Those of you who know me know that I have a particular love/fascination with Toxoplasma gondii. And why shouldn't I? It's an amazingly complicated for a single-celled organism. Capable of manipulating hosts in ways worthy of gruesome science fiction, this parasite has captivated many a parasitologist. I knew that this parasite was capable of infecting a variety of hosts. It is most well-known for infecting cats, mice/rats, and humans. However, as I delve deeper into the literature for my dissertation, I'm finding that this parasite infects quite a WIDE range of hosts. While it steers clear of amphibians and reptiles, Toxoplasma gondii has been found rampantly among birds and mammals. I've started finding reports of this parasite infecting everything from rabbits to racoons, to ferrets and flying squirrels. My most current awe of this parasite came today as I scoured the literature and found that this parasite isn't restricted by the bounds of land...it has actually taken to the sea as well.
Putting the obvious correlations of this parasite to a pirate aside, let us look at what we know about Toxoplasma gondii's relation to the sea. I had read previously that the parasite had been isolated from sea otters. No one really understands how the parasite could infect this kind of animal. The current theory is that feral cats are defecating near shorelines and that the parasites are being swept into the tides, where they are being picked up by a mysterious paratenic host. This mystery host is then eaten by sea otters and the parasites find a new home in their sea-dwelling host. The biggest question is what is this paratenic host? Also, if we do find the paratenic host, how can we prove that cats pooping along the shore is really the way that this parasite is cycling? Perhaps there is an alternative seafaring life cycle at play? I suppose we won't know until someone takes the time to find out.
People have started trying to take the time. A group of researchers made an attempt to experimentally infect bivalves (molluscs with two shells...things like clams, oysters, etc.) with Toxoplasma gondii. These experimental infections have proved to be successful. Thus, we have learned that bivalves have the ability to become infected and to pass the infection on to animals that eat them. However, this has not yet been demonstrated to be the case in a natural setting. I'm not sure if anyone is already working on this, but I sure hope so!
Sea otters aren't the only marine animals that have ever been infected by this crafty little parasite. It turns out that a great deal of other marine mammals have produced isolates following testing for this parasite. Many different kinds of seals have been shown to harbor the parasite, though not all of them demonstrated clinical symptoms of toxoplasmosis. This includes fur seals, elephant seals, harbor seals, and sea lions.
Of course pinnipeds can't have all the fun. Toxo has also found its way into a number of cetaceans and sea cows. It has been known to cause congenital toxoplasmosis in various species of dolphins. It's also popped up in beluga whales and a few different species of manatees. How could these animals, these exclusively marine animals, be picking up this parasite that normally goes through a cat-rat cycle?
So many questions with so few answers. We clearly have a lot to learn about the incredible adaptability of this uniquely amazing parasite. I love that every paper I read about this parasite brings up new ideas and questions that push the bounds of our understanding of something that seems so simple superficially. This is why I love this parasite. I can't wait to see what we discover next! I hope that my own research will help shed some light on the origins of this parasite...someday...