Giardia species in people

Among the species and assemblages (similar to genotypes) of Giardia infecting mammals, birds (G. ardeae and G. psittaci), amphibians (G. agilis) or reptiles (G. varani), almost all human infections are associated with G. duodenalis Assemblage A or Assemblage B (newly named G. enteritica), both of which also infect a wide range of domestic animals and wildlife. Infection of people with Giardia is thought to be much more common than is clinical disease. Studies from many developed and emerging countries indicate that overall Assemblage B is somewhat more common than Assemblage A in people, and that mixed infections are very rare. Based on molecular data, Assemblage A can be subdivided into AI and AII, the latter being the more common in people.  The other assemblages within G. duodenalis (C and D [G. canis?] - in domestic and free-ranging canids; E [G. bovis?] in domestic ruminants and pigs; F [G. cati?] in cats: G [G. simondi?] in mice and rats; and H in seals) are much more host-specific in domestic animal and wildlife species, and although assemblages C, D, E and F have been reported rarely from people, their occurrence and significance as causes of human disease are unknown.

Some indication of the zoonotic potential of Giardia in animals is provided by data for the occurrence of the various species and assemblages in these hosts.  In dogs, most Giardia infections are G. canis, although the potentially zoonotic G. duodenalis, particularly Assemblage AI, and G. enteritica have also been reported.  Giardia felis is the predominant species in cats, although infections with G. duodenalis also occur.  Cattle, sheep, goats and pigs in most parts of the world for which there are data are most commonly infected with G. bovis, although G. duodenalis, particularly Assemblage AI, also occurs.  Wildlife are most commonly infected with G. duodenalis or G. enteritica, although other species/assemblages are found occasionally.  As molecular analyses of Giardia develop, however, and more animals are tested, it is possible that our understanding of the taxonomy and host-specificity of Giardia will change.  Also, while spread of the parasite from animals to people is the major public health concern, it is important to realize that spread from people to animals also occurs.

The cysts of Giardia in the faeces of hosts are immediately infective and are relatively resistant to adverse environmental conditions. Also, very few cysts (<5) are required to establish an infection, even in an immunocompetent host. Although G. duodenalis Assemblages A and B infect domestic animals and wildlife, the major source of Giardia cysts for people is human faeces. People can acquire the parasite by contact with (the faeces of) infected people, especially children in settings such as day cares, very occasionally other mammals, and from the ingestion of contaminated foods, drinking water, other drinks, or recreational water, especially swimming pools.  Also, there is evidence that sometimes heavy rains will wash the oocysts into streams, rivers and lakes used as sources of drinking water.  

In Canada, both Giardia and Cryptosporidium in people are federally notifiable diseases and so there are data from (primarily passive) surveillance for the occurrence of the parasite in people in this country. National data from the Public Health Agency of Canada (PHAC) indicate 3,965 human cases of giardiasis in 2006 (the most recent year for which data are available) - approximately 8.2 cases per 100,000 population, of which an unstated number were likely travel-related. No data are available on the assemblages of G. duodenalis involved in these cases. The incidence rate varies with season, location and age of the affected people. Recent data from the US indicate that giardiasis is more common in children one to 9 years old and in adults 35 to 44 years old than in other age classes, and in late summer and fall.

When interpreting passive surveillance data of this type it is important to realize that each case reported likely represents many more cases of illness that do not have contact with a physician, or are not accurately diagnosed, and, therefore, are not captured by the surveillance system. A recent published estimate from the US Centers for Disease Control and Prevention (CDC) indicate that the raw surveillance data for Giardia should be multiplied by approximately 45 and those for Cryptosporidium by approximately 100 to better reflect the actual occurrence of the diseases. These calculations suggest that in Canada in 2006 there were 178,425 cases of human giardiasis, about one seventh of the number estimated in the US.

More information about Giardia is available under Dogs .