GI Infections: Immunocompromised-Host Enteric Infections

0:00Welcome to Board Pearls. This is episode five of five of the GI Infections chapter, in the Special Populations and Acute or Supportive Care module. In this episode we cover the enteric infections that target the immunocompromised host: CMV colitis with owl-eye intranuclear inclusions on biopsy of the ulcer base and ganciclovir as primary therapy, MAC enterocolitis with macrolide-containing regimens, microsporidia, and Cyclospora with trimethoprim-sulfamethoxazole as treatment of choice.

0:29The immunocompromised gut expands the pathogen list in a way the community-acquired episodes did not. Each of these organisms lives behind a specific recognition cue, and each cue points to a specific drug. The cue and the drug travel together. If you can name the host, the stain, and the histology, you can name the therapy.

0:50Start with CMV colitis, because it is the one whose host scenario the boards return to most often. Cytomegalovirus establishes lifelong latent infection after primary exposure and reactivates under immunosuppression. The hosts in whom CMV colitis emerges fall into four groups. The first is solid organ transplant recipients, especially in the first one to six months after transplant when immunosuppression is most intense. The second is HIV patients with CD4 below one hundred, with disseminated disease most often below fifty. The third is patients on steroid therapy or biologics for inflammatory bowel disease. The fourth is patients receiving chemotherapy or other long-term immunosuppression.

1:35The presentation is bloody diarrhea, fever, abdominal pain, and weight loss. Endoscopy typically shows deep punched-out or serpiginous ulcers, often in the right colon and terminal ileum, with the mucosa between ulcers either grossly normal or inflamed depending on the underlying host. That ulcer morphology is what makes the next decision important.

1:55The most-tested host scenario is acute severe ulcerative colitis that fails to respond to intravenous corticosteroids. CMV colitis is found in roughly a third of these refractory cases on appropriate biopsy testing. Identifying it changes the next-step decision from biologic escalation to antiviral therapy combined with reduction of immunosuppression where the underlying disease allows. The reason this matters is that the stem looks like steroid-refractory UC. The treating instinct is to reach for infliximab or cyclosporine. The right move is to look for CMV first.

2:30Now to the diagnostic point that drives the most question stems. Biopsy must be taken from the ulcer base, meaning the floor of the ulcer, rather than from the ulcer edge or grossly normal mucosa. CMV-infected fibroblasts and capillary endothelial cells live in the submucosa at the ulcer floor. The edge and the normal-appearing mucosa do not concentrate the virus, so biopsies from those sites yield false negatives. If a stem describes a colonoscopy with random biopsies returning negative in a patient who clinically looks like CMV, the issue is sampling, not absence of disease.

3:08Histology with hematoxylin and eosin shows large basophilic intranuclear inclusions surrounded by a clear halo. This is the owl's-eye Cowdry type B inclusion. There is also cytomegaly with cytoplasmic inclusions. Hematoxylin and eosin alone, though, has sensitivity below fifty percent for CMV in colonic biopsies because the inclusions can be sparse and easy to miss. That is why immunohistochemistry for CMV antigens is the recommended adjunct on every biopsy where CMV is in the differential. Tissue PCR for CMV DNA is an additional adjunct in specialized centers.

3:46Blood CMV PCR supports systemic disease and is the appropriate first systemic test. But a positive blood result does not establish tissue invasion, and a negative blood result does not exclude tissue-localized colitis. The diagnosis is made in the tissue.

4:02Treatment is intravenous ganciclovir five milligrams per kilogram every twelve hours, renal-dose-adjusted, for fourteen to twenty-one days for induction. The transition is to oral valganciclovir nine hundred milligrams twice daily until clinical and virologic resolution.

4:18The resistance pattern is the part of CMV pharmacology that gets tested directly, and it has to be understood at the level of viral enzymes. Ganciclovir is a prodrug. It requires phosphorylation by the viral UL97 kinase to become active. UL97 mutations block this activation and produce ganciclovir resistance. Foscarnet is a pyrophosphate analog that directly inhibits the viral UL54 DNA polymerase without requiring activation. Because foscarnet does not depend on UL97 phosphorylation, UL97 mutations do not affect it, and foscarnet remains active in UL97-mutant CMV. That is why foscarnet is the salvage agent when ganciclovir fails despite adherence and adequate dosing.

5:09UL54 mutations affect the polymerase itself and produce resistance to both ganciclovir and foscarnet. Few options remain at that point. Maribavir, four hundred milligrams orally twice daily, directly inhibits the UL97 kinase enzymatic activity rather than requiring it for activation. UL97 mutations that block phosphorylation of ganciclovir do not necessarily prevent maribavir binding to the same kinase. That is why maribavir works in UL97-resistant disease. The absence of myelosuppression compared with ganciclovir also reflects this mechanism, because maribavir does not require host-cell phosphorylation.

5:48Reduction of immunosuppression is paired with antiviral therapy where the underlying disease state allows. The reason is that restoring CD8 T-cell control of CMV is the durable mechanism of clearance. Antivirals alone do not achieve it. The drug suppresses replication, and the immune system clears what is left.

6:09Move now to MAC enterocolitis, where the recognition cue is histologic and the drug logic is combination by design. Mycobacterium avium complex affects HIV patients with CD4 below fifty cells per microliter. The presentation is fever, night sweats, weight loss, abdominal pain, chronic diarrhea, hepatosplenomegaly, and abdominal lymphadenopathy. Endoscopy in the duodenum shows pale yellow nodules with villus blunting. Small bowel biopsy shows lamina propria packed with macrophages.

6:44The histologic discriminator from Whipple disease is acid-fastness, and this distinction is testable directly on a biopsy-image stem. MAC-laden macrophages are PAS-positive and acid-fast positive on Ziehl-Neelsen or Kinyoun staining. Whipple macrophages are PAS-positive and acid-fast negative. Two diseases, same macrophage-stuffed lamina propria, same PAS positivity. The acid-fast stain breaks the tie. If the image shows pink rods on a Kinyoun stain alongside the PAS-positive macrophages, the diagnosis is MAC.

7:16Treatment of disseminated MAC is a macrolide plus ethambutol, with rifabutin added in advanced disease or when macrolide resistance is suspected. The macrolide is either clarithromycin five hundred milligrams orally twice daily, or azithromycin five hundred to six hundred milligrams orally daily. Ethambutol is fifteen milligrams per kilogram orally daily. Rifabutin is three hundred milligrams orally daily when added.

7:41The combination logic is mechanistic and it predicts the stem about why monotherapy fails. MAC is an intracellular pathogen, living within macrophages. Macrolides accumulate intracellularly to high concentrations, which is what makes them the active backbone. But macrolide monotherapy selects for macrolide resistance rapidly, because residual organisms persist after partial killing and the surviving population is enriched for resistance. Ethambutol acts on a different target. It inhibits mycobacterial cell-wall arabinogalactan synthesis through arabinosyl transferase, eliminating residual organisms before they can develop macrolide resistance. Rifabutin is an RNA polymerase inhibitor, adding a third non-overlapping mechanism for advanced disease or documented macrolide resistance.

8:30Therapy continues until immune reconstitution is achieved on antiretroviral therapy, defined as CD4 above one hundred cells per microliter for at least six months. The reason is the same one that applies to CMV. Intracellular killing of residual organisms requires functional T-cell-mediated macrophage activation. Antibiotics alone cannot replace it. The drug holds the line. The immune system finishes the job.

9:00Primary prophylaxis is a board-relevant historical fact. Patients with CD4 below fifty historically received azithromycin twelve hundred milligrams orally weekly, or clarithromycin five hundred milligrams orally twice daily. Current US guidelines no longer routinely recommend primary prophylaxis when patients are initiating effective antiretroviral therapy, because MAC risk falls quickly with viral suppression. The twelve-hundred-milligram weekly azithromycin regimen remains the historical standard and shows up in stems framed in the older guideline era.

9:33A second MAC-adjacent stem the boards write is paradoxical worsening after starting antiretroviral therapy. A patient with newly diagnosed HIV and disseminated MAC starts a macrolide-containing regimen alongside antiretroviral therapy and improves over two weeks. The patient then deteriorates with fever, lymphadenitis, and worsening abdominal pain despite microbiologic improvement on the MAC drugs. That is immune reconstitution inflammatory syndrome. It reflects a reactivated immune response to pathogen antigens, not failure of antimycobacterial therapy. Management is to continue the MAC regimen, continue antiretroviral therapy, and add anti-inflammatory therapy where severity demands it.

10:18Move to microsporidia, where the teaching point lives at the level of beta-tubulin pharmacology. Microsporidia cause chronic watery diarrhea with weight loss in HIV at low CD4 counts and in transplant patients. Enterocytozoon bieneusi is the most common species. Encephalitozoon intestinalis is less common but matters because of the drug-response difference.

10:41Diagnosis is by chromotrope or modified trichrome stain on stool, also called the modified Weber stain, or by PCR. The spores are one to two micrometers, which is small enough that careful microscopy and the right stain are both needed; an ordinary ova-and-parasite exam misses them.

10:59The drug logic separates the two species. Albendazole binds beta-tubulin to inhibit microtubule polymerization. Encephalitozoon intestinalis has an albendazole-sensitive tubulin and is treated with albendazole four hundred milligrams orally twice daily for twenty-one days. Enterocytozoon bieneusi has a tubulin variant that albendazole binds poorly, so the drug is unreliable against this species. Fumagillin is the alternative. It inhibits methionine aminopeptidase two, an entirely different target that is not affected by the tubulin variant. Fumagillin has limited US availability, which is itself a testable point. In HIV, antiretroviral therapy is the dominant intervention, because immune reconstitution clears the infection where antiparasitic therapy alone is incomplete.

11:50Move to Cyclospora cayetanensis, where the recognition cue is epidemiologic and the drug is trimethoprim-sulfamethoxazole for a reason that is worth understanding. Cyclospora is a coccidian protozoan. It produces self-limited diarrhea in immunocompetent hosts and prolonged diarrhea in HIV. The classic board stem is imported produce in summer months: raspberries, basil, cilantro, snow peas. The other classic stem is a returning traveler from Nepal or Peru.

12:22Diagnosis is by modified acid-fast stain on stool. The oocysts are eight to ten micrometers, with ultraviolet autofluorescence as a confirmatory feature. The size and the autofluorescence both distinguish Cyclospora from Cryptosporidium, which is four to six micrometers and does not autofluoresce.

12:40Treatment is trimethoprim-sulfamethoxazole, one double-strength tablet orally twice daily for seven to ten days, with longer courses in HIV. The reason TMP-SMX is the drug of choice, and the reason it is more or less the only drug of choice, lives in the parasite's folate biology. Cyclospora is a coccidian parasite that depends on de novo folate synthesis without a salvage pathway. Trimethoprim inhibits dihydrofolate reductase. Sulfamethoxazole inhibits dihydropteroate synthase. The two-step block is synthetically lethal to the organism, because there is no alternative folate supply to fall back on. Nitazoxanide and metronidazole, which target anaerobic protozoa, do not act on coccidia. That is why the stem about a traveler with Cyclospora and a sulfa allergy is genuinely a hard problem and not just a guideline lookup.

13:33One more coccidian protozoan lives on the same workup pathway, and the boards link it to Cyclospora because the same drug treats it for the same reason. Cystoisospora belli, formerly Isospora belli, causes chronic diarrhea most often in immunosuppression but also as chronic travelers' diarrhea in immunocompetent hosts, with the highest endemic incidence in Haiti and other resource-limited regions. Diagnosis is by detection of large oocysts, ten by twenty micrometers, on stool examination with modified acid-fast staining.

14:04The pathognomonic feature on the boards is that Cystoisospora is the only protozoan to cause peripheral eosinophilia along with Charcot-Leyden crystals in stool. That combination is the stem-level fingerprint. Treatment is trimethoprim-sulfamethoxazole one double-strength tablet orally twice daily for ten days, with longer courses in HIV. Secondary prophylaxis is indicated when CD4 is below two hundred cells per microliter, because that is the level below which CD4-mediated control of intracellular protozoa fails, mirroring the threshold for Pneumocystis prophylaxis.

14:41Step back and look at what the four organisms have in common. Each one targets the immunocompromised host, and each one is recognized by a specific cue that points to a specific drug. CMV gives you the owl-eye intranuclear inclusion at the ulcer base, treated with ganciclovir, with foscarnet as the salvage when UL97 mutations break the prodrug pathway. MAC gives you the PAS-positive, acid-fast-positive macrophages packed in the duodenal lamina propria, treated with a macrolide plus ethambutol because intracellular killing requires high macrolide concentrations and a second drug to prevent resistance. Microsporidia give you the one-to-two-micrometer spores on a modified trichrome stain, treated with albendazole if the species is Encephalitozoon intestinalis and fumagillin if it is Enterocytozoon bieneusi, because the tubulin variant decides drug binding.

15:36Cyclospora gives you the eight-to-ten-micrometer autofluorescent oocyst on modified acid-fast stain, with the imported-produce or Nepal-or-Peru stem, treated with trimethoprim-sulfamethoxazole because coccidian folate biology has no salvage pathway. And Cystoisospora gives you the large oocyst plus the eosinophilia plus Charcot-Leyden crystals signature, treated with the same TMP-SMX for the same coccidian-folate reason.

16:04The unifying thread underneath all of it is that antimicrobial therapy holds the infection in check while immune reconstitution provides durable clearance. Ganciclovir suppresses CMV replication and CD8 T-cells finish it. Macrolides and ethambutol suppress MAC and CD4-mediated macrophage activation finishes it. Albendazole or fumagillin reduces microsporidia burden and antiretroviral therapy clears it. TMP-SMX kills Cyclospora and Cystoisospora, and a CD4 above two hundred keeps them from coming back. The drug and the host work together. Neither one alone is sufficient.

16:43That sets up Chapter thirty four, GI in the Immunocompromised Host, which takes the broader frame this chapter only gestured at. The next chapter covers the solid organ transplant GI disease workup, including acute gut graft-versus-host disease. It covers the HIV host framework with CD4 stratification and the immune reconstitution inflammatory syndrome pattern. And it covers the checkpoint inhibitor colitis and hepatitis irAEs that have become a routine part of the GI consult service. The organizing question there is what host state the patient is in, because the host state changes what the workup looks for and what the treatment risks.

17:22For the full chapter, the practice vignettes, and the topic-tagged question bank, head to board pearls dot com. You'll find the rest of the series on Apple Podcasts, Spotify, or wherever you listen to podcasts. That brings us to the end of episode five of five of chapter thirty three, and I'll see you in the next one.