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Giardia as a zoonosis (or not)

Gerald L. McLaughlin, Ph.D gmclaugh at INDYVAX.IUPUI.EDU
Wed Sep 10 08:55:09 EST 1997

Graham Clark-

The few fingerprinting studies I've seen on Giardia seemed somewhat
inconclusive, and infectivity studies from humans to animals have been
negative, in general, and animal-to-human challenges weren't cleared by
human use committees, I believe.  My impression from the literature is that
animal-human transmission of Giardia is probably very rare, relative to
human-human transmission.  I'm also less certain that water contamination is
the main route of transmission, based partly on the distribution of cases.
Several authors have favored fecal contamination in day care and other
setings.  As the most common intestinal parasite, it's surprising how little
is firmly known about Giardia, especially the zoonosis question.


At 03:06 AM 9/10/97 -0700, Graham Clark wrote:
>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
Gerald McLaughlin, Ph.D.
Associate Professor
Department of Pathology and Laboratory Medicine
635 Barnhill Dr., MS A128
Indianapolis, IN  46202-5113
Ph 317-274-2651; FAX 317-278-0643
e-mail:  gmclaugh at iupui.edu

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