While the haemoflagellates just discussed are of great medical and veterinary importance, many other flagellates are found in the digestive, urinary and genital canals of animals. Only two of these, Giardia intestinalis and Trichomonas vaginalis, are important as human parasites.
Giardia intestinalis (G. lamblia) is a universal parasite of the upper small intestine of man, monkey and pig. It is characterised by having two nuclei and three or four pairs of flagella, arranged bilaterally. The trophozoites are motile and measure about 7 and 14 Âµm. From the hostâs point of view, the most significant feature of this organism is a shallow sucking disc on the anterior ventral surface, by which the animal attaches itself firmly to cells of the hostâs intestinal epithelium.
Giardia: trophozoite (left) and cyst (right)
Giardia forms cysts, the stage usually seen in faecal examinations. The cysts are ellipsoidal, have a well-separated cyst membrane, contain four nuclei when mature, and show several bundles of curved axonemes (roots of the flagellar system). The cysts form as the liquid content of the bowel thickens and becomes dehydrated. Cysts are usually not present in unformed or diarrhoeic stools, and trophozoites are absent from normal faeces.
Transmission of Giardia is by ingestion of cysts in food or water contaminated with infected human faeces. Infection may also be acquired by contact with contaminated hands or clothes. In addition, flies can take up the cysts in their mouths and body surfaces, carry them away and deposit them on food.
The incubation period is 1-2 weeks. People, dogs and cats suffer acute gastroenteritis with diarrhoea. Gastroenteritis may be followed by chronic malabsorption, weight loss, abdominal pain that waxes and wanes for a period of many months. At first, the stools may be watery, progress to soft, semi-formed stools with steatorrhea and a rancid foul odour. Patients may also exhibit malaise, nausea, bloating and flatulence. Diarrhoea may continue indefinitely if not treated. Diarrhoea is primarily due to disruption of the enterocyte brush borders and loss of disaccharidase enzymes.
The incidence of giardiasis is clearly related to age. In children, the incidence increases gradually before it peaks at puberty. Then it declines until the incidence in the elderly is almost nonexistent. The loss of infection in older groups probably is due to the development of some kind of immunity because of repeated encounters with the parasite. It may also be a reflection of age-related physiological change in the host.
While the infection rates in the tropics in adolescents or preadolescents can be as high as 25 %, the figure for the same age groups in the temperate zones is only about 2 %, an indication that the parasite does better under warmer conditions.
Dogs and cats are reservoir hosts that contaminate the environment with their infected faeces and are a potential risk to humans. The same may be true for cattle, goats and pigs.
Treatment with metronidazole is effective against giardiasis. In addition, patients should be given a protein rich diet and supplements of vitamin A and B complex.