Toxoplasma gondii is an intracellular protozoan. The life-cycle of T. gondii is divided into five stages: trophozoite, cyst, schizont, gametophyte, and oocyst. Final maturation to sexual maturity occurs only in the intestines of cats. Most infections have no clinical signs, others are acute. Among healthy people, toxoplasmosis is generally mild, but among newborn babies this zoonotic infection is serious, often fatal. Toxoplasmosis is fatal in the immunocompromised individuals such as cancer patients with chemotherapy, HIV/AIDS patients and organ transplant recipients. Toxoplasmosis has been reported to increase the fatality rate of leukemia, myeloma, melanoma, AIDS, and brain cancer and squamous cell lung cancer.
Toxoplasma gondii can infect virtually all warm-blooded animals, including humans. About fifty percent of stray dogs and cats carry Toxoplasma gondii. It occurs in most areas of the world and up to one-third of the human population is chronically infected. The frequency of Toxoplasma gondii infection varies in different countries. The lower prevalence is usually associated the the almost total absence of cats and the lack of meat in their diet. The infection rate in US is estimated to be 10–15%; in the regions of tropical Africa and Latin America it reaches 60%, and in Ethiopia 87%. About 40% in Europe carry the dormant, cystic form. In Brazil, toxoplasmic encephalitis might reach up to 40% in patients with AIDS, among them 10-30% die. The lack of feral cat population control and ineffective sanitation in meat production sites are playing a role that has led to the wide spread of the disease in Mexico, a large country with a human population in 2010 of more than 112 million. Areas with the highest rate of toxoplasmosis are wet coastal regions of the Gulf of Mexico and the Pacific (64%). Studies have reported the highest prevalence rate in coastal areas (40-65%), in people with low socioeconomic status and also in women of reproductive age (70.8% of their cats were Toxoplasma gondii positive). People who eat out of home and in places where hygiene and cooking practices are uncertain (such as long distance truck drivers) may have an increased risk of infection with T. gondii.
Food-borne transmission of T. gondii is considered to be the most important route for human infection, which occurs through the ingestion of raw or inadequately cooked meat containing tissue cysts, or of food or water contaminated by oocysts shed by cats. Moreover, infection acquired during pregnancy can be transmitted to the fetus, sometimes leading to serious consequences. Toxoplasmosis is currently incurable because the parasite can change from the rapidly replicating tachyzoite stage to the dormant bradyzoite stage. Bradyzoites avoid detection and elimination by the immune system and cannot be killed with drugs.
Although the infection is usually believed to be harmless and have a benign course in immunocompetent persons, it could be indirectly responsible for deaths due to its effects on the traffic and workplace accidents, and also suicides. Moreover, latent toxoplasmosis is probably one of the most important risk factors for schizophrenia.
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Recently, an exploratory study performed with a data mining technique on electronic records of 1.3 million patients of the University of Michigan Health System showed the existence of a strong association between dog and cat-bite injuries and the probability of being diagnosed with depression at some point in life. The association was stronger for cat bites than dog bites. While only 9 % of all patients of the data set were ever diagnosed with depression, this diagnosi was found in 41 % of those with cat bites and 28 % of those with dog bites.4
What Happens When Toxoplasma Get Cancer?
Cancer also occurs in parasitic protozoans such as Trypanosoma brucei, a blood parasite, and Toxoplasma gondii. Without treatment, protozoan cancers may cause severe disease and death in mammals, including humans. The parasite goes through distinct stages enabling it to complete its life cycle and cell multiplication, and to retain a permanent presence of the parasite in the intermediate host.The lack of differentiation in some parasite strains results in the parasite’s losing the ability to form cysts in the intermediate hosts or to generate oocysts in cat. It has been suggested that a genetic mutationa prevents conversion of the key stage from acute (tachyzoite) to chronic (bradyzoite) infection in the host, a process that normally limits infection in the hosts, and the RH strain of T. gondii behaves as the equivalent of a T. gondii cancer killing both the host and the parasite when an animal host is infected with the RH strain.
The genomes of these single-cell organisms, in combination with their complex life cycles and fascinating life cycle, may help in the future to better understand the origins of cancers and, in particular, leukemias.5
- Seroprevalence and genetic characterization of Toxoplasma gondii in cancer patients in Anhui Province, Eastern China. Lin Wang et al.
- Toxoplasmosis complicating lung cancer: a case report. Nianhong Lu et al.
- Toxoplasma gondii infection in interstate truck drivers: a case–control seroprevalence study. Cosme Alvarado-Esquivel et al.
- Cat scratches, not bites, are associated with unipolar depression - cross-sectional study. Jaroslav Flegrcorresponding author and Zdeněk Hodný
- Cancer in the parasitic protozoans Trypanosoma brucei and Toxoplasma gondii
Zhao-Rong Lun,a,b,c,1 De-Hua Lai,a Yan-Zi Wen,a Ling-Ling Zheng,a Ji-Long Shen,d Ting-Bo Yang,a Wen-Liang Zhou,a Liang-Hu Qu,a Geoff Hide,c,1 and Francisco J. Ayalae,1 Proc Natl Acad Sci U S A. 2015 Jul 21; 112(29): 8835–8842.