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Giardia as a zoonosis (or not)

Gerald L. McLaughlin, Ph.D gmclaugh at INDYVAX.IUPUI.EDU
Thu Sep 11 15:07:58 EST 1997

When I saw that Australian study, I did think that it was unusual relative
to previous literature, perhaps suggesting a founder population of unusually
broad-spectrum Giardia on that continent, and unusually limited/local water
supplies.  The fingerprinting studies I recall included one study by Nash
and McCutchan, about 1987, but I don't have the references at hand.  The EPA
is putting some effort toward detecting and/or removing Giardia and
Cryptosporidium in water, but this has been problematic without proper
speciation or viability assays.  I remain more inclined toward old-fashioned
fecal contamination as a major route for giardiasis in the US.  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


At 09:51 AM 9/11/97 -0700, Graham Clark wrote:
>I guess from Jerry's response there must be less of a consensus than 
>I thought! My response to Andy's posting was more that his conclusions 
>were inconsistent with my reading of the posted replies. However, Jerry
>raises some interesting points. Yes, human-human direct transmission
>is very common in some settings, like day-care centres. However, 
>epidemiological studies have strongly linked water supply with infection
>in a number of countries across the world and socioeconomic spectrum. In
>a number of cases contamination of water is unlikely to be due to human
>waste, implying a zoonotic origin for some outbreaks. The relative 
>likelihood of an animal vs. a human source for infection is unclear and 
>probably varies. Certainly water treatment companies are investing an
>awful lot of effort in detecting and eliminating Giardia cysts if water 
>is not a significant transmission route. I'm not sure what fingerprinting
>studies Jerry refers to but certainly using isoenzyme markers, Australian
>workers found genetically identical isolates in humans and other mammals 
>(see medline entry below).
>Graham Clark
>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.
>Meloni BP, Lymbery AJ, Thompson RC
>Genetic characterization of isolates of Giardia duodenalis by enzyme
>electrophoresis: implications for reproductive biology, population 
>structure, taxonomy, and epidemiology.
>J Parasitol 1995 Jun;81(3):368-383 
>The nature and extent of genetic variation in Giardia was used to 
>infer its mode of reproduction, population structure, taxonomy, and
>zoonotic potential. Ninety-seven isolates of Giardia duodenalis, 
>from a defined area in Western Australia and throughout Australia
>and overseas, were obtained from humans, cats, cattle, sheep, 
>dogs, goat, beaver, and rats. Enzyme electrophoresis revealed
>extensive genetic variation with 47 different zymodemes. The 
>widespread occurrence of certain zymodemes and the similarity of
>relationships among isolates inferred from independent genetic 
>markers suggests a clonal population structure for G. duodenalis,
>although occasional bouts of genetic exchange may occur. The 
>47 zymodemes clustered similarly in phenetic (UPGMA) and
>phylogenetic (Fitch-Margoliash) analyses. The level of genetic
> diversity in isolates from a defined geographical area in Western
>Australia was similar to the level of diversity in isolates 
>from throughout Australia. These data suggest that clonal lineages 
>within G. duodenalis are evolutionarily independent. Although 
>there was a significant overall correlation between genetic distance 
>separating zymodemes and occurrence in different host species, we f
>ound genetically identical isolates from humans and other animals and
>extensive genetic diversity between isolates from humans. We 
>interpret this as evidence for zoonotic transmission of the parasite. 
>>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.
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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