Giardiasis
USMLE Step 1Difficulty: easyLast updated:
A 31-year-old man presents with 3 weeks of foul-smelling, watery diarrhea, abdominal cramps, bloating, flatulence, and belching. Symptoms began 10 days after a backpacking trip during which he drank untreated water from mountain streams. He has no fever and no blood in the stool. Stool antigen testing confirms an intestinal protozoan whose cysts remain viable in surface water.
Which of the following is the most appropriate treatment?
Answer and explanation
Correct answer: A. Tinidazole
Bloating, flatulence, belching, and malodorous watery diarrhea without blood after drinking untreated stream water are typical of giardiasis. The source explains that Giardia cysts remain viable in surface water and resist routine chlorination, which is why mountain streams have been implicated in outbreaks. It lists backpackers, hikers, and campers who drink unsafe water as a risk group. Symptoms are usually mild and include watery malodorous diarrhea, cramps, distention, flatulence, eructation, and nausea. Characteristic trophozoites or cysts in stool, or stool antigen and molecular tests, establish the diagnosis. Treatment is with oral metronidazole, tinidazole, secnidazole, or nitazoxanide. Of the options listed, only tinidazole is an appropriate antigiardial agent.
Each option explained
- A. TinidazoleCorrect. Tinidazole is a recommended treatment for giardiasis, along with metronidazole, secnidazole, and nitazoxanide.
- B. CiprofloxacinIncorrect. Ciprofloxacin treats bacterial diarrhea and is not active against Giardia.
- C. Pyrantel pamoateIncorrect. Pyrantel pamoate treats intestinal nematodes such as pinworm, not protozoa.
- D. PraziquantelIncorrect. Praziquantel treats trematode (fluke) and cestode (tapeworm) infections, not Giardia.
- E. IvermectinIncorrect. Ivermectin treats Strongyloides and other nematode or ectoparasite infections, not protozoa.
Key point
Giardiasis after drinking untreated stream water is treated with metronidazole, tinidazole, secnidazole, or nitazoxanide.
From the source
“Treatment is with oral metronidazole, tinidazole, secnidazole, or nitazoxanide.”