JPC SYSTEMIC PATHOLOGY
SIGNALMENT (JPC #1368773): Dog
Heart: Multifocally affecting approximately 40% of this section of heart, there is disruption of the myocardial architecture with cardiomyocyte degeneration, necrosis loss and replacement with eosinophilic cellular and karyorrhectic debris (lytic necrosis), admixed with moderate numbers of viable and degenerate neutrophils, fewer macrophages, hemorrhage, fibrin and edema. Multifocally, many cardiomyocytes are fragmented and hypereosinophilic with loss of cross striations, disorganization of myofibrils or hyalinization of the sarcoplasm (necrosis) and others are swollen, pale and vacuolated (degeneration). Within areas of inflammation and necrosis, there are scattered, 10-20 um diameter, irregularly round amoebic trophozoites with abundant granular amphophilic cytoplasm, a single 7 um eccentric magenta nucleus and a 1-2 um dense basophilic nucleolus surrounded by a thin, clear rim. Cysts are smaller, irregular in shape and have a thick 1-2 um, wavy outer wall. The endocardium is thickened up to 3 times normal by edema, hemorrhage, fibrin, and low numbers of lymphocytes, neutrophils, and macrophages.
MORPHOLOGIC DIAGNOSIS: Heart: Myocarditis, necrotizing, subacute, multifocal, marked, with amoebic trophozoites and cysts, breed unspecified, canine.
ETIOLOGIC DIAGNOSIS: Myocardial acanthamoebiasis
CAUSE: Acanthamoeba sp.
- Pathogenic free living amoebas include Acanthamoeba spp., Naegleria fowleri, Balamuthia mandrillaris and Sappinia diploidea
- Habitats include fresh and salt water, soil, dust, sewage
- Usually solitary opportunistic infection or often associated with immune suppression
- Typically cause pneumonia, skin infection and/or encephalitis, but can disseminate to many organs
- Young and immune suppressed dogs are most susceptible
- 19 genotypes described for Acanthamoeba (T1 to T19), the T4 genotype is overrepresented in human
- Dormant free-living cyst phase is able to resist adverse environmental conditions
- Vegetative replicating trophozoite feeds on bacteria in the environment and host tissues in the body
- Transmission from environment via inhalation, resulting in colonization of nasal mucosa
- Cerebral and meningeal infection occurs hematogenously (Acanthamoeba and B. mandrillaris) or by direct penetration of the olfactory neuroepithelium (Naegleria)
- Disseminated form usually involves the central nervous system and lungs
TYPICAL CLINICAL FINDINGS:
- Similar to canine distemper
- Early clinical signs: Mild oculonasal discharge, anorexia, lethargy
- Later symptoms: Respiratory distress, neurologic symptoms
TYPICAL GROSS LESIONS:
- Multifocal to coalescing, mottled red to tan/yellow areas in lung, brain, heart, liver, or pancreas
- Scattered hemorrhage in myocardium and on serosal surfaces
TYPICAL LIGHT MICROSCOPIC LESIONS:
- Well circumscribed, unencapsulated, necrohemorrhagic, fibrinonecrotic, or necrogranulomatous foci containing many trophozoites and cysts
- Thrombosis, occasionally with enmeshed ameba
- Trophozoites and cysts have a single round nucleus with a large, dense, centrally located nucleolus
- Trophozoites are 10-30 um in diameter, with an eccentric nucleus and eosinophilic cytoplasm containing glycogen vacuoles and occasional ingested erythrocytes
- Trophozoites resemble macrophages
- Cysts are generally rare and are more prevalent in CNS lesions than in extraneural lesions
ADDITIONAL DIAGNOSTIC TESTS:
- Organisms are PAS positive
- Cysts can be demonstrated with silver stains
- Identification of species: Immunofluorescence, culture, transmission electron microscopy
- Trophozoites of Acanthamoeba and Naegleria are indistinguishable by light microscopy
- Naegleria fowleri generally does not produce cysts in tissue and is often associated with acute neurologic infections
- Balamuthia: Slightly larger and more pleomorphic trophozoites than other amoebas; often multiple nucleoli
- Balamuthia mandrillaris: Free-living amoeba that causes fatal meningoencephalitis in humans and animals, including gorillas and Old World primates
- Acanthamoeba spp. cause disease in immune suppressed nonhuman primates: Acute necrotizing meningoencephalitis with a granulomatous pneumonia
- Reported in a non-immunosuppressed lemur
- Balamuthia mandrillaris and Acanthamoeba have been reported to cause disease in horses
- Invasive amoebiasis in non-human primates occurs as gastrointestinal and systemic diseases due to enteroinvasive Entamoeba histolytica, which causes ulcerative colitis in New World monkeys, Old World monkeys and the great apes
- Hematogenous spread to the liver, lung and brain reported
- Gastric amoebiasis due to histolytica also occurs in leaf-eating primates (colobus monkey, silver-leafed monkey) due to higher stomach pH that is conducive to survival of amoebae; erosive and ulcerative gastritis
- Amoebic gill disease (Neoparamoeba spp.): Proliferative condition affecting the gills characterized by hyperplasia and fusion of gill lamellae and filaments; most often of farmed fish
- Entamoeba invadens: causes ulcerative colitis and hepatitis with high morbidity and mortality in snakes and lizards
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