Giardia as a zoonosis (or not)

Graham Clark graham.clark at LSHTM.AC.UK
Thu Sep 11 11:51:46 EST 1997

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.

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