Wednesday, 20 July 2011

Where Did Toxoplasma gondii Come From?

Toxoplasma gondii, one assumes, evolved as a parasite of cats. Is this a safe assumption? All the texts tell us that the cat is the only animal in which T. gondii completes the sexual phase of its life cycle, which is strong evidence for the cat being the original host. It is possible that there are other hosts in which T. gondii can produce gametocytes and reproduce sexually – maybe we just haven’t found them yet. Maybe we haven’t looked exhaustively either. Nonetheless, in the absence of evidence to the contrary, I’ll stick with the assumption that T. gondii evolved in cats.

T. gondii was first discovered in Tunisia in 1908. By coincidence, Northern Africa is also one of the places where the cat is thought to have been domesticated (there or present day Iraq) and this explains my somewhat illogical but long held assumption that T. gondii probably evolved in Africa. There is actually no reason why the parasite couldn’t have evolved in some other feline species and spread to domestic cats later.

One theory has it that T. gondii evolved in South America.

[caption id="attachment_309" align="alignleft" width="162" caption="Did prehistoric jaguars have T. gondii?"]Prehistoric cat statue from Guatemala, Simon Burchell[/caption]

The paper “Globalization and the Population Structure of Toxoplasma gondii,” reports an odd distribution of genotypes: one is found worldwide, one is found everywhere but South America, and several more are found only in South America. The authors' (Lehmann et al, PNAS July 25, 2006) interpretation is that there was an early split, originating in South America and leaving two populations to evolve in isolation from each other. One evolved in the “old world” and today is found everywhere but South America. The other is found everywhere, but apparently only spread very recently from its origins. Several more never left South America.

So maybe Toxoplasma gondii came from a prehistoric cat species in South America. It's a place to start.

(Image by Simon Burchell. Creative Commons)

Friday, 15 July 2011

Must We Hate Worms?

Is the revulsion we feel toward intestinal worms (in fact, anything called a parasite) innate or learned? If we hadn’t been surrounded by the “yuck factor” all our lives, grossed out by anything that wriggles or crawls, would we view them with disgust or curiosity? I think it would be more in the realm of curiosity.

In his book Parasites and Parasitic Infections in Early Medicine and Science, R. Hoeppli describes early attitudes toward parasites. Even within the last few hundred years, many people believed they arose spontaneously from intestinal contents, blood, even dust and ashes. And “in China,” he writes, “there existed from ancient times the widespread belief that one should have at least three worms in order to remain in good health” (p. 164).

Robin M. Overstreet has investigated the deliberate ingestion of parasites for various reasons by humans and found that parasites are often deliberately eaten and sometimes even regarded as delicacies (“Flavor Bugs and other Delights,” Journal of Parasitology: 89(6)). Overstreet describes a boy “open[ing] the intestine (of a possum) where a lump existed to allow the tapewoms to squirt out, remov[ing] the feces from the worms, and drop[ping] each writhing organism straight into his mouth.”

A former co-worker described an encounter with a small boy who was playing with a rather large roundworm, whirling it around while holding on to one end. When asked where he got it, he calmly indicated that it had come out of his nose.

These things suggest that a horror of worms is not a natural characteristic of humans. Similarly, Hans Zinsser writes that “as wise a man as Linnaeus suggested that children were protected by their lice from a number of diseases” (Rats, Lice and History, p. 139).

[caption id="attachment_301" align="alignleft" width="249" caption="Portrait of Carolus Linnaeus by Alexander Roslin, 1775"]Linnaeus, by Alexander Roslin[/caption]

I’ve often suggested that if it were possible to keep mosquitoes out of your back yard, it would be socially unacceptable to have any there. I believe that’s what’s happened to parasites. If they were unavoidable, we’d accept them as part of life, like mosquitoes and mud puddles. In the handful of decades since we’ve been able to avoid having parasites in developed countries, we’ve also learned to abhor them.

Wednesday, 13 July 2011

Getting Parasites from Animals

An article by Peter Michael on (“Dingo Poo Spreading Deadly Parasites to Humans,” July 13) interests me for several reasons. First, the “deadly parasite” involved is Ecinococcus granulosus, which is by no means a new parasite for people in many parts of the world. It was once a big problem anywhere sheep were raised because sheep dogs could pass the worm’s eggs to people. Today, mostly because sheep dogs typically get better preventative care, the parasite is slowly losing ground.

It’s a bitter twist, then, that wild canines that have lost their fear of humans and come into human communities are now a source of infection. Therein lies the other thing that interests me. A few of the people who commented on that article have it right: it’s not that these wild animals are “encroaching” on us; it’s that we have encroached on them, and we’ve been doing that ever since the first human settlements with the beginning of agriculture and domestic animals.

[caption id="attachment_296" align="alignleft" width="200" caption="Dingos by Joshposh, Creative Commons 3.0"]Dingos by Joshposh, Creative Commons 3.0[/caption]

Before we grew our own food and raised animals, how often would we have come in close enough contact with an animal to catch a disease? Sporadically: when an animal was killed for food, when we shared the same cave perhaps, and by accident. Now we breed them and invite them in by carelessly providing food: cattle, pigs, birds, dogs, cats, fish, raccoons, dingos, rats, mice, and lots more. Meanwhile, we destroy their habitat and oblige them to adjust. Civilization is bad for our health: so many devastating diseases would be rare if we had not settled down and brought animals, and their parasites, into our space, one way or another.

Of course, without civilization, people would be rare as well. Most of us wouldn’t be here, I wouldn’t be writing this, and there wouldn’t be an internet. So one wonders, was it all worth it, and where does it stop?

Monday, 11 July 2011

Babesia in the Blood Supply

Blood transfusions have saved a lot of lives, my own included, but gone are the days when we thought that the chief concern with blood transfusions was transfusion reactions. Units of blood today are subjected to a growing battery of tests to detect infectious agents, because many pathogens are found in the blood stream and can easily be passed from donor to recipient, setting up a new infection. Blood transfusion can pass along HIV, hepatitis, malaria, trypanosomiasis, and a host of other things. Now, Babesia is recognized as being common enough to be of concern for the blood supply.

Babesia is a tiny blood parasite usually transmitted by a tick bite. The tick responsible is the same species that transmits Lyme disease, and babesiosis appears to be following in the footsteps of Lyme, appearing in places where Lyme has emerged, notably the northeastern United States.

[caption id="attachment_290" align="alignleft" width="300" caption="Babesia in ticks, mice, and humans"]Babesia Life Cycle Diagram[/caption]

In some places, studies show, about one in ten people have antibodies to Babesia,  indicating that they have been infected. Because the diagnosis is often missed, and because, while fatal in some cases, the infection goes unnoticed in many people, the infection is likely considerably more common that we think.

It looks as though we’ll need to add a screen for Babesia to the processing of blood units for transfusion, at least in places where Lyme disease is common. Though babesiosis is mild in many cases, anyone receiving a blood transfusion is likely more vulnerable to serious illness. Reliable, cost effective, large scale screening methods, however, take time to develop and implement.