Classification

Taxonomy

• Within the Excavata. phylum Metamonada, and the order Diplozoa.
• The species G. duodenalis (syn. Giardia lamblia, Giardia intestinalis) is broken into 8 groups called assemblages based on various molecular genetic markers. These assemblages are designated by letters:
  • The one most commonly found in cats is assemblage F.
  • Occasionally, cats are also found infected with assemblage A that has been reported from many different animals and humans.
  • Assemblage B is for the most part restricted to human hosts.
  • 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.
• There are other distinct species recognized in other hosts: Giardia microti and Giardia muris in rodents; Giardia psittaci and Giardia ardeae in birds; and Giardia agilis in amphibians.

Distribution

• 4-10% or more of cats infected.
• Prevalence and clinical problems highest in catteries.

Significance

• Commonly asymptomatic.
• Can be clinical: a) chronic intermittent or continuous diarrhea (not watery) with steatorrhea and malabsorption; b) more common in young animals Giardiasis.
• 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 cats are infected with F, and most dogs with C and D, and these assemblages cannot be considered as zoonotic forms except under special conditions.
• Molecular genotyping from individual animals may help assess zoonotic potential.