Q&A: The truth about Toxoplasma gondii risk from household cats

Toxoplasma gondii is a common protozoal parasite with a particularly important relationship to cats, which serve as its definitive host. Most mammals, including people, can serve as intermediate hosts, and infection is often asymptomatic. However, the parasite can cause clinically significant disease in certain circumstances, including neurologic disease in cats and reproductive losses in some livestock.

For veterinary professionals, Toxoplasma gondii also presents a familiar communication challenge: although cats are central to the parasite's life cycle, exposure can occur through several routes, including contaminated food, soil and water.

In this interview, Christopher Lee, DVM, MPH, DACVPM, DACVM (Parasitology), discusses when clinicians should consider Toxoplasma gondii, how suspected infections may be evaluated and what veterinary teams should communicate to households experiencing pregnancy.

Below is a transcript of the conversation, lightly edited for clarity.

dvm360: Tell us a bit about Toxoplasma gondii.

Lee: It's one of my favorite parasites. It's a single-celled protozoan organism. The definitive host is cats, and the intermediate host is kind of the rest of the world. It's almost everybody. Ruminants, like cattle specifically, are a bit resistant to it, but most mammals are considered intermediate hosts for this, including people.

dvm360: How often do veterinary professionals see Toxoplasma gondii in their practices?

Lee: So, we see it all the time. But recognize it, probably not so much. And that's not a [criticism of] veterinary professionals. It's the fact that we're dealing with dogs and cats, and they're largely going to be asymptomatic. If you're on the large animal side, you probably will see it a bit more if you're dealing with smaller ruminants. So, in sheep and goats, there is a concern about abortion.

So, if you're on the protein production side, you may be seeing a bit more. If you're on the small animal side, which is where I've spent most of my time, then it would be something that we look at, we consider it in certain cases like cats with neurologic disease. We'll include it on PCR panels. But as a general rule, we're going to not see it as much.

dvm360: Because it is asymptomatic, what would trigger a veterinary professional to test for it?

Lee: Sometimes when we have cats with neurologic [issues], like a cat will come in for seizures, you can go ahead and send out a panel. Neuro panels will include PCR for Toxoplasma. When we're dealing with the concern about someone that's pregnant, there could also be tests that you could run to see if the cat was previously exposed, but that's not always common.

dvm360: What steps should be taken when there is a suspected case?

Lee: The damage, very often, not always, but many times when we think about protozoans, is the asexual magnification that causes a lot of the damage. And so, when we think about this, the sexual portion, that recombination of the genetic material, that's really only happening within the definitive host, which is the cat. But that asexual phase will also occur within the cat, and sometimes that can lead to disease. And if you have a cat, say, coming in for seizures, that may be a reason to be running a polymerase chain reaction (PCR) panel, and they will look at a variety of different things that can cause that.

So, often animals come in [and we perform] blood work, radiographs, urinalysis; those are all going to be part of the early diagnostics. Neurologic panels are going to be added on top of that. And then ultimately, if the case proceeds, then we're looking at imaging, MRI, CT scans.

dvm360: Are there additional concerns in households experiencing pregnancy?

Lee: This is something that we get asked a lot as veterinarians. So, Toxoplasma in our minds is a very common thing. And I think part of it is the fact that it's a very sticky message. Something may just easily stick in your mind as an image, and it may be true, it may not be, but it's hard for us to not think about. And sometimes the truth can be a lot more complex.

So, when we think about it, because cats are the definitive host, all of the oocysts, your infective agent, are going to end up being from cats. If, after 24, 48 hours, those oocysts become infective, then if you ended up accidentally ingesting it, you could become infected. And if it occurs while a person is pregnant, then that can lead to miscarriage.

So, that absolutely is a realistic possibility. And it's why both MDs and DVMs talk to people about it as a risk. That's why pregnant women should not be changing out the litter box. If for some reason, they're in a situation where they have to change out the litter box, you would want to not only use PPE—personal protective equipment—but you would also want to be doing it every day because it takes at least a day or more for those oocysts to become infective.

So, when we take a look at all of that, yes, it's important that we still talk about it. I'm not saying that we do not, but it is not the most common situation. There is far greater risk with uncooked meat, unwashed vegetables, unwashed fruit, and soil. Those are your much higher risks, so that is where we need to put our focus. But at the same time, have somebody else change the litter box.

Stay tuned for the full video version of the above interview.