Recent research has demonstrated that wildlife reservoirs can be established through parasite infections transmitted directly from human hosts.
The concept that parasites for which humans are a natural reservoir can spill-over directly to wildlife is not widely recognized; neither is the fact that when this happens, new reservoirs of potential public health significance may be established in wildlife.
3.2. Giardia/Beaver Fever
Molecular typing of the common enteric protozoan Giardia
) has drastically altered our thinking of this parasite as a wildlife pathogen that is spilling over into humans. Most evidence to date suggests that more often than not the parasite spills over from domestic cycles into wildlife populations. Further, once infected, these wildlife populations may maintain the parasites and serve as an ongoing spill-back reservoir for humans.
It is often a common ‘knee-jerk’ reaction when parasites with zoonotic potential are found in wildlife that they represent a threat to public health [12
] as a reservoir and potential source of infection for humans [13
]. Indeed, this was the case when WHO initially listed the common enteric protozoan parasite Giardia
as a zoonosis over 25 years ago as a result of epidemiological observations suggesting that giardiasis in campers in Canada was caused by drinking stream water contaminated with Giardia
from beavers [13
]. No one thought to ask the question of where the beavers got their Giardia
infections from until only beavers downstream from a sewage works were found to be infected. With the subsequent application of molecular tools, it has been confirmed that beavers are susceptible to zoonotic strains of Giardia
]. The question now is: are they victim or villain with respect to human giardiasis?
A similar situation has been reported in non-human primates for which there is a growing literature of the invasion of human pathogens into wild populations [14
]. For example, it was suggested that the finding of Giardia
and the co-habiting enteric protozoan Cryptosporidium
in mountain gorillas in the Bwindi Impenetrable National Park Uganda was thought to indicate enhanced contact with humans and/or domestic livestock. This was confirmed when rangers and their cattle were found to be infected with Giardia
and that the genotype was the same as that recovered from the gorillas [15
Muskoxen (Ovibos moschatus
) are indigenous to the arctic tundra of Canada and Greenland and have been translocated to areas in Alaska, USA, Russia, Norway and Sweden. These animals are well adapted to their northern environment, and tend to have a relatively simple parasite fauna. Recent surveys on the biodiversity and impacts of parasites in Arctic ungulates described Giardia duodenalis
, Assemblage A, the zoonotic genotype, in muskoxen [16
]. This unexpected finding (a novel strain, or the livestock strain, was predicted) raises many interesting questions regarding the origin and epidemiology of this parasite in humans and wildlife in this Arctic ecosystem. In particular, is this a pathogen initially introduced to muskoxen by humans? Is Giardia
now maintained as a sylvatic cycle in muskoxen (or other wildlife species on the island) independent of humans? Does the Giardia
from muskoxen spill-back into humans?
The permanent human population of Banks Island is restricted to one small community of ~ 120 humans, many of whom spend extended periods of time ‘on-the-land’ hunting, fishing and drinking directly from the water bodies. Additionally, < 100 tourists visit the island annually for outdoor recreational opportunities. Muskoxen and humans tend to concentrate around the lush river valleys, an ideal setting for interspecies sharing of a faecal-orally water-borne parasite. Other sources of ongoing dispersal of the parasite include the disposal of offal from commercial muskox harvests on the land (in the past) and, more recently, on the sea ice, raising questions about the strain and source of the Giardia
detected in seals in this region which are known to be susceptible to zoonotic strains of Giardia
of human origin [16
Another little explored area with respect to Giardia
in wildlife are the impacts at individual and population levels. In experimentally and naturally infected sheep, Giardia
reduced rates of weight gain, impaired feed efficiency and decreased carcass weight [17
]. In cattle, Giardia
is commonly found alone or in combination with other pathogens as a cause of calf diarrhoea, which can have economic significance [19
]. The impact of Giardia
on the health and production (body condition, fecundity and pelage) of free-ranging ungulates, including muskoxen, remains unknown.
Similarly, in Australia, marsupials are commonly infected with Giardia
but until recently, it was not known to what species or strain(s) of Giardia
they were susceptible. Studies on the Quenda (Isoodon obesulus
), a common widespread species of bandicoot in southern Australia, demonstrated that they were infected with a novel, genetically distinct form of Giardia
, so different to what has been described from humans and other animals, that it probably represents a distinct species [20
]. The Giardia
isolates genotyped from Quenda in their natural habitats have all proved to be the novel strain. However, when Quenda were trapped and examined on a farm, they were found to be infected with ‘domestic’ strains of Giardia
normally found in livestock and humans. Presumably this reflects the susceptibility of Quenda to other strains of Giardia
, as with the case of beavers in North America. This case study raises questions regarding the pathogenicity of non-host adapted strains of Giardia
in naïve wildlife hosts. Additionally, it also raises the question of competition between co-habiting ‘strains’ of Giardia
] and whether in this case, and perhaps in other species of wildlife, zoonotic strains of Giardia
can out-compete the host-specific wildlife strains.