A bit of respect for toxoplasmosis
Toby is my 17lb ball of feline lovin’. He loves to cuddle and has taken a special interest to sleeping across my stomach these last few months.
I like to say to my husband “Hey, look! Toby is already trying to smother the baby!”
For some reason he doesn’t find that as funny as I do.
One of the best things about being a pregnant cat owner is that you are expected to lay off the dirty chore of litter cleaning. Fear of a parasite living in the poo called Toxoplasma gondii. And while I actually wanted to be very afraid of little Toxo, and continue to get out of litter duty, from what little I knew about the life cycle of this parasite, I realized that I might not have a valid excuse.
Before I dive into why Toxoplasmosis is a hard infection to catch in my Toby-specific scenario and why Toxoplasma gondii could possibly be one of the coolest freaking parasites on the planet, I should probably start with the scary part.
Being exposed to toxoplasmosis during pregnancy is very bad. In a 2005, American Journal of Medicine review entitled “Toxoplasmosis in Pregnancy”, authors Kravetz and Federman, lay out the risks and the recommendations for pregnant women. The risks include miscarriage, mental retardation, microcephaly, hydrocephalus, and seizures. Yikes.
Enough scary stuff, onto the cool part.
First, here is the life cycle of Toxo:
Quick summary of this pretty confusing figure:
Let’s start with the rodent (or really any small warm-blooded creature) – once an animal is infected, the parasite infects cells across the body and replicates until the host’s immune system realizes what is going on. At that point, Toxo puts up the shields and lives on as cysts throughout the body. Although the parasite can hang out in the host for quite some time, it cannot complete its life cycle until it is inside the intestines of a feline. Toxo can only sexually reproduce inside a cat so in order to spread its genes (every creature’s basic goal in the grand scheme of evolution), it must get itself into a feline digestive tract.
It’s not a stretch to expect rats to be ok eating parasite-laden cat poo, or that rats will occasionally be eaten by a cat, but from the perspective of Toxo, wouldn’t be a hell of a lot more convenient if the rat you hitched a ride on and built lots and lots of copies of yourself within, was guaranteed to be eaten?
And this is exactly what Toxo does.
In normal rodents, there is an instinct to avoid cats at all costs. However, in Toxo infected rodents this aversion to feline predators disappears. Even more fascinating is the mode by which it does this. Toxo infection appears to target specific signaling pathways in the brain to switch the lovely and pungent scent of cat urine from a “holy crap, stay away!” signal to a “ooh, that’s a sexy smell!” signal . Yes, the infected rodents are actually attracted to cat pee! The authors who first made this finding actually called this a “fatal attraction”  – the parasite essentially manipulates its host’s brain chemistry to make that host more likely to be eaten by the one creature that will house and nurture the part of its parasitic life cycle required to spread its genes.
But wait, there’s more! Toxo is also a sexually transmitted disease, spread between infected males and uninfected females and their resulting pups. Normally, females have a sense to steer clear of sick males. However, in a stroke of evolutionary brilliance, Toxo makes these infected males more sexually attractive to females . What? That is bananas!
Ah, the wonders of evolution.
So, why am I not afraid of Toby’s litter box? Well, the way I see it, the only way I could get infected would be if this indoor cat, who is pretty lazy and not a very good mouser, manages to get out and back in without my realizing it, have amazing luck to come upon a Toxo infected rodent, and eat it without leaving any evidence behind. And at that point, I would still have to get his poo into my mouth somehow. Yum.
Even Kravetz and Federman would agree with me. When the authors of the “Toxoplasmosis in Pregnancy” article carefully evaluated the findings from the scientific literature as to the causes for Toxoplasmosis, they note that it is really raw meat that we should be afraid of. Toxo is lurking in our beef (8%), pork (20%), and lamb (20%) but luckily heating and freezing kills it quite effectively. Of the seven recommendations the authors offer about how to avoid Toxoplasmosis, the top three are related to raw meat handling, the fourth is about washing vegetables, the fifth is about working in soil, and the last two are about house cats. But even these last two, do not state “Get rid of your cat! Never touch a litter box!” just advise keeping them inside and washing your hands or wearing gloves when handling litter.
(Important update from 7/3: As noted by Dr. Tell in the comments, we are only at risk for toxoplasmosis infection if this is the first encounter with the Toxo parasite. A good number of us are actually immune to toxoplasmosis since we have probably already been infected in our lifetime! Kravetz and Federman note that 15% of women of childbearing age actually have signs of previous Toxo infection and Dr. Tell notes that the CDC says 22.5% of adults have been previously infected. Since after a primary infection, your immune system knows what it is looking for and is able to fight it very effectively, it is pretty unlikely that the second/third/fourth time Toxo comes around you will actually get toxoplasmosis. I figure I am probably carrying around quite a few anti-Toxo antibodies given my childhood love for cats and sandboxes…another reason I probably don’t have to worry!)
So, I’ll cook my meat, wash my veggies, garden with gloves on but I’ll continue to change the litter box… and make sure to wash my hands really well afterwards, cause, well, you never know.
I still get the “Oh, you have a cat!? Well, you shouldn’t be cleaning the litter” and my reply to that is often “Actually, I’m not that concerned. Wanna hear about how freaking cool the Toxo parasite is?”
This usually ends in puzzlement.
And then acknowledgement that I am a total nerd.
1. Vyas A, Kim S-K, Giacomini N, Boothroyd JC, Sapolsky RM (2007) Behavioral changes induced by Toxoplasma infection of rodents are highly specific to aversion of cat odors. Proc Natl Acad Sci USA 104: 6442–6447.
2. Berdoy, M., Webster, J. P. and Macdonald, D. W. (2000). Fatal attraction in rats infected with Toxoplasma gondii. Proc. Biol. Sci. 267, 1591-1594.
3. Dass, S. A.,Vasudevan, A., Dutta, D., Soh, L. J., Sapolsky, R. M. and Vyas, A. (2011). Protozoan parasite Toxoplasma gondii manipulates mate choice in rats by enhancing attractiveness of males. PLoS ONE 6, e27229.
I’m really happy to have found your blog! All in all, you have a pretty great coverage of toxo and its relation to pregnancy in this post, much better than most people get from their physicians. (Now, if they’ve talked to a veterinarian about it, their picture would look a lot more like what you present here).
I would add, though, that it isn’t merely being exposed to Toxoplasma during pregnancy that is bad, it is being exposed /for the first time/ that is bad slash potentially dangerous to the fetus being carried. This is an important distinction, considering that most of us have already been exposed earlier in life and without profound immunosuppression will suffer no ill effects from that previous exposure or subsequent exposures.
Dr. Tell, this is such a great point!
As the Kravetz and Federman review points out, in the US, 15% of women of child-bearing age actually show evidence of prior Toxo infection. And as you point out, it is really rare to be reinfected the next time Toxo comes around because the immune system knows what it is fighting off. Perhaps because of this, congenetical toxoplasmosis (read – baby getting infected in the womb) is actually pretty rare – 400-4000 cases per year in the US. I’d also put my bets on the heightened hygienic awareness that comes with pregnancy.
So, if you’ve already managed to eat cat poop in your lifetime, the immune system is kick ass enough to stop Toxo before it becomes a problem the second/third/fourth time Toxo comes around.
Personally, I probably have quite the anti-Toxo antibody stockpile given my affection for cats and sandboxes as a child.
I would propose that you could do a great service to science and public health if you amend your post to reflect the “for the first time” topic of our comment conversation here. People who google the topic and come to your blog may not read the comments (I know that I often do not read blog comments, especially when I have come to the piece via a search) and, as we both agree, that piece of information on prior infection is pretty darned important.
Admittedly, in vet school and since then I have always heard higher estimates as to already infected adults, over 20% (CDC currently says estimated 22.5% of adults) and up to 95% of some populations (especially in hotter, more humid areas).
Please heed the plea of this humble veterinarian and immunologist. 🙂
Not only is toxo only a problem if the human pregnant lady gets infected for the first time, it’s also only a problem if the *cat* is infected for the first time. I have a older cat, and she has antibodies for toxo from a previous time in her life. Even if she were to go outside and be exposed to toxo now, she would *not* shed toxo in her poop. It’s kittens and young cats that pose the most risk to pregnant women, because they are likely to be getting infected with toxo for the first time. This is why toxo infection is so low for any one point in time, and why eating undercooked meat or gardening are much higher risk activities. This information about the cat only being infectious after their first infection is frequently not mentioned in warnings about toxo.