriever puppy. Neospora caninum is a protozoan parasite of animals and can cause serious neuromuscular and polysystemic disease in dogs,(2,6) as well as significant disease in cattle,(6,9) small ruminants, horses, and wildlife.(5) Infection can be fatal in dogs of any age, but disease is most severe in puppies less than 6 months of age and those that are congenitally infected.(5,7,8) Based on the literature, there is an apparent age-related variation in lesion distribution in the canine. Young dogs and puppies typically develop lesions in the skeletal muscles and spinal nerve roots with resultant ascending paralysis, which tends to be most severe in the hind limbs.(1) Lesions may also be found in multiple organs including the central nervous system, lungs, heart, and liver.(1,8)
Infection in adult dogs can result in polymyositis, polysystemic infection, or multifocal central nervous system involvement.(8,10) Several reports describe necrotizing cerebellitis and cerebellar atrophy as a significant lesion associated with Neospora encephalitis in adult dogs.(3,11,12) This particular case is unique in that it is significant the necrotizing cerebellar lesions were found in a puppy. Transmission of N. caninum in dogs may occur both vertically and horizontally. Vertical transmission is well recognized in the dog with data suggesting that transmission from the dam to puppies occurs transplacentally, during the terminal stages of gestation, or postnatally via milk.(9) Horizontal transmission also occurs in the dog through ingestion of infected tissues.(9) Further communications with the breeder revealed that the dam of this puppy, as well as 10 other dogs from the same kennel, was fed a diet of raw beef and deer. Additional serological testing revealed that dam, as well as 4 other dogs, tested positive for Neospora caninum. The diet of raw tissues were the likely source of infection in the dam with subsequent horizontal transmission this puppy. Given that most puppies do not develop clinical signs until over 4 weeks of age,(8) it is difficult to determine if transplacental or postnatal infection occurred in this case.
Cerebellum and brainstem: Necrotizing polioencephalitis, subacute, multifocal to coalescing, moderate with lymphocytic and neutrophilic meningitis and apicomplexan schizonts.
Central nervous system (CNS) changes in the slide due to Neospora infection were profound and conference participants noted the protozoal schizonts were often found in less affected areas of the section; the precise reason for this discordant finding is unclear but not uncommon. Conference participants agreed that the most prominent change in the cerebellum was necrosis and loss of cerebellar grey matter extending into the adjacent white matter. The molecular and granular cell layers were reduced in thickness and there was loss of Purkinje cells with occasional empty baskets in the most severely affected areas. Glial nodules and prominent spheroids were described, as well as inflammatory infiltration of the meninges with extension into, and prominent expansion of, Virchow-Robin spaces. The inflammatory infiltrate was primarily mononuclear, dominated by lymphocytes, plasma cells and gitter cells; the latter being most noticeable in areas of necrosis and neuroparenchymal loss.
Differential diagnosis considered for this case included Toxoplasma gondii as well as Sarcocystis spp. Many features of toxoplasmosis and neosporosis are similar including the presence of the proliferative tachyzoite and tissue cyst phases. However, N. caninum does not develop in a parasitophorus vacuole and has a thicker cyst wall than T. gondii. The differences cannot be reliably differentiated by light microscopy, and require the use of electron microscopy or immunohistochemistry in many cases.(14) Ultrastructural differences include greater number of micronemes and rhoptries with N. caninum, in addition to a thicker cyst wall.(12) N. caninum is more commonly reported in the CNS where the tissue cysts are most commonly found; although T. gondii has a CNS form causing similar histologic changes. N. caninum seems to have an affinity for cells of the monocyte macrophage system, although many cell types can be infected, and it likely spreads to the CNS via leukocyte trafficking.(14) Toxoplasmosis seems to affect a wider variety of mammalian species
Although very uncommon, both Sarcocystis canis and Sarcocystis neurona have been documented to cause CNS disease in dogs,(12) In one documented case of S. neurona the affected dog was receiving immunosuppressive therapy and developed widespread encephalitis, predominantly in the grey matter, with brainstem, cerebellum and cerebrum being involved. The lesions also consisted of intense areas of inflammation, which was most pronounced in the cerebellum. (4) The dividing schizonts of S. neurona form distinct rosettes of merozoites, arranged around a prominent residual body and their schizonts differ from other protozoa in that the merozoites lack rhoptries.(14) Encephalo-myelitis due to S. neurona infection has also been described in cats, which along with many other mammals including harbor seals and nonhuman primates, can serve as intermediate hosts for S. neurona.(12)
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