Recently, there was a post on this subject at Neatorama:

http://www.neatorama.com/2007/04/04/parasite-can-alter-hosts-fear

I felt the need to reply to one of the comments there with a speculation on the evolution of such specialized brain localization. My guesstimate was that the parasites may target themselves to different tissue types by binding to tissue-specific markers.

Some years ago (six? seven?), an experiment was done in which filamentous bacteriophage were used to create a library of random peptide motifs that were “displayed” on their coat protesin. Each virion would express a different motif. This “phage display library” was then injected into the bloodstream of a mouse. After some incubation, different organs were taken, and bacteriophage were recovered from them. The bacteriophage population found in a particular organ was presumed to be enriched for phage that display peptides that confer tissue-specific localization.

After successive rounds of clonal isolation, propagation, selection by injection into more mice and harvesting; clonal isolates were found that could be targetted to specific tissues. The authors proposed that this type of targeting might be used to deliver drugs to specific organs or tissues; or even deliver drugs to tumors for chemotherapy.

So I wonder if there is any evidence of this kind of tissue-targeting in T. gondii. Phage display library enrichment is a laboratory-based evolutionary process, and is illustrative of the way that a parasite might evolve the kind of characteristics we are seeing now in Toxoplasma.

I set down the book and found your blog. Looking forward to more scientific surprises…

It would be interesting to know when that ability to survive in hosts other than its two first choices (rats and cats) developed and whether that correlates with this evolutionary sweet spot indicated by this research.

Would this time relate well to the time that cats were first domesticed, following the availability of bulk grain from agriculture? Domestication brings many unrelated cats together for social contact and would allow greater spread of toxoplasma.

I guess it goes to show that as much as the world’s changed, from our perspective, in the last 10,000 years — for example, oh, that entire “civilisation” thing — the life of toxoplasma has been remarkably consistent.

So I guess there’s two things to draw from this. First, that toxoplasma is amazingly successful. Second, they target an amazingly successful host, i.e. rats. If anything, that ties into the development of civilisation that I mentioned above. We humans have gradually created an environment that is simply perfect for rats, so why would either the rats or their incredible parasites bother to change in all that time?