JPC SYSTEMIC PATHOLOGY
Signalment (JPC #2133906): Cow
HISTOPATHOLOGIC DESCRIPTION: Lung: Affecting up to 75% of this section, replacing normal pulmonary parenchyma, there are multifocal to coalescing areas of coagulative and lytic necrosis with a center of eosinophilic karryorhectic and cellular debris and fragmented alveolar septa and bronchiolar walls that are surrounded by a dense cellular infiltrate consisting of large numbers of degenerate and non-degenerate neutrophils and macrophages, with few eosinophils, admixed with abundant fibrin, edema, hemorrhage, and additional necrotic debris that fills and expands surrounding alveolar lumina. Surrounding alveolar septa are thickened up to 3 times normal by macrophages, neutrophils, and fibrous connective tissue (fibrosis). Also present within the necrotic centers, within alveoli and alveolar septa, are numerous amoebic trophozoites and cysts. Trophozoites are up to 25um, have vacuolated cytoplasm, a 4-7um karyosome with a single endosome. Cysts are up to 20um with a clear halo separating an outer ectocyst and inner endocyst. Bronchiolar epithelium is multifocally hyperplastic and thrown into folds into the airway lumen which is filled with an exudate composed of necrotic debris, viable and degenerate neutrophils, macrophages, hemorrhage and fibrin with additional trophozoites. Multifocally, the tunica media and adventitia of vessels is expanded and infiltrated by cysts, trophozoites, neutrophils, macrophages, fibrin, hemorrhage, and karyorrhectic debris (necrotizing vasculitis). There is perivascular hemorrhage low to moderate numbers of macrophages, plasma cells and lymphocytes. The pleura is expanded by fibrin and edema.
MORPHOLOGIC DIAGNOSIS: Lung: Pneumonia, fibrinous, necrotizing and pyogranulomatous, chronic-active, diffuse, severe, with amoebic trophozoites and cysts, breed unspecified, bovine.
ETIOLOGIC DIAGNOSIS: Pulmonary amebiasis
CAUSE: Acanthamoeba sp.
- Amoebae (subphylum Sarcodina, superclass Rhizopodea) belonging to genera Acanthamoeba, Sappinia, Balamuthia, and Naegleria are free-living, opportunistic, saprophytic organisms found in soil and water that are occasionally pathogenic for animals and man
- Amoebae can cause two types of CNS disease: primary amoebic meningoencephalitis by N. fowleri and chronic granulomatous amoebic encephalitis by Acanthamoeba sp. or B. mandrillaris
- For Acanthamoeba spp. primary sites of infection include respiratory tract and skin
- Immunosuppression is a common predisposing condition in amoebic infections
- Proposed routes of pulmonary infection for acanthamebiasis include inhalation of organisms from air or soil or exposure of nasal cavity to contaminated water, followed by colonization of nasal passages, subsequent organism replication, and descent into lower respiratory tract
- In the case of encephalitis caused by Acanthamoeba, hematogenous infection from the lung or cutaneous lesions is most frequently described; alternatively, infection of the nasal passages can lead to retrograde spread to the CNS via olfactory nerves or penetration through cribiform plate, but this route is most frequently described for Naegleria fowleri
- Two stage life cycle: Dormant free-living cyst in environment and vegetative feeding form (trophozoites) in host tissues
- Cyst inhaled or ingested > excystation to amoeba form (trophozoite) > invade other tissues > binary fission > cyst formation
TYPICAL CLINICAL FINDINGS:
- Signs are dependent upon organs involved and reflect chronic granulomatous disease
- In dogs, clinical manifestations of acanthamebiasis are similar to canine distemper: fever, occulonasal discharge, anorexia, lethargy, respiratory distress (coughing and dyspnea), and CNS signs (dysmetria, head tilt, seizures, etc.)
- Clinical laboratory findings are non-specific
TYPICAL GROSS FINDINGS:
- Lung: pale tan to deep-red, raised, semisolid nodules distributed in all lung lobes
- Nodules may coalesce, with central cavitation
- Brain: large, multifocal, and vary from red to tan-brown due to hemorrhage and necrosis of parenchyma
- Other organs affected include skin, heart, liver, kidney, adrenal gland, and pancreas
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Necrohemorrhagic with purulent pyogranulomatous or granulomatous inflammation with intralesional ameobic trophozoites and cysts, sometimes within macrophages; in brain often seen in perivascular and subarachnoid spaces
- Trophozoites measure 15-50 um in diameter and have a round, eccentric karyosome (nucleus) and a single, centrally located, prominent endosome (nucleolus) surrounded by a thin, clear rim; the cytoplasm is eosinophilic and contains glycogen vacuoles
- Trophozoites can be mistaken for macrophages, but are differentiated by the presence of the karyosome, endosome, and vacuolated cytoplasm
- Cysts (ectocyst) are 15-25um with a wrinkled outer wall and PAS-positive inner wall (endocyst) separated by a clear space
ADDITIONAL DIAGNOSTIC TESTS:
- Organisms are PAS and GMS positive
- Can be cultured using potato dextrose agar
- Species may be identified by immunofluorescence and PCR
- Balamuthia mandrillaris: Difficult to distinguish from Acanthamoeba on H&E; reported veterinary cases have been in old world primates (gorilla, gibbon, mandrill baboon, colobus monkey)
- Naegleria fowleri: Causes acute hemorrhagic necrosis of the cerebral cortex and/or olfactory bulbs; pockets of amoebic trophozoites are present within edematous and hemorrhagic neuropil or occur deep in Virchow-Robin spaces and around blood vessels; cyst forms do not occur in host tissue
- Entamoeba histolytica: Less distinct or invisible endosome; clumped, marginated chromatin; and lesions bound by a variably thick fibrous capsule; causes amebic abscesses in liver, lung, and brain, with typical, flask-shaped ulcers in the intestin
- Blastomyces dermatitidis: Budding forms are usually present in blastomycosis; amoebic cysts may resemble B. dermatitidis, but lack budding forms
- Prototheca sp. and Chlorella: Uninucleated forms of these algal organisms are similar to Acanthamoeba cysts but can be distinguished by presence of endosporulated forms (triangular or wedge-shaped endospores)
- Acanthamoeba infections have been described in multiple nonhuman primate species and other mammals
- Balamuthia mandrillaris infections have been described in Old World and New World nonhuman primates
- Naegleria fowleri has flagella when free-living in water, has caused disease in a South American tapir; intranasal injection of amoebae causes meningoencephalitis in mice and monkeys
- Entamoeba invadens causes severe amebic dysentery in certain groups of snakes and reptiles; other groups of reptiles (e.g. garter snake, box turtles) carry the organism as a commensal, do not develop dysentery, and transmit to susceptible species
- Entamoeba histolytica causes amebic dysentery in humans, nonhuman primates (especially old world) and rarely in other species (dogs, cats, pigs); many infections are asymptomatic; erosive/ulcerative colitis that is classically flask-shaped
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