Giardia lamblia is a flagellate parasite also known as Giardia intestinalis. About 200,000,000 people are infected with Giardia lamblia worldwide. The parasite is very common in dogs and is also fairly common in the U.S. among day care centers and hikers, although it is frequently misdiagnosed. Giardia lamblia are two celled organisms that have 4-8 copies of their 5 chromosomes. They reproduce by asexual reproduction and have direct transmission via cysts. They live in the small intestine of humans where they feed on top of villi, disrupting fat metabolism. The infection causes long-lasting intestinal problems, including cramping, diarrhea, yellow/white stool, and foul flatulence, although pathology varies greatly among infections.
Giardia lamblia reproduce through asexual reproduction and are transmitted directly via cysts. The cysts are ingested by humans and excyst in the duodenum. There they multiply by binary fission and the trophozoites feed on the mucosa of the duodenum. They feed on top of and damage the villi, affecting fat metabolism. The trophozoites and cysts leave the body in feces where the trophozoite disintegrates and the environmentally resistant cysts can contaminate water supplies and soil and infect other humans.
Monis et al. (2009) reviewed the literature pertaining to the taxonomy of Giardia and suggest that the taxonomy needs to be revised to recognize the distinct genetic groupings within the genus. They suggest that an earlier taxonomy, which recognizes host specificity of different lineages, be re-adopted since newer molecular data largely supports those groupings. Adopting the taxonomic framework they advance will help us to better understand epidemiology of Giardia (e.g. that domestic dogs may be reservoirs for the disease in certain parts of the world).
Monis et al. also summarize studies that provide evidence of genetic exchange in the parasite, suggesting sexual reproduction is taking place. Giardia lamblia has been considered to reproduce asexually by simple binary fission, but there is increasing evidence from epidemiological and molecular genetic studies that they are capable of sexual reproduction. Studies have shown that Giardia has maintained part of the meiotic machinery and ability of chromosomes to cross over, as well as evidence of recombination events. Researchers have demonstrated in several isolates of Giardia banding patterns that would seem to indicate that Giardia is functionally diploid and that sexual reproduction must have occurred to produce the apparent heterozygotes. Recombination would give Giardia an evolutionary advantage of ability to respond to adversity, making it more resistant to antigiardial drugs or other competing strains.
Castillo-Romero et al. studies the role of enolase in Giardia differentiation. Enolase is a glycolytic enzyme that is present in the cyst wall of the parasite. Differentiation from cyst to trophozoite and trophozoite to cyst is essential for the transmission of Giardia. Recent evidence shows that enolase may play a role in pathogenesis and differentiation in several organisms, and enolase is located on the surface of several parasites. In Giardia enolase is secreted by trophozoites in the presence of intestinal epithelial cells. Because of this, enolase is believed to be a new surface-associated virulence factor. The researchers tested the role and localization of enolase in Giardia in vitro by using a genetic marker. They found that enolase localizes in the plasma membrane, cytoplasm, and caudal flagella and that the insertion of enolase mutants impaired excystation in the parasite.
Monis, Paul T., Caccio, Simone M. and Thompson, R.C. Andrew. "Variation in Giardia: towards a taxonomic revision of the genus." Trends in Parasitology. Vol. 25 No. 2. Elsevier Inc. 2008.
Castillo-Romero, Araceli, Davids, Barbara J., Lauwaet, Tineke, and Gillin, Frances D. "Importance of enolase in Giardia lamblia differentiation." Molecular and Biochemical Parasitology. Vol. 184. Issue 2. pg 122-125. August 2012. http://www.sciencedirect.com.ezproxy.uvm.edu/science/article/pii/S0166685112001041