key: cord-0771573-hhfis05b
authors: Westerhoff, Maria
title: Histologic features of colonic infections
date: 2021-11-12
journal: Pathologe
DOI: 10.1007/s00292-021-01015-7
sha: d07b7e1107754a3411730b2f0b99c56a782f6747
doc_id: 771573
cord_uid: hhfis05b

BACKGROUND: The histopathologic diagnosis of infectious colitis remains relevant despite recent advances in microbiologic techniques. OBJECTIVE: This article aims to describe the histologic features of selected infectious diseases of the colon. MATERIALS AND METHODS: Existing reports on histopathologic and clinical aspects of colonic infectious agents were reviewed. RESULTS: While histology alone may not be as sensitive as current microbiologic methods, tissue identification of infectious agents still plays an important role in patient care. Infectious colitis can have a variety of clinical manifestations, ranging from strongyloidiasis, which can cause a smoldering, subclinical infection for decades, to syphilis, which can clinically mimic cancer or inflammatory bowel disease. Therefore, the histopathologic identification of infection as the cause of a patient’s colitis has a considerable impact on treatment decisions. Morphologic overlap can occur between infection and other diseases, however. Moreover, some infections can elicit various tissue responses beyond acute colitis. Immunosuppressed patients may not mount an inflammatory response to pathogens such as cytomegalovirus or adenovirus. Sexually transmitted proctocolitis can cause plasma-cell-rich inflammation. Gastrointestinal histoplasmosis is more likely to cause diffuse histiocyte infiltration rather than the expected granuloma formation. In some cases, ancillary tests are useful, but equivocal results can cause diagnostic dilemmas. CONCLUSION: Given the range with which colonic infectious disorders can manifest, pathologists should be aware of the typical features of infectious colitis, as well as findings beyond the classic morphologies.

Infectious colitis can exhibit a range of manifestations and mimic diseases treated by immunosuppression. A variety of diagnostic techniques are available beyond that of histology, such as immunohistochemistry (IHC) and molecular analysis. Nevertheless, it is still important to be able to recognize the tissue appearance and clinical significance of infectious agents, as well as the types of inflammatory responses they may elicit. The fact that diagnoses may be time sensitive and that differential diagnoses may include conditions with treatments that can worsen infection underscores the importance of proper histologic recognition.

Cytomegalovirus Cytomegalovirus (CMV) is of particular concern in chronic idiopathic inflammatory bowel disease (IBD) and transplant pa- 1 9 Cytomegalovirus (CMV)-infected cells (H&E stain). a Two infected cells (arrows) are seen: one exhibits enlargement and the classic glassy nuclear inclusion, and another displays subtler features with coarse, eosinophilic cytoplasmic inclusions. b Immunosuppressed patients may not mount much of an inflammatory response to CMV infection as seen in this biopsy. Two enlarged, CMV-infected endothelial cells are present (arrows). H&E hematoxylin and eosin tients. Corticosteroid-dependent IBD patients have a higher risk for CMV than those taking biologic agents [1] . Moreover, as there is a high risk of colectomy in treatment-refractory IBD patients presenting with severe flare, it is critical to search for CMV in their biopsies. Identifying CMV allows for a treatable, alternative cause of the symptoms besides refractory IBD, which may allow the patient to avoid surgery. Microscopically, infected colonic biopsies may show ulcer tissue or severely active colitis with viral inclusions. CMV-infected cells are enlarged and can have both nuclear and cytoplasmic inclusions. The nuclei classically exhibit eosinophilic, glassy inclusions surrounded by a halo of empty space and then rimmed with condensed chromatin, imparting an "owl's eye" appearance. The cytoplasmic inclusions are brightly eosinophilic granules (. Fig. 1) . The viral inclusions can be seen in endothelial cells, stromal cells, epithelial cells, and macrophages. Intact, infected colonic mucosa may show crypt abscesses or apoptotic bodies.

Although CMV inclusions can be recognized on routine hematoxylin and eosin (H&E) staining, several issues may pose challenges to their identification. It can be difficult to see viral inclusions in the cellular background of heavily inflamed tissue, particularly in IBD patients presenting with flare. The opposite can also be true. Pathologists usually scan slides on low-power magnification for foci of in-flammation to indicate where to look on higher-power magnification for abnormalities, such as viral inclusions. Transplant patients' biopsies, however, may lack an inflammatory response to infection and may appear normal, causing pathologists to overlook virally infected cells (. Table 1 ). Moreover, clinicians do not always inform pathologists that the biopsies are from immunosuppressed patients, increasing this risk of misdiagnosis. Other difficulties in diagnosing CMV on H&E include inadequacy of sampling and morphologic mimicry. CMV-infected cells may be quite focal and missed if only a few sites are sampled or only a few sections of tissue are examined [2] . Ganglion cells or reactive myofibroblasts in ulcers can mimic the appearance of viral inclusions. Finally, infected cells may not display the classic morphology of CMV [3] . IHC can aid in the identification of CMV and is particularly important to utilize in high-risk settings such as steroiddependent IBD patients and immunosuppressed transplant patients suspected of CMV infection.

There has been controversy over whether CMV inclusions in biopsies represent true pathogens or "innocent bystanders" in the background of severe IBD. One study considered clinical improvement upon antiviral treatment to indicate that the virus was pathogenic. They found that IBD patients with 5 or more CMVpositive cells in a single biopsy fragment benefited from antiviral therapy and had a reduced risk of surgery. This suggests that the burden of viral inclusions signifies whether CMV is causative of the patient's symptoms or whether it was a bystander in severe IBD [4] . Another report, however, found that patients with 5 or more CMVpositive cells per biopsy section were at a higher risk of colectomy than those with less than 5, despite antiviral therapy [5] . As CMV was associated with higher scores of histologic inflammatory activity, these authors considered the presence of CMV to be an indicator of severe inflammation. Nevertheless, because CMV infection is a treatable reason to avoid colectomy, current clinical guidelines from the US and the UK call for biopsies to be evaluated for CMV by H&E and IHC in patients with severe flares.

Finally, pathologists may question whether finding rare CMV cells on H&E is clinically important or whether positive cells on IHC without an H&E correlate should be reported. Regarding the latter, CMV IHC can certainly cause spurious focal staining with certain cells, such as plasma cells. This is of dubious significance. On the other hand, the presence of IHCpositive cells matching the size and shape of true inclusions, but without H&E correlates, can pose challenges as to whether this is clinically actionable. A small study showed that the majority of patients with CMV inclusions identified by IHC alone did improve with antiviral therapy [3] . As to the clinical significance of rare CMV cells seen on H&E, this may be condition dependent. One report evaluated IBD, human Table 1 List of organisms that may not be surrounded by an inflammatory response Infectious organisms not always associated with a tissue inflammatory response Cytomegalovirus (CMV) Adenovirus (ADV) Schistosoma eggs Aspergillosis Bacterial infections Strongyloidiasis immunodeficiency virus (HIV), and bone marrow transplant patients with rare CMV inclusions. As a single-institution study, it revealed that IBD patients with rare CMV in their practice were likely to receive treatment if the blood polymerase chain reaction (PCR) result was also positive for CMV, whereas HIV and transplant patients with rare inclusions would receive treatment regardless of the blood PCR result [6] .

Adenoviruses (ADV) are nonenveloped, double-stranded DNA viruses that can cause clinically significant diarrhea. Gastrointestinal (GI) ADV infections have been described mainly in HIV, solid organ transplantation, intestinal transplantation, and hematopoietic stem cell transplant (HSCT) patients [7] . Pediatric patients are particularly prone to infection, with 32% of pediatric HSCT recipients affected compared to 6% of adult HSCT patients [8] . In fact, after CMV, ADV is the most frequent DNA virus to infect pediatric patients in the early phase of allogenic HSCT [9] . Mortality rates of disseminated infection are as high as 26%, but anti-viral treatment with proven efficacy against ADV is lacking [10] .

The infection causes nuclear inclusions that can be subtle on microscopy due to minimal nuclear changes and little inflammatory response. Unlike CMV, which causes both nuclear and cytoplasmic inclusions, ADV affects only the nucleus and changes are characterized by a smudgy or glassy-appearing inclusion within the nucleus, minimally enlarging it, and rimmed by a compressed ring of darker chromatin (. Table 2 ). Infected goblet cells may have a slightly enlarged, crescent-shaped nu- Ranges from minimal inflammatory response to mild active or chronic inflammation Ranges from minimal inflammatory response to ulcer cleus due to indentation by the nearby cytoplasmic mucin vacuoles (. Fig. 2) . The epithelium may take on a subtly disorganized and regenerative appearance due to slight enlargement of nuclei by the inclusions and depleted mucin of the affected cells. Apoptotic bodies can be seen in association with the infected cells; as such, in HSCT patients' colonic biopsies, graft versus host disease (GVHD) may not be confidently diagnosed in the presence of apoptosis near viral inclusions. Immunohistochemistry is helpful for highlighting the inclusions. Coinfection with CMV and other pathogens can occur [7] .

Biopsies for acute bacterial colitis are infrequent specimens, especially given the availability of PCR testing for gastrointestinal pathogens. Patients may be biopsied if they have severe or unusual clinical presentations or if they have prolonged symptoms that do not improve with treatment. The morphologic features seen in biopsy depend on the timing of the biopsy procedure in relation to the duration of the patient's symptoms. If biopsies are taken within the first week of symptoms, the classic "acute self-limited colitis" pattern of neutrophilic inflammation with cryptitis and crypt abscesses predominates. If the biopsy is taken later in the course of the illness, plasma cells and eosinophils increase in the lamina propria. Infectious colitis tends to be patchy rather than diffuse in distribution. Certain infectious agents have a propensity for affecting specific segments of the colon. Campylobacter, Yersinia, tuberculosis, and Salmonella have a predilection for the proximal colon, whereas gonorrhea, syphilis, and lymphogranuloma venereum (LGV) tend to affect the distal colon. The histology of bacterial infection varies depending on the organism and can range from minimal inflammation, active colitis without crypt architectural distortion, to features mimicking inflammatory bowel disease (IBD). Granulomas, histiocytic collections, pseudomembranous colitis, and ischemic change can also occur with specific bacterial entities.

Pseudomembranous colitis can be caused by infectious agents such as Clostridioides difficile, Shigella, Escherichia coli O157:H7, and other Shiga toxin-producing E coli. Watery diarrhea in the setting of antibiotic use is the classic clinical scenario for C. difficile infection. Severe disease can lead to fulminant colitis with shock or toxic megacolon. The diagnosis in a pa-a b tient experiencing acute diarrhea can be established via positive molecular, culture, or immunoassay studies. Diffuse yellow plaques and nodules can be seen on endoscopy in pseudomembranous colitis (. Fig. 3 ), but some patients may present without pseudomembranes and merely show erythema and friability. These atypical endoscopic manifestations still have the characteristic histologic findings. Histologically, pseudomembranous colitis has a mix of inflammatory and ischemic features. Fibrin, neutrophils, necrotic epithelium, and mucus make up the inflammatory exudates of the pseudomembranes. These originate from necrotic crypts and protrude into the lumen with the appearance of a volcanic eruption. The surface epithelium demonstrates an ischemic appearance, characterized by attenuated epithelium lining the crypts (. Fig. 4) . The adjacent epithelium may show active colitis or hemorrhage. The same organisms that can cause pseudomembranous colitis can also cause a predominantly ischemic appearance that can be indistinguishable from noninfectious causes of ischemia in the colon.

There are typhoid and nontyphoid Salmonella species. Nontyphoid Salmonella in-clude S. enteritidis and S. typhimurium; these classically cause the acute colitis pattern, although crypt distortion and plasma cells may be seen as well (. Fig. 5a ). S. typhi and S. paratyphi cause typhoid fever. Patients present with acute-onset abdominal pain and fever, followed by watery diarrhea that progresses to bloody diarrhea. The terminal ileum and right colon are most affected. Endoscopically, prominent, nodular Peyer patches can be seen in the terminal ileum. Aphthous ulcers mimicking Crohn's disease occur when the mucosa overlying Peyer patches ulcerates. In advanced cases, deep, discoid, or linear ulcers form and can progress to perforation [11] .

The microscopic findings in typhoid fever differ from the classic acute colitis pattern of bacterial infection. Ulcers along the Peyer patches show lymphocytes, plasma cells, and histiocytes, along with necrosis. These histiocytes, also referred to as "typhoid cells" or "Mallorycells," are the main cell type. The classic typhoid fever lesion is the ulcerated Peyer patch, with numerous macrophages containing phagocytized bacteria and red blood cells. lymph nodes. It can mimic Crohn's disease because of the organism's predilection for the right colon and its ability to cause aphthous ulcers, granulomas, and transmural inflammatory involvement with lymphoid aggregates (. Fig. 5b ). Both acute colitis and granulomas may be seen on mi-croscopy, although granulomas with neutrophilic abscesses are more associated with Y. pseudotuberculosis. PCR may be necessary to assist in distinguishing between yersiniosis and Crohn's disease [12] .

Granulomas may be seen more in ileocecal involvement than in the rest of the colon,

where the disease can take on a nonspecific, inflammatory appearance.

Abdominal tuberculosis (TB), caused by Mycobacterium tuberculosis, accounts Der Pathologe 5 Fig. 6 ). On the other hand, granuloma formation can be seen in immunocompetent patients [15] . The organisms are not only positive on stains for acid fast bacteria but also on periodic acid-Schiff (PAS) staining and Grocott methenamine silver (GMS) [16] .

LGV is caused by the obligate intracellular bacterium Chlamydia trachomatis, serovars L1, L2, L3. These serovars infect macrophages and monocytes, which take the organism to the lymph nodes and may disseminate the disease. Although anal intercourse is a common cause of LGV proctitis, rectal disease can also occur via lymphatic spread of the organism from the vagina. HIV patients are particularly at risk; however, HIV positivity is not necessary to contract sexually transmitted proctocolitis [17] . Coinfection with other Table 5 RBC red blood cell, PCR polymerase chain rection, GMS Grocott methenamine silver, LGV lymphogranuloma venereum infectious agents, such as syphilis and/or gonorrhea, is frequent. Diagnosis is confirmed by rectal swab for C. trachomatis nucleic acid amplification, culture, or direct immunofluorescence testing. Three disease stages occur in LGV. Regional lymph node involvement and anorectal manifestations occur in the second stage. This includes inflammatory masses, proctitis, and hemorrhagic proctocolitis (. Table 3 ). Theanorectum is mostcommonlyaffected, but more proximal involvement can occur [18] . Without treatment, the disease progresses to scarring with stricture and fistula formation, representing the late, tertiary stage.

The histologic features of LGV proctitis are mild to moderate active inflammation and lamina propria plasmacytosis. The ulcers, granulomas, fibrosis, and strictures that may also be seen with this disease can make distinction from IBD difficult. Unlike IBD, however, LGV proctitis tends to have mostly intact crypt architecture, a lack of Paneth cells, and fewer eosinophils than IBD ( [19] ; . Table 4 ). Early LGV proctitis may not show granulomas; neutrophilic cryptitis and crypt abscesses are present and pseudomembranes can form if the inflammation is severe [20] . Soon after, however, lymphoplasmacytic and histiocytic inflammation predominates, with plasmacell-rich inflammation in the lamina propria and submucosa (. Fig. 7) . Swollen endothelial cells and perivascular plasma cell cuffing can also be seen [19] . An RNA in situ hybridization test for C. trachomatis exists for formalin-fixed paraffin-embedded tissue samples. Positivity consists of punctate, dot-like staining in the cytoplasm of affected cells. It is reported to have 84% sensitivity and 100% specificity [21] .

Syphilis is caused by the spirochete Treponema pallidum. The anorectal area is the most affected portion of the lower GI tract. Initial screening is with a nontreponemal test such as rapid plasma reagin, and, if reactive, followed by a treponemal test, such as fluorescent treponemal antibody absorption. The diagnosis can also be made on anorectal mucosal or ulcer swab samples via PCR [17] . Like LGV, syphilitic proctitis can clinically mimic IBD or rectal cancer. The microscopic appearance is similar to LGV: the mucosa exhibits intense lamina propria plasmacytosis with mostly intact crypt architecture and occasional granulomas (. Fig. 8) . Not all cases may show abundant plasma cells, however. Additional patterns reported of lower GI syphilis include histiocytic-predominant infiltration of the lamina propria along with loose granulomas and a lymphoma-like pattern with atypical appearing lymphocytes [22] . Notably, plasma cells may also be seen cuffing submucosal nerves. IHC for T. pallidum highlights the organisms in affected areas such as the epithelium and around vessels. Spirochetosis due to Brachyspira spp. can also be positive for this stain and should not be misinterpreted as syphilis [23] . Histologically, the Brachyspira organisms are present as an accentuated brush border on top of a b 

Fungal infections of the lower GI tract are uncommon and often occur in immunocompromised hosts. Identification methods such as molecular assays and culture are important in confirming the type of fungal organism. Although stains such as GMS may be helpful, histopathologic identification is insensitive, especially as fungal forms can swell and change their morphology with necrosis or treatment [24] . As some organisms are resistant to, or may require a different drug besides, ampho-a b 

Colonic candidiasis occurs exclusively in immunocompromised patients and accounts for 20% of GI candidiasis. Colon is the third most common GI organ to be involved, following esophagus and stomach [25] . Gross findings include mucosal flecks that resemble pseudomembranous colitis, irregular ulcers, segmental infarcts, and masses. Microscopically, the organisms display a mix of budding yeasts, pseudohyphae, and occasional septate hyphae. They may involve the superficial mucosa and submucosa, elicit an inflammatory mass, or cause transmural infarcts with extensive angioinvasion.

Gastrointestinal aspergillosis is often accompanied by lung involvement. Like candidiasis, it also occurs exclusively in immunocompromised patients and can cause infarction and angioinvasion. Gross abnormalities include mucosal flecks resembling pseudomembranous colitis, linear or oval ulcers, and infarcts. Microscopically, the organisms have septate hyphae with parallel walls and exhibit acute angle branching (. Fig. 9a ). Angioinvasion is characterized by fibrin thrombi mixed with hyphae that radiate out of vessels and extend into surrounding tissues. Hyphae may also be seen embedded in the mucosal surface. The Splendore-Hoeppli phenomenon, in which intensely eosinophilic, proteinaceous deposits of antigen-antibody complexes surround the organisms, can occur. Other fungi can have overlapping features with Aspergillus. Mucor, which only rarely affects the colon, also tends to cause angioinvasion [26] . Unlike Aspergillus, Mucor are pauciseptate and exhibit broad, ribbon-like hyphae with branching at various angles and optically clear centers. Basidiobolus ranarum, like Mucor, is also in the zygomycetes family. It is the causative agent of basidiobolomycosis; most cases are reported from desert regions of the US and the Middle East. This disease can cause severe infection in both immunocompetent and immunosuppressed individuals. The organisms have broad, occasionally septate hyphae with an appearance resembling crinkled cellophane [27] . The organisms are typically surrounded by a striking Splendore-Hoeppli reaction, which occurs less frequently with Mucor ( [28, 29] ;

. 

Histoplasma capsulatum, a soil saprophyte that grows well in bat and avian guano, is prevalent in the Ohio and Mississippi river valleys; it is also endemic in areas of Central and South America. Infection occurs when this soil fungus is inhaled, ingested by macrophages, and spread to the lymph nodes. GI histoplasmosis can affect both immunocompetent and immunocompromised patients. While isolated GI involvement does occur, the GI tract is frequently involved in the setting of disseminated disease. The colon, specifically, is involved in over 55% of GI histoplasmosis [30] . The most common sites of involvement include right colon, followed by the rectum and descending colon. Gross manifestations include ulcers, erosions, obstructive masses, or polyps [31] . Microscopically, the lamina propria is diffusely expanded by histiocytes with intracellular clusters of small (2-5 μm), nar-row-base budding yeasts with thin walls resembling a halo (. Fig. 10) . The protoplasm retracts during fixation, resulting in a clear pseudocapsule that appears not to stain. They are positive for GMS and PAS, but may also pick up Ziehl-Neelsen [32] . The organisms can be easily missed within ulcer debris and, rarely, may be extracellular. Granulomas are infrequent in the GI tract, occurring in only 8.5% of cases [30] . Intracellular Candida, particularly C. glabrata, can mimic Histoplasma. C. glabrata tends to have variably sized yeast cells, more frequent buds, and lack the halo seen around Histoplasma in tissue sections. They are positive on Gram stain and are more often extracellular.

Candida elicits a neutrophilic response, whereas Histoplasma manifests with histiocytes or granulomas. Pneumocystis jirovecii and Talormyces marneffei resemble histoplasmosis, but these organisms only rarely involve the colon. Pneumocystis jirovecii organisms are obligate extracellular organisms resembling crushed ping pong balls and lacking buds. T. marneffei infection occurs mostly in Southeast Asia in HIV patients. Like H. capsulatum, T. marneffei organisms are also small, 2-5 μm, and intracellular within macrophages; however, they do not bud and occasionally have larger forms with midline septa causing the organisms to resemble pill capsules. Other potential mimics of histoplasmosis include leishmaniasis and malakoplakia (. Fig. 11) . Leishmania organisms can also be seen within macrophages; however, they are negative on GMS, positive on Giemsa staining, and can also be extracellular. They are small (2-4 μm) round to oval organisms, with a bar-shaped paranuclear kinetoplast [33] .

Paracoccidioidomycosis, which occurs most commonly in Central and South America, tends to affect the entire colon. Half of cases involving the GI tract are in the context of disseminated disease, although no risk factors such as an immunocompromised status are associated with it [34] . Patients present with diarrhea and have ulcers, masses, or strictures identified endoscopically. The infection a b c d Fig. 12 8 Amebiasis and balantidiasis (H&E stain). a In this case of amebiasis, the colonic mucosa shows mild, superficial epithelial damage. The overlying exudate contains the E. histolytica trophozoites admixed with fibrin, mucus, and inflammatory cells. b E. histolytica trophozoites are shown here; note the ingested red blood cells.c,d The trophozoite of Balantidium coli is much larger than E. histolytica, has a ciliated surface, and a kidney-shaped macronucleus. H&E hematoxylin and eosin mimics IBD and may also cause malakoplakia [35] . The organisms are variably sized, round, budding yeast cells ranging from 4 to 40 μm. The classic finding is that of a large mother cell surrounded by budding cells, resembling a ship's wheel. Without this distinctive feature, however, it may be easy to confuse these organisms with other budding yeasts.

Coccidioides immitis infection is referred to as Valley fever and only rarely affects the colon [36, 37] . When it does, it can mimic carcinomatosis bystuddingtheperitoneum. In the tissue, the fungi appear as large, 10-80 μm double-walled refractile spherules filled with endospores. The diagnosis should only be made if at least one unequivocal, intact spherule containing endospores is seen [24] . Vague, circular structures resembling empty Coccidioides spherules should not be interpreted as the organism. These can represent cross sections of bulbous parts of other types of fungi such as Aspergillus. The opposite is also true: expelled Coccidioides endospores can mimic other fungi, such as Cryptococcus.

Cryptococcus neoformans, whichis found in pigeon droppings, can cause a granulomatous colitis that simulates Crohn's disease. It can present as a mass, stricture, or abscess, and affect any part of the colon. Most patients are immunocompromised and GI involvement is asso-ciated with disseminated disease. In fact, GI cryptococcosis may be the first manifestation of disseminated disease [38] . Cryptococcus organisms measure 4-7 μm and show notable variation in size and frequent budding. They have an area of clearing resembling a soap bubble around the organism. This represents a poorly stained capsule on H&E sections that, on the other hand, is intensely positive with mucicarmine. Fontana-Masson staining is useful in capsule-deficient Cryptococcus.

The protozoan Entamoeba histolytica causes amebiasis, which is transmitted mainly through fecal-oral spread, but also a b c d Fig. 13 8 Strongyloidiasis (H&E stain). a Adult females have visible internal organs.b Strongyloides stercoralis within a crypt lumen without much inflammation. c Brightly eosinophilic Splendore-Hoeppli phenomenon is seen around a larval form. d Degenerated larvae can be difficult to visualize and may mimic other infectious agents. H&E hematoxylin and eosin through anal intercourse. Antigen detection and PCR are sensitive and specific diagnostic tools, but biopsy can raise the first diagnosis [39, 40] . E. histolytica has trophozoite and cyst forms. The cysts are highly infectious and, unlike trophozoites, can resist both gastric acid and chlorine concentrations used in sewage. When ingested, the cysts reach the ileocecal valve area and release the trophozoites. The trophozoites adhere to colonic epithelial mucin and colonize the large intestine. They also initiate tissue damage by secreting enzymes that lyse cells and eventually give rise to the classic, flask-shaped ulcers. Chronic infection can last for years, be limited to the GI tract, or spread to other organs, such as liver. The infection can be asymptomatic, cause mild diarrhea, and blood and mucus in the stool, or severe, acute fulminant necrotizing colitis with toxic megacolon and death. Recent literature includes a report of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection aggravating preexisting amoebic infection, inducing fulminant colitis [41] . Endoscopic findings are nonspecific and show friable and diffusely inflamed mucosa. Mass formation, stricture, and perforation can also occur due to amebiasis. The proximal colon is commonly involved, but multiple sites of the colon are usually affected [42] . The classic amoebic lesion is that of a flask-shaped ulcer. As the organisms induce ulcers, they invade both laterally as well as deeply. They undermine intact mucosa, leaving overhanging edges that form the flask-like appearance. Trophozoites are numerous at the edges of the ulcer, undermining the epithelium and causing necrosis and tissue disruption. They are also found in the overlying exudate admixed with fibrin, mucin, and inflammatory cells. Invasion of trophozoites into or through the muscularis propria is a risk factor associated with death in fulminant amebic colitis [42] . The organisms measure up to 40 μm and have abundant foamy or vacuolated cytoplasm. It is important to see the ingested red blood cells, which helps distinguish E. histolytica from mimics such as macrophages and nonpathogenic amoebas. The amoebae have subtle nuclear features characterized by fine peripheral chromatin and a central karyosome (. Fig. 12) . The organisms are intensely PAS positive; Fig. 12d ).

The nematode Strongyloides stercoralis can complete its entire life cycle within a human being. As a result, strongyloidiasis carries the potential for chronic, asymptomatic disease via low levels of autoinfection. Immunocompetent and immunosuppressed persons can be infected alike, and initially become exposed via skin contact with contaminated soil in tropical and subtropical endemic areas. Other, less frequent, modes of transmission include fecal-oral spread and donor-derived infections in transplant recipients [44] . The larvae can reach the small intestine by various means. This includes migrating from the skin to the pulmonary and tracheobronchial system and being swallowed. Maturation to adult worms occurs in the small bowel. Females produce eggs that develop into noninfectious larvae (rhabditiform larvae), which are shed into the stool. Autoinfection occurs when some of the rhabditiform larvae transform into the infective, filariform larvae instead of being excreted [45] . These filariform larvae penetrate the bowel mucosa or the perianal skin and again make their way to the lungs to repeat the cycle of autoinfection.

Clinical diagnosis is difficult due to the lack of specific symptoms and low detection rates of common tests. The low levels of autoinfection can cause persistent, undetected disease over decades, with symptoms that may emerge long after the original exposure. Immunosuppression with even short courses of corticosteroids-regardless of dose, immune status, or remoteness of initial infection-can cause increased autoinfection and more overt symptoms can emerge. The change in immune milieu causes accelerated transformation of the organisms into the infective larval form or increased parasitic burden by means of asexual reproduction. Hyperinfection syndrome refers to the sequelae of accelerated autoinfection. It is often related to a change in immune status and carries a high mortality rate, especially with dissemination of the organisms to organs beyond the GI tract and lungs.

Serologic testing, stool examination, culture, and PCR are some methods employed for diagnosis. Detection rates, however, may be low unless the patients are experiencing accelerated autoinfection. Colonoscopy may exhibit an appearance and distribution mimicking IBD. Other endoscopic features include erythema, yellow-white nodules, edema, loss of vascular pattern, and ulcers [46] . Therefore, due to the lack of specific symptoms and low sensitivity of common testing modalities, a biopsy identification of the organisms may be the first, and unexpected, way that strongyloidiasis is diagnosed.

Histologically, the organisms may be seen in the small bowel, colon, and stomach. Both worms and eggs can be found in GI samples in strongyloidiasis. Site-dependent differences exist: as the small bowel is the site of maturation to adults, adult females and eggs are usually visualized in the small bowel rather than the colon, unless the patient is immunosuppressed. Cross sections of the larvae measure 12 to 18 μm in diameter [47] . They exhibit a cuticle of 1 μm in thickness, surrounding collections of nuclei that measure 1 μm each. These nuclei correspond to the alimentary canal of the organism, and, on longitudinal section, are arranged in 1-2 rows measuring 1 μm each. Adult females are larger, exhibiting cross-sectional diameters of 30 to 45 μm as well as visible internal organs and sharply pointed tails (. Fig. 13 ). Adults and larvae may be seen occupying crypt lumens without an inflammatory response, whereas the presence of larvae in the lamina propria is usually accompanied by inflammation.

The background bowel mucosa may show varying degrees of eosinophilic and neutrophilic inflammation, crypt distortion, and granulomas, mimicking Crohn's disease. The degree of tissue eosinophilia is related to the burden of parasitic infection [47] . The Splenodore-Hoeppli phenomenon can occur around the organisms [48] . It is important to suspect and search for strongyloidiasis in eosinophilic aggregates, particularly as degenerating larvae can easily be overlooked. One of the differential diagnoses of eosinophilia in the bowel includes idiopathic eosinophilic gastroenteritis, a diagnosis that may be treated with steroids. In the context of overlooked strongyloidiasis, a misdiagnosis of eosinophilic gastroenteritis with ensuing steroid treatment may result in hyperinfection syndrome and fatal consequences.

Intestinal schistosomiasis (IS) can be a chronic condition acquired first by skin contact with cercarial larvae in contaminated freshwater. Species of this trematode fluke that involve the intestines include S. japonicum, S. mansoni, and S. mekongi, as opposed to S. haemato-a b Fig. 14 8 Schistosoma ova. a A multinucleated giant cell is seen surrounding a refractile S. mansoni egg with a lateral spine. b Calcified Schistosoma eggs are seen with minimal inflammatory response bium, which tends to affect the urinary tract and is only occasionally found in the GI tract. The schistosomes penetrate the dermis and blood vessel walls, eventually reaching the liver, where they feed on red blood cells, mature, and mate in the liver vasculature. Females lay eggs in the vessels, which migrate to the intestine, infiltrate the wall to reach the lumen, and are shed in the stool [49] . Stool ova and parasite studies may be negative in IS patients and peripheral blood eosinophils may not be elevated in chronic disease as compared to acute schistosomiasis [50] [51] [52] Colonoscopically, IS tends to cause yellow, granular or "sandy" appearing mucosa or polyp formation. Ulcers may be seen, but not the cobblestoned appearance or longitudinal fissuring ulcers typical of Crohn's disease. Compared to Crohn's disease, IS tends to involve the left colon and rectum, whereas Crohn's disease affects small intestine and ascending colon more frequently.

Strictures and transmural involvement are uncommon but can occur (. Table 5 ).

Resection specimens of IS may show submucosal fibrosis, noncaseating granulomas, and multinucleated giant cells with eggs. However, muscularis mucosa thickening, pyloric metaplasia, and crypt abscesses typical of Crohn's disease were not seen in IS according to one comparative study, and lymphoid aggregates were rare. Interestingly, contrary to what is expected, Crohn's disease patients may have more eosinophils in the lamina propria than is seen in IS. The worms may be seen in mesenteric veins.

On biopsy, granulomas and multinucleated giant cells may be present, but deeper sections may be necessary to reveal the eggs within the granulomas. The ova measure up to 180 μm with a width of approximately 70 μm, and may be calcified (. Fig. 14) . Inflammatory polyps and ulcers may be seen. Sometimes, little inflammatory response may be apparent surrounding the eggs. Crypt architectural distortion is uncommon, but can be seen in 25% of IS cases, mostly at the edges of ulcers. It is thought to be ischemia related from vascular occlusion by eggs.

By identifying the presence of CMV and other infectious causes for colitis, pathologic diagnoses can change patients' treatment. In addition to immunohistochemical and molecular tests, artificial intelligence (AI) may be a resource to aid pathologists in the future. AI could help direct pathologists to focus on the areas of tissue most suspicious for infectious organisms, particularly when dealing with vastly inflamed or necrotic tissue. It may also be useful in urgent clinical situations when immunohistochemical or molecular test results are not readily available.

The colon is subject to a wide variety of infectious agents. Accurate identification of the pathogen often requires definitive laboratory studies. The morphologic appearance of infectious agents in tissue is important to recognize, as some diseases, e.g., strongyloidiasis, schistosomiasis, and amebiasis, may go undetected as chronic disease and only be diagnosed by biopsy.

Histopathologists play an important role in the management of immunocompromised patients, such as those with IBD, HIV, or transplants.

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Effects of antiviral therapy for patients with inflammatory bowel disease and a positive intestinal biopsy for cytomegalovirus

Cytomegalovirus infection in ulcerative colitis is related to severe inflammation and a high count of cytomegaloviruspositive cells in biopsy is a risk factor for colectomy

Immunostaining detection of Cytomegalovirus in gastrointestinal biopsies: clinicopathological correlation at a large academic health system

Adenovirus infection of the large bowel in HIV positive patients

Incidence of adenovirus infection in hematopoietic stem cell transplantation recipients: findings from the AdVance study

A survey on incidence and management of adenovirus infection after allogeneic HSCT

Adenovirus infections in adult recipients of blood and marrow transplants

Comparison of clinical features and pathologic findings in fatal cases of typhoid fever during the initial and later stages of the disease

The role of Yersinia enterocolitica and Yersinia pseudotuberculosis in granulomatous appendicitis: a histologic and molecular study

Abdominal tuberculosis of the gastrointestinal tract: revisited

Intestinal tuberculosis: return of an old disease

Pathologic findings in disseminated Mycobacterium avium-intracellulare infection. A report of 11 cases

A novel presentation of Mycobacterium avium complex in arecipientofalungtransplant: acasereport

European Guideline on the management of proctitis, proctocolitis and enteritis caused by sexually transmissible pathogens

Rectal pseudotumor secondary to Chlamydia trachomatis: not everything is what it seems

Sexually transmitted infectious colitis vs inflammatory boweldisease: distinguishingfeaturesfromacasecontrolled study

Case records of the Massachusetts General Hospital. Case 2-2006. A 31-year-old, HIV-positive man with rectal pain

Lymphogranuloma venereum (LGV) of the anorectum: evaluation of clinicopathological associations and the utility of a novel RNA in-situ hybridisation stain

Syphilis of the aerodigestive tract

Immunohistochemical detection of human intestinal spirochetosis

Challenges and pitfalls of morphologic identification of fungal infections in histologic and cytologic specimens: a ten-year retrospective review at a single institution

Fungal infections of the small and large intestine

Gastrointestinal mucormycosis

Zygomycetes in human disease

Gastrointestinal basidiobolomycosis: an unusual fungal infection mimicking colon cancer

Basidiobolomycosis of right colon mimicking as carcinoma of colon

Thepathologicspectrumofgastrointestinal and hepatic histoplasmosis

Gastrointestinal histoplasmosis

Acid-fastness of histoplasma in surgical pathology practice

Colonic leishmaniasis followed by liver transplantation

Fungal infection of the colon

Intestinalmalakoplakiaassociated with paracoccidiodomycosis: a new association

A case of the hiccups in the setting of colonic ulcers

Abdominal and pelvic coccidioidomycosis

Gastrointestinal cryptococcosis

Clinical manifestations and endoscopic findings of amebic colitis in a United States-Mexico border city: a case series

Fatal attraction: intestinal amebiasis and COVID-19 as risk factors for colonic perforation

Fulminant amebic colitis: analysis of 55 cases

Detection of entamoeba histolyticainexperimentallyinducedamoebicliver abscess: comparison of three staining methods

Human infection with Strongyloides stercoralis and other related Strongyloides species

Colonoscopic findings and pathologic characteristics of Strongyloides colitis: a case series

Histopathology of gastric and duodenal Strongyloides stercoralis locations in fifteen immunocompromised subjects

Strongyloides stercoralis mesenteric lymphadenopathy: clue to the etiopathogenesis of intestinal pseudo-obstruction in HIV-infected patients

Bilharzia: pathology, diagnosis, management and control

Clinicopathologic features of chronic intestinal Schistosomiasis and itsdistinctionfromCrohndisease

Gastrointestinal manifestations of schistosomiasis