The Stages of Toxoplasma gondii

T. gondii constructing daughter scaffolds with...

Image via Wikipedia

As stated in Toxoplasma gondii, today I’m going to break down the stages of the parasite. Those stages are:

  • Sporulated oocysts (sporozoites) from environment
  • Tissue cysts containing bradyzoites in raw/undercooked meat
  • Tachyzoites in tissues
  • Entero-epithelial stages (only in cats)

Oocysts are produced non-sporulated but will sporulate in about 2 days. They are about 10×10 µm and millions can be produced within days. Cats can develop strong immunity to this stage.

In the intermediate host, the life cycle is as follows:

  • Excystation of sporozoites
  • Sporozoites –> Tachyzoites
  • Tachyzoites disseminate infection and eventually become bradyzoites or tissue cysts
  • Latent infection
  • Endodyogeny (a form of sexual reproduction)

Tachyzoites have a fondness for neural and cardiac tissue but can infect any nucleated cell in the body. This is the stage that crosses into the placenta and causes congenital infections. Rapid division of tachyzoites is what causes tissue destruction, the spread of infection and lesions.

Bradyzoites (“brady” = slow) are responsible for tissue cysts. They can be present in any organ and survive for the life of the infected animal since they are resistant to the drugs used to treat Toxoplasmosis. If the host is immunocompromised, infection can be reactivated and intermediate hosts can infect one another. Bradyzoites are at fault for initiating meat induced infection, on top of being the only stage to give rise to entero-epithelial stages.

Once again, the etero-epithelial stages are only found in cats. There are 5 types of entero-epithelial schizonts (which will not be listed). This is where the sexual stages/oocysts are produced.

Overall, Toxoplasma gondii was the most successful parasite discussed in my Intro Parasitology course.

:: Information from this post is from my notes from an Introductory Parasitology class taught by Dr. Zajac at Virginia Tech ::

Trichinella spiralis

Trichinella LifeCycle

Image via Wikipedia

If you read Feral Pigs: From Piglets to Problems, you might recall that Trichinella spiralis was one of the parasites mentioned. I was fortunate enough to have an Introductory Parasitology class at Virginia Tech (full of really interesting and disgusting things that induce paranoia) where this and Toxoplasma gondii were discussed, so most of this post will be thanks to notes from Dr. Zajac’s incredible class. Many of the terms included can be found defined here.

– Scientific Classification –

The definitive and intermediate host of Trichinella are in the same animal (in this diagram, a rodent). The parasite is looking for striated muscle, and therefore as a histiotrophic parasite, spends part of its life cycle within a cell or tissue where it waits for the muscle to be eaten. ANY warm-blooded animal can serve as a host.

Trichinella spiralis

Image via Wikipedia

Pigs can get infected by eating garbage, rodents or each other. As pigs are omnivorous, they may eat rats which can be loaded with Trichinella. Once infected, they may not show any sign of disease.

Human infection, on the other hand, can result in enteritis, myalgia, fever, and kidney and respiratory failure if the parasites are prevalent enough. 5 larvae per gram of muscle is fatal.

To prevent infection, cook pork thoroughly or freeze it at -15˚C for 20 days or more, depending on the size.

Parasites like Trichinella have added to the perception that pork is unsafe. In the U.S., it is less of an issue now in confined pigs since they tend to be grain fed instead of fed with raw garbage. Organic and pasture raised operations have brought about concern of the re-emergence in swine. A couple of other hosts from which one can acquire infection are bears and horses. Current Trichinella cases are mostly from undercooked bear meat and the parasites can survive freezing. There have been several outbreaks of the parasite in horse meat in France and even some U.S. horses.

:: Next post: Toxoplasma gondii ::