GI Infections

This episode follows one Clostridioides difficile infection from the first loose stool to the operating room. The organizing idea is that each step has a gate: test only the right patient, read a two-step result correctly, grade severity from two laboratory numbers, then match the drug and route to that grade. The order is the testing threshold, NAAT with toxin EIA, a second patient whose positive NAAT may be colonization, severity, first-line therapy with fidaxomicin or oral vancomycin, why metronidazole ranks last, fulminant disease with rectal vancomycin and early surgical consultation, the choice of operation, and why a recovered patient is not retested.

Recurrent Clostridioides difficile infection is a problem of surviving spores, so every second-line plan aims to keep drug in the colon across several rounds of germination. The episode follows one woman through three steps. At her first recurrence, the drug that failed last time decides the next drug: fidaxomicin after metronidazole or vancomycin, and a vancomycin taper and pulse after fidaxomicin. Bezlotoxumab is no longer available. At her second recurrence, microbiota therapy replaces a third antibiotic course, as screened-donor FMT, rectal Rebyota, or oral Vowst. Her next antibiotic brings oral vancomycin prophylaxis, because she has prior CDI. Last, a recurrence is defined by symptoms before any testing, since post-infection irritable bowel syndrome can mimic it.

The organizing idea is that the right move in acute diarrhea depends on the organism, the host, and the clock. The episode starts with a traveler's bloody diarrhea and asks when to give an antibiotic, which one fits the region, why rifaximin fails in invasive disease, and when loperamide is safe. It then separates Campylobacter, Salmonella, Shigella, and Shiga toxin-producing E. coli, where one must get no antibiotic. Next come hosts at special risk, with Vibrio and Yersinia in iron overload and Listeria in pregnancy. It closes with preformed toxin syndromes recognized by onset time, and the sequelae that follow infection.

Each gut parasite is matched to its own test and its own drug, and the episode works through that idea in two patients and a final case. A woman who drank stream water shows when to suspect a parasite, why antigen or PCR testing beats stool microscopy, and how Giardia, pregnancy, recurrence, and Cryptosporidium change management. A traveler with a liver abscess shows why amebiasis needs a tissue agent followed by a luminal agent. A man from rural Ecuador facing prednisone shows how Strongyloides is screened by serology and treated with ivermectin to prevent hyperinfection, and why hookworm needs one dose of albendazole. The episode ends with Anisakis, which is removed at endoscopy rather than treated with a drug.

This final episode of the GI Infections chapter covers infections that a weakened immune system lets in, then the virus that needs no such help. It begins with CMV colitis in steroid-refractory ulcerative colitis, where the ulcer base is biopsied with immunohistochemistry and ganciclovir is given with immunosuppression reduced. A transplant recipient then shows how UL97 and UL54 mutations decide the next drug. Next comes advanced HIV: separating MAC from Whipple disease with an acid-fast stain, treating MAC with several drugs, and finding and treating microsporidia, Cyclospora, and Cystoisospora. The episode closes with a norovirus outbreak on a care-home floor, managed with supportive care, soap and water, and bleach.