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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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Pretty In Pink
I've written a new Parasite of the Day blog post! This one is about a very unique type of parasitic fluke with the tongue-twisting name of Polypipapiliotrema. It infects coral polyps, turning them bright pink and swollen. But to what ends? Follow the link to the blog post below to find out!
Polypipapiliotrema stenometra
Polypipapiliotrema stenometra
dailyparasite.blogspot.com
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I have written an end-of-2018 round-up post for the Parasite of the Day blog! A look back on 2018 and some of the fascinating parasite stories that were covered on the blog this year. From tadpole pinworms, to nasty hookworms, to cave-dwelling leeches, and fungus that disintegrate cicada butt - follow the link below to find out more!
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Fully Filled Fish Fillet
I've written a new Parasite of the Day blog post! This one is about a monkfish that was found to be infected with over one thousand tapeworm larvae - but what tapeworms are those? And where did they come from? (and where are they going to go?) Follow the link to the blog post below to find out!
Grillotia sp.
Grillotia sp.
dailyparasite.blogspot.com
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Pin The Worm In The Cockroach
I've written a new Parasite of the Day blog post! Pinworms are found in a wide variety of animals, and some pinworms which infect cockroaches - such as Leidynema appendiculatum - which is found in cockroaches all over the world. Even though it is widely distributed and infects many different cockroach species, not all cockroaches has this parasite. So why is it that some cockroaches are host to this pinworms while some are not? Follow the link to the blog post below to find out
Leidynema appendiculatum
Leidynema appendiculatum
dailyparasite.blogspot.com
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Blood Seal
I've written a new Parasite of the Day blog post! A species of hookworm that infect seal pups - but it enters the host through a rather unusual pathway, and its effects on seal pups can be deadly. To find out more, follow the link below!
Uncinaria sp.
Uncinaria sp.
dailyparasite.blogspot.com
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Fluffy Attachment
I've written a new Parasite of the Day blog post! This post is about a rather unusual-looking caterpillar which has an even more unusual life style. Instead of munching on leaves, the caterpillar of Epipomponia nawai lives as an ectoparasite of cicadas! To find out more, follow the link below!
Epipomponia nawai
Epipomponia nawai
dailyparasite.blogspot.com
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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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