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Tommy Leung
Parasitologist, Evolutionary Biologist, Researcher, Lecturer
Parasitologist, Evolutionary Biologist, Researcher, Lecturer
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Of Chimps, Leopards, And Toxoplasma
Some of you may be familiar with the story about a little cat parasite call Toxoplasma gondii. It seems to be able to alter rodent behaviour so that they are more likely to be eaten by a cat, but it can also infect humans (and any warm-blooded vertebrate animal) and supposedly mess with human behaviour as well. Spoooooky. At least that's how the story goes. Like any other story, there is some grain of truth to it, but it is buried within a whole mass of (more sensationalised) dross. Any studies into Toxoplasma and host behaviour manipulation has the potential to go viral as it includes all the elements that makes a good headline - it contains cats, brain parasites, and zombies (in the form of host behaviour manipulation).

The literature on Toxoplasma and host behaviour is MASSIVE - some of it is good science, others are more like tabbies dressed as tigers. But for this post, I'm going to focusing on one story within a larger narrative, I want to talk about a paper recently published in Current Biology which had whipped the media into a frenzy (again) about how human behaviour is affected by Toxoplasma. http://www.cell.com/current-biology/abstract/S0960-9822%2815%2901517-1

Here's a tl;dr version of the study. The study found that compared with uninfected chimps, chimpanzees infected by Toxoplasma are not as averse to the odour of leopard (their natural predator) urine. The researchers concluded that this is because Toxoplasma is manipulating the chimps' behaviour so that they will be more likely to be eaten by a leopard (the final host for Toxoplasma are felines).

1) While the media coverage seems to be focused on how the parasite affects human behaviour, this experiment was done on chimps, and the media is extrapolating the conclusion of that study to humans. Humans and chimps may be genetically similar on some level, we have been separated by 5-7 million years of evolution, and our ancestors evolved in very different environments. There are some very key differences in the behaviour of chimps versus humans.
 
2) The study was not only correlative in nature, it was based on testing chimps for presence of Toxoplasma antibodies - not the parasites themselves, just a potential indicator of the parasites presence (having antibodies for something doesn't guarantee the presence of the said thing in the body). The researchers didn't confirm the presence of the parasites themselves. I understand they can't exactly do the latter for ethical reasons, in which case, maybe don't cannonball your way into such sensationalised conclusions?

3) The study tested how chimps response to the odour of urine and other big cats - the question is, just how much of a role does the sense of smell play in chimpanzees' predator avoidance repertoire? There is surprisingly little research on that. Is the sense of smell that important for predator avoidance compared with their other senses? Also, considering that chimpanzees are social animals, they would also rely upon other individuals in the group to warn of the presence of predators - you can't consider the vulnerability of a chimp to predation without the context of its social structure.

4) They mention potential behaviour variations between individuals (i.e. personalities) which may account for different level of aversion towards leopard urine odour which are pre-existing, regardless of the parasite. Good. But then, they just dismiss that possibility outright, by citing a single study that has found Toxoplasma is associated with disrupted fear response - in rat. Studies in other animals have shown that propensity for "recklessness" varies between individuals, even without the influence of parasites. So they're essentially saying Toxoplasma is the only possible explanation for why those chimps behaved slightly differently (in one aspects - response to leopard urine odour), even after bringing up the possibility that these behaviour variations exists regardless of parasitism, and discounting the dozens of other equally valid potential explanations. Not Wow.

5) Furthermore, when I dig into the methods, I found that the study was conducted on captive chimpanzees. Captive animals (especially behaviourally complex animals such as chimps) are known to exhibits behaviour which deviate significantly from their wild relatives. So we have no way of establishing whether such behaviour is representative of how they would behave in a natural setting (let alone extrapolating it to humans as the media has done). Once again, I understand that it would be extremely difficult to conduct such a study on wild chimps, in which case, the point I bring up in (2) still applies - don't jump to such sensationalised conclusions

6) Given the correlative nature of the study, we have no way of establishing how these chimps would have behaved before getting infected with Toxoplasma. So you can't rule out that maybe the chimps that behaved "oddly" are simply more likely to pick up Toxoplasma. They did mention this possibility, but they dismissed it just as quickly in the same manner as I described for (4).

7) The paper has 10 references in total (the supplementary material has 2 additional reference, but they were for methodological techniques), but did not cite a review recently published in 2014 in Advances in Parasitology which discussed at length the wide array of inconsistencies and seeming contradictory results from rodent-toxoplasmosis behavioural studies.
https://books.google.com.au/books?hl=en&lr=&id=ftnEAgAAQBAJ&oi=fnd&pg=PA109

And that's all I have to say about that. Peace out.
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Insane In The Ant Brain
I've written a new Parasite of the Day blog post! This post is about the infamous lancet fluke - the parasite that makes parasitised ant climb up a blade of grass so it'll get eating by a grazing mammal. But how does this little parasite perform such a feat of ant-jacking and what sacrifices does it involve? Find out more in the link below!
Dicroceolium dendriticum (revisited)
Dicroceolium dendriticum (revisited)
dailyparasite.blogspot.com
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Caving With Leeches
I've written a new Parasite of the Day blog post! This post is about leeches that live on European cave salamanders. This leech is the only known ectoparasite of these unique lungless salamander, but what effects are those blood-suckers having on their host? Follow the link below to find out more about these leeches and their cave-dwelling hosts!
Batracobdella algira
Batracobdella algira
dailyparasite.blogspot.com
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Snake Eaters
I've written a new Parasite of the Day blog post! This post is about Burmese pythons, Floridan snakes some peculiar lung parasites called "tongue worms". To find out more these snake parasites, follow the link below.
Raillietiella orientalis
Raillietiella orientalis
dailyparasite.blogspot.com
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Fungal Booty-Snatcher
I've written a new Parasite of the Day blog post! This post is about a parasitic fungus that infects periodical cicadas and turn their abdomen into a chalky mass of spores. But that's not all they do - infected cicadas also behave differently from uninfected ones, but as an added twist, it depends on what type of infection that manifests from their exposure to the fungal spore. To find out more about this fungal booty-snatcher, follow the link below.
Massospora cicadina
Massospora cicadina
dailyparasite.blogspot.com
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Taking Time In Tadpoles
I've written a new Parasite of the Day blog post! This post is about a pinworm that lives inside the gut of tadpoles, but it can only infect the host at this stage - when the tadpole turns into a frog, it becomes uninhabitable for the pinworm. So how does this little parasite make the most of its host while it still has the chance to do so? Follow the link below to find out more about this gut-dwelling nematode!
Gyrinicola batrachiensis
Gyrinicola batrachiensis
dailyparasite.blogspot.com
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It's What's Inside That Matters
I've written a new Parasite of the Day blog post! You've heard of chest-bursters and tongue-biters, but have you heard of belly dwellers? For the first post of 2018, meet Riggia puyensis - a plump parasitic isopod that lives in the belly of suckermouth catfish. Follow the link below to find out more about this parasitic crustacean!
Riggia puyensis
Riggia puyensis
dailyparasite.blogspot.co.nz
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Parasite of the Day blog 2017 in review
I've written a Parasite of the Day blog post featuring a summary of some of the most interesting parasites that were featured in the year 2017. Follow the link below to see the year 2017 through the lens of parasitology.
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What What In The Butt
I've written a new Parasite of the Day blog post! This post is about about a parasitic snail that spends its adult life living the cloaca of a sea cucumber. But there are some challenges associated with life in a sea cucumber's butt, so how does this snail overcome them? Follow the link below to find out.
Megadenus atrae
Megadenus atrae
dailyparasite.blogspot.com
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I've written a new Parasite of the Day blog post! This post is about prickly parasite that usually live in the guts of seals and sea lions, but apparently also Magellanic penguins? So how does a seal parasite end up in the gut of penguins? Follow the link below to find out.
Corynosoma australe
Corynosoma australe
dailyparasite.blogspot.com
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