We report a 60-year-old immunocompetent patient with chronic biliary isospo
riasis who failed to respond to orally administered cotrimoxazole prophylax
is and orally administered treatment with nitazoxanide, a 5-nitrothiazole b
enzamide compound. Severe malabsorption was regarded as responsible for the
subtherapeutic levels of nitazoxanide in plasma and bile, resulting in tre
atment failure. Intravenously administered cotrimoxazole stopped the sheddi
ng of Isospora belli oocysts in bile within 5 days, excluding initially sus
pected resistance to cotrimoxazole. Patients with malabsorption and cholang
itis due to Coccidia such as Isospora belli and Cryptosporidium spp. or due
to protozoa that cause microsporidiasis seem to be predisposed to fail to
respond to otherwise effective treatment.