Classification

Taxonomy

• Within the Excavata. phylum Metamonada, and the order Diplozoa.
• Giardia taxonomy remains in a state of flux. There are several species recognized: 3 in mammals, 2 in birds, and one in amphibians.
  • Giardia duodenalis  (syn. Giardia lamblia, Giardia intestinalis) in humans, livestock, other domestic animals, and wildlife.
  • Giardia microti and Giardia muris in rodents.
  • Giardia psittaci and Giardia ardeae in birds.
  • Giardia agilis in amphibians.
• The species G. duodenalis  is broken into 8 groups called assemblages based on various molecular genetic markers. These assemblages are designated by letters:
  • The ones most commonly found in dogs are assemblages C and D.
  • The one most commonly found in cats is assemblage F.
  • Ungulates and pigs tend to be infected with E, rats are infected with G, and seals are infected with H.
  • Assemblage B is found mainly in people.
  • Assemblage A has been reported from a number of different animal types and is the only assemblage that appears to be readily shared by hosts in disparate mammalian groups.

Distribution

• Worldwide.

Significance

• Commonly asymptomatic.
• Can be clinical:
  • Chronic intermittent or continuous diarrhea (not watery) with steatorrhea and malabsorption.
  • More common in young animals.
• The zoonotic potential of the Giardia  observed in companion animals remains under considerable debate.  The common assemblages of dogs and cats (C, D, and F) have only rarely been identified in humans and when found have been in the immunocompromised or in children living in squalid conditions. However, dogs and cats have been identified on occasions as passing the cysts of Giardia that fall in assemblages A and B. It is unclear how much of this is due to coprophagy versus actual infections. Also, studies showing that the same Assemblages are shared between people and their pets have been far from conclusive at this point. Again, most dogs are infected with C and D  and most cats are infected with F, and these cannot be considered as zoonotic forms except under special conditions.
• Molecular genotyping from individual animals may help assess zoonotic potential.