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Giardia as a zoonosis (or not)

Graham Clark graham.clark at LSHTM.AC.UK
Fri Sep 12 03:51:23 EST 1997

Jerry - 

>The fingerprinting studies I recall included one study by Nash
>and McCutchan, about 1987, but I don't have the references at hand. 

The paper you refer to is cited below. As you can see the number of 
isolates was much lower than in the Australian study but in essence the 
results were the same - some animal and human isolates are indistinguishable
and there is evidence for two genetic subtypes infective to humans.

>From the clinical microbiologists with whom I've discussed this, cases tend
>to be in mini-outbreaks not linked by single water supplies, but by close
>human-human contact.

This may be true in the US but there is evidence from as close by as 
Vancouver for repeated water-borne outbreaks (Judy Isaac-Renton's work).

David - 

>I have seen no documented report(s) of dog to human transmission of 
>Giardia. I am also skeptical that beavers transmit Giardia to humans.

I agree that most of the literature indicates that there is little if
any transmission from dogs to humans. Indeed with a few exceptions dog
Giardia seem to be genetically distinct from those in humans living in 
the same area. This does not rule out the possibility of transmission
but indicates that it is probably rare. However, with regard to beavers
the Nash paper below indicates that their Giardia can be indistinguishable
from human isolates implying that transmission is quite possible, although
I doubt that transmission has been demonstrated directly.

C. Graham Clark, Ph.D.
Department of Infectious and Tropical Diseases,
London School of Hygiene and Tropical Medicine,
Keppel Street, London WC1E 7HT, England, G.B.

Nash TE, McCutchan T, Keister D, Dame JB, Conrad JD, Gillin FD

Restriction-endonuclease analysis of DNA from 15 Giardia isolates obtained 
from humans and animals.

J Infect Dis 1985 Jul;152(1):64-73 

The DNA banding pattern of 11 human and four animal isolates (two beaver,
one cat, and one guinea pig) of Giardia were compared by using two related
techniques. Patterns were compared after endonuclease restriction of DNA
followed by agarose gel electrophoresis and ethidium bromide staining and
after Southern blot analysis using recombinant plasmids containing Giardia
DNA as probes. Two major groups could be distinguished with ethidium 
bromide staining of eight isolates. Southern blot analysis, however, could
distinguish nine different patterns among the 15 isolates studied. One 
common banding pattern was seen in six isolates (two animal and four human);
the remainder of the isolates were unique, with the exception of two 
identical isolates from sisters. Three isolates (one from a beaver and two 
from humans) were markedly different from Giardia with the common banding 
pattern, whereas the other six unique isolates varied moderately. Beavers 
and other mammals do not seem to possess their own species of Giardia. 
This methodology introduces a way of distinguishing one species of Giardia
isolate from another and promises to be helpful in epidemiological 

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