(p. 522) Giardia infections 

Giardia is a ubiquitous intestinal protozoan parasite of vertebrates and the most common intestinal pathogen of humans and domestic animals with a worldwide distribution including both temperate and tropical regions.

Giardia was first observed in 1681 by Antony van Leeuwenhoek in his own faeces (Dobell 1920), and the organism has intrigued biologists and clinicians ever since. However, the first detailed description of the parasite was not given until two centuries later by Lambl (1859). Koch’s postulation was proven by Rendtorff in 1954 when he successfully transmitted symptomatic Giardia infection to human volunteers following orally administered cysts. The first symptoms of clinical giardiasis were reported in the early 1920s, although the significance of Giardia as a cause of diarrhoeal disease was controversial for many years (see Farthing 1994; Cox 1998), and it is only recently that the significance of Giardia as a cause of chronic disease in children and its association with failure to thrive, wasting and malabsorption syndromes has been fully realised (reviewed in Farthing 1994; Hall 1994; Gracey 1994; Rabbani and Islam 1994; Hesham et al. 2005; Savioli et al. 2006; Thompson 2008).

The question of Giardia ’s role as a source of zoonotically transmitted disease again has been controversial. The World Health Organization (WHO) recommended that Giardia should be considered as a zoonotic agent in 1979 (Anon. 1979). Since that time, increasing circumstantial epidemiological evidence from waterborne outbreaks, the results of some cross-infection experiments and molecular characterization studies of Giardia isolates from humans and other animals has led most authorities to conclude that Giardia should be considered a zoonotic parasite (Acha and Szyfres 2003; Savioli et al. 2006; and reviewed in Thompson 2004). However, as discussed below, the frequency of zoonotic transmission is uncertain.