Saving Parasites

[Guest guest]

Dear Dr Wedderburn,

The attached article is not exclusively

Asian but I found it so interesting that I thought it worth posting.

Regards,

 

Sincerely,

 

 

http://www.africanconservation.org/dcforum/DCForumID5/184.html

New Scientist

unregistered user

18-Jun-04, 10:18 AM (GMT)

 

" Save the rhino maggot! "

 

 

 

New Scientist vol 181 issue 2440 - 27 March 2004, page 40

 

 

Well, should we? Even if the rhino itself is at risk? It's the kind of

problem that conservation biologists are starting to find increasingly

troubling, reports Matt Kaplan

 

 

IN OCTOBER 1995 a white rhino, recently arrived in England from South

Africa, excreted an insect larva in its dung. Concerned about what this

maggot was doing inside their endangered animal, the rhino's new owners

called in entomologist Martin Hall of the Natural History Museum (NHM) in

London to have a look. The larva pupated, hatched and Hall identified it as

an endangered parasitic fly that feeds exclusively on rhinos. With neither

rhino to dine on nor mate to breed with, it was left to die - one less

parasite in the world. Only Hall mourned its passing.

" Save the rhino maggot! " doesn't have much appeal as a conservation slogan.

But the fact is that for every charismatic animal facing extinction there is

an entire world of other species living on and inside it that may be equally

endangered. From lice that suck blood at the surface and tapeworms that grow

fat inside, to gut bacteria that aid digestion, almost every creature,

especially in the wild, has hundreds of parasites that call it home. Many of

these depend on one or two closely related host species for their survival:

if their hosts become extinct, so do they. Yet, while the world's zoos spend

vast amounts of time and money trying to save rhinos, pandas and the like,

their parasites have never been considered... until now.

 

The breakthrough for the world's endangered parasites came last year when a

genomics corporation called Diversa in San Diego, California, approached

researchers at the Center for Reproduction of Endangered Species (CRES) at

San Diego Zoo and Wild Animal Park with the idea of exploring microbial

habitats within their rare and endangered animals. For Diversa this would be

an amazing opportunity to do some bioprospecting in uncharted territory. The

CRES researchers were equally keen, but for different reasons. They could

see the potential benefits of comparing the parasitic load of captive

animals at the zoo with that carried by their wild counterparts. It seemed

like a happy coincidence of interests that would give new insights into

hidden worlds, but the project has already ruffled some feathers.

 

" This all started with gastrointestinal problems in our douc langurs, the

most endangered primates in the world, " says Mark Schrenzel, an expert in

animal diseases at CRES. Douc langurs are likely to become extinct in the

wild within a decade, yet they do not thrive in zoos. Wherever they are kept

in captivity it is the same story: constant digestive problems and frequent

episodes of vomiting and diarrhoea. The CRES researchers were desperate to

find the source of the problem. They suspected that some of the microbes

living in the guts of their captive langurs might be different from those

that would normally be present in the wild. To discover whether they were

right, they needed to analyse the microbial environment inside their

primates.

 

By a stroke of luck, it was then that Diversa contacted CRES geneticist

Oliver Ryder, and it was agreed that Diversa researchers would work with the

CRES to collect samples of microbial communities from endangered species at

San Diego Zoo and compare them with similar samples taken from healthy

individuals living in the wild. The hope was that they might be able to

identify beneficial microbes that were missing from the captive animals.

" This is an incredible opportunity to learn about the microbial diversity

and symbiosis in and around endangered species and get a better

understanding of the role played by organisms at the micro level, " says

Ryder. The plan is to study the parasites associated with a whole range of

animals, including proboscis monkeys, tree frogs and dozens of ruminants.

But the first candidate is, of course, the douc langur.

 

Research began in mid-November 2003, and the team reckons it will take about

18 months to analyse DNA from the gut of captive and wild douc langurs and

compare the genetic sequences with those taken from known organisms. CRES

then hopes to take organisms found in the healthy wild animals and

reintroduce them to the animals in captivity. With a bit of luck, this

should help to improve the balance of the captive animals' gut flora. And

the langurs are not the only ones who stand to benefit. Eric Mathur of

Diversa points to just one of the possible commercial spin-offs.

" Discovering enzymes to improve animal feed is a major part of what Diversa

does, and understanding the digestive fauna of the douc langurs helps us to

gain a lot of insight in this area, " he says.

 

In other species, the relationships between parasites and their hosts are

often highly specific, so the chances of finding new and exotic organisms

look good. " We used to think that roughly 50 per cent of the parasitic

flatworms in the fish we study were species specific, " says parasitologist

Rod Bray, also from the NHM. But it seems even that figure may be too low.

" Recent DNA work is showing that there is much more specificity than we

originally thought, " he says. And while he cautions against applying this

finding to other creatures, he adds: " When it comes to parasites in fish, I

think it is reasonable to say that practically every species has specific

parasites. "

 

Exclusive relationships

 

Research by Bray's colleagues highlights the sorts of barriers that can

preserve an exclusive relationship between a parasite and its host. The

tapeworm Taenia olngojinei, for example, only infests hyenas, even though

its larvae are carried by antelopes, which are eaten by both lions and

hyenas. But this particular tapeworm is not picked up by lions because it

forms cysts within the pelvic girdle of the antelope, explains Arlene Jones,

a parasitologist at the NHM. Hyenas are the only animals on the savannah

with jaws strong enough to crunch through the bone and release the tapeworm

larvae. " That's the one parasite we do understand, " says Jones. But for most

others, the reasons behind the exclusive relationship with a host remain a

mystery.

 

If endangered animals also have unique parasites - and it seems a pretty

safe bet that they do - the parasites' fate must be as uncertain as the fate

of their hosts. And this then raises a tricky ethical question: should

conservationists be as concerned about the fate of these parasites as they

are about the survival of their hosts?

 

This has certainly not been a consideration until now, as the story of the

California condor lice reveals. In the mid-1980s, with the California condor

population down to 25 individuals, conservationists took the drastic step of

bringing in all the remaining birds for captive breeding at San Diego Zoo

and Wild Animal Park. Most birds have lice, so one of the first things the

researchers did was to treat them with a pesticide dust to remove lice and

other external parasites. Team leader Mike Wallace recalls only one or two

birds having any lice at all and, as far as he and his team could tell, they

were just " typical lice " .

 

In light of what we have since learned about parasites and their hosts, that

assessment seems outdated. " There are in fact no such things as 'typical

lice', " says Chris Lyal, a louse specialist at the NHM, although when the

California condors were captured this was not widely realised. The Chewing

Lice: World checklist and biological overview, recently published by the

Illinois Natural History Survey, lists three species of louse as parasites

of the California condor. Because two of them have been found nowhere else,

they are seemingly extinct, says Lyal. It is impossible to say whether the

San Diego Zoo's pesticide application actually caused their demise because,

as Lyal points out, lice do not infest all of their hosts equally. The two

species in question may have become extinct long before the birds were taken

into captivity, as the population of the host birds slumped. We will never

know.

 

Thanks to the phenomenal efforts of CRES researchers, there are now more

than 200 California condors, many of which are back in the wild. But can

this breeding programme truly be considered a success if two other species

became extinct in the process? Lyal, for one, is saddened by the loss. " What

I don't understand is why the pesticide was used at all if the birds only

had a few lice. They would not have been very damaging, " he says. " In the

future, we need to think very carefully about the costs and benefits of the

treatments we give to endangered animals in captivity. "

 

Not all parasites are as benign as a few lice, and those that pose a health

risk to their host are certainly not going to be championed by

conservationists. But where do you draw the line? What about Hall's

endangered rhino maggot? This larva of the rhino botfly lives most of its

life in the gut of its host, just as its close relative does in horses. " A

standard riding horse can easily live a long and healthy life with a light

infestation of maggots in its guts, " says Hall. And if the infestation gets

out of control it can be treated with one of the family of chemicals known

as macrocyclic lactones, such as ivermectin. Hall sees no reason not to try

and breed the rhino botfly in captivity inside its host. " It would be a

challenge, but the reality is that these flies are really just a minor

nuisance to the rhinos, " he says. But he realises this is probably not going

to happen because of the time and money it would take. While resources can

be found to keep animals like condors and pandas around, who is going to

stump up the cash to protect an insect whose larvae chew on the insides of

rhinos?

 

If this seems like discrimination against species that humans find

distasteful, conservationists disagree. " Our conservation efforts are guided

by the needs of the species rather than by the popularity of the animal

itself, " says Sharon Dewar of San Diego Zoo. She points out that the zoo

puts plenty of effort into conserving less popular species, including the

Visayan warty pigs, red river hogs, bearded pigs and endangered cattle such

as the lowland anoa and Javan banteng.

 

But inevitably, finance is a factor. " We believe a tapeworm has a right to

exist just as any other animal does, " Schrenzel says. " But we do not have

the resources to save everything. Our work here is focused on vertebrates

and as a result they are prioritised. " For Schrenzel and the other CRES

scientists, the partnership with Diversa is a sideline, and the health of

endangered animals remains the top priority. " We have no choice but to

actively eradicate harmful parasites and bacteria from our captive

populations, " he says.

 

Parasites aren't all bad

 

It would be hard to disagree, except that in many cases we don't actually

know enough about the relationships between particular parasites and their

hosts to say whether they are harmful, benign or even beneficial. Even

deciding what is or is not a parasite may not be straightforward. " There is

no unambiguous definition, " says David Johnston from the NHM. When pressed,

he recites: " A parasite is an organism that lives in or on another... " But

then, he adds, that leads you straight into problems with species like the

cuckoo, which lays its eggs in other birds' nests and fools them into

raising its young. And what about the klepto-parasitic birds that harass

other birds until they produce vomit, which they then eat? Or the species of

fish where a male bites a female, fuses with her body and then extracts

nutrients from her blood, donating only his sperm in return. The issue is

very complicated, says Johnston. We call most disease-causing

micro-organisms " pathogenic " , he points out. " Why don't we call harmful

bacteria parasitic? What about viruses? Prions? Is there such a thing as a

parasitic chemical? "

 

And there is a further problem: organisms can often fulfil different roles

in relation to their host at different times of their lives and under

different conditions. What can be a pathogen in some circumstances may be a

commensal organism (doing neither harm nor good) or even a symbiont

(benefiting the host) if circumstances change. " People almost immediately

classify a tapeworm as a pathogen, yet in well-fed individuals many species

are probably functioning more like commensals, " says Johnston. " It is only

when the going gets tough and food is scarce that the worm can start to

exert a negative effect. "

 

Then there's the finding that inflammatory bowel disease (IBD) is rare in

countries where intestinal nematode worm infections are common. In a study

published last year (Infection and Immunity, vol 70, p 5931), Steve Collins

and colleagues at McMaster University in Ontario, Canada, suggest that the

worms themselves may play a protective role by inducing an immunological

response that essentially distracts the immune system from the inflammatory

responses associated with IBD.

 

If the relationship between organisms is so complex, has CRES set itself an

impossible task in its search for douc langur symbionts? " I don't think so, "

says Schrenzel. " While we say that we are looking for symbionts and avoiding

pathogens, we know that in reality things will not be a simple matter of

black and white. " The researchers believe that they have no choice but to

try, given this particular primate's closeness to extinction.

 

But even if the beneficial gut bacteria prove elusive, the collaboration

between CRES and Diversa could lead to far-reaching changes in the way

conservationists think. Each endangered animal may come to be seen as an

ecosystem in its own right, home to a large community of parasites that live

on and in it. " Ecosystems are such complex things, " says Bray. " Remove

something from the system and inevitably there will be some effect. This is

true of herbivores, carnivores, scavengers and even parasites. "

 

This way of thinking makes conservation an even more difficult balancing act

than it already is. But it may also offer new opportunities for the survival

of animals like the douc langur. And perhaps even for some of the parasites

themselves.

 

 

Matt Kaplan

Matt Kaplan reports from London and Los Angeles on conservation and ecology

issues