Introduction: Cystoisospora belli infection is one of the most important causes of watery diarrhoea in patients with HIV and causes high rates of morbidity and mortality. The introduction of highly active antiretroviral therapy (HAART) in recent years has improved the ability of immune response and decreased viral load.

Methods A prospective study was performed among HIV patients admitted to hospital of Lambayeque. Herein we describe seven clinical cases of diarrhoea caused by C. belli infection in HIV patients, who showed different evolution and response to treatment.

Results Five were males, with a mean age of 32 years and chronic diarrhoea. Four patients had recurrent diarrhoea despite receiving secondary prophylaxis with cotrimoxazole and good viral and immunological response to HAART in addition to specific treatment. While others were not receiving HAART and prophylaxis, but responded well to treatment.

Conclusion: C. belli is an important cause of diarrhoea in HIV patients on HAART and prophylaxis. In this study, 7 cases of patients with HIV infection and diarrhoea caused by C. belli are presented. Three of those were newly diagnosed, so they did not receive HAART and secondary cotrimoxazole prophylaxis and their CD4+ levels were below 200/uL. However, they responded favourably to C. belli treatment, with no recurrences. Meanwhile, the other four patients were receiving HAART, secondary prophylaxis and had evidence of immune restoration (>200 CD4+/uL), but the standard treatment failed to eradicate the parasite. This clinical contradiction has been reported previously with some particularities.

We suggest that persistent infection may be due to drug failure by intrinsic or extrinsic to the parasite causes, or to defects in restoration of the intestinal immune system, or both.