ABSTRACT
Giardia is a protozoan consisting of six species that infect mammals, amphibians and birds. The only species that infects humans is Giarda intestinalis (alt. lamblia and duodenalis). It has two stages to its life cycle, a flagellated trophozoite and a cyst with a highly resistant wall that enables it to remain viable outside the body of the host for long periods. G. intestinalis has two genotypes A and B with B-genotype accounting for most cases worldwide. Cysts are ingested from contaminated water or food. Excystation releases trophozoites into the intestine lumen, which then divide within 12 hours. These attach to the intestinal wall causing diarrhea and damage to the epithelium. Penetration of the epithelium by the trophozoites is rare. Some of the trophozoites form cysts and both trophozoites and cysts pass out of the body in the feces. Spread is mainly through water but can occur through food and fomites. G. intestinalis is a common enteric protozoal infection worldwide, with 280 million cases a year and up to 33% of individuals infected. It affects nearly 2% of adults and 8% of children in developed countries and is a cause of “traveler’s diarrhea”, also called “backpackers diarrhea”. Host defenses against G. intestinalis are not well characterized but involve both nonimmunologic mucosal processes and immune mechanisms. Antimicrobial peptides are important, and IL-6 derived from dendritic cells is important. CD4+T cells, especially Th-17, play a major role together with secretory IgA. Variant specific-surface proteins (VSP) are important and changing of these gives rise to antigenic variation and evasion of the immune system. CD8+ T cells do not play a role in defense but might be involved in epithelial damage.
Infection leads to malabsorption and is serious in children. Post-infectious complications include irritable bowel syndrome and chronic fatigue syndrome, and changes in the intestinal microbiota profile in children with Giardia might contribute to some of these complications. The “gold standard” for diagnosis is fecal examination under the microscope to identify trophozoites and cysts. PCR testing is mainly laboratory based and used for genotyping. String tests can be performed. Most frequent drugs used are nitroimidazole derivatives, especially metronidazole; benzimidazoles (albendazole and mebendazole): nitrofuran derivatives; acridine compounds (mepacrine and quinacrine): aminoglycoside: the oral aminoglycoside paromomycin is the drug of choice for pregnant women; nitazoxanide. For prevention, good hygiene is important and boiling water before consumption. There is no known chemoprophylaxis or a vaccine for humans.
