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Giardia as a zoonosis (or not)

Graham Clark graham.clark at LSHTM.AC.UK
Wed Sep 10 05:06:24 EST 1997

Andy Fell wrote:

>Thanks for all your responses. I wanted to know whether there was a
>consensus of opinion on Giardia, so I have a consistent story for my 
>I think I can summarize it as: 
>1. There is no reason to divide the species occuring in different mammals,
>including humans.

Ouch! I'm not sure where you got this view and the other 'consensus'
points, but they certainly do not reflect either the view in the Giardia
field or the responses I have seen posted.

1. There are certainly multiple species in mammals. In addition to
G. intestinalis/lamblia/duodenalis, G. muris is a valid species (as
pointed out already by Barry Lifland and Omar Barriga) and several
others have been described that are likely to be valid also but have not 
been studied as much. There are at least two genetic subtypes of Giardia 
intestinalis found in humans but they both appear to cause the same

>2. But most infection is probably human to human, so there is some
>restriction on gene flow between the populations. 

2. I'm not sure what you mean by 'restriction on gene flow between the 
populations' - populations of Giardia or humans? Also most infection of
humans is via faecally contaminated water not human contact as you seem 
to imply. It is in contaminated water that the zoonotic origins come 
into play as the faeces can be of non-human origin and still give rise 
to human infection. The relative contribution of humans and non-humans
to new infections is unclear. These views were stated by Gordon Reynolds
and Omar Barriga.

>3. Widespread infection of dogs and cats does not seem to pose a risk 
>to their owners. 

3. This is the opposite of what Barry Liflind said.

Graham Clark

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