Toxoplasma gondii: Mode of Infection and Effect on Neurological Cells

By [Alexander S. McQuiston]

Introduction

The left shows a bradyzoite cyst with the dense cyst wall surrounding the bradyzoites. The right shows tachyzoites inside the parasitophorous vacuole. Bradyzoites will slowly reproduce asexually in the cyst and tachyzoites will rapidly reproduce asexually CDC.

Toxoplasma gondii is one of the most common cyst parasites around the world due to its ability to infect almost all warm blooded organisms, including humans (Gaskell et al. 2009). Some estimates of T. gondii infections are reported to be as high as a third of the world’s population (Tenter et al. 2000; Weiss et al. 2009). More specifically, T. gondii is an intracellular heteroxenous parasite belonging to the apicomplexan phylum. The apicomplexan phylum is a large group of parasitic protists that contain specialized organelles required for target host invasion starting at the apical pole (Dubey et al. 1998; Weiss 2011)). Infection of T. gondii causes toxoplasmosis in both healthy and immunocompromised organisms. When the parasite infects healthy organisms the disease is normally asymptomatic, but if the parasite infects an immunocompromised organism, such as an AIDS patient, diseases like encephalitis can occur (Carruthers et al. 2007). Along with medical diseases, T. gondii has also been hypothesized to cause a variety of neurological disorders such as schizophrenia (Weiss et al. 2009).

T. gondii has both intermediate and definitive hosts. The intermediate hosts include a wide variety of warm blooded animals from rodents to humans, but only has one definitive host, felines (cats) (Tenter et al. 2010). T. gondii is also capable of both asexual and sexual reproduction. When the parasite is in an intermediate host it reproduces asexually, and when the parasite is in its definitive host it is able to reproduce sexually. The ultimate goal of T. gondii is to infect its definitive host so it can sexually reproduce and then go on to infect other organisms (Webster et al. 2007). An interesting way T. gondii has been able to facilitate its transmission from an intermediate host to its definitive host is by invading target host cells and then manipulating host cellular behavior and physiological processes (Prandovsky et al. 2011; Webster et al. 2007).

Life Stages

There are three main infectious stages, all intracellular, of T. gondii: tachyzoites, bradyzoites (cyst forming), and sporozoites. Both tachyzoites and bradyzoites asexually multiply while oocysts sexually multiply (Dubey et al. 1998).

Tachyzoites

Tachyzoites are responsible for the acute infection by rapidly multiplying inside a host which causes the overall population of the parasite to grow. It can also be considered the more aggressive parasite stage because this stage of the parasite moves around the body and invades target host cells (Carruthers et al. 2007). Sporozoites go through differentiation into tachyzoites once the infection is ready to take place. Tachyzoites will multiply within a cell until cell lysis and the new parasites can move on to infect more target host cells. Tachyzoites are mainly found within intestine epithelial cells (Dubey at al. 1998).

Bradyzoites

Bradyzoites are considered to be slower metabolic and reproductive parasites. Following the invasion by tachyzoites, tachyzoites can go through differentiation in to bradyzoites if certain environmental factors are met such as a decrease in reproductive success or if the tachyzoite can no longer manipulate host cellular processes (Kamerkar et al. 2012). Once differentiated in to bradyzoites and the formation of a cyst wall forms around the bradyzoites, the bradyzoites slowly divide and fill up the cyst. Cysts are mainly found in neurological tissue and muscle tissues. Bradyzoites were originally thought to be static structures that caused chronic toxoplasmosis, but recent studies have shown that they play a role in behavioral manipulation and also break down host cells in order to invade new cells (Dubey et al. 1998).

Sporozoites

Sporozoites are the only T. gondii stage that sexual reproduce. When bradyzoites reach a definitive host (a cat), they differentiate into sporozoites. Sporozoites are found within oocysts that reside in the intestinal tract of cats (Tenter et al. 2000). Sporozoites are expelled from the definitive host in fecal matter. This stage of T. gondii is spore-like and can survive outside of the definitive host in soil and water for about a year. Sporozoites contribute to infection of intermediate hosts via ingestion (Dubey et al. 1998).

How T. gondii Infects Host Cells

This displays a tachyzoite invading a target cell. The apical pole is already inside the host, but the rest of the cell has not been encapsulated. Constriction can be seen in the middle of the tachyzoite which makes invasion slightly more difficult NATURE.

T. gondii's Effect on the Brain

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