A useful initial approach to evaluating diarrhea in an individual infected with human immunodeficiency virus (HIV) is to distinguish acute from chronic diarrhea, and small from large bowel involvement. In addition, particular consideration should be given to the stage of HIV disease, current medications, and sexual history, as these factors help determine likely pathogens. Finally, evaluating the degree of systemic illness is essential to assessing the need for hospital admission.
Acute diarrhea is defined as the presence of three or more loose or watery stools per day for less than 2 weeks. Diarrhea is defined as persistent if it has been present between 2 and 4 weeks and is considered chronic when present for 4 weeks or more. Pathogens infecting the small bowel affect the secretory and nutritional absorption functions of the gastrointestinal (GI) tract and typically present with large volumes of watery stool, often accompanied by cramps, bloating, and abdominal gas (Table 16.1). Severe or prolonged diarrhea may result in dehydration, malnutrition, and weight loss. Large bowel involvement primarily affects water resorptive capacity and typically causes frequent, small-volume diarrhea that may be bloody or mucoid and is often accompanied by pain.
Overall, up to 40% of patients with HIV infection report at least one episode of diarrhea in any given month, and approximately one quarter of patients experience chronic diarrhea at some point.