Giardia lamblia: Epidemiology and Molecular Pathogenesis

Giardia lamblia (synonyms: G. duodenalis and G. intestinalis) is a widespread intestinal protozoan parasite of humans, particularly in settings with inadequate sanitation and poorly protected water sources. This overview examines its transmission, intestinal pathogenesis, clinical manifestations, genetic diversity, host immune responses, and diagnostic approaches. Infection occurs through ingestion of environmentally resistant cysts in contaminated water or food or through direct fecal–oral transmission. Following excystation in the small intestine, trophozoites attach to the intestinal epithelium without typically invading host tissues. Colonization can disrupt epithelial barrier function and impair nutrient absorption. The clinical spectrum ranges from asymptomatic infection to acute or chronic diarrhea, abdominal discomfort, malabsorption, weight loss, and impaired growth in children. The parasite exhibits substantial genetic diversity, with human infections predominantly associated with assemblages A and B, contributing to complex transmission patterns. Parasite persistence and pathogenicity involve variant surface proteins, cysteine proteases, and metabolic adaptations that support immune evasion and intestinal colonization. Host defense depends on coordinated innate and adaptive immune responses, including secretory immunoglobulin A production and CD4+ T-cell-mediated immunity. Diagnostic approaches include stool microscopy, antigen detection assays, and polymerase chain reaction-based methods. Molecular techniques also support parasite genotyping and epidemiological investigations, extending their application beyond the detection of infection. Together, these approaches provide complementary information for examining the occurrence and transmission of giardiasis. Understanding interactions among parasite genetic diversity, virulence mechanisms, and host defenses provides a basis for interpreting clinical variability and infection persistence. The overview highlights the value of integrating parasitological, immunological, and molecular perspectives to strengthen understanding of giardiasis and guide further diagnostic and epidemiological research.