Intestinal protozoa are single-celled eukaryotic parasites that infect the gastrointestinal tract, causing significant morbidity worldwide. The most important species are Giardia lamblia, Cryptosporidium parvum, and Entamoeba histolytica, each with distinct biology, epidemiology, and clinical manifestations.
Giardia lamblia (Giardiasis)
Giardia lamblia is a flagellated protozoan that colonizes the small intestine. The parasite exists in two forms: the trophozoite, which is pear-shaped with two nuclei and four pairs of flagella, and the cyst, which is the infectious and environmentally resistant form. Transmission occurs through the fecal-oral route, often via contaminated water or food.
After ingestion, excystation in the duodenum releases trophozoites that attach to the intestinal epithelium via a ventral sucking disc. Attachment causes enterocyte damage, microvillous atrophy, and malabsorption. Symptoms include diarrhea, abdominal cramps, bloating, steatorrhea, and weight loss. Many infections are self-limiting, but chronic cases can lead to growth retardation in children and persistent gastrointestinal symptoms.
Laboratory diagnosis relies on microscopic identification of cysts and trophozoites in stool specimens. Direct wet mounts, iodine-stained preparations, and permanent stains such as trichrome or iron hematoxylin are used. Antigen detection by ELISA or immunofluorescence provides higher sensitivity. Molecular methods including PCR can detect Giardia DNA and are useful for confirmatory testing and genotyping.
Treatment is with antiparasitic drugs such as metronidazole, tinidazole, or nitazoxanide.
Cryptosporidium parvum (Cryptosporidiosis)
Cryptosporidium parvum is an apicomplexan parasite that infects epithelial cells of the small intestine. The oocyst stage is highly resistant to chlorine and can survive in chlorinated water, making it a common cause of waterborne outbreaks. The species C. hominis primarily infects humans, while C. parvum is zoonotic, transmitted from cattle and other animals.
The life cycle includes both sexual and asexual stages within a single host. Sporozoites released from ingested oocysts invade enterocytes, where they undergo merogony, gametogony, and sporogony. Autoinfection occurs through thin-walled oocysts, perpetuating the infection in immunocompromised hosts.
In immunocompetent individuals, cryptosporidiosis causes self-limited watery diarrhea that resolves within 1–2 weeks. In immunocompromised patients, particularly those with HIV/AIDS or undergoing transplantation, the infection can become chronic, severe, and life-threatening, with biliary tract involvement leading to cholangitis.
Diagnosis is by modified acid-fast staining of stool specimens, which reveals pink-red oocysts 4–6 µm in diameter. Antigen detection by ELISA or immunofluorescence is standard. PCR provides species identification and is more sensitive than microscopy. Cryptosporidium is often included in multiplex gastrointestinal PCR panels.
Nitazoxanide is the only FDA-approved treatment, though it has limited efficacy in immunocompromised patients. Immune restoration is essential for managing cryptosporidiosis in HIV patients.
Entamoeba histolytica (Amebiasis)
Entamoeba histolytica is an ameboid protozoan that causes intestinal and extraintestinal amebiasis. It exists as an invasive trophozoite and an infectious cyst. The cyst form is resistant to gastric acidity and environmental conditions. Transmission occurs through ingestion of cysts in contaminated food or water.
In the colon, trophozoites can invade the intestinal mucosa, causing flask-shaped ulcers and amebic colitis. Symptoms range from mild diarrhea to severe dysentery with blood and mucus. Extra-intestinal infection most commonly involves the liver, resulting in amebic liver abscess, which presents with right upper quadrant pain, fever, and leukocytosis.
Differentiation of E. histolytica from the non-pathogenic commensal E. dispar is essential. Microscopy cannot reliably distinguish them. Antigen detection by ELISA specific for E. histolytica, PCR, or isoenzyme analysis is required. Invasive amebiasis is treated with tinidazole or metronidazole followed by a luminal agent such as paromomycin to eliminate cysts.
