The Discovery of Giardia1

The pear-shaped flagellate Giardia duodenalis is a cosmopolitan protozoan parasite of humans. It has a global distribution, and indeed is ranked by Farthing in the top ten of human parasitic diseases (1). Infection with G. duodenalis is normally self-limiting. Human giardiasis can be divided into two disease phases: acute and chronic. The acute phase is usually short-lived, characterised by flatulence with sometimes sulphurous belching and abdominal distension with cramps. Diarrhoea is initially frequent and watery but later becomes bulky, sometimes frothy, greasy and offensive. The gas may cause the stools to float. In chronic giardiasis, malaise, weight loss and other features of malabsorption become prominent and stools are usually pale or yellow, frequent and of small volume. Malabsorption of vitamins and D-xylose can occur, while disaccharidase deficiencies (most commonly lactase) are frequently detected in chronic cases. In young children, ‘failure to thrive’ is frequently due to giardiasis, and all infants being investigated for causes of malabsorption should have a diagnosis of giardiasis excluded (2). The parasite exists in two distinct morphological forms. The reproductive phase is a trophozoite which parasitizes the cells lining the upper small intestine. There is also a resistant, resting phase, an environmentally-resistant cyst which is voided in the faeces. This is the infective and disseminating stage. Giardia is the most commonly detected intestinal protozoan parasite in the world, and the prevalence of giardiasis in developing countries is approximately 20% comThe Discovery of Giardia1 Brian J. Ford Gonville & Caius College, University of Cambridge, UK.*