Field Studies of Amebiasis in Bangladesh

Entamoeba histolytica infection results from ingestion of the cyst of Entamoeba histolytica from fecally-contaminated food or water. Local invasion results in amebic dysentery and metastasis to amebic liver abscess. The diagnosis of intestinal amebiasis is ideally made using an E. histolytica-specific stool antigen detection test or using real-time polymerase chain reaction (PCR). The two major clinical syndromes of amebiasis are amebic colitis and amebic liver abscess. Together they are estimated to result in 50 million cases of colitis and liver abscess and 100,000 deaths worldwide each year. In developing countries, colonization with E. histolytica has been observed in 5% or more of poor children. Patients with amebic colitis typically present with a several week history of gradual onset of abdominal pain and tenderness, diarrhea and bloody stools (dysentery). The pathological lesions in the colon include ulceration of the intestinal epithelium and invasion into the lamina propria by trophozoites. Inflammation, with infiltrating neutrophils and mononuclear lymphocytes is pronounced, but inflammatory cells near the amebae are killed with pyknotic nuclei characteristic of apoptotic death. Amebic liver abscess is a rare form of the disease that is almost exclusively limited to adult males.

Cryptosporidiosis in humans is caused primarily by two species, Cryptosporidium parvum and C. hominis. There are annually approximately 1.4 cases/100,000 in the United States reported to the Centers for Disease Control. Infection results from ingestion of fecally contaminated water or food containing the infectious oocyst form. Sporozoites are released from the oocyst in the small intestine and attach to the epithelial cell surface. Upon invasion of the epithelial cells the parasite undergoes both the sexual and asexual stages of the life cycle. Infection with C. parvum in a normal host leads to days to several weeks of non-bloody diarrhea.

In many people E. histolytica and Cryptosporidia infections resolve without symptoms. A major emphasis of this study will be to study the host, environment, and parasite factors controlling susceptibility to E. histolytica and Cryptosporidia infection and disease, to begin to answer the question of why all infections do not cause disease. At the same time, want to improve diagnostic techniques for these diseases.

This study will continue a cohort of 420 children (average age now 10.5 years) that the research team has been following since birth and measure the incidence of amebiasis and cryptosporidiosis prospectively. In addition, 500 live births will be enrolled into an existing cohort.

This work has several objectives. The first objective is to compare current "gold-standard" diagnostic techniques for these diseases with newer antigen detection and polymerase chain reaction techniques. The second objective is to delineate the protective role of innate and acquired immunity to E. histolytica. The investigators hypothesize that protective immunity is mediated both by innate immune responses initiated via Toll-like Receptor stimulation and acquired responses including mucosal immunoglobulin A (IgA) and systemic Interferon-γ. Acquired immune responses (peripheral blood mononuclear cell cytokine production and fecal IgA anti-cross reacting determinant) during amebic and cryptosporidial infection and disease will be determined in the children in the cohort. The third objective is to test for the association of genetic polymorphisms in innate and acquired immune genes with incidence of amebiasis. The investigators hypothesize that common genetic polymorphisms in genes of the innate and acquired immune system influence susceptibility to E. histolytica and Cryptosporidia infection or disease. The final objective is to test the role of human genetic polymorphisms and microbiome in protection from undernutrition. Genome wide scans and microbiome compositional analyses will be performed on children to determine if the nutritional status of the child correlates with these measurements.