Toxoplasma gondii – Protozoa (CNS Infection)

00:01 Welcome to Parasites: Toxoplasma. This is one of the series of videos where we explore in detail individual parasites and the diseases they cause. And after you've listened to this video, you will understand the lifecycle of toxoplasma gondii, including how it's transmitted to humans. You will be familiar with pathogenesis of toxoplasmosis and why pregnant mothers should avoid infection. And finally you will understand how to prevent and treat toxoplasmosis.

00:39 Now remember these latter videos are considering protozoan parasites, these are single celled eukaryotes, one cell only not multiple cells and they include parasites of medical importance.

00:54 We've talked about Plasmodium, we are going to be talking now about Toxoplasma.

00:58 Here is a lovely photograph of a toxoplasma species. There are four cells in this photograph in sort of pairs, and they’re stained with a dye that makes them glow in a beautiful way. But you can see that they are composed of single cells and each one has nuclei.

01:20 The organism that we’re going to focus on today is Toxoplasma gondii, the causative agent of toxoplasmosis. Now it turns out that much of the world is infected with toxoplasma, between you and I, one of us is probably infected and we don't even know it, because fewer than 1% of infected individuals develop disease. This is a very successful parasite. It manages to cause so many infections without causing any harm or very little harm. Now I think a great proportion of the population is infected with toxoplasmosis, if they like to eat raw or undercooked meat as you will see, and there are certain countries in the world that like to do that more than others. Strains of Toxoplasma infect all the animals on the planet, so not only are humans largely infected, but all the animals around us as well. So as I said this is an incredibly successful parasite. Remember the definitive hosts for parasites that we defined in our beginning lecture on parasites, well for toxoplasma gondii, the cat is the definitive host. Your little cat at home might be harboring a toxoplasma.

02:46 How do cats get infected? When they eat another animal by predation like a mouse that they catch in the wild. Now if you keep your cat inside and you never feed it wild animals and just feed it canned foods, they won’t develop toxoplasmosis, although depending what's in the food they might develop a spongiform encephalopathy, but that's the subject of a different video of course.

03:10 Let's talk a little bit about the cat cycle of toxoplasmosis. Cats acquire toxoplasma by eating mice, who in turn are infected. Mice are often eaten by cats and it is said, although it's probably not true, that the presence of toxoplasma in the mouse makes them fear cats less than they normally would and therefore they're caught. Well that sounds like a good story, but the real truth is that the presence of toxoplasma in the mouse probably makes them less averse to cat urine, that is what scares them away in the first place.

03:46 So the mouse is eaten by the cat, the toxoplasma then enters the cat where it stays for the life of the animal and cats then will shed toxoplasma in their feces and that's one way that humans may be infected. Now you may say, how would I ever be infected by something in my cat's feces? Well you know you have to clean the cat's litter box frequently and in doing so you may contaminate yourself. Cats also may go outside and they may defecate in your backyard and then your children will go out and play in the yard and pick up the parasites that way. You know in the end the world is a dangerous place and you just need to know microbiology to know how to take care of yourself, that's why we’re teaching you this. So humans can get infected from cats or as I said, by ingesting raw or undercooked meat which harbors the toxoplasma parasite. The parasite will then enter our circulation where it will reproduce and cause problems or perhaps not. So we can acquire the parasite either from cats or by eating undercooked meat, including lamb, mutton and beef containing the cysts of the parasite. These are sort of the stable form that will persist for a long time. And as I've said, cats acquire the infection by eating mice, they shed this oocyst in their feces and we can also get it from cats in that manner as well. When we ingest a parasite, it goes into our gut, the parasites penetrate through the gut wall, they drill through your intestine and enter the bloodstream, where they then of course go to the liver. One of the first places anything goes when it gets into your blood is the liver and there it encounters macrophages and it will infect them and move about your body in macrophages and the parasite will even reproduce in the macrophages, at some point it will burst out and spread more within your body. Within 4 to 6 weeks after your initial ingestion of this parasite by whatever means, your host immune system is finally kicked in. It will start to control infection and in fact that leads to the production of dormant cysts.

06:10 This is a very large cyst form, a stable form. This is produced in response to the immunity that you put up towards the parasite infection and those cysts can stay with you forever.

06:22 So once you're infected with toxoplasma, you harbor it for the rest of your life. And it may or may not cause a serious initial illness, but then of course if you are immunosuppressed, if you develop AIDS or if you get an organ transplant and you need to take immunosuppressive drugs, those dormant cysts will start to come alive again and replicate, make more toxoplasma and they can cause serious problems by invading various organs as shown here, and they can cause serious brain infections. So AIDS is one way that you can reactivate the cysts, any form of immunosuppression.