Of Brain and Toxoplasma

Posted by Kasra

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This  wonderful comic had stayed in my drafts folder for a very long time and I had decided finally not to post since Calamaties of Nature had stopped publishing comics. But then again, a recent PLOS ONE paper reminded me of it again.  Toxoplasma gondii is one of my favorite parasites. It is one of the most common parasites of humans and in majority of cases lays dormant throughout life, making it one of the most successful parasites in Nature. After infection (eating poorly cooked infected meat or contact with feces of an infected cat), T. gondii escapes the gut and migrates to the brain. At first glance, it does not seem to do much over there, at least nothing drastic. But studies including current work by Ingram et al. have shown that T. gondii infection can permanently alter animal behavior, permanently meaning the behavioral change stays even after infection has been cleared.  This recent study is another example demonstrating this ability of the parasite:

Ingram et al. checked how the behavior of an infected or uninfected mouse differ when exposed to predators, in this case Bobcat urine. As you can see in the figure below (Part A), they set up a field, one part of which was spotted with Bobcat urine or Rabbit urine (controlling for effect of just urine versus predator urine). Infected, uninfected and infected with a attenuated parasite (which could successfully clear the infection) mice were let in the area to see which spots they spend more or less time at, translated to which areas they avoid and which areas they do not. As you can see in sections Bi and Bii, the mouse partly avoided the section with rabbit urine (hence the importance of controls!) but this avoidance was way stronger when exposed to Bobcat urine. Surprisingly this behavior almost vanishes in infected mice, regardless of the strain, in other words, regardless of presence or absence of parasites in the brain.

From Ingram et al. PLOS ONE, 2013

From Ingram et al. PLOS ONE, 2013

There is a body of research on possible effects of T. gondii infection on human brain and behavior and correlations to increased suicide and schizophrenia have been suggested, although not confirmed. Still we all keep in mind that correlation does not mean causation.

Could this alteration of animal behavior also have an evolutionary explanation/impact on Toxoplasma’s life cycle? In other words, is there more to this phenomenon than just parasite infects brain, brain acts strange?

There is a difference between cats (big and small, domestic and wild) and the rest of the animals when it comes to Toxoplasma. In other animals, as I mentioned above, Toxoplasma escapes the gut after ingestion and moves  to muscle  and brain tissue and just stays there. So basically the infection is kind of a dead-end. It cannot be transferred to the next host until the current host dies and gets eaten up by another one. However,cats are the main hosts for Toxoplasma. That is where the parasite goes through the sexual stage of its life cycle. Also, Toxoplasma cysts are constantly shed from an infected cat through its feces. Not a dead-end infection. Now the loss of predator in Toxo-infected mice makes sense! It helps the parasite get back to its main host where it can complete the cycle! This is a trait that would have been highly favored by natural selection whenever it evolved. Because it would strongly increase the chance of the parasite being passed around to the next host.

At the end, what the comic made me think of was, could those parasites also have parasites that would alter their behavior in their own benefit? Is this is a classic example of the Extended Phenotype idea introduced by Richard Dawkins?


Ingram WM, Goodrich LM, Robey EA, & Eisen MB (2013). Mice Infected with Low-Virulence Strains of Toxoplasma gondii Lose Their Innate Aversion to Cat Urine, Even after Extensive Parasite Clearance. PloS one, 8 (9) PMID: 24058668


The host as the parasite niche

Posted by: Kasra Hassani

I remember some years ago when I used to read about ecology, behavior and evolution, how fascinated I was by the classic book of G. Evelyn Hutchinson, The ecological theater and the evolutionary play (1965) and by the way that book was depicting niches in ecology and evolution. Separation of niches and consequent speciation occurs constantly and could look simple in the first look. On the other hand, the resulting diversity could be also beyond imagination. Different depths in the cork of an old tree in a tropic jungle can provide different micro-environments to the species living in them, both micro and macroscopic. Various tiny species of insects and other invertebrates could live in different depths of the cork of the tree; different species of birds could feed from them using specialized beaks that penetrate different depths of the cork, so that they would not enter the other species’ niche. It does not just end there. Species distribution varies by height of the tree as so do the mico-environments. And yet even more, in some cases speciation can be temporal with different species of birds coming to the tree at different times during the day. Thus, a tree could act as countless niches for countless species and thus countless biologic diversity.

Years later, I stumbled upon the same phenomenon in the world of parasites. As I was looking through the second chapter of “Foundations of Parasitology” by Schmidt and Roberts (2005), I realized that the vertebrate intestinal tract could be looked at the same way as a tree. Along the intestinal tract, different environments exist, in terms of physical conditions, nutrients, pH and enzymes. Therefore, we should not be surprised to see the same trend and observe different parasites adapting to different micro-environments along the tract and staying away from each other. Below is a graph of distribution of different intestinal worms along the intestine tract of a bird. The bar shows distance from the stomach. Specialization and separation of niches among different parasites can be readily seen (Stock and Holmes 1988).

Adapted from Stock and Holmes, J Parasitology 1988

Another study that the book chapter mentions is about 8 different species of nematodes that were found to be distributed in the intestine of a turtle, not only longitudinally but also radially with preferences towards either lumen or the ring of the intestine (Schad, 1965).

Sometimes we go in so deep into our molecules that we miss the big picture of biology of parasites. How they interact with each other and with their hosts in an ecological and evolutionary point of view. I guess the beauty of biology is that no matter how diverse and complicated the organisms and their relationships can get, still the same principles and patterns apply.