JPC SYSTEMIC PATHOLOGY
SIGNALMENT (JPC #2461138): A Saint Bernard
HISTORY: The dog presented with a one-week history of ataxia and showed mild obtundation and severe tetraparesis. Postural reactions were decreased to absent in the hindlimbs. Muscle tone was decreased in the hindlimbs and normal in the forelimbs. The cranial nerve reflexes were within normal limits.
HISTOPATHOLOGIC DESCRIPTION: Cerebellum: Multifocally expanding the leptomeninges and perivascular spaces and infiltrating the adjacent neuropil of the molecular layer are numerous macrophages, lymphocytes, fewer plasma cells and neutrophils admixed with eosinophilic cellular and karyorrhectic debris (necrosis), increased numbers of glial cells (gliosis) and numerous small caliber blood vessels lined by reactive endothelium. Multifocally, admixed with the inflammatory cells are numerous protozoal cysts up to 70 um in diameter with a 1 um thick cyst wall that contain numerous 2 x 2 um basophilic bradyzoites as well as moderate numbers of 2 x 6 um crescentic tachyzoites arranged in small clusters.
Skeletal muscle: Separating, surrounding and replacing approximately 60% of myocytes are numerous lymphocytes, plasma cells, macrophages, fewer neutrophils and reactive fibroblasts admixed with hemorrhage. Remaining myocytes have either a pale, swollen, vacuolated sarcoplasm (degenerate) or have a hypereosinophilic, fragmented sarcoplasm with loss of cross-striations and a pyknotic nucleus (necrotic). Rare myocytes have basophilic sarcoplasm and multiple nuclei that are internalized and row (regeneration). Scattered within and around muscle fibers are few clusters of previously described protozoal cysts and tachyzoites.
MORPHOLOGIC DIAGNOSIS: 1. Cerebellum: Meningoencephalitis, necrotizing, lymphohistiocytic, multifocal, moderate, with protozoal cysts and extracellular tachyzoites, Saint Bernard, canine.
2. Skeletal muscle: Myositis, necrotizing and lymphohistiocytic, multifocal, moderate, with myocyte degeneration and regeneration and intra- and extra-cellular protozoal tachyzoites.
ETIOLOGIC DIAGNOSIS: Cerebellar and intramuscular neosporosis
CAUSE: Neospora caninum
· Neospora is a cyst-forming coccidian organism in the phylum Apicomplexa that is an important cause of bovine abortion and neuromuscular disease in dogs
· Clinical neosporosis has also been reported infrequently in sheep, goats, deer, and horses
· Dogs and coyotes are the only known definitive hosts and can also be intermediate hosts; herbivores are more common intermediate hosts
· Three infectious stages: Tachyzoites, tissue cysts and oocysts
o Oocysts are present in the definitive host
o Tachyzoites and tissue cysts are present in the intermediate and definitive host
· The likely method of spread is through leukocyte trafficking
· Cell death is caused by active intracellular replication
· The character of lesions in the CNS are consistent with endothelial tropism, vascular injury, tissue ischemia, and infarction
· Pups may be subclinically infected with later reactivation associated with immunosuppressive illnesses and administration of modified live-virus vaccines or glucocorticoids
· Similar life cycle to Toxoplasma gondii
· Tissue cysts ingested by definitive host > oocytes produced, passed in feces, and sporulate > intermediate hosts ingest sporulated oocysts > zoite release and penetration of cells > proliferation by endodyogeny as tachyzoites > encyst in CNS and other tissues as bradyzoites
· Within the intermediate host, vertical transmission of tachyzoites can occur across the placenta; repeated transplacental infections can also occur in subsequent pregnancies
TYPICAL CLINICAL FINDINGS:
· Young dogs infected in utero are most severely affected
· Ataxia, head tilt, hind limb paresis and rigidity that ascends, progressive paralysis
· Muscle atrophy, dysphagia, heart failure
TYPICAL GROSS FINDINGS:
· Foci of necrosis and hemorrhage in the central nervous system with vascular pattern
· White streaks in muscle
· Granulomas in visceral organs
· Multifocal to generalized, ulcerative papulonodular dermatitis
· Necrosis and fibrinohemorrhagic enteritis
TYPICAL LIGHT MICROSCOPIC FINDINGS:
· Necrotizing granulomatous, lymphoplasmacytic and occasionally eosinophilic meningoencephalitis with diffuse gliosis, +/- axonal swelling, digestion chamber formation
· Tachyzoites: 2 x 6 um crescentic organisms with small nucleus and small amounts of cytoplasm, in groups or clusters, in neurons, endothelium or epithelial cells or extracellular
· Cysts: Round to oval, up to approximately 100 um long, with 2-4 um thick wall enclosing bradyzoites which are 8 x 2 um; occur within CNS and less distinctly, in skeletal muscle
o Little host reaction to intact tissue cysts
· Oocysts: Round, 10-12 um diameter, and contain two sporocysts with four sporozoites each
· Lymphohistiocytic and occasionally eosinophilic polyradiculoneuritis with secondary degeneration and necrosis in muscles or nerves
· Pyogranulomatous, occasionally eosinophilic to necrotizing and hemorrhagic dermatitis with tachyzoites in macrophages, keratinocytes, and neutrophils
· Fibrinohemorrhagic enteritis
o Severity of epithelial necrosis is related to the number of schizonts present
· Does not develop within a parasitophorous vacuole
· Moderately electron dense cytoplasm; single plasmalemma and double inner membrane complex; a conoid consisting of 2 microtubules arranged in a helix; 11 or more rhoptries; micronemes arranged perpendicular to the pellicle; organelles include a Golgi complex; smooth and rough endoplasmic reticulum; ribosomes; one or two mitochondria
· Tissue cyst wall - 1 to 4 um thick, amorphous, non-septate
ADDITIONAL DIAGNOSTIC TESTS:
· Specific diagnosis made by immunohistochemistry
· PCR, various ELISAs, indirect fluorescent antibody test (IFAT), and neospora agglutination test (NAT)
· Tachyzoites are PAS positive
o Indistinguishable from Neospora by light microscopy
o Thin-walled cysts typically less than 1 um thick (thinner than Neospora)
o Rhoptries are honeycombed; micronemes are less numerous than in Neospora
· Sarcocystis spp.:
o 4 x12 um zoites
o Large, thin- or thick-walled cysts with septa
o Tachyzoites lack a parasitophorous vacuole & have a multilobed nucleus
o In adults, the only clinical sign is abortion in dairy and beef cattle, typically at five to six months gestation (mid-term); fetus may be mummified, stillborn or born alive without clinical signs
§ Lesions in fetus: lymphocytic, plasmacytic and to a lesser extent histiocytic, hepatitis, pancarditis or myocarditis, myositis and placentitis
§ Pathognomonic CNS lesion in the fetusis the presence of multifocal discrete foci of necrosis (100-300 um in diameter) particularly in the brain and lesser extent in the spinal cord; advanced lesions the necrosis can be replaced by macrophages and a few glial cells making the lesion appear as a granuloma
§ Difficult to recognize N. caninum tachyzoites and tissue cysts in aborted fetal brain
o Only < 2-month-old calves have been reported with neurologic signs
· Sheep and Goats: Infrequent cause of abortions and congenital infections of lambs and kids
· Horse: Rare abortions and congenital infections of foals by N. hughesi; smaller cysts with thin (<1 um thick) walls and smaller bradyzoites than N. caninum; similar to S. neurona;
o Meningoencephalomyelitis, variable vasculitis and necrosis with microgliosis; perivascular cuffing with macrophages, multinucleated giant cells, lymphocytes, plasma cells or neutrophils
o Most common in the gray and white matter of the spinal cord
· Experimental infections: Cats, mice, rats, gerbils, and rhesus macaques (Macaca mulatta); mice often used to study mechanism of infection
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