|T. gondii tachyzoites under 100x magnification with oil|
(Nicolle & Manceaux, 1908)
Toxoplasma gondii (tŏk'sə-plāz'mə gŏn'dē-ī') is an obligate, intracellular, parasitic protozoan that causes the disease toxoplasmosis. Found worldwide, T. gondii is capable of infecting virtually all warm-blooded animals, although felids such as domestic cats are the only known definitive hosts in which the parasite can undergo sexual reproduction.
In humans, T. gondii is one of the most common parasites in developed countries; serological studies estimate that 30–50% of the global population has been exposed to and may be chronically infected with T. gondii, although infection rates differ significantly from country to country. For example, previous estimates have shown the highest prevalence of persons infected to be in France, at 84%. Although mild, flu-like symptoms occasionally occur during the first few weeks following exposure, infection with T. gondii produces no readily observable symptoms in healthy human adults. This asymptomatic state of infection is referred to as a latent infection and has recently been associated with numerous subtle adverse or pathological behavioral alterations in humans. In infants, HIV/AIDS patients, and others with weakened immunity, infection can cause serious and occasionally fatal illness (toxoplasmosis).
T. gondii has been shown to alter the behavior of infected rodents in ways thought to increase the rodents' chances of being preyed upon by cats. Support for this "Manipulation Hypothesis" stems from studies showing T. gondii infected rats have a decreased aversion to cat urine and increased reaction time. Because cats are the only hosts within which T. gondii can sexually reproduce to complete and begin its lifecycle, such behavioral manipulations are thought to be evolutionary adaptations to increase the parasite's reproductive success. The primary mechanisms of T. gondii–induced behavioral changes in rodents is now known to occur through epigenetic remodeling in neurons which govern the associated behaviors; for example, it modifies epigenetic methylation to cause hypomethylation of arginine vasopressin-related genes in the medial amygdala to greatly decrease predator aversion. Widespread histone-lysine acetylation in cortical astrocytes appears to be another epigenetic mechanism employed by T. gondii. Differences in aversion to cat urine are observed between non-infected and infected humans and sex differences within these groups were apparent as well.
A number of studies have suggested that subtle behavioral or personality changes may occur in infected humans, and infection with the parasite has recently been associated with a number of neurological disorders, particularly schizophrenia. A 2015 study also found cognitive deficits in adults to be associated with joint infection by both T. gondii and Helicobacter pylori in a regression model with controls for race-ethnicity and educational attainment. However, although a causal relationship between latent toxoplasmosis with these neurological phenomena has not yet been established, preliminary evidence suggests that T. gondii infection can induce some of the same alterations in the human brain as those observed in mice.
- 1 Lifecycle
- 2 Cellular stages
- 3 Risk factors for human infection
- 4 Preventing infection
- 5 History
- 6 Behavioral differences of infected hosts
- 7 References
- 8 Bibliography
- 9 External links
The lifecycle of T. gondii can be broadly summarized into two components: 1) a sexual component that occurs only within cats (felids, wild or domestic), and 2) an asexual component that can occur within virtually all warm-blooded animals, including humans, cats, and birds. Because T. gondii can sexually reproduce only within cats, they are defined as the definitive host of T. gondii. All other hosts – hosts in which only asexual reproduction can occur – are defined as intermediate hosts.
Sexual reproduction in the feline definitive host
When a member of the cat family is infected with T. gondii (e.g. by consuming an infected mouse laden with the parasite's tissue cysts), the parasite survives passage through the stomach, eventually infecting epithelial cells of the cat's small intestine. Inside these intestinal cells, the parasites undergo sexual development and reproduction, producing millions of thick-walled, zygote-containing cysts known as oocysts.
Feline shedding of oocysts
Infected epithelial cells eventually rupture and release oocysts into the intestinal lumen, whereupon they are shed in the cat's feces. Oocysts can then spread to soil, water, food, or anything potentially contaminated with the feces. Highly resilient, oocysts can survive and remain infective for many months in cold and dry climates.
Ingestion of oocysts by humans or other warm-blooded animals is one of the common routes of infection. Humans can be exposed to oocysts by, for example, consuming unwashed vegetables or contaminated water, or by handling the feces (litter) of an infected cat. Although cats can also be infected by ingesting oocysts, they are much less sensitive to oocyst infection than are intermediate hosts.
Initial infection of the intermediate host
T. gondii is considered to have three stages of infection; the tachyzoite stage of rapid division, the bradyzoite stage of slow division within tissue cysts, and the oocyst environmental stage. When an oocyst or tissue cyst is ingested by a human or other warm-blooded animal, the resilient cyst wall is dissolved by proteolytic enzymes in the stomach and small intestine, freeing sporozoites from within the oocyst. The parasites first invade cells in and surrounding the intestinal epithelium, and inside these cells, the parasites differentiate into tachyzoites, the motile and quickly multiplying cellular stage of T. gondii. Tissue cysts in tissues such as brain and muscle tissue, form approximately 7–10 days after initial infection.
Asexual reproduction in the intermediate host
Inside host cells, the tachyzoites replicate inside specialized vacuoles (called the parasitophorous vacuoles) created during parasitic entry into the cell. Tachyzoites multiply inside this vacuole until the host cell dies and ruptures, releasing and spreading the tachyzoites via the bloodstream to all organs and tissues of the body, including the brain.
Formation of tissue cysts
Following the initial period of infection characterized by tachyzoite proliferation throughout the body, pressure from the host's immune system causes T. gondii tachyzoites to convert into bradyzoites, the semidormant, slowly dividing cellular stage of the parasite. Inside host cells, clusters of these bradyzoites are known as tissue cysts. The cyst wall is formed by the parasitophorous vacuole membrane. Although bradyzoite-containing tissue cysts can form in virtually any organ, tissue cysts predominantly form and persist in the brain, the eyes, and striated muscle (including the heart). However, specific tissue tropisms can vary between species; in pigs, the majority of tissue cysts are found in muscle tissue, whereas in mice, the majority of cysts are found in the brain.
Consumption of tissue cysts in meat is one of the primary means of T. gondii infection, both for humans and for meat-eating, warm-blooded animals. Humans consume tissue cysts when eating raw or undercooked meat (particularly pork and lamb). Tissue cyst consumption is also the primary means by which cats are infected.
Tissue cysts can be maintained in host tissue for the lifetime of the animal. However, the perpetual presence of cysts appears to be due to a periodic process of cyst rupturing and re-encysting, rather than a perpetual lifespan of individual cysts or bradyzoites. At any given time in a chronically infected host, a very small percentage of cysts are rupturing, although the exact cause of this tissue cysts rupture is, as of 2010, not yet known.
Theoretically, T. gondii can be passed between intermediate hosts indefinitely via a cycle of consumption of tissue cysts in meat. However, the parasite's lifecycle begins and completes only when the parasite is passed to a feline host, the only host within which the parasite can again undergo sexual development and reproduction.
Population structure in the wild
Khan et al. reviewed evidence that despite the occurrence of a sexual phase in its life cycle, T. gondii has an unusual population structure dominated by three clonal lineages (Types I, II and III) that occur in North America and Europe. They estimated that a common ancestor founded these clonal lineages about 10,000 years ago. In a further and larger study (with 196 isolates from diverse sources including T. gondii found in the bald eagle, gray wolves, Arctic foxes and sea otters), Dubey et al. also found that T. gondii strains infecting North American wildlife have limited genetic diversity with the occurrence of only a few major clonal types. They found that 85% of strains in North America were of one of three widespread genotypes (Types II, III and Type 12). Thus T. gondii has retained the capability for sex in North America over many generations, producing largely clonal populations, and matings have generated little genetic diversity.
During different periods of its life cycle, individual parasites convert into various cellular stages, with each stage characterized by a distinct cellular morphology, biochemistry, and behavior. These stages include the tachyzoites, merozoites, bradyzoites (found in tissue cysts), and sporozoites (found in oocysts).
Motile, and quickly multiplying, tachyzoites are responsible for expanding the population of the parasite in the host. When a host consumes a tissue cyst (containing bradyzoites) or an oocyst (containing sporozoites), the bradyzoites or sporozoites stage-convert into tachyzoites upon infecting the intestinal epithelium of the host. During the initial, acute period of infection, tachyzoites spread throughout the body via the blood stream. During the later, latent (chronic) stages of infection, tachyzoites stage-convert to bradyzoites to form tissue cysts.
Like tachyzoites, merozoites divide quickly, and are responsible for expanding the population of the parasite inside the cat intestine prior to sexual reproduction. When a feline definitive host consumes a tissue cyst (containing bradyzoites), bradyzoites convert into merozoites inside intestinal epithelial cells. Following a brief period of rapid population growth in the intestinal epithelium, merozoites convert into the noninfectious sexual stages of the parasite to undergo sexual reproduction, eventually resulting in the formation of zygote-containing oocysts.
Bradyzoites are the slowly dividing stage of the parasite that make up tissue cysts. When an uninfected host consumes a tissue cyst, bradyzoites released from the cyst infect intestinal epithelial cells before converting to the proliferative tachyzoite stage. Following the initial period of proliferation throughout the host body, tachyzoites then convert back to bradyzoites, which reproduce inside host cells to form tissue cysts in the new host.
Sporozoites are the stage of the parasite residing within oocysts. When a human or other warm-blooded host consumes an oocyst, sporozoites are released from it, infecting epithelial cells before converting to the proliferative tachyzoite stage.
Risk factors for human infection
The following have been identified as being risk factors for T. gondii infection:
Infection in humans and other warm-blooded animals can occur:
- by consuming raw or undercooked meat containing T. gondii tissue cysts The most common threat to citizens in the United States is from eating raw or undercooked pork. It is possible, though unlikely, to ingest the parasite through other products:
- by ingesting water, soil, vegetables, or anything contaminated with oocysts shed in the feces of an infected animal. Cat fecal matter is particularly dangerous: Just one cyst consumed by a cat can result in thousands of oocysts. This is why physicians recommend pregnant or ill persons do not clean the cat's litter box at home. These oocysts are resilient to harsh environmental conditions and can survive over a year in contaminated soil.
- from a blood transfusion or organ transplant
- from transplacental transmission from mother to fetus, particularly when T. gondii is contracted during pregnancy
- from drinking unpasteurized goat milk
- by contact with soil
- from eating unwashed raw vegetables or fruits
Cleaning cat litter boxes is a potential route of infection; however, numerous studies have shown living in a household with a cat is not a significant risk factor for T. gondii infection, though living with several kittens has some significance.
In warm-blooded animals, such as brown rats, sheep, and dogs, T. gondii has also been shown to be sexually transmitted, and it is hypothesized that it may be sexually transmitted in humans, although not yet proven. Although T. gondii can infect, be transmitted by, and asexually reproduce within humans and virtually all other warm-blooded animals, the parasite can sexually reproduce only within the intestines of members of the cat family (felids). Felids are therefore defined as the definitive hosts of T. gondii, with all other hosts defined as intermediate hosts like human or other mammals.
The following precautions are recommended to prevent or greatly reduce the chances of becoming infected with T. gondii. This information has been adapted from the websites of United States Centers for Disease Control and Prevention and the Mayo Clinic.
Basic food handling safety practices can prevent or reduce the chances of becoming infected with T. gondii, such as washing unwashed fruits and vegetable and avoiding raw or undercooked meat, poultry, and seafood. Other unsafe practices such as drinking unpasteurized milk or untreated water can increase odds of infection. Because T. gondii is typically transmitted through cysts that reside in the tissues of infected animals, meat that is not properly prepared can present an increased risk of infection. Freezing meat for several days at subzero temperatures (0 °F or −18 °C) before cooking eliminates tissue cysts, which can rarely survive these temperatures. During cooking, whole cuts of red meat should be cooked to an internal temperature of 145 °F (63 °C). Medium rare meat is generally cooked between 130 and 140 °F (55 and 60 °C), so cooking whole cuts of meat to medium is recommended. After cooking, a rest period of 3 min should be allowed before consumption. However, ground meat should be cooked to an internal temperature of at least 160 °F (71 °C) with no rest period. All poultry should be cooked to an internal temperature of at least 165 °F (74 °C). After cooking, a rest period of 3 min should be allowed before consumption.
Oocysts in cat feces take at least a day to sporulate and become infectious after they are shed, so disposing of cat litter daily greatly reduces the chances of infectious oocysts being present in litter. As infectious oocysts from cat feces can spread and survive in the environment for months, humans should wear gloves when gardening or working with soil, and should wash their hands promptly after disposing of cat litter. The same precautions apply to outdoor sandboxes, which should be covered when not in use.
Furthermore, pregnant or immunocompromised people are at higher risk of becoming infected or transmitting the parasite to their fetus. Because of this, they should not change or handle cat litter boxes. Ideally, cats should be kept indoors and only fed food that has low to no risk of carrying oocysts, such as commercial cat food or well-cooked table food.
In 1908, while working at the Pasteur Institute in Tunis, Charles Nicolle and Louis Manceaux discovered a protozoan organism in the tissues of a hamster-like rodent known as the gundi, Ctenodactylus gundi. Although Nicolle and Manceaux initially believed the organism to be a member of the genus Leishmania that they described as "Leishmania gondii", they soon realized they had discovered a new organism entirely. They named it Toxoplasma gondii, a reference to its morphology (Toxo, from Greek τόξον (toxon); arc, bow, and πλάσμα (plasma); i.e., anything shaped or molded) and the host in which it was discovered, the gundi (gondii). The same year Nicolle and Mancaeux discovered T. gondii, Alfonso Splendore identified the same organism in a rabbit in Brazil. However, he did not give it a name.
The first conclusive identification of T. gondii in humans was in an infant girl delivered full term by Caesarean section on May 23, 1938, at Babies' Hospital in New York City. The girl began having seizures at three days of age, and doctors identified lesions in the maculae of both of her eyes. When she died at one month of age, an autopsy was performed. Lesions discovered in her brain and eye tissue were found to have both free and intracellular T. gondii'. Infected tissue from the girl was homogenized and inoculated intracerebrally into rabbits and mice; the animals subsequently developed encephalitis. Later, congenital transmission was found to occur in numerous other species, particularly in sheep and rodents.
The possibility of T. gondii transmission via consumption of undercooked meat was first proposed by D. Weinman and A.H Chandler in 1954. In 1960, the cyst wall of tissue cysts was shown to dissolve in the proteolytic enzymes found in the stomach, releasing infectious bradyzoites into the stomach (and subsequently into the intestine). The hypothesis of transmission via consumption of undercooked meat was tested in an orphanage in Paris in 1965; yearly acquisition rates of T. gondii rose from 10% to 50% after adding two portions of barely cooked beef or horse meat to the orphans' daily diets, and to 100% after adding barely cooked lamb chops.
In 1959, a study in Bombay found the prevalence of T. gondii in strict vegetarians to be similar to that found in nonvegetarians. This raised the possibility of a third major route of infection, beyond congenital and carnivorous transmission. In 1970, the existence of oocysts was discovered in cat feces, and the fecal-oral route of infection via oocysts was demonstrated.
Throughout the 1970s and 1980s, a vast number of species were tested for the ability to shed oocysts upon infection. Whereas at least 17 different species of felids have been confirmed to shed oocysts, no nonfelid has been shown to be permissive for T. gondii sexual reproduction and subsequent oocyst shedding.
Behavioral differences of infected hosts
The differences in behavior observed in infected hosts compared to non-infected individuals have been shown to be sex dependent. In humans for example, studies using the Cattell’s 16 Personality Factor questionnaire, found that infected men scored lower on Factor G (superego strength/rule consciousness) and higher on Factor L (vigilance) while the opposite pattern was observed for infected women. In 9 out of 11 studies, sex differences within personality traits were observed using Cattell’s Personality Factor questionnaire. However, human studies have not been able to show causation as they have all been observational studies. Mice infected with T. gondii have motor performance worse than non-infected mice. In a human study with volunteer blood donors, reaction times and the amount of time the subject remained focused were worse for the infected group than for the control group. However, the infection status was found to only explain less than 10% of the variability in motor performance, making this a weak correlation. A few observational studies on human subjects have also found the risk of traffic accidents to be significantly greater in infected persons than non-infected controls. One of these studies concluded that this risk was 2.65 times greater for infected persons, and it is hypothesized that the patterns of decreased psychomotor performance could be responsible for the increased prevalence of traffic accidents among infected persons.
- Dardé, ML; Ajzenberg, D; Smith, J (2011). "3 – Population structure and epidemiology of Toxoplasma gondii". In Weiss, LM; Kim, K. Toxoplasma Gondii: The Model Apicomplexan. Perspectives and Methods. London: Academic Press/Elsevier. pp. 49–80. doi:10.1016/B978-012369542-0/50005-2. ISBN 978-0-12-369542-0.
- J.P Dubey (2010) p. 1
- "CDC - Toxoplasmosis - Biology". 17 March 2015. Retrieved 14 June 2015.
- "Cat parasite linked to mental illness, schizophrenia". CBS. Retrieved 23 September 2015.
- "CDC – About Parasites". Retrieved 12 March 2013.
- Flegr J, Prandota J, Sovičková M, Israili ZH (March 2014). "Toxoplasmosis--a global threat. Correlation of latent toxoplasmosis with specific disease burden in a set of 88 countries". PLoS ONE 9 (3): e90203. doi:10.1371/journal.pone.0090203. PMC 3963851. PMID 24662942.
Toxoplasmosis is becoming a global health hazard as it infects 30-50% of the world human population. Clinically, the life-long presence of the parasite in tissues of a majority of infected individuals is usually considered asymptomatic. However, a number of studies show that this 'asymptomatic infection' may also lead to development of other human pathologies. ... The seroprevalence of toxoplasmosis correlated with various disease burden. Statistical associations does not necessarily mean causality. The precautionary principle suggests however that possible role of toxoplasmosis as a triggering factor responsible for development of several clinical entities deserves much more attention and financial support both in everyday medical practice and future clinical research.
- Pappas, G; Roussos, N; Falagas, ME (October 2009). "Toxoplasmosis snapshots: global status of Toxoplasma gondii seroprevalence and implications for pregnancy and congenital toxoplasmosis.". International Journal for Parasitology 39 (12): 1385–94. doi:10.1016/j.ijpara.2009.04.003. PMID 19433092.
- Berdoy, M; Webster, J; Macdonald, D (2000). "Fatal attraction in rats infected with Toxoplasma gondii". Proceedings Biological Sciences 267 (1452): 1591–1594. doi:10.1098/rspb.2000.1182.
- "CDC Parasites – Toxoplasmosis (Toxoplasma infection) – Disease". Retrieved 12 March 2013.
- J.P Dubey (2010) p. 77
- Cook TB, Brenner LA, Cloninger CR, et al. (2015). ""Latent" infection with Toxoplasma gondii: association with trait aggression and impulsivity in healthy adults". J Psychiatr Res 60: 87–94. doi:10.1016/j.jpsychires.2014.09.019. PMID 25306262.
- Webster, JP (May 2007). "The effect of Toxoplasma gondii on animal behavior: playing cat and mouse". Schizophrenia Bulletin 33 (3): 752–6. doi:10.1093/schbul/sbl073. PMC 2526137. PMID 17218613.
- Webster, JP; Kaushik, M; Bristow, GC; McConkey, GA (Jan 2013). "Toxoplasma gondii infection, from predation to schizophrenia: can animal behaviour help us understand human behaviour?". The Journal of Experimental Biology 216 (Pt 1): 99–112. doi:10.1242/jeb.074716. PMC 3515034. PMID 23225872.
- Hari Dass SA, Vyas A (December 2014). "Toxoplasma gondii infection reduces predator aversion in rats through epigenetic modulation in the host medial amygdala". Mol. Ecol. 23 (24): 6114–6122. doi:10.1111/mec.12888. PMID 25142402.
- Flegr J, Markoš A (December 2014). "Masterpiece of epigenetic engineering – how Toxoplasma gondii reprogrammes host brains to change fear to sexual attraction". Mol. Ecol. 23 (24): 5934–5936. doi:10.1111/mec.13006. PMID 25532868.
- Vanagas L, Jeffers V, Bogado SS, Dalmasso MC, Sullivan WJ, Angel SO (October 2012). "Toxoplasma histone acetylation remodelers as novel drug targets". Expert Rev Anti Infect Ther 10 (10): 1189–1201. doi:10.1586/eri.12.100. PMC 3581047. PMID 23199404.
- Bouchut A, Chawla AR, Jeffers V, Hudmon A, Sullivan WJ (2015). "Proteome-wide lysine acetylation in cortical astrocytes and alterations that occur during infection with brain parasite Toxoplasma gondii". PLoS ONE 10 (3): e0117966. doi:10.1371/journal.pone.0117966. PMC 4364782. PMID 25786129.
- Flegr, J; Lenochová, P; Hodný, Z; Vondrová, M (2011). "Fatal attraction phenomenon in humans – cat odour attractiveness increased for Toxoplasma-infected men while decreased for infected women". PLoS Neglected Tropical Diseases 5 (11): e1389. doi:10.1371/journal.pntd.0001389. PMID 22087345.
- Flegr, J (Jan 2013). "Influence of latent Toxoplasma infection on human personality, physiology and morphology: Pros and cons of the Toxoplasma-human model in studying the manipulation hypothesis". The Journal of Experimental Biology 216 (Pt 1): 127–33. doi:10.1242/jeb.073635. PMID 23225875.
- Gale SD, Erickson LD, Brown BL, Hedges DW (2015). "Interaction between Helicobacter pylori and latent toxoplasmosis and demographic variables on cognitive function in young to middle-aged adults". PLoS ONE 10 (1): e0116874. doi:10.1371/journal.pone.0116874. PMC 4295891. PMID 25590622.
- Parlog A, Schlüter D, Dunay IR (March 2015). "Toxoplasma gondii-induced neuronal alterations". Parasite Immunol. 37 (3): 159–170. doi:10.1111/pim.12157. PMID 25376390.
- Blanchard N, Dunay IR, Schlüter D (2015). "Persistence of Toxoplasma gondii in the central nervous system: a fine-tuned balance between the parasite, the brain and the immune system". Parasite Immunol. 37 (3): 150–158. doi:10.1111/pim.12173. PMID 25573476.
- Louis M Weiss, Kami Kim (2011) p. 2
- Louis M Weiss, Kami Kim (2011) p. 39
- J.P Dubey (2010) p. 22
- Dubey, JP; Ferreira, LR; Martins, J; Jones, JL (October 2011). "Sporulation and survival of Toxoplasma gondii oocysts in different types of commercial cat litter". The Journal of Parasitology 97 (5): 751–4. doi:10.1645/GE-2774.1. PMID 21539466.
- Dubey, JP (Jul 2009). "History of the discovery of the life cycle of Toxoplasma gondii". International Journal for Parasitology 39 (8): 877–82. doi:10.1016/j.ijpara.2009.01.005. PMID 19630138.
- Kapperud, G; Jenum, PA; Stray-Pedersen, B; Melby, KK; Eskild, A; Eng, J (Aug 15, 1996). "Risk factors for Toxoplasma gondii infection in pregnancy. Results of a prospective case-control study in Norway". American Journal of Epidemiology 144 (4): 405–12. doi:10.1093/oxfordjournals.aje.a008942. PMID 8712198.
- Dubey, JP (July 1998). "Advances in the life cycle of Toxoplasma gondii". International Journal for Parasitology 28 (7): 1019–24. doi:10.1016/S0020-7519(98)00023-X. PMID 9724872.
- J.P Dubey (2010) p. 107
- Robert-Gangneux, Florence; Dardé, Marie Laure (2012). "Epidemiology of and diagnostic strategies for toxoplasmosis". Clinical Microbiology Reviews 25 (2): 264–296. doi:10.1128/CMR.05013-11. PMID 22491772.
- Louis M Weiss, Kami Kim (2011) pp. 23–29
- Louis M Weiss, Kami Kim (2011) pp. 39–40
- Miller, CM; Boulter, NR; Ikin, RJ; Smith, NC (January 2009). "The immunobiology of the innate response to Toxoplasma gondii". International Journal for Parasitology 39 (1): 23–39. doi:10.1016/j.ijpara.2008.08.002. PMID 18775432.
- Louis M Weiss, Kami Kim (2011) p. 343
- Louis M Weiss, Kami Kim (2011) p. 41
- "CDC Toxoplasmosis – Microscopy Findings". Retrieved 13 March 2013.
- Clarence R. Robbins (24 February 2012). Chemical and Physical Behavior of Human Hair. Springer. p. 585. ISBN 978-3-642-25610-3. Retrieved 12 March 2013.
- Louis M Weiss, Kami Kim (2011) p. 3
- Jones, JL; Dubey, JP (Sep 2012). "Foodborne Toxoplasmosis". Clinical Infectious Diseases 55 (6): 845–51. doi:10.1093/cid/cis508. PMID 22618566.
- J.P Dubey (2010) p. 46
- Louis M Weiss, Kami Kim (2011) p. 580
- Louis M Weiss, Kami Kim (2011) p. 45
- J.P Dubey (2010) p. 47
- Khan A, Böhme U, Kelly KA, Adlem E, Brooks K, Simmonds M, Mungall K, Quail MA, Arrowsmith C, Chillingworth T, Churcher C, Harris D, Collins M, Fosker N, Fraser A, Hance Z, Jagels K, Moule S, Murphy L, O'Neil S, Rajandream MA, Saunders D, Seeger K, Whitehead S, Mayr T, Xuan X, Watanabe J, Suzuki Y, Wakaguri H, Sugano S, Sugimoto C, Paulsen I, Mackey AJ, Roos DS, Hall N, Berriman M, Barrell B, Sibley LD, Ajioka JW; Böhme; Kelly; Adlem; Brooks; Simmonds; Mungall; Quail; Arrowsmith; Chillingworth; Churcher; Harris; Collins; Fosker; Fraser; Hance; Jagels; Moule; Murphy; O'Neil; Rajandream; Saunders; Seeger; Whitehead; Mayr; Xuan; Watanabe; Suzuki; Wakaguri; et al. (September 2006). "Common inheritance of chromosome Ia associated with clonal expansion of Toxoplasma gondii". Genome Res. 16 (9): 1119–25. doi:10.1101/gr.5318106. PMC 1557770. PMID 16902086.
- Dubey JP, Velmurugan GV, Rajendran C, Yabsley MJ, Thomas NJ, Beckmen KB, Sinnett D, Ruid D, Hart J, Fair PA, McFee WE, Shearn-Bochsler V, Kwok OC, Ferreira LR, Choudhary S, Faria EB, Zhou H, Felix TA, Su C; Velmurugan; Rajendran; Yabsley; Thomas; Beckmen; Sinnett; Ruid; Hart; Fair; McFee; Shearn-Bochsler; Kwok; Ferreira; Choudhary; Faria; Zhou; Felix; Su (September 2011). "Genetic characterisation of Toxoplasma gondii in wildlife from North America revealed widespread and high prevalence of the fourth clonal type". Int. J. Parasitol. 41 (11): 1139–47. doi:10.1016/j.ijpara.2011.06.005. PMID 21802422.
- Rigoulet, Jacques; Hennache, Alain; Lagourette, Pierre; George, Catherine; Longeart, Loïc; Le Net, Jean-Loïc; Dubey, Jitender P. (2014). "Toxoplasmosis in a bar-shouldered dove (Geopelia humeralis) from the Zoo of Clères, France". Parasite 21: 62. doi:10.1051/parasite/2014062. ISSN 1776-1042. PMC 4236686. PMID 25407506.
- Louis M Weiss, Kami Kim (2011) p. 19
- Louis M Weiss, Kami Kim (2011) p. 359
- Louis M Weiss, Kami Kim (2011) p. 306
- Tenter, AM; Heckeroth, AR; Weiss, LM (Nov 2000). "Toxoplasma gondii: from animals to humans". International Journal for Parasitology 30 (12–13): 1217–58. doi:10.1016/S0020-7519(00)00124-7. PMC 3109627. PMID 11113252.
- Jones, JL; Dargelas, V; Roberts, J; Press, C; Remington, JS; Montoya, JG (Sep 2009). "Risk factors for Toxoplasma gondii infection in the United States". Clinical Infectious Diseases 49 (6): 878–84. doi:10.1086/605433. PMID 19663709.
- Cook, AJ; Gilbert, RE; Buffolano, W; Zufferey, J; Petersen, E; Jenum, PA; Foulon, W; Semprini, AE; Dunn, DT (Jul 2000). "Sources of toxoplasma infection in pregnant women: European multicentre case-control study. European Research Network on Congenital Toxoplasmosis". BMJ 321 (7254): 142–7. doi:10.1136/bmj.321.7254.142. PMC 27431. PMID 10894691.
- Sakikawa, M; Noda, S; Hanaoka, M; Nakayama, H; Hojo, S; Kakinoki, S; Nakata, M; Yasuda, T; Ikenoue, T; Kojima, T (Mar 2012). "Anti-Toxoplasma antibody prevalence, primary infection rate, and risk factors in a study of toxoplasmosis in 4,466 pregnant women in Japan". Clinical and Vaccine Immunology 19 (3): 365–7. doi:10.1128/CVI.05486-11. PMC 3294603. PMID 22205659.
- Dubey, J. P., et al. "Prevalence of Viable Toxoplasma Gondii in Beef, Chicken, and Pork from Retail Meat Stores in the United States: Risk Assessment to Consumers." The Journal of Parasitology Vol. 91.5 (2005): 1082-093. JSTOR. The American Society of Parasitologists. Web. 1 Dec. 2014.
- Mai, K; Sharman, P; Walker, R; Katrib, M; de Souza, D; McConville, M; Wallach, M; Belli, S; Ferguson, D; Smith, N (2009). "Oocyst wall formation and composition in coccidian parasites". Memorias do Instituto Oswaldo Cruz 104 (2): 281–289. doi:10.1590/S0074-02762009000200022. PMID 19430654.
- Siegel, S; Lunde, M; Gelderman, A; Halterman, R; Brown, J; Levine, A; Graw, R (1971). "Transmission of Toxoplasmosis by Leukocyte Transfusion". Blood 37 (4): 388–394. PMID 4927414.
- Bobić, B; Jevremović, I; Marinković, J; Sibalić, D; Djurković-Djaković, O (Sep 1998). "Risk factors for Toxoplasma infection in a reproductive age female population in the area of Belgrade, Yugoslavia". European Journal of Epidemiology 14 (6): 605–10. doi:10.1023/A:1007461225944. PMID 9794128.
- Jones, J. L.; Dargelas, V.; Roberts, J.; Press, C.; Remington, J. S.; Montoya, J. G. (2009). "Risk Factors forToxoplasma gondiiInfection in the United States". Clinical Infectious Diseases 49 (6): 878–884. doi:10.1086/605433. PMID 19663709.
- Dass, S; Vasudevan, A; Dutta, D; Soh, L; Sapolsky, R; Vyas, A (2011). "Protozoan parasite Toxoplasma gondii manipulates mate choice in rats by enhancing attractiveness of males". PLoS ONE 6 (11): 1–6. Bibcode:2011PLoSO...627229D. doi:10.1371/journal.pone.0027229.
- Arantes, T; Lopes, W; Ferreira, R; Pieroni, J; Pinto, V; Sakamoto, C; Costa, J (2009). "Toxoplasma gondii: Evidence for the transmission by semen in dogs". Experimental Parasitology 123 (2): 190–194. doi:10.1016/j.exppara.2009.07.003. PMID 19622353.
- Gutierrez, J; O'Donovan, J; Williams, E; Proctor, E; Brady, C; Marques, P; Worrall, S; Nally, J; McElroy, M; Bassett, H; Sammin, D; Buxton, D; Maley, S; Markey, B (2010). "Detection and quantification of Toxoplasma gondii in ovine maternal and foetal tissues from experimentally infected pregnant ewes using real-time PCR". Veterinary Parasitology 172 (1–2): 8–15. doi:10.1016/j.vetpar.2010.04.035. PMID 20510517.
- Flegr, J; Klapilová, K; Kaňková, S (2014). "Toxoplasmosis can be a sexually transmitted infection with serious clinical consequences. Not all routes of infection are created equal". Medical Hypotheses 83 (3): 286–289. doi:10.1016/j.mehy.2014.05.019. PMID 24986706.
- Gallas-Lindemann, C.; Sotiriadou, I.; Mahmoodi, M. R.; Karanis, P. (2013). "Detection of Toxoplasma gondii oocysts in different water resources by Loop Mediated Isothermal Amplification (LAMP)". Acta Tropica 125 (2): 231–236. doi:10.1016/j.actatropica.2012.10.007. PMID 23088835.
- Alvarado-Esquivel, C.; Liesenfeld, O.; Márquez-Conde, J. A.; Estrada-Martínez, S.; Dubey, J. P. (2010). "Seroepidemiology of Infection with Toxoplasma gondii in Workers Occupationally Exposed to Water, Sewage, and Soil in Durango, Mexico". Journal of Parasitology 96 (5): 847–850. doi:10.1645/GE-2453.1. PMID 20950091.
- Esmerini, P. C. O.; Gennari, S. M.; Pena, H. F. J. (2010). "Analysis of marine bivalve shellfish from the fish market in Santos city, São Paulo state, Brazil, for Toxoplasma gondii". Veterinary Parasitology 170 (1–2): 8–13. doi:10.1016/j.vetpar.2010.01.036. PMID 20197214.
- Dattoli, V. C. C.; Veiga, R. V.; Cunha, S. S.; Pontes-De-Carvalho, L.; Barreto, M. L.; Alcantara-Neves, N. M. (2011). "Oocyst Ingestion As an Important Transmission Route of Toxoplasma gondii in Brazilian Urban Children". Journal of Parasitology 97 (6): 1080–1084. doi:10.1645/GE-2836.1. PMID 21740247.
- "CDC: Parasites – Toxoplasmosis (Toxoplasma infection) – Prevention & Control". Retrieved 13 March 2013.
- "Mayo Clinic – Toxoplasmosis – Prevention". Retrieved 13 March 2013.
- J.P Dubey (2010) p. 45
- Green, Aliza (2005). Field Guide to Meat. Philadelphia, PA: Quirk Books. pp. 294–295. ISBN 1-59474-017-8.
- Flegr, J; Lindova, J; Kodym, P (2008). "Sex-dependent toxoplasmosis-associated differences in testosterone concentration in humans". Parasitology 135 (4): 427–431. doi:10.1017/S0031182007004064. PMID 18205984.
- Flegr, J (2007). "Effects of Toxoplasma on Human Behavior". Schizophrenia Bulletin 33 (3): 757–760. doi:10.1093/schbul/sbl074. PMC 2526142. PMID 17218612.
- Hrda, S; Votypka, J; Kodym, P; Flegr, J (2000). "Transient Nature of Toxoplasma gondii-infected induced behavioral changes in mice". Journal of Parasitology 86 (4): 657–663. doi:10.1645/0022-3395(2000)086[0657:TNOTGI]2.0.CO;2. PMID 10958436.
- Hutchison, W; Aitken, P; Wells, B (1980). "Chronic Toxoplasma infections and motor performance in the mouse". Annals of Tropical Medicine and Parasitology 74: 505–510.
- Havlícek, J; Gasová, Z; Smith, P; Zvára, K; Flegr, J (2001). "Decrease of psychomotor performance in subjects with latent 'asymptomatic' toxoplasmosis". Parasitology 122 (5): 515–520. doi:10.1017/S0031182001007624.
- Flegr, J; Klose, J; Novotná, M; Berenreitterová, M; Havlícek, J (2009). "Increased incidence of traffic accidents in Toxoplasma-infected military drivers and protective effect RhD molecule revealed by a large-scale prospective cohort study". BMC Infectious Fiseases 9: 72. doi:10.1186/1471-2334-9-72. PMC 2692860. PMID 19470165.
- Flegr, J; Havlícek, J; Kodym, P; Malý, M; Smahel, Z (2002). "Increased risk of traffic accidents in subjects with latent toxoplasmosis: a retrospective case-control study". BMC Infectious Fiseases 2: 11. doi:10.1186/1471-2334-2-11. PMC 117239. PMID 12095427.
- Kocazeybek, B; Oner, Y; Turksoy, R; Babur, C; Cakan, H; Sahip, N; Unal, A; Ozaslan, A; Kilic, S; Saribas, S; Aslan, M; Taylan, A; Koc, S; Dirican, A; Uner, H; Oz, V; Ertekin, C; Kucukbasmaci, O; Torun, M (2009). "Higher prevalence of toxoplasmosis in victims of traffic accidents suggest increased risk of traffic accident in Toxoplasma-infected inhabitants of Istanbul and its suburbs". Forensic Science International 187 (1–3): 103–108. doi:10.1016/j.forsciint.2009.03.007. PMID 19356869.
- Louis M Weiss; Kami Kim (28 April 2011). Toxoplasma Gondii: The Model Apicomplexan. Perspectives and Methods. Academic Press. ISBN 978-0-08-047501-1. Retrieved 12 March 2013.
- J. P. Dubey (15 April 2010). Toxoplasmosis of Animals and Humans, Second Edition. CRC Press. ISBN 978-1-4200-9237-0. Retrieved 12 March 2013.
- ToxoDB : The Toxoplasma gondii genome resource
- Anti-Toxo : A Toxoplasma news blog and list of research laboratories
- Toxoplasma images, from CDC's DPDx, in the public domain
- Toxoplasmosis Research Institute & Center
- Cytoskeletal Components of an Invasion Machine – The Apical Complex of Toxoplasma gondii
- The Culture-Shaping Parasites, in Seed Magazine
- Sneaky Parasite Attracts Rats to Cats, All Things Considered, April 14, 2007
- Toxoplasma overview, developmental stages, life cycle image at MetaPathogen
- Toxoplasma lecture, Robert Sapolsky
- Could a brain parasite found in cats help soccer teams win at the World Cup?, – By Patrick House – Slate Magazine
- How Your Cat Is Making You Crazy, the Atlantic Magazine, March 2012
- Mystery Marine Mammal Deaths, CosmosMagazine.com, June 2008
- Toxoplasma gondii in the Subarctic and Arctic
- Toxoplasmosis – Recent advances, Open access book published in September 2012
- Toxoplasma gondii, the Immune System, and Suicidal Behaviour