Giardia

Classification: Taxonomic ranks under review (cf. Illustrated Guide to Protozoa, 2000. Allen Press)

Protista (unicellular eukaryotes)
Sarcomastigophora (with pseudopodia and/or flagella)
Mastigophora (flagellates)
Zoomastigophora (zooflagellates, without chloroplasts)
Diplomonadida (zoites with two nuclei)

Family: Hexamitidae
Diplomonads have complex nucleus-associated karyomastigonts and are defined by the possession of two nuclei and unique bilateral symmetry (diplozoic appearance). They typically form reproductive cysts (each containing four nuclei) which facilitate their transmission between hosts. Hexamitids are parasites or commensals in mammals, birds, reptiles and fish. Several parasitic species cause severe clinical disease in man and domestic animals. All species are characterized by their pyriform shape and bilateral symmetry with two equal nuclei lying side by side, two intracytoplasmic granular axonemes and 6-8 flagella.

Giardia duodenalis [this species causes giardiasis (diarrhoea) in vertebrates]

Parasite morphology: The parasite forms two developmental stages: trophozoites and cysts. The trophozoites are pyriform (10-30µm long) and have 8 flagella (2 anterior, 2 lateral, 2 ventral and 2 caudal), a prominent ventral adhesive disc, 2 longitudinal axonemes and 2 tangential curved median bodies. Cysts are ovoid to ellipsoid (11-14µm in length by 7-10µm in width), membrane-bound (sometimes imparting a halo-appearance) and contain 4 nuclei, axonemes and median bodies.

Host range: Infections have been recorded in many human and animal populations throughout the world. Some 40 species have been described on the basis of host occurrence but most are morphologically indistinguishable. Three species groups have been recognized on the basis of trophozoite morphology; thin elongate trophozoites assigned to the G. agilis group found mainly in birds and frogs, short stout trophozoites belonging to the G. muris group found mainly in rodents, and medium-sized trophozoites belonging to the G. duodenalis group found mainly in mammals. Host specificity within each group remains contested. Epidemiological studies have frequently suggested zoonotic and water-borne transmission between mammals. Indeed, common regional names for the disease often suggest animal or water sources of human infection (e.g. beaver fever, backpacker’s malady, traveller’s diarrhoea). While cross-transmission studies are difficult to perform due to biological, technical and ethical concerns, experimental studies with G. duodenalis isolates have not confirmed broad host specificity. Instead, molecular characterization techniques have identified a range of genotypes which vary in their host specificity and zoonotic potential. Genes used to characterize types have included small subunit ribosomal DNA, glutamate dehydrogenase, triose-phosphate isomerase and beta giardin. Three main zoonotic genotypes have been found in humans (designated A1, A2, B3) while a growing number of other genotypes have been found in domestic animals and wildlife.

Site of infection: Flagellated trophozoites are found in the small intestines of their hosts, especially the duodenum. Trophozoites have been observed swimming with a distinct corkscrew motion in luminal content as well as adhering to the gut mucosal surface with their ventral adhesive discs (when they detach, they leave distinct oval impressions in the microvillous layer).

Pathogenesis: Infections interfere with the normal absorptive functioning of the small intestines, thereby causing osmotic overload of the large intestines resulting in watery diarrhoea. Attached parasites may physically blanket the small intestinal mucosa significantly reducing the surface area for absorption. It is also thought some parasite molecular products may exert a chemical action on mucosal cells. Infections apparently damage and increase the turnover rate of epithelial cells culminating in villous atrophy which further reduces the surface area for absorption. These factors contribute to malabsorption of fats and other nutrients resulting in watery diarrhoea and steatorrhea accompanied by dehydration, intestinal pain and flatulence. Most clinical infections are self-limiting and resolve spontaneously but some persist leading to chronic weight loss, retarded growth and ‘failure-to-thrive’ syndrome. Young individuals are most susceptible to clinical infections and focal outbreaks are common in child day-care centres and among intensively-reared and housed young animals. Not all infected individuals, however, develop clinical signs but may remain asymptomatic carriers.

Mode of transmission: Infections are passed between hosts by the faecal-oral transmission of encysted parasite stages. When trophozoites pass through the colon, they form nonflagellated cysts which are excreted and contaminate the environment. The cysts are said to be reproductive in that they undergo nuclear division as they mature becoming quadrinucleate. Following their ingestion by a new host, they excyst in the small intestine releasing two trophozoites. Excystation stimuli include various post-gastric digestive conditions (bile salts, enzymes, pH, microaerophilic conditions, etc). Most infections are transmitted accidentally by ‘hand-to mouth’ contact whereby objects contaminated with faecal material are placed in the mouth (e.g. contaminated fingers, utensils, clothing, etc). The cysts are quite resistant to external environmental conditions and can survive for some time, particularly in cool moist conditions. The cysts also contaminate water supplies and cause infections when subsequently ingested with drinking water or the consumption of food-stuffs diluted or washed with contaminated water. Infections have also been associated with recreational water use, including swimming pools, lakes and water-theme parks. Conventional water treatment procedures (filtration and chlorination) are not wholly effective against Giardia cysts as they are quite small and hardy.

Differential diagnosis: Faecal cysts may be detected by routine coprological examinations (stained smears, or sedimentation/flotation concentration techniques) but test sensitivity is poor due to intermittent cyst excretion. Endoscopic techniques (gastroscopy through to duodenum) have been used in chronic cases to detect trophozoites in intestinal biopsy material. More recently, sensitive and specific immunological techniques have been developed to detect parasite antigens in faecal preparations (copro-antigen tests). Similar monoclonal antibody immunoreagents are also used in many countries to detect cysts in water samples using immuno-magnetic separation techniques.

Treatment and control: Flagyl (metronidazole) is the drug of choice for giardiasis despite mild side-effects (such as nausea). However, there are growing problems with metronidazole-resistant parasite strains. Other nitroimidazole derivatives (tinidazole), nitrofurans (furazolidone), acridine drugs (quinacrine) and microtubule inhibitor anthelmintics (albendazole) have been reported effective. Control depends largely on good sanitation, proper effluent disposal and effective water treatment (well-maintained sand filtration or microfiltration, optimum chlorination or ozonation).