Libratto

WE ARE NOT ALONE

Imagine if alien invaders had the power to turn our women into spendthrift hussies and our men into unkempt scofflaws. That they had the ability to decrease our reaction times, and increase the rate at which we are injured in accidents. That they could cause birth defects and mental retardation among newborns. That they could even cause the fraction of male births to rise above 70%.

Such an alien not only lives among us, it lives in us… and a lot of us. The toxoplasma gondii (or t. gondii) parasite infects 20% to 80% of the world’s population, and there’s reasonably good evidence – most of it flowing out of a single group in Prague – that t. gondii exhibits these effects among its human hosts. [1]

The latest study from Dr. Flegr and colleagues published this week in the journal “Naturwissenschaften” shows a clear association between the concentration of t. gondii antibodies and the fraction of male births: the stronger the woman’s immune system’s response (indicating prior infection), the more likely she is to give birth to a boy.

Although infection with t. gondii can cause some flu-like symptoms (that’s your immune system kicking in), those symptoms generally disappear pretty quickly. But instead of being eradicated, t. gondii settles quietly into your muscle and nerve tissue – including your brain – in tiny cysts, waiting silently as you go about with your life.

But how does this dormant parasite alter the rate of male births?

Believe it or not, at the earliest stages of pregnancy, the deck is strongly stacked in favor of males. In humans the implantation success rate is far higher for male zygotes than female, and at 5 – 7 weeks, about 62% of embryos are male. This imbalance early on is crucial to achieving sex balance at birth because the maternal immune system because, as Legr notes:

“The maternal immunological reaction against male-specific H-Y antigens is an important cause of selective mortality of the Y-chromosome-bearing embryos, and, consequently, of the secondary sex ratio adjustment.”

In other words, it may be a man’s world, baby, but the womb is a completely different story. Just because you’re paranoid doesn’t mean someone’s not out to get you, and in many cases that someone is your mom.

No one really knows why t. gondii infection might alter the reaction of the maternal immune system to male-specific antigens, but we do know that with toxoplasmosis comes production of interleukin-10 and transforming growth beta factor, both of which can cause immunosuppression. Furthermore, we know that women acutely affected with t. gondii during pregnancy are at significantly increased risk of giving birth to infants with developmental defects. Surprisingly, another Czech researcher [2] reported way back in 1957 that the prevalence of toxoplasmosis in 94 mothers of Down Syndrome children was 84%; the prevalence in a control population matched on age and other factors associated with Down Syndrome was only 32%

What this all adds up to is that infection with t. gondii relaxes the quality control mechanism that’s applied during gestation. In short, the rate of spontaneous abortion probably drops as a result of t. gondii.

If you put it all together, you get more boys. The sex ratio is stacked in favor of males at implantation as a means to balance out the selective mortality rate for male embryos. Toxoplasmosis turns down that rejection process, the ratio is held more or less constant, and presto – you end up with more male births.

What’s really interesting is whether the increased rate of male births is simply an accident or the beautifully sneaky result of millions of years of evolution. T. gondii, for example, cannot complete its life cycle in humans. For completion of the circle of life from the perspective of t. gondii, you need a cat. Only in a cat can the t. gondii parasite complete all five stages of its existence, which concludes with the dispersal of “eggs” in feline feces (i.e., kitty poop). [3]

But trying to get from one cat’s droppings into the innards of another cat is not easily accomplished, especially given the fastidious habits of cats. (It’s not unlikely, by the way, that cats developed such habits as a defense to t. gondii – in the presence of the parasite, cats that practiced better potty habits were more likely to survive and procreate than those who did not.)

No, if you want to get back into a cat, you’ll need something else: a rat, for example. And indeed, t. gandii infects rodents very effectively.

Okay, now you’ve successfully made your way into a rat. You need to find your way back into a cat, which is not a remote possibility. A cat catches your rat host, eats it up (without cooking or freezing it), and you’re all set.

But if you really wanted to increase your chances of getting into a cat, better that your host have as many encounters with cats as possible. It turns out that male rodents are more exploratory, have a larger home range, and are more migratory than female rodents.

Getting the picture? If you’re a crafty t. gondii, you’ll do everything you can to make sure you get yourself into a male rodent – including increasing the birth rate of male offspring among mother rats from whom you will transmit yourself vertically. And while you’re at it, you’d might as well make the male rat you now call home more aggressive and less nimble – all the better to get eaten by that cat.

This constellation of actions is known as the parasites manipulation activities. Such activities alter the behavior of the intermediate host (here, the rat) to increase the chance of transmission to the definitive host (here, the cat).

Saber-toothed tigers aside, there’s not not much of an opportunity for humans to be manipulated in a manner that helps t. gondii make its way back into their favored cats. In short, these changes in our behavior and sex ratio at birth are simply collateral damage in the war between cats, rats and t. gondii.

For some very interesting commentaries on manipulation and the possible effects of toxoplasmosis in humans, check an online copy of a presentation given by Dr. Flegr, a commentary by Carl Zimmer (author of Parasite Rex), a 2003 Future Pundit blog, and a thorough review of the evidence at Signal Plus Noise.

1. You’ll find Flegr’s manuscripts on his website.

2. Hostomska et al. The role of toxoplasmosis in the mother in the development of mongolism in the child. Ceskoslov Pediatr 12:713-723, 1957

3. Hence the warnings to pregnant women to avoid cat litter boxes.

Opinions expressed are those of the author alone.