Introduction

Parasitic diseases can have a significant impact on wildlife. This is a major concern, particularly for the conservation of threatened species.2 Parasites can cause wildlife populations to decline either temporarily or permanently.3 For example, about 14 species of frogs in Australian rainforests are thought to have become extinct because of the parasitic fungal disease, chytridiomycosis.4

There are at least as many species of parasites as there are other organisms.5,6 Parasites are an important part of biodiversity and a healthy ecosystem. This means that there is no simple way to deal with parasite infections.

Research has primarily focused on parasitic diseases that threaten humans or domestic animals.7 Insufficient attention has been given to the potential impacts of parasitic diseases on wildlife.

Humans and domestic animals can spread parasites to wildlife. This is expected to increase as wildlife interact more with humans and domestic animals due to urban expansion and land clearing. Recent advances in science have meant that researchers are now gaining an insight into diseases spread to wildlife from people and domestic animals.

Wildlife as a source of parasite infection

Wildlife are often seen as the cause of parasitic diseases that spread to humans or livestock. In many cases though, there is evidence that these parasitic diseases were originally introduced to wildlife populations by humans. Examples include Giardia (think ‘Bali Belly’), sarcoptic mange (scabies) and Echinococcus granulosus, a tapeworm parasite (‘hydatids’).

Giardia was registered by the World Health Organization as a zoonotic parasite (one that can be spread from people to animals and vice versa) over 30 years ago. At the time, campers in Canada had been diagnosed with giardiasis, a disease caused by Giardia. Investigators suggested that this disease was caused when campers drank stream water contaminated with Giardia from beavers.8

It was assumed that beavers spread the parasite to humans but this was not the case. Only beavers downstream from sewage works were found to be infected and later studies confirmed that beavers are prone to infection with zoonotic strains of Giardia.8 Beavers were infected as a result of accidentally ingesting the human strain of Giardia. This infection then increases the number of Giardia cysts in stream water. It is not known whether there is a beaver strain of Giardia.

Giardia has been found in many mammals, however not much is known about the strains that occur naturally in wildlife.9,10,11 A new species of Giardia has been found in quenda (Isoodon obesulus fusciventer).12 In the majority of cases, the type of Giardia found in mammals has usually proved to be from humans. Giardia has been found in wildlife living in pristine and/or remote areas. This includes primates in Africa, muskoxen in the Arctic, house mice on remote islands, Australian marsupials and marine mammals in various parts of the world.9,13,14,15,16,17,18,19 In all of these cases, humans were found to be the source of infection through environmental contamination, either directly or indirectly via domestic animals.

Sarcoptic mange, or scabies, is a well-known threat to the health of endangered or isolated wildlife populations.51 In southeast Australia, Common Wombats (Vombatus ursinus) are under threat from Sarcoptes scabei var. wombati, a variant of scabies which occurs throughout their home range.52 This has the potential to severely reduce local wombat numbers, and threaten the survival of small isolated populations. Scientists have strong evidence that this variant of scabies came from humans and domestic dogs.53

Echinococcus granulosus, a tapeworm parasite introduced into Australia from sheep during early European settlement, is now widespread. This parasite affects the health of wildlife, humans and livestock.14,20 Infection with Echinococcus can cause hydatid disease which might be fatal.

Many species of kangaroos and wallabies throughout Australia have been infected by the tapeworm parasite. This particularly threatens the survival of endangered small macropod species that exist in isolated colonies with small home ranges.21 The parasite causes major cysts in the lungs. Studies have found that this reduces effective lung volume in wallabies by approximately 55% in males and 70-80% in females.21 This makes the animal weak and easy for predators to catch.22,23 Dingoes then become infected when they prey on kangaroos and wallabies.

Impact of parasite infection on wildlife

Wildlife conservation efforts often neglect the potential impacts of parasites on wildlife populations. Humans know very little about the ecological relationships between parasites and hosts and their impact on wildlife health. Some targeted investigations have produced valuable and often unexpected data. Examples are investigations into Leishmania, Trypanosoma and Toxoplasma in Australia.

Another major concern to wildlife is the unintentional introduction of diseases through the relocation or release of captive bred animals. 24 Wildlife living in areas where the release occurs may not have been exposed to the same diseases and might not have any immunity.24 For example, parasites introduced by exotic fish species, such as anchor worm, also poses a threat to native fishes. This can occur when exotic fish, such as goldfish, are released into waterways.25

Leishmania is an intracellular parasite, (one that lives in cells in the skin or different organs in the body), and is spread by sandflies. There are numerous species of Leishmania that affect a variety of mammals. Humans and domestic dogs are susceptible to infection, which often results in serious disease. Leishmania had not been detected in Australia until it was discovered recently in kangaroos in the Northern Territory.26,27 Initially, the media and government speculated that the kangaroos could spread the infection to humans.28 An investigation showed that the infection was a new species of Leishmania.26 This new species may have evolved over a long time and adapted to macropods.

All of the infected kangaroos were in captivity. The stress associated with captivity may have caused the development of lesions that would be unlikely to develop in animals in the wild. There have been reports of Leishmania entering Australia in infected humans or dogs from other parts of the world, but it has always been assumed that this could not spread because we lack the insect vectors which transmit the parasite from one host to another.28,29,30 The discovery of the parasite in kangaroos shows that vectors are present in Australia and that Leishmania introduced from overseas does pose a risk to humans, pets and wildlife.

There are a number of species of Trypanosoma, which cause diseases such as sleeping sickness in humans, nagana in animals in Africa, and Chagas disease in South and Central America. Studies have now demonstrated that Trypanosoma infection is widespread in Australia and occurs in many different host species, often in a high percentage of animals in a particular population.31 Humans have almost certainly spread the infection from one wildlife population to another by relocating animals.1

In Western Australia, different species of Trypanosoma infect the woylie (Bettongia penicillata) and the closely related boodie (Bettongia lesueur). It is not known whether cross-infection can occur and the effect of increased disease risk if it happened. 31 Populations of the same native species in different locations can also have different species or strains of Trypanosoma and will infect other animals if the animals are relocated. 31

There is evidence to suggest that some species in the same location can be infected by multiple species or strains of Trypanosoma. In one isolated conservation park, two strains of the parasite were found in the chuditch (Dasyurus geoffroii). 31 Some strains of Trypanosoma are geographically widespread and occur in multiple species, while others appear to be specific to one species and/or confined to certain locations. 31

The virulence of Trypanosoma infection may be affected by whether the animal has other infections or by the general condition of the animal.32,33 One study found a higher frequency and intensity of infection in a declining population of woylies compared with a stable population in the southwest of Western Australia.34 Animals in the declining population were also exposed to high levels of the parasite Toxoplasma gondii.35

A better understanding of Trypanosoma and how it is spread in Australia is needed to contribute to conservation of endangered species and relocation programs.

Toxoplasma gondii is a common zoonotic parasite of mammals, including people, and birds. Studies have found that Toxoplasma is common and widely distributed among native animals in Western Australia.1 The parasite is genetically highly variable with many different strains that vary in how much damage they cause to the host.

Infections with Toxoplasma probably do not cause many adverse effects in healthy wildlife. The severity of infection is thought to increase if animals are in captivity, live in areas where there are introduced predators such as cats and foxes, where there is competition for food and resources or in degraded habitats.36,37,38

Anchor worm (Lernaea cyprinacea) is a parasite of the skin and gills of freshwater fishes. Infections in native fishes cause large wounds, bleeding, castration and high death rates. In 2007, anchor worm was found on four native freshwater fish species in the Canning River, Western Australia.54 These included Western Minnow (Galaxias occidentalis), Western Pygmy Perch (Nannoperca vittata), Nightfish (Bostockia porosa) and Freshwater Cobbler (Tandanus bostocki).54 The parasite was also found on three introduced fish species – goldfish (Carassius auratus), Eastern Mosquitofish (Gambusia holbrooki) and Speckled Mosquitofish (Phalloceros caudimaculatus).54

Anchor worm is not native to Australia and had previously only been found on native and exotic fish species in New South Wales and Victoria.55 The parasite was probably introduced by infected exotic fish, such as goldfish (Carassius auratus), being released into waterways.54 Goldfish and common carp (Cyprinus carpio) have been found in many streams, irrigation drains and lakes near Perth, Western Australia, and in waterways between the Moore and Vasse Rivers on the Swan Coastal Plain.56 Introduced fishes cause harm to native freshwater fishes through predation, competition and habitat change.56 They also spread disease, which is an increasing concern for the health of freshwater habitats worldwide. 61,62, 63, 64

Parasites have conservation value

While parasites can be a threat to wildlife, they also have conservation value. Parasites are important components of ecosystems and can alter the stability of food webs.39,40 Parasites may influence the behaviour of individual hosts,41 regulate population sizes of the host by changing birth and death rates42 and act as ecosystem engineers.43 Therefore, reducing the parasite population and losing parasite species may have important impacts on ecosystem function and affect biodiversity. 44,45

When species become extinct, this often causes the extinction of many parasite species, especially those that are specific to a particular host.44,46,47 Researchers generally have incomplete knowledge about parasite species that exist in these hosts.48,49 In many cases it is not known how many parasites species are lost through extinction. For example, a study into freshwater fishes of south-western Australia found 44 parasite species. All but two of these species appeared to be native and most were previously unknown to science.49

 

Conclusion

Treatment of parasitic infection is essential where there is a threat to endangered wildlife. In some cases however, parasites may be important to the health of individual animals to develop immunity, or to the health of a wildlife population or the broader ecosystem. 1

The number of threatened species is increasing throughout the world. This is being exacerbated by human encroachment into wildlife habitats, land degradation and climate change.1 Understanding the diversity and ecology of parasites in ecosystems, and the relationship between hosts and parasites, is very important for wildlife conservation.

 

 

References

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3 Daszak et al., 2000; Harvell et al., 2002; Smith et al., 2006.

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