II. Transmission of T. gondii
T. gondii can be acquired by humans in several different ways. It may be transmitted as a tissue cyst, usually by eating undercooked meat from an infected animal, or as an oocyst.
- Ingestion of Tissue cysts
infection of mammals with T. gondii is widespread. Such infections occur when farm
animals ingest feed containing cat feces; when grazing animals inhale or ingest
dried cat feces deposited on the ground; and when an animal eats a smaller
animal, such as a mouse or rat that is infected. T. gondii then invades many
parts of the body, especially muscles, where it becomes tissue cysts and
remains for life. When the muscle is eaten as meat, especially if it has not
been thoroughly cooked, the person becomes infected.
and pork are thought to be the most common source of T. gondii tissue cysts for
humans, although cysts also occur in beef, chicken, and wild animal meat (e.g.,
deer, moose, bear). There have even been epidemics of adult toxoplasmosis among
individuals who ate undercooked meat, such as hamburger, from a common source
(Kean BH, Kimball AC, Christenson WN. An epidemic of acute toxoplasmosis. JAMA.
- Ingestion or Inhalation of Oocysts
previously noted, approximately 1.5 million cats (1 percent of 150 million) in
the United States are excreting oocysts on any given day; they may excrete up
to 20 million oocysts per day, and the oocysts may live for a year or longer.
Thus, wherever cats defecate is likely to be a source of contamination.
Children’s play areas and sandboxes are common places for cats to defecate
because they can use the area’s loose soil or sand to bury their feces. A study
in Brazil reported that school playgrounds in elementary schools were heavily
contaminated by T. gondii oocysts (dos Santos TR, Nunes CM, Luvizotto
MCR, et al. Detection of Toxoplasma
gondii oocysts in environmental
samples from public schools. Vet Parasitol. 2010;171:53–57). Children may
become infected by putting dirty hands, including oocysts, in their mouths. A
family epidemic was described as having occurred this way (Stagno S, Dykes AC,
Amos CS, et al. An outbreak of toxoplasmosis linked to cats. Pediatrics.
1980;65:758–762, copyright 1980, the American Academy of Pediatrics; linked to
PDF file with permission).
the cat feces dry, the oocysts may become aerosolized. They can thus be inhaled
by a person changing cat litter or just walking in an area where cats have
defecated. An outbreak of toxoplasmosis among patrons of a riding stable was
thought to have occurred in this manner (Teutsch SM, Juranek DD, Sulzer A, et
al. Epidemic toxoplasmosis associated with infected cats. N Engl J Med.
(also called sandpits) are of special interest. Studies of sandboxes in public
parks have been carried out in Japan. In one study, 12 of the 13 sandboxes were
contaminated with animal feces; the “mean number of feces found in 1 square
meter of the sandpits was 35” (Uga S. Prevalence of Toxocara eggs and number of
faecal deposits from dogs and cats in sandpits of public parks in Japan. J Helminthol.
1993;67:78–82). In another study of three public sandboxes observed over 140
days, an average of 2.3 cat defecations occurred each day in each sandbox (Uga
S. Minami T, Nagata K. Defecation habits of cats and dogs and contamination by
Toxocara eggs in public park sandpits. Am J Trop Med Hyg. 1996;54:122–126).
that 1 percent of the cats were infected, that each infected cat excreted 10
million oocysts each time it defecated, and that the oocysts remained viable
for one year, each sandbox would contain approximately 85 million viable
oocysts at any given time. For children playing in such a sandbox, the chances
of inhaling or ingesting (e.g., by putting fingers in mouth) T. gondii oocysts would appear to be high.
are also commonly used by cats for defecation and are also thought to be a
common source of infection by inhalation for gardeners. Unwashed vegetables
from gardens can also carry oocysts. Studies have also shown that cockroaches
and flies can carry oocysts from cat feces to fruits and vegetables (Wallace GD.
Experimental transmission of Toxoplasma
gondii by cockroaches. J Infect Dis.
1972;126:545–547; Wallace GD. Experimental transmission of Toxoplasma gondii by
filth-flies. Am J Trop Med Hyg. 1971;20:411–413). Another possible mode of
transmission is by dogs that roll in cat feces. One study reported that 23
percent of dogs did this, suggesting “the contamination of fur, after rolling
in cat feces containing oocysts, might make these accessible to children who
pet dogs” (Frenkel JK, Parker BB. An apparent role of dogs in the transmission
of Toxoplasma gondii : the probable
importance of xenosmophilia. Ann NY Acad Sci. 1996;791:402–407).
water infected with T. gondii oocysts is increasingly suspected of being a
major source of transmission (Dubey JP. Toxoplasmosis—a waterborne zoonosis.
Vet Parisit. 2004;126:57–72). The water is thought to become contaminated by
runoff from areas where cats defecate. Several epidemics of toxoplasmosis have
been reported due to contaminated water, most notably a 1995 epidemic in
Victoria, British Columbia, due to the contamination of the city water supply
by cat feces (Bowie WR, King AS, Werker DH, et al. Outbreak of toxoplasmosis
associated with municipal drinking water. Lancet. 1997;350:173–177, copyright
1997; linked to PDF file with permission from Elsevier).
relative importance of different modes of T.
gondii transmission has been widely
debated but minimally studied. In countries like France, which has a high rate
of T. gondii -infected individuals,
the most important source of transmission is thought to be cysts in undercooked
meat. Studies of pregnant women in Europe have identified the eating of raw or
undercooked meat as the most likely source of transmission (Kapperud G, Jenum
PA, Stray-Pedersen B, et al. Risk factors for Toxoplasma gondii infection
in pregnancy: results of a prospective case-control study in Norway. Am J
Epidemiology. 1996;144:405–412; Baril L, Ancelle T, Goulet V, et al. Risk
factors for Toxoplasma infection in
pregnancy: a case-control study in France. Scand J Infect Dis. 1999;31:305–309;
Cook AJC, Gilbert RE, Buffolano W, et al. Sources of Toxoplasma infection in pregnant women: European multicentre
case-control study. Br Med J. 2000;321:142–147). In countries like the United
States, in which meat is generally well cooked, direct transmission of oocysts from
cats especially via water, and contaminated fruits and vegetables, is thought
to be more important (Boyer K, Hill D, Mui, E, et al. Unrecognized ingestion of
Toxoplasma gondii oocysts leads to congenital toxoplasmosis and
causes epidemics in North America. Clin Infect Dis. 2011;53:1081-9).
question has been raised whether the clinical outcome is different if a human
becomes infected by a tissue cyst or an oocyst. In mice, infection by oocysts
appears to be more pathogenic. In humans, “circumstantial evidence suggests
that oocyst-induced infections...are clinically more severe than tissue cyst-acquired
infections” (Dubey JP. Toxoplasmosis—a waterborne zoonosis. Vet Parisit.
2004;126:57–72). There are also suggestions that reinfection can occur with
different strains of T. gondii (Elbez-Rubinstein A, Ajzenberg D, Dardé M-L,
et al. Congenital toxoplasmosis and reinfection during pregnancy: case report,
strain characterization, experimental model of reinfection, and review. J
Infect Dis. 2009;199:280–285).
- Vertical Transmission from Infected Mother to Offspring
In humans, it is well known that if a previously uninfected woman is infected with T. gondii while pregnant, the T. gondii
may cross the placenta and cause brain damage (e.g., cysts, seizures,
mental retardation) in the offspring. This is why women are cautioned to
not change cat litter while they are pregnant.
In female mice, it is known that a mouse which has been infected with T. gondii, even before becoming pregnant, may pass along the T. gondii
to their offspring. This may occur in subsequent litters as well so
that the infected mouse mother may infect many offspring. [Owen MR,
Trees AJ. Vertical transmission of Toxoplasma gondii from chronically infected house (Mus musculus) and field (Apodemus sylvaticus)
mice determined by polymerase chain reaction. Parasitology.
1998;116:299-304]. More surprising is the vertical transmission of T. gondii from
an infected mouse to its offspring, then from this offspring to its
offspring, and on for up to five generations (Beverly JKA. Congenital
transmission of Toxoplasmosis through successive generations of mice.
Nature. 1959;183:1348-1349). This would give the appearance of being a
genetic disease with maternal inheritance, but in reality, would simply
be the vertical transmission of an infectious agent.
- Sexual Transmission of T. gondii
A recent study in dogs demonstrated that T. gondii can be transmitted sexually in that species. Male dogs were infected by T. gondii ; it was
then found in their semen. The infected semen was then used to
artificially inseminate four uninfected female dogs. Seven days after
insemination, all four dogs had antibodies to T. gondii. Two of
the pregnant dogs had miscarriages; the other two delivered four
puppies, none of whom lived longer than three weeks and all of which had
cysts containing T. gondii in their brains (Arantes TP, Lopes WD, Ferreira RM, et al. Toxoplasma gondii : evidence for the transmission by semen in dogs.
Exp Parasitol. 2009;123:190–194). Another recent study demonstrated that T. gondii
can be sexually transmitted in rats; 43 of 69 rat pup offspring,
following sexual transmission, were found to be infected (Dass SAH,
Vasudevan A, Dutta D, et al. Protozoan parasite Toxoplasma gondii
manipulate mate choice in rats by enhancing attractiveness of males.
PLoS One. 2011;11:e27229. doi:10.1371/journal.pone.0027229).
There is also evidence that T. gondii
can be sexually transmitted in humans. How often this actually happens
and its clinical importance are unknown. A study in Germany examined
semen collected from 125 men who were being examined for possible
infertility. Among the 125, 3 men had evidence of T. gondii in their semen. Two of the men also had blood antibodies to T. gondii,
but the third did not. One man had urethritis and another had gonorrhea
(Disko R. Braveny I, Vogel P. Untersuchungen zum Vorkommen von
Toxoplasma gondii im menschlichen Ejakulat. [Studies on the occurrence
of Toxoplasma gondii in the human ejaculate]. Z. Tropenmed. Parasitol.
An American study examined the testes of 10 men who had died from AIDS and T. gondii
opportunistic infection. In 6 of the 10 cases bradyzoite-filled cysts
were identified in the testes. In 4 of these 6 cases, the only other
organ in which T. gondii was found was the brain (De Paepe M,
Guerrieri C, Waxman M. Opportunistic infections of the testis in the
Acquired Immunodeficiency Syndrome. Mt Sinai J Med. 1990;57:25-29).
- Do people with close cat contact have a greater chance of being infected with T. gondii?
noted previously, T. gondii can be transmitted from cats to humans in many
different ways. Being bitten by a cat, however, apparently does not cause the
transmission of T. gondii (Westling K, Jorup-Ronstrom C, Evengard B.
Toxoplasmosis not transmitted by cat bite, but high prevalence of antibodies to
Toxoplasma gondii in patients bitten by their own cat. Scand J
Infect Dis. 2010;42:687–690). Some of the means of transmission require no
contact whatever between cats and humans, e.g., through tissue cysts in
undercooked lamb, drinking water infected with oocysts, oocysts deposited by a
neighborhood cat in your garden. For this reason, attempts to show a
correlation between having antibodies to T.
gondii and past contact with cats
have yielded very inconsistent results.
review of 30 such studies reported that half of them found a correlation, but
half did not (Hall S, Ryan M, Buxton D. The epidemiology of Toxoplasma infection. In Joynson DHM,
Wreghitt TG, eds, Toxoplasmosis: A Comprehensive Clinical Guide, Cambridge: Cambridge
University Press; 2001:85–91). Those studies that were negative were more
likely to have been studies of adults, e.g., pregnant women who were asked if
they presently owned a cat. Those studies that were positive were more likely
to have included children and teenagers, such as studies done in Costa Rica and
Panama (Sousa OE, Saenz RD, Frenkel JK. Toxoplasmosis in Panama: a 10-year
study. Am J Trop Med Hyg. 1988;38:315–322; Frenkel JK, Ruiz A. Human
toxoplasmosis and cat contact in Costa Rica. Am J Trop Med Hyg. 1980;29:1167–1180).
The results varied depending on how the question was asked, with cat ownership
in childhood more likely to yield a positive correlation with T. gondii antibodies than cat ownership in adulthood.
The complexity of studying human–cat contact was also illustrated by a
Norwegian study that asked about cat contact in great detail. Becoming infected
with T. gondii was not statistically related to “living in a
neighborhood with a cat” (p=0.71) or “living in a household with a cat”
(p=0.13) but was statistically significantly related to “living in a household
with a kitten less than 1 year old” (p=0.04) (Kapperud G, Jenum PA,
Stray-Pedersen, et al. Risk factors for Toxoplasma
gondii infection in pregnancy:
results of a prospective case-control study in Norway. Am J Epidemiol.
1966;144:405–412). An American study reported that living in a household with
one or two kittens was not a significant risk factor for becoming infected with
T. gondii, but living in a household
with three or more kittens was a highly significant risk factor (OR 35.4)
(Jones JL, Dargelas V, Roberts J, et al. Risk factors for Toxoplasma gondii infection
in the United States. Clin Infect Dis. 2009;49:878–884).
- Do children with close cat contact have a greater chance of developing schizophrenia?
view of the above, it is of interest that two studies that have assessed cat
contact during childhood reported that it was significantly more common in
individuals with schizophrenia than in controls. In the first study of 165
parents of individuals with schizophrenia and bipolar disorder, 51 percent
reported that they owned a cat during pregnancy or during the first 10 years of
life of the affected individuals, compared to 38 percent among matched controls
(p=0.02, chi square; however, this was not corrected for the number of
questions asked, which would require a p<0.01 using a Bonferroni
correction). The question was asked for four different periods, and the results
were as follows:
to 1 yr
the largest difference in the ages for cat ownership was for ages 6–10. Dog or
other pet ownership was not included in this questionnaire (Torrey EF, Yolken
RH. Could schizophrenia be a viral zoonosis transmitted from house cats?
Schizophr Bull. 1995;21:167–171).
second study included 264 mothers of individuals with schizophrenia or bipolar
disorder and 528 matched controls and included questions on both cat and dog
ownership as follows:
to age 13
to age 13
in which the individual later developed schizophrenia or bipolar disorder were
significantly more likely to have owned a cat, but not a dog, between birth and
age 13 (p=0.0072) but not during the pregnancy (Torrey EF, Rawlings R, Yolken
RH. The antecedents of psychoses: a case-control study of selected risk factors.
Schizophr Res. 2000;46:17–23).
the mixed results of previous studies of antibodies to T. gondii and history of cat
contact, the results of these two studies suggest that if T. gondii is associated
etiologically with some cases of schizophrenia, then transmission of the
protozoa is most likely to be via oocysts, not tissue cysts, and to take place
few of the T. gondii antibody studies (see section V) have also
collected information on cat exposure. For example, a Turkish study of 73
patients with first-episode schizophrenia and 40 controls reported that 15/73
(28%) of the patients had a cat in their house compared to 1/40 (3%) of the
controls (p=0.006) (Tanyuksel M, Uzun O, Araz E, et al. Possible role of
toxoplasmosis in patients with first-episode schizophrenia. Turk J Med Sci.
OTHER SPECIFIC TOPICS
I. All about Cats
II. Transmission of T. gondii
III. Epidemiological Similarities and Differences between Toxoplasmosis and Schizophrenia
IV. Effects of T. gondii on Behavior and Psychiatric Symptoms
V. Studies of T. gondii Antibodies in Schizophrenia
VI. Neurotransmitters and T. gondii
VII. Neuropathology of T. gondii
VIII. Treatment Approaches to Toxoplasmosis and Schizophrenia