Cosmos Q&A: Why we must value parasites

Parasites don’t get great PR or a lot of consideration when we’re talking conservation. An international team hopes to change that, however, and has published a paper in the journal Biological Conservation laying out a global plan to protect them, including 12 specific goals for the next decade.

Cosmos spoke to Melinda Moir from the University of Western Australia, an entomologist with a background in invertebrate conservation, who was invited to be a part of the working group.

You appear to have joined a support group for parasites. Please explain yourself.

Unfortunately, parasites are vastly under-represented in the conservation sphere, with only a handful represented on official conservation lists like the IUCN red list. This is despite published estimates of one in every three parasite species at potential risk of extinction in the next 50 years.

Our group includes people from many different institutions (museums, universities, research centres, government agencies) and countries (US, Panama, Italy, New Zealand, Australia, Singapore) who are all interested in advancing parasite conservation.

While we all recognise that the greatest risk to parasites is co-extinction – loss with their host species or with some change in the host species’ populations – parasite conservation is more complicated than it at first appears. There are many different drivers that are increasing the risk faced by parasites that we have addressed in our 12-step plan to counter parasite extinctions.

From a scientific perspective, what are “parasites”? Do we use that term in the same way that we do “insects” or “birds”?

“Parasite” can encompass a much broader range of taxa than terms such as “bird” or “insect”, as these latter terms are based on taxonomy (the classification of living organisms), whereas “parasite” is a term based on ecology or lifestyle. The term parasite is most often associated with an invertebrate animal that lives on, or in, another animal while feeding from it, such as lice on birds or roundworms in mammals.

However, parasite can actually encompass a much wider range of taxa, from parasitic plants like mistletoe utilising the nutrients of trees, herbivorous insects that eat plants, fungal infections of humans, to vampire bats feeding on mammals. Basically, a parasite is anything that feeds off another living organism (the host), without killing the host organism when the parasite load is low.

Do we fully understand what they do?

Parasites are a large and important part of global biodiversity. We do know that they play important roles in wildlife population control, ecosystem stability and flow, nutrient cycling, and potentially even buffering against the emergence of virulent diseases. There is some evidence of parasites being beneficial for their hosts, such as intestinal parasites possibly helping humans with disorders caused by inflammation of the intestines (e.g., Crohn’s disease), by activating the immune system.

Certainly, those parasites which alter the behaviour of one of their hosts can then benefit other organisms. For example a Nematomorph parasite has been found to alter their host cricket’s behaviour to seek water, which creates a large food supply for an endangered trout in rivers in Japan, accounting for 60% of the trout’s annual energy intake.   

Parasites have been described as one of the last frontiers in the world of biodiversity because there are so many species yet to be discovered and aspects of their ecology are still being uncovered. So although we don’t fully understand or even know about the existence of most parasitic species, we do know that most are an essential component of ecosystems.  

Why have so few be identified and named?

There are a number of problems here, but one major issue is the lack of parasite taxonomists (scientists who describe species). It isn’t exactly the sexiest job in the world to be discovering new mites, lice or internal worms! So attracting undergraduate students into this realm and away from the cute, the furry and the fluffy animals is difficult. The students most likely to encounter parasites do so from the medical or veterinary or agricultural perspective, where the aim is to eradicate the parasite, to improve the health of the host.

Most parasites are difficult to collect, as collecting them may require harming the host individual. Parasites are often hard to see, and their taxonomy can be very difficult, usually requiring specialist equipment and curation techniques to be able to identify any differences between species. It takes years of study to work in this world, and then there are few jobs worldwide that allow people a full time job of describing new species. A consequence of this is that there are few established mentors in academia grooming the next generation of parasitologists. So the cycle of neglect continues.

Do parasites tend to affect us directly, or indirectly via their impact on other parts of the environment?

An extremely small percentage of parasite species directly impact humans either by parasitising us or our livestock and pets. An even smaller percentage act as vectors to viruses and bacteria, which tend to have more detrimental effects on the host than the parasites themselves do by simply feeding on the host. For example, the Brown Dog tick (Rhipicephalus sanguineus) is host to the bacterium Ehrlichia canis, which has just recently arrived in the Northern Territory and Western Australia. Although the tick is present across northern Australia and has limited impact on dog health if in low numbers, the bacterium causes Ehrlichiosis, which can be  fatal.  

The extinction of parasites can imbalance the system and then impact humans. A good example of this is general host-tick coextinction in the tropical rainforests of Panama reducing competition. One tick, Amblyomma oblongoguttatum, a vector of spotted fever group rickettsiosis (causing Rickettsia in humans) has become dominant as a consequence. This means that there is more potential for us to catch Rickettsia in these forests because of the lack of a diversity of tick  species.

Do we affect them as much as they affect us? Is that changing?

Our impacts on parasites, either directly or indirectly, far outweigh their negative impacts on us. And unfortunately, this impact is increasing. Up to 10% of parasite species could be threatened by climate change alone, and when you add in co-extinction risk (the loss of the parasite with the extinction of the host species), this rate jumps to one in every three parasite species being threatened with extinction.

Evidence of the threat faced by parasites is clear in a study that compared seagulls in the 1960s versus today. The scientists found that two thirds of flatworm parasites are now missing, representing a “systematic collapse” of the parasite ecosystem. Parasites are highly threatened because they face a double threat: they’re sensitive on their own to disturbances like climate change or habitat loss, but are also highly threatened with extinction if their host experiences reductions in population size or density. But parasites are still mostly neglected by conservation research and practice. Furthermore, as some recent cases have uncovered, we are actually to blame for increasing the impacts of parasites on humans, as in the example of the coextinction of ticks in Panama  above.

Can you conserve or save parasites in the same way that you do endangered wildlife? Do we need to save them, or just get out of their way?

Parasites need active management on several fronts, and we need to stop actively trying to eliminate them from the ecosystem. Currently, the management of many threatened species of potential host (plant, vertebrate) includes actively removing the host species from their natural environment and/or actively removing the parasites through pesticides or anti-parasitic drugs. For instance, to conserve the Tasmanian devil, insurance populations against the deadly Devil Facial Tumour Disease have been necessary, but this management has also included the routine use of anthelmintics. Luckily, this practice has now been stopped in recognition of the need to also conserve the devils’ parasitic fauna.

Every effort should be made to survey the parasite diversity on these highly threatened hosts. Once a parasite is identified as being threatened, it should feature on conservation lists such as the IUCN Red List. Threatened parasites may need adaptive management just as their threatened host do. For example, the tuatara tick has been translocated with its host the New Zealand tuatara lizard. However, scientists subsequently found that tick populations were declining despite the tuartara population remaining stable. The problem appeared to be that the density of tuatara was too sparse for the tick. This example is quite typical; the loss of the parasite occurs before the host because of a change in the host species population [The genome of the tuatara lizard has just been sequenced. Read the Cosmos report here.]

Our new global plan encompasses such remedial efforts within 12 goals for the next decade that could advance parasite biodiversity conservation through an ambitious mix of research, advocacy, and management. These goals fall within four categories: data collection and synthesis, risk assessment and prioritisation, conservation practice, and outreach and education.

What is the appeal in studying parasites? What’s your elevator pitch to the next generation of scientists?

Parasites are one of the few great unknowns in biodiversity where young scientists can really make their mark. It is almost impossible to discover a new bird, mammal or amphibian in the world today; however, there are likely to be numerous undescribed parasites just in your backyard – whether that be microscopic parasitic wasps laying eggs in other insects feeding on your vegies in the garden, or the internal worm of the little brown honeyeater nesting in the backyard tree. And many parasites are weird and wonderful, changing their host’s behaviour, morphology or colour – if you like being at the forefront of science, this field is definitely for you.

Is this work part of what we now hear called disease ecology?

Parasitism is a large component of disease ecology, but work on parasites is called parasitology and is a world unto itself. Disease ecology largely evolved around infectious disease and, set against a backdrop of pandemics, veterinarians and treatment, often has negative connotations associated with parasites. So although the theory of disease ecology can help to inform mechanisms of transmission of parasites across host individuals, it is at odds with parasite conservation because it comes from a different perspective of eliminating the parasite rather than conserving it.

What’s your favourite parasite story or memory?

Recently I was told by a colleague of the South American nematode Myrmeconema neotropicum,which lives inside the black ant Cephalotes atratus. The nematode requires two hosts to complete its lifecycle; an ant and a bird. To move from the ant to the bird, the nematode changes the ant’s black abdomen to resemble a red berry, and the bird consumes the berry-abdomen plus nematode.

Much closer to home, I’m worried for the Numbat acanthocephalan or spiny-headed worm (Multisentis myrmecobius). It requires two hosts to complete its lifecycle, the numbat and an invertebrate, most likely termites, as these are the main food source of numbats. Currently, all translocated numbats are treated for this spiny-headed worm, which has only been found in one population of the endangered numbat at Dryandra in southwest Australia.

Although the worm isn’t recognised on any conservation lists, it is highly likely that it is either critically endangered or extinct. No one has, however, assessed the threat status of the worm, despite one of its hosts being an iconic threatened Australian marsupial. This particular example highlights the complex, unseen and perilous nature of parasite  existence.