Read-Only Case Details Reviewed: Oct 2008

JPC SYSTEMIC PATHOLOGY

RESPIRATORY SYSTEM

September 2017

P-P06

Signalment (JPC #2133906): Cow

HISTORY: None

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.

CONDITION: Acanthamebiasis

GENERAL DISCUSSION:

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

PATHOGENESIS:

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

LIFE CYCLE:

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

DIFFERENTIAL DIAGNOSIS

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)

COMPARATIVE PATHOLOGY:

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

REFERENCES:

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