Wednesday, March 2, 2011

Toxoplasma gondii: Parasitic Mind-Control

Toxoplasmosis is a leading cause of death attributed to food-borne illness in the United States. While a large number of people carry this disease, few actually experience symptoms, as a result of strong immunity. Nonetheless, in the case of the immunodeficient, it can be devastating. Most cases are passed from mother to child; when contracted during pregnancy, the infection can be detrimental and even fatal to the fetus.

This infection is caused by Toxoplasma gondii (T. gondii). A single-celled protozoan parasite, it can be a symbiont of most warm-blooded animals, but its definitive host is the cat. It is commonly contracted by humans through consumption of undercooked meat which have been an intermediate host to the parasite. [1] Image: [1]


T. gondii is the only species belonging to the genus Toxoplasma. It is a single-celled protozoan parasite. A member of the phylum Apicomplexa, this parasite has an apical complex used for borrowing into host cells and for reproduction. The apical complex can also be used for secretion of compounds that aid in attachment and penetration of the host. While T. gondii lacks flagella or cilia, it is able to move by way of cell crawling. The parasite uses a few methods of manipulation in order to protect itself and to ensure progression from intermediate hosts, like mice, to the definitive host, the cat. [2] image: [2]


T. gondii lives inside mammal hosts throughout most of its life.
It can be hosted by most warm-blooded animals. The parasite completes its sexual stage inside its definitive host, the cat. Here, sporozoites are spread from intestinal epithelial cells to virtually all cells of the cat's body. Oocysts are excreted in the fecal matter of the cat. Intermediate hosts include other animals like birds, rodents, and humans; animals bred for human consumption or wild game can also be infected by ingestion of oocysts found in the environment. [3] image: [4]

Life Cycle

T. gondii oocysts are found in the feces of cats. They are usually only shed for one or two weeks. It takes between one and five days for the oocysts to sporulate and become infective. Intermediate hosts (birds, rodents, or animals bred for human consumption/wild game) ingest material that has been contaminated by feline waste, including soil, water, or plants. The oocysts become tachyzoites after ingested, and concentrate in muscle and neural tissue, where they form cysts. Intermediate hosts can be consumed by cats or humans. Cats and humans can also directly ingest sporulated oocysts from contaminated sources of water, food, or soil in the environment. Humans are highly at risk when they come in contact with feline waste by changing a litter box. The parasite can also be transmitted through blood transfusion, organ transplant, and most commonly, from mother to child at birth. [1] image: [1]


T. gondii causes the disease Toxoplasmosis, which is found worldwide. Over 60 million Americans are infected [2]; between 30% and 50% of people worldwide are infected. It is most commonly passed along through pregnancy. [4] There is a higher prevalence in European countries like France and Germany, with around 88% of the population is infected. This is believed to be due to higher consumption of raw/undercooked meat in those countries. [5] There is also a higher prevalence in Central America, which may be due to a higher occurrence of stray cats combined with a climate that favors oocysts and soil exposure. [6]

While there is such a high prevalence of infection, very few people actually have symptoms. Most people who show symptoms have low immunity; toxoplasmosis is usually self-limiting. It can, however, be seriously detrimental or even fatal to the immunocompromised, or to fetuses of a mother who first contracts the illness during pregnancy. [1] In the case of congenitally infected children, it can lead to blindness or mental retardation. [7]

This infection is commonly associated with impoverished people, and is thus considered a "Neglected Infection of Poverty" (NIP).[1]

From a socio-economic standpoint, the cost of care of sick children is exceedingly expensive. In some European countries it is required that pregnant women are tested for this infection. This raises the debate of the potential cost and benefit of mandatory testing. If a woman is tested and finds she has not yet contracted this infection, she can learn precautions to take in order to prevent infection while pregnant. [7] image: [4]

While most people will not experience them, symptoms are usually similar to those of the flu or mononucleosis. In patients with low immunity, more severe symptoms can be found, such as headache, confusion, poor coordination, seizures, blurred vision, and a lung problem similar to tuberculosis or Pneumocystis carinii pneumonia (an opportunistic infection effecting people with AIDS).

When pregnant women are infected, the fetus is most at risk of contracting an infection during the third trimester. However, the earlier the infection is contracted, the worse the symptoms will be. Most early infections lead to stillbirths or miscarriages. If the baby survives, symptoms can include seizures, enlarged liver and spleen, jaundice, and eye infection.

When contracted from their mothers, many babies will not show symptoms until later in life (usually teenage years). These symptoms include hearing loss, mental retardation, and eye infections that could lead to blindness. [8]

In order to prevent infection through ingestion of meat, all meat should be fully cooked to recommended temperatures, and even frozen for several days ahead of time. Fruits and vegetables should be washed and peeled before eaten. Kitchenware should be kept clean and sanitary, and as always, normal hygiene rules, such as washing your hands, apply. It is recommended that you do not drink untreated water in order to avoid infection. When gardening or working with soil, gloves should be worn. Litter boxes should be changed daily, since the parasite becomes infectious from one to five days after eggs are shed. Pregnant women should avoid changing litter boxes or handling stray cats. [1]

Pyrimethamine (Daraprim) is an antimalarial medication. When used in conjuction with Sulfadazine, it can be used to treat toxoplasmosis. In most cases, patients will have to take these medications for life. If a pregnant woman becomes infected, Spiramycin is an available medication for treatment and can possibly prevent fetal contraction of the infection. [8]

An Example of Host Manipulation

T. gondii can manipulate it's host in order to both protect itself, and ensure progression to its next host. For protection, it encysts itself and then releases a substance that increases the host's concentration of T cells. This improves the host's immune response, killing other T. gondii parasites, except those that are protected by cysts. This ensures that the host will not become overrun and die from the parasite, thus acting as a "home-security" strategy for the parasite.

This parasite can also alter its host's behavior. It causes mice to be less afraid of cats' scents- mice are ordinarily programmed to avoid these scents. T. gondii can sometimes even cause mice to be attracted to or seek out the scents. This ensures that the parasite makes it to the definitive host. [2]

video: [9]




  1. It is interesting that billions of years ago the parasite was a single-celled free living organism, and now it is one of the most strategic parasites that exist (in the sense that once a human is infected, our immune system is unable to actually kill and destroy Toxoplasma). This reminds me of the discussion in Parasite Rex- the author describes how humans facilitated the dispersal of Toxoplasma by spreading cats around the world, for instance domestication, which indirectly increased the distribution of Toxoplasma. In an essence, it’s as if we’ve brought this upon ourselves.