JPC SYSTEMIC PATHOLOGY

CARDIOVASCULAR SYSTEM

March 2025

C-P05 (NP)

 

SIGNALMENT (JPC #1368773): Dog

 

HISTORY: None

 

HISTOPATHOLOGIC DESCRIPTION: Heart: Affecting 40% of this section, there is multifocal loss of regular myocardial architecture with cardiomyocyte degeneration (swollen cardiomyocytes with pale and vacuolated sarcoplasm), necrosis (fragmented and hypereosinophilic cardiomyocytes with loss of cross striations, disorganization of myofibrils or hyalinization of the sarcoplasm, and nuclear pyknosis or karyorrhexis), and loss with replacement by eosinophilic cellular and karyorrhectic debris (lytic necrosis) admixed with moderate numbers of viable and degenerate neutrophils, fewer macrophages, hemorrhage, fibrin, and edema. Within areas of inflammation and necrosis, there are scattered, 10-20 µm diameter, irregularly round amoebic trophozoites with abundant granular, amphophilic cytoplasm, and a single 7 µm eccentric magenta nucleus with a 1-2 µm dense basophilic karyosome surrounded by a thin, clear rim. There are also amoebic cysts which 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.

 

GENERAL DISCUSSION:

• 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 (P-P06), skin infection, and/or encephalitis, but can disseminate to many organs
• Young and immune suppressed dogs are most susceptible
• 19 genotypes described for Acanthamoeba spp. (T1 to T19), the T4 genotype is overrepresented in human

 

PATHOGENESIS:

• 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 trophozoites and cysts
• Thrombosis, occasionally with enmeshed ameba 
• Trophozoites and cysts have a single round nucleus with a large, dense, centrally located karyosome
• Trophozoites are 10-30 µm 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 PAS and silver stains (e.g. Gomori’s methenamine silver)
• Identification of species: Immunofluorescence, culture, transmission electron microscopy (presence of two cell wall layers can help differentiate acanthamoeba spp. from other amoebae) 
• Cytology: Cysts (10 – 25 µm) will present with a thin (1-3 µm) non-staining wall, coarsely granular internal structures, and may or may not present with a discernable nucleus; trophozoites (15-50 µm) will contain abundant vacuolated and granular cytoplasm with a small round nucleus and a single large nucleolus

 

DIFFERENTIAL DIAGNOSIS:

• 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

 

COMPARATIVE PATHOLOGY:

Pathogenic amoebas in other species:

• Nonhuman primates:
• Balamuthia mandrillaris: Free-living amoeba that causes fatal meningoencephalitis in humans and animals, including gorillas and other Old World primates
• Acanthamoeba spp. cause necrotizing disease in immune suppressed nonhuman primates
• Enteroinvasive Entamoeba histolytica (D-P03C, D) causes ulcerative colitis in New World monkeys, Old World monkeys, and great apes
• Hematogenous spread to the liver, lung and brain reported
• Gastric amoebiasis due to E. histolytica often 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
• Horses: Balamuthia mandrillaris and Acanthamoeba spp. have been reported to cause disease 
• 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 
• Snakes, lizards: Entamoeba invadens (D-P03A, B): Causes ulcerative colitis and hepatitis with high morbidity and mortality in snakes and lizards

 

REFERENCES:

1. Boes KM. Chapter 5: Respiratory System. In: Raskin RE, Meyer DJ, & Boes KM eds. Canine and Feline Cytopathology: A Color Atlas and Interpretation Guide. 4th ed. St. Louis, MO: Elsevier; 2022:224.
2. Cantile C, Youssef S. Nervous system. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier; 2016: 386.
3. Caswell JL, Williams KJ. Respiratory system. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier; 2016: 574.
4. Fahey MA, Westmoreland SV. Nervous system disorders of nonhuman primates and research models. In: Abee CR, Mansfield K, Tardif S, Morris T, eds. Nonhuman Primates in Biomedical Research: Diseases. Vol 2. 2nd ed. San Diego, CA: Elsevier; 2012:746-748.
5. Greene CE. Infectious Diseases of the Dog and Cat. 4^th ed. St. Louis, MO: Elsevier Saunders; 2012.
6. Grimes CN, Fry MM, LeBlanc CJ, Hecht S. The Lung and Intrathoracic Structures. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and Hematology of the Dog and Cat. 5th ed. St. Louis, MO: Elsevier Mosby; 2014:300f. 
7. Strait K, Else JG, Eberhard ML. Parasitic diseases of non-human primates. In: Abee CR, Mansfield K, Tardif S, Morris T, eds. Nonhuman Primates in Biomedical Research: Diseases. Vol 2. 2nd ed. San Diego, CA: Elsevier; 2012:206-208. 
8. Uzal FA, Plattner BL, Hostetter JM. Alimentary system. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier; 2016: 242.

 

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