The diagnosis and clinical importance of Giardiasis

tic animals and wildlife throughout the world. The parasite is a flagellate, which was discovered by the inventor of the microscope, Antoni van Leeuwenhoek. In a letter written to Hooke at the British Royal Society of Parasitology in 1681 he accurately described the motile trophozoite of G. lamblia, which he observed in a sample of his own stool as "very prettily moving animacules, some rather larger, others somewhat smaller than a blood corpuscle and all of one and the same structure ...." "their bodies were somewhat longer than broad, and their belly, which was flatlike, furnished with sundry little paws....". The descriptive information of this historic letter was not widely distributed and it was Vilem Lambl who comprehensively described the trophozoites, which he found in the stools of paediatric patients in Prague in 1859. His published observations were accompanied by beautiful drawings of the organism based on his many microscopic observations. The life cycle The life cycle of Giardia is well-known and comprises two developmental stages; the trophozoite and the cyst [Figure 1]. The most common location of the former stage is in the crypts within the duodenum of the host (e.g. human) where trophozoites live closely applied to the mucosa. There, binary fission repeatedly takes place, the eventual result of which is the establishment of the protozoan, often in enormous numbers. The binucleate trophozoite is usually described as being teardrop-shaped with the posterior end being pointed; it measures 10-20 μm [Figures 2 and 3]. The anterior portion of the ventral surface of the organism is modified to form a sucking disc, which serves to attach the organism to the mucosa. The transformation of trophozoites into cysts takes place in the host intestinal tract, when the trophozoites, together with the faecal mass, move down through the colon. The cysts, which are shed into the environment by the host, may be either round or oval, and they contain four nuclei [Figure 4]. The cyst represents the resting stage of the organism; its rigid outer wall protects the parasite against changes in environmental temperature, dehydration, and such disinfectants as chlorine, all of which would destroy the trophozoite. The cyst is also the infective stage; the cycle continues when a suitable host ingests the cyst. Stomach acidity and other factors trigger the excystation process, which usually takes places in the new host’s small intestine.