GI Infections: GI Infections: Gut Parasites and Drugs

0:00Welcome to Board Pearls. This is episode four of five of the GI Infections chapter, in the Special Populations and Acute or Supportive Care module. This episode is the gut parasites, each matched to its test and its drug. The first patient is a woman who drank stream water. Her case shows when a parasite is suspected, why antigen testing has replaced stool microscopy, why tinidazole is one dose while metronidazole takes a week, what to do when symptoms outlast the infection, and how Cryptosporidium, carried by the same water, differs. A traveler with a liver abscess shows why amebiasis needs two drugs in sequence. The second patient is a man from rural Ecuador about to start prednisone, and his case shows which parasite can kill a patient the week he starts it and how that's prevented, along with why hookworm needs a single dose. The episode ends with the parasite that needs a scope instead of a drug.

0:52Here's the first patient. A forty-four-year-old woman went backpacking and drank untreated mountain stream water. Ten days after returning, she developed foul-smelling greasy diarrhea, bloating, cramping, and heavy gas. Two weeks later she's still sick and has lost three kilograms.

1:09She most likely has giardiasis, a protozoal infection of the small intestine. A parasite is suspected for one of three reasons: an exposure, such as untreated water, daycare, recent travel, or immunocompromise; diarrhea that persists beyond seven to fourteen days; or a specific pattern. Steatorrhea with weight loss suggests Giardia, dysentery with a liver abscess suggests Entamoeba histolytica, hyperinfection when steroids start suggests Strongyloides, and peripheral eosinophilia suggests Strongyloides or Cystoisospora. She has the exposure, the duration, and the pattern of Giardia.

1:44The workup is chosen for the suspected organism rather than a generic ova-and-parasite exam, because antigen tests and polymerase chain reaction, PCR, are more sensitive than microscopy for many of these organisms. Giardia is the most commonly diagnosed enteric protozoal infection in the United States, with risk from untreated surface water, daycare, sexually active men who have sex with men, freshwater swimming, and immunoglobulin A deficiency or common variable immunodeficiency. The test is a stool antigen enzyme immunoassay or stool PCR, both highly sensitive. A single ova-and-parasite exam misses a third to a half of infections, because cysts are shed intermittently. Her antigen is positive.

2:28Her drug is chosen by half-life. Tinidazole, two grams as a single oral dose, is preferred, and metronidazole, two hundred fifty milligrams three times daily for five to seven days, is the alternative. Tinidazole's longer half-life sustains drug exposure across the parasite's life cycle from one dose, while metronidazole's shorter half-life needs repeated doses, with more cumulative gastrointestinal and neurologic side effects. Nitazoxanide, five hundred milligrams twice daily for three days, is another option, and the only drug approved by the Food and Drug Administration for both giardiasis and cryptosporidiosis. She takes one dose of tinidazole.

3:08Three weeks later her antigen is negative, but she has bloating and loose stools after dairy. Lactose intolerance often persists for weeks after Giardia is cleared, and it's the usual explanation for apparent treatment failure once the cysts are gone. So she isn't retreated, and cutting back on lactose while her gut recovers should help.

3:27Two situations would change her care. In pregnancy, paromomycin, twenty-five to thirty-five milligrams per kilogram per day divided three times daily for five to ten days, is the treatment of choice, because it's poorly absorbed, so fetal exposure is negligible while it still reaches the parasite in the gut. And recurrent giardiasis, or recurrent enteric infections with sinopulmonary infections, prompts evaluation for common variable immunodeficiency.

3:54Her stream water could also have carried Cryptosporidium, which resists chlorine. It's acquired from contaminated water, daycare, contact with calves and lambs, and recreational water. In a healthy host like her, it causes self-limited watery diarrhea over one to two weeks. In human immunodeficiency virus, HIV, infection with a CD4 count below one hundred to two hundred, it causes voluminous chronic watery diarrhea with weight loss, and it's the leading cause of acquired immunodeficiency syndrome, AIDS, cholangiopathy, covered in chapter thirty-four.

4:27Cryptosporidium is diagnosed by modified acid-fast stool stain, stool antigen, or PCR, and treated with nitazoxanide, five hundred milligrams twice daily for three days, which works less well in the immunocompromised. In HIV, antiretroviral therapy is the main treatment, because immune recovery clears what antiparasitic drugs often can't.

4:47A traveler with a different parasite shows why some infections need two drugs. A man back from India has fever and right upper quadrant pain, and imaging shows a right lobe liver abscess with positive amebic serology. He has invasive amebiasis from Entamoeba histolytica. Its spectrum runs from asymptomatic infection of the gut lumen to bloody dysentery to amebic liver abscess, most often in the right lobe, with the classic anchovy-paste aspirate. It comes from contaminated food and water in endemic regions, Mexico, India, sub-Saharan Africa, and Latin America, and from fecal-oral spread, including sexual exposure.

5:27The colonic ulcer is flask-shaped on histology. A stool antigen test separates pathogenic E. histolytica from non-pathogenic Entamoeba dispar, which look alike under the microscope. Serology is highly sensitive in invasive disease like his, but less useful in people from endemic regions, where many are seropositive from past exposure.

5:49His treatment comes in two stages, because the parasite lives in two places. Metronidazole and tinidazole are well absorbed and reach the liver and colonic wall, killing the invasive trophozoites that cause symptoms, but that same absorption leaves little drug in the gut lumen, so cysts survive. Paromomycin, or the alternatives iodoquinol or diloxanide furoate, isn't absorbed, so it acts in the lumen and kills the cysts that would cause a relapse.

6:15His regimen is metronidazole, five hundred to seven hundred fifty milligrams three times daily for seven to ten days, or tinidazole, two grams daily for three to five days, followed by paromomycin for seven days. Most amebic liver abscesses respond to medicine alone, because metronidazole reaches the trophozoites in the liver, and drainage is reserved for large abscesses, peripheral abscesses at risk of rupture, and abscesses that don't respond. If he stopped after metronidazole, he'd feel better while cysts in his gut waited to relapse, so the luminal agent is added.

6:49Now the second patient. A sixty-seven-year-old man grew up on a farm in rural Ecuador and moved to the United States thirty years ago. He's just been diagnosed with vasculitis, and high-dose prednisone is planned for tomorrow. His eosinophil count is mildly high, and he mentions an itchy, wandering rash on his trunk that comes and goes.

7:09He may carry chronic Strongyloides infection, suggested by his origin, his eosinophilia, and his wandering rash, and that makes his prednisone the most important decision in this episode. Strongyloides stercoralis is acquired from soil in tropical and subtropical regions, Latin America, sub-Saharan Africa, Southeast Asia, the Caribbean, and parts of the rural southeastern United States, including Appalachia. It persists for decades by autoinfection, so thirty years away doesn't protect him. Chronic infection causes eosinophilia, intermittent abdominal pain and diarrhea, and larva currens, a migrating serpiginous rash on the trunk and perianal skin.

7:46Immunosuppression is what makes it lethal. Glucocorticoids, anti-tumor necrosis factor, TNF, agents, transplant immunosuppression, or chemotherapy in a carrier can trigger hyperinfection and dissemination, with mortality up to seventy percent, as filariform larvae penetrate the bowel wall carrying gut bacteria with them and cause gram-negative bacteremia or meningitis. So anyone with past endemic exposure has Strongyloides serology before planned immunosuppression. Stool microscopy isn't adequate, because larvae are shed intermittently and in small numbers.

8:21His timing decides his treatment. Ivermectin, two hundred micrograms per kilogram daily for one to two days, treats chronic infection, with longer courses for hyperinfection or disseminated disease. It's preferred over albendazole because it paralyzes the worm through glutamate-activated chloride channels that mammals lack, while albendazole acts on tubulin and is weaker against the autoinfective larvae that keep the infection going, so albendazole is second-line. When immunosuppression can't wait, ivermectin is given empirically before confirmation, because missing chronic infection before steroids risks hyperinfection. His prednisone can't wait, so he gets ivermectin now, alongside his serology.

9:04His farm childhood also raises hookworm, and it shows why a different worm needs only one dose. Hookworm enters through bare skin, classically from barefoot soil contact, causing ground itch at the entry site, then cough with transient lung infiltrates as larvae pass through the lungs, then attachment of adult worms to the small bowel, causing diarrhea, pain, and chronic blood loss. Each worm consumes up to half a milliliter of blood a day, so chronic infection causes iron deficiency anemia. Stool microscopy turns positive about two months after infection, and eosinophilia supports the diagnosis.

9:41Hookworm is treated with albendazole, four hundred milligrams once. A single dose suffices because adult hookworms live in the gut lumen and there's no autoinfective cycle to interrupt, unlike Strongyloides, whose tissue-migrating larvae and autoinfection need multi-day dosing. Footwear in endemic regions is the lasting prevention.

10:01One parasite is treated with a scope rather than a drug. A woman has sudden severe epigastric pain, nausea, and vomiting a few hours after eating sushi. She has anisakiasis, acquired from raw or undercooked saltwater fish, sushi, sashimi, ceviche, or pickled herring, when Anisakis larvae penetrate the wall of the stomach or bowel. It sometimes brings eosinophilia, and allergic reactions, urticaria or anaphylaxis, can accompany or replace the GI symptoms.

10:35There's no reliably effective antihelminthic for Anisakis. Endoscopy shows the larva embedded in the gastric or duodenal wall with surrounding edema and redness, and removing it is both the diagnosis and the cure. Surgery is reserved for penetration through the wall causing an ulcer, perforation, or an eosinophilic granuloma. Severe epigastric pain hours after raw fish means a scope.

11:00Here are the traps the boards set in these patients. Relying on a single ova-and-parasite exam for Giardia. Using a week of metronidazole when one dose of tinidazole works. Retreating lactose intolerance after Giardia. Using metronidazole in pregnancy instead of paromomycin. Missing common variable immunodeficiency in recurrent giardiasis. Relying on nitazoxanide instead of antiretroviral therapy for Cryptosporidium in HIV. Stopping after metronidazole in amebiasis without a luminal agent. Starting steroids before screening for Strongyloides, or screening with stool microscopy instead of serology. And trying drugs for Anisakis instead of removing it endoscopically.

11:44So, start to finish. The test and the drug are chosen by parasite. Giardia is diagnosed by stool antigen or PCR and treated with a single dose of tinidazole, or paromomycin in pregnancy, and recurrence raises common variable immunodeficiency. Cryptosporidium is treated with nitazoxanide, but antiretroviral therapy is what clears it in HIV. Entamoeba histolytica needs a tissue agent, metronidazole or tinidazole, followed by the luminal agent paromomycin.

12:15Strongyloides is screened by serology before immunosuppression in anyone with endemic exposure, however long ago, and treated with ivermectin to prevent fatal hyperinfection. Hookworm needs a single dose of albendazole. And Anisakis is removed at endoscopy.

12:30Episode five, the last of the chapter, is the opportunistic infections: cytomegalovirus colitis, Mycobacterium avium complex, microsporidia, the coccidia, and norovirus.

12:44For 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 four of five of chapter thirty-three, and I'll see you in the next one.