2-year-old, female, Guereza monkey (Colobus guereza).The monkey was housed in an open-range recreation park together with numerous other monkeys of the same species and in close contact to other animal species and human visitors. Clinically, a poor body condition, dyspnoea and abdominal discomfort were noticed for two days prior to death.

Gross Description:  

Necropsy revealed a poor body condition and moderate hyperplasia of hepatic and gastric lymph nodes. Approximately 30% of the liver tissue was replaced by multifocal to coalescing, variably demarcated, irregularly shaped, pale yellow, occasionally slightly elevated nodules measuring 0.5 - 4.0 cm in diameter (Fig. 1). On the cut surface, they showed a pale yellow coloration and a dry, elastic consistency. In addition, a moderate multifocal fibrino-necrotizing gastritis, and mild acute diffuse catarrhal enteritis were observed.

Histopathologic Description:

Liver: There are randomly arranged, multifocal to coalescing, irregular to round, pale eosinophilic, hypocellular areas surrounded by a poorly demarcated hypercellular rim (Fig. 2). The centrally located pale eosinophilic, floccular mass with an irregularly distributed scant amount of karyorrhectic debris gradually changes into a peripheral zone of dissociated hepatic cells with pale, irregularly vacuolated cytoplasm, loss of cellular detail, ruptured outer membranes, and karyorrhexis and karyolysis. This area is surrounded by a zone composed of an inner layer of macrophages, epithelioid macrophages and few neutrophilic granulocytes, gradually changing into an outer layer consisting of macrophages, lymphocytes, plasma cells and fibroblasts embedded in an extracellular matrix exhibiting a moderate to high amount of collagen fibres that irregularly extends into the neighbouring hepatic tissue. Within the center of the lesions there is a moderate amount of multifocal, oval, indistinct, unicellular structures of approximately 20-30 μm diameter with clear borders, abundant finely vacuolated, lightly eosinophilic cytoplasm, and a single, eccentrically located, small, round, pale amphophilic nucleus (interpreted as protozoan trophozoites) (Fig. 3). These protozoal organisms are labelled brightly red in periodicacid- Schiff (PAS)-stained sections (Fig. 4, 5). The surrounding hepatic tissue displays a mild to moderately increased amount of periportal fibroblasts and collagen fibre-rich extracellular matrix (periportal fibrosis), a mildly increased amount of periportal bile ducts, and moderately increased amount of intravascular and intrasinusoidal erythrocytes (congestion).

Morphologic Diagnosis:  

Liver: Hepatitis, granulomatous and necrotizing, multifocal to coalescing, chronic, severe with intralesional protozoal trophozoites consistent with Entamoeba histolytica.

Lab Results:  

Urinalysis revealed a pH value of 8.0; 300 mg protein/liter; a normal amount of glucose and urobilinogen; and no leukocytes, nitrites, ketones or bilirubin.

The liquid of the anterior eye chamber contained a urea concentration of 19.98 mmol/L (120 mg/dL). Aerobic and anaerobic microbiological cultures of the liver resulted in a mild amount of α-hemolytic Streptococcus spp. and coagulase-negative Staphylococcus spp. Aerobic and anaerobic microbiological cultures of the stomach revealed a mild amount of Enterococcus species, coagulase-negative Staphylococcus spp., coryneform bacteria, α- hemolytic Streptococcus spp., Geotrichium species, Prevotella bivia and Prevotella intermedia. Both organs were culturally negative for Listeria monocytogenes under cold enrichment.

PCR of the liver was negative for human hepatitis virus A, B, C, and Francisella tularensis.

Immunohistology of the liver was positive for Entamoeba histolytica and negative for Toxoplasma gondii, Coxiella burnetti and Listeria monocytogenes.


Entamoeba histolytica

Contributor Comment:  

Entamoeba (E.) histolytica is a protozoan parasite belonging to the phylum Sarcomastigophora, subphylum Sarcodina (Rhizopoda), order Amoebida, family Entamoebidae, genus Entamoeba. It is the etiologic agent of human amoebiasis, with an incidence of up to 50 million clinical cases per year, including up to 100,000 fatalities.(13,19,21,23) Entamoeba histolytica is distributed worldwide among human beings, but is also reported to occur in a wide range of New and Old World monkeys.(5,13,20) It is rarely found in domestic animals, including dogs, cats, cattle and captive macropods.(2,14,15,20) Furthermore, guinea pigs and hamsters are susceptible to experimental infection.(2,3,17) In addition to E. histolytica, multiple non-pathogenic species of the genus Entamoeba, including E. dispar and E. moshkovskii, are frequently detected in faeces and the intestinal tract of humans and nonhuman primates.(13,16,20) The only other pathogenic species of the genus Entamoeba is E. invadens, which occurs in reptiles.(6) Notably, it is impossible to differentiate the cysts of the highly related species E. histolytica, E. dispar, and E. moshkovskii by light microscopic investigation of fecal smears.(13,16,19,21,23) However, morphological differences, including a slightly larger size of E. histolytica trophozoites (20-30 μm) as compared to E. dispar trophozoites (12-15 μm), have been described. Furthermore, only E. histolytica is erythrophagocytic and invasive and therefore can be detected within tissues upon histological examination.(19) In histological sections, E. histolytica trophozoites exhibit a granular and lightly stained cytoplasm and a round nucleus with chromatin plaques at the periphery and a small endosome.(6)

Infection with E. histolytica occurs via the fecaloral route. In human patients, excystation takes place in the large intestine. The trophozoites are usually commensals within the intestinal lumen, reproducing by binary fission and encysting as they move further down the digestive tract. Cysts are shed for many months to years in untreated humans. The cysts remain infective for weeks to months in a moist environment.(13,19) A complete intra-intestinal life cycle, including cyst formation, is only reported to occur in humans and non-human primates; therefore, the zoonotic potential of infected non-primate mammals is thought to be limited.(18)

Pathogenicity of E. histolytica is affected by the strain of organism, host species infected, nutritional status, environmental factors and bacterial flora of the gastrointestinal tract.(20) Only in cases of mucosal invasion, which happens in less than 10% of human patients, does E. histolytica become pathogenic and lead to amoebic dysentery. (13,19) Clinical signs in affected monkeys include apathy, lethargy, weakness, dehydration, gradual weight loss, anorexia, vomiting, and severe diarrhea, which may be catarrhalic or hemorrhagic.20 Amoebiasis commonly presents as necrotizing colitis in humans and many non-human primates.(5,13,19,20) However, fibrino-necrotizing gastritis seems to be the principal lesion in certain species of leaf-eating monkeys of the subfamily colobinae [old world monkeys (Cercopithecoidea), family cercopithecidae], including colobus monkeys (Colobus guereza), silvered leaf monkeys (Presbytis cristatus), dusky leaf monkeys (Presbytis obscurus), and proboscic monkeys (Nasalis larvatus).(4,8-11) The stomach of the colobinae is divided into four parts: presaccular, saccular, tubular, and pyloric portion, with the first two portions serving as enlarged fermentation chambers as a special adaption to the leaf-eating lifestyle. It is suggested that the normal neutral pH within these gastric compartments provides a favourable environment for excystation of ingested E. histolytica cysts, followed by tissue invasion.(4,8) Invasive trophozoites are regularly detected within the ulcerative gastric lesions.(4,8-11) In rare cases, and independently of whether the primary lesion is in the stomach or in the colon, some trophozoites are thought to enter the vessels of the mesenteric vasculature, thereby leading to metastatic foci of amoebic infection in distant parts of the body.(20) Fatal amoebiasis with abscess formation, particularly in the liver and more infrequently the lung and the central nervous system, is reported in man, baboons, chimpanzees, orang-utans, spider monkeys, douc langurs, and several colobus monkeys.(4,5,8,11,13,18-20) Amoebic hepatic lesions may become clinically obvious many years after the initial exposure and without concurrent intestinal lesions and fecal cyst shedding.(8,13,18) Similar to the presented case, most reported cases of amoebic hepatic lesions in non-human primates were characterized by a multifocal distribution and granulomatous and necrotizing lesions.(4,8,11,18) The central necrotic areas of the lesions are thought to represent foci of caseous necrosis due to the action of lytic substances produced by the trophozoites.(8) In human patients, however, the liver lesions are mainly situated in the right liver lobe and consist of a single abscess in most patients (65-75%). (13)

The differential diagnosis of granulomatous and necrotizing hepatic lesions in primates includes yersiniosis, salmonellosis, listeriosis, tularemia, tuberculosis, necrobacillosis, Q-fever, histoplasmosis, yellow fever, infections with lymphocytic choriomeningitis virus (callitrichid hepatitis), human hepatitis B virus, herpesvirus simiae (herpesvirus B), herpesvirus saimiri, simian varicella virus, herpesvirus tamarinus, herpes simplex virus, cytomegalovirus, ebola virus, toxoplasmosis, Capillaria hepatica, and schistosomiasis. Non-human primates are possible reservoir hosts of human hepatitis A virus; however, they display only slight hepatocellular degeneration, necrosis and inflammation and are most likely clinically asymptomatic. Furthermore, chimpanzees have been experimentally infected with the human hepatitis C virus; however, spontaneous infections of non-human primates have not been reported.(1) Due to the zoonotic potential of most of these diseases, increased personal protection and a thorough etiologic work-up is suggested in cases of hepatitis in non-human primates.

JPC Diagnosis:  

Liver: Hepatitis, random, necrotizing and granulomatous, multifocal to coalescing, severe, with amoebic trophozoites.

Conference Comment:  

Conference participants discussed the virulence factors in Entamoeba histolytica infection. The first of three virulence factors is a multifunctional lectin (Gal/GalNAc lectin); in addition to binding to glycoprotein residues of the target cell, it also plays a role in cytolysis, invasion, resistance to complement and possibly the process of encystation. The second virulence factor is the amoebapore, a channel-forming protein analogous to T-cell perforins and NK-lysin from Natural Killer (NK) cells; once the channel is inserted into the host cell membrane, extracellular water and ions rush into the cell causing cell lysis. The final virulence factor is a family of cysteine proteases produced by E. histolytica. These proteins function to break down the extracellular matrix, facilitating invasion. Not only does this aid organism invasion, the breakdown of the extracellular matrix also results in loss of cellular adhesion, possibly contributing to further cellular death.(12)

These virulence factors must enable E. histolytica to overcome a variety of innate host defenses. The first and most extensive host barrier is the layer of mucin extending from the oral cavity to the rectum. The glycoproteins found in host mucin competitively bind to residues used in amoebic attachment; resident intestinal bacteria also bind to mucin, further reducing available binding sites to the protozoan. The complement system plays a role in the innate immune response to E. histolytica. The neutral cysteine proteinase elaborated by the parasite has been shown to activate complement via the classical and alternative pathways, thus activating the immune response. However, the Gal/GalNAc lectin possessed by the organism binds C8 and C9, thus preventing formation of the membrane attack complex.(12)

In addition to lectin-mediated evasion of the complement system, E. histolytica has several methods by which it evades the host immune system. The cysteine proteinases and hydrolytic enzymes not only degrade extracellular matrix proteins, they also degrade IgA and IgG antibodies, thereby facilitating initial invasion of the intestinal epithelium as well as aiding in systemic dissemination.(12) When immune complexes bind to the amoeba, a unique process of capping takes place whereby the bound antibodies are quickly moved to the posterior pole of the organism, the cap is released and the plasma membrane is regenerated;(7) thus, immune complexes are essentially shed from the protozoal cell surface. E. histolytica is also able to alter the host acute-phase immune response, primarily via an unknown mechanism of T-cell- modulated macrophage function, especially Th1.(12) The amoeba also produce monocyte locomotion-inhibitory factor which inhibits the respiratory burst in macrophages. Research also has demonstrated the presence of surface perioxiredoxin which neutralizes host-generated reactive oxygen species and nitric oxide.(7) Serine proteases secreted by E. histolytica bind to cathepsin G secreted by neutrophils, and protozoal arginase consumes host L-arginine, a precursor for the nitric oxide production by host macrophages.(7)


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