Toxoplasma gondii: A Review of Parasitology, Pathology, and Epidemiology
Toxoplasma gondii: A Review of Parasitology, Pathology, and Epidemiology Parasites are naturally “clever,” yet insidious organisms. You may have a parasite inside you right now, without even knowing it. An effective parasite lives without being detected because if it is, then measures can be taken to eradicate it. One such parasite is known as Toxoplasma gondii. T. gondii is an obligate intracellular protozoan first discovered by Nicolle and Manceaux in 1908 (Nguyen, 2006). Since its discovery, T. gondii has been implicated as the causative agent of toxoplasmosis, an infectious disease that may result in a spectrum of consequences. T. gondii continues to be an important disease in the modern world, especially in pregnant women and immunocompromised patients. In order to better understand how T. gondii is able to infect its human hosts, we will explore the parasite’s cellular properties and survival strategies. Parasitic Characteristics
Parasites are amazing organisms because they must develop and survive in unusual and often hostile environments. In the face of such aggressive conditions, parasites have adapted various complex strategies that allow them to survive. Common characteristics of successful parasitic organisms include methods that allow ease of entry into the host, methods to avoid detection by the host immune system, the ability to develop eggs/cysts, the ability to locate a new host, and avoidance of killing the host (Camus & Zalis, 1995). Toxoplasma gondii employs several of these strategies in order to complete its life cycle. T. gondii is a single-celled eukaryotic protozoan parasite has a characteristic crescent shape. This shape serves a critical function in breeching host cells. One end of the cell is more conical and is believed to be the region that initially breeches the host’s cell membrane (CDC, 2008). The opposite end of the cell is more pointed and is responsible for holding the organelles that function in attachment and infiltration of host cells (Huynh, Rabenau, Harper & Beatty, 2003). Although it is not entirely understood how T. gondii crosses into its host cell, it has been hypothesized that organelles called mirconemes provide adhesive protein expression. The M2AP-MIC2 complex of proteins found in this organelle appears to be responsible for these adhesive secretions (Huynh et. al, 2003). In addition to aiding in adhesion and attachment, this complex appears to facilitate the organism’s motility as well, especially since no other means of motility such as flagella have been discovered (Camus & Zalis, 1995). T. gondii also has rhoptries, which are regulated secretory organelles involved in the invasion of host cells. These rhoptry proteins secrete chemicals upon entering the host cell to alter the cell’s vacuole for maximum survival upon to assist in the replication process (Bradley & Boothroyd, 2001). A site on the rhoptry protein ROP1 has been indicated as paving the pro region in rhoptry protein function (Bradley & Boothroyd, 2001). This ROP1 site has been the target of much research for potential vaccines to T. gondii, which will be discussed further in a later section. T. gondii also employs several mechanisms to help avoid its detection by the host’s immune system. This organism has the ability to seclude itself from the environment when stressed by immunologic factors. For example, the parasitic cell is able to insert and present specific antigens on its surface to conceal it from immune attack (Nguyen, 2006). Toxoplasma is also able to invade immunologically competent macrophages in order to “defuse” the killing mechanisms of these...
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