James Samsom

Toxoplasmosis is a parasitic disease which predominantly affects the central nervous system and eyes; it is caused by the protozoan Toxoplasma gondii. Serological evidence shows that toxoplasmosis affects a large proportion of the world population [1]. T. gondii can only sexually reproduce in the gut of felids; however, it is capable of infecting most birds and mammals, including humans, as intermediate hosts in order to transmit to its primary host [2]. The infection can be transmitted via contact with contaminated food or soil or congenitally from mother to child. The parasite forms tissue cysts, mostly in the central nervous system and eyes of hosts, and in immunosuppressed individuals or unborn children it can lead to serious complications such as: hydrocephalus, retinochoroiditisor encephalitis [3]. In healthy individuals the disease is latent and does not cause any apparent detrimental symptoms. There is new evidence that suggests T. gondii may be capable of altering the behaviour of the intermediate host in order to favour transmission to its feline primary host [4]. It has always been assumed that toxoplasmosis is asymptomatic in healthy people and is only a threat to immunosuppressed individuals and unborn children; however, there is recent evidence which links the disease to various neuropsychiatric disorders such as schizophrenia [5, 6]. Toxoplasmosis is a globally distributed major zoonosis and research is currently being done on effective diagnosis and treatment options in both humans and cats. Important new research is also being done on the pathophysiology and possible neuropsychiatric effects of this disease.


1. Cause: Toxoplasma gondii
2. Signs and symptoms
3. Diagnosis
4. Pathophysiology
5. Treatment
6. Epidemiology
7. References

Cause: Toxoplasma gondii

Life cycle

Life_cycle.pngThe life cycle of T. gondii has a sexual phase and an asexual phase [7]. The sexual phase only occurs within members of the family felidae whom are the primary hosts of T. gondii [2]. The asexual phase occurs in an intermediate host which can be almost any warm blooded animal, including: pigs, sheep, rats, birds, and humans. However, the typical intermediate hosts of T. gondii are species which cats are likely to consume such as rodents.

T. gondii invades host cells by forming a space called a parasitophorous vacuole [8]. Slowly replicating versions of the parasite called bradyzoites form within the vacuole. Bradyzoites replicate by endodyogeny: a type of binary fission. Reproductive bradyzoite containing vacuoles form tissue cysts, primarily in the muscles and brain [9]. Encysted T. gondii does not activate the immune system as it is contained within host cells. Cysts are not static and regularly rupture and reinvade host cells [10, 11]. Released parasites from ruptured cells can differentiate into tachyzoites: fast replicating motile versions of the asexually reproducing which reinvade tissue. These will trigger an immune response and are normally quickly cleared by the immune system. Infection has two phases: acute and chronic [12]. Acute (primary) infections are characterized by widespread invasion by tachyzoites resulting in an immune response and other symptoms. Chronic (latent) infections are characterised by encysted bradyzoites and can be asymptomatic in healthy individuals.

When an infectious cyst or oocyst is ingested by the feline primary host, T. gondii begins its sexual phase. Cysts are dissolved in the feline’s digestive system thereby releasing bradyzoites which invade the intestinal epithelium [11]. The parasite will divide and differentiate within the intestinal tissue by a process termed schizogony before it begins forming gametes [13]. Female “macrogametes” form within intestinal cells and male “microgametes” are released. Microgametes use flagella to swim to and penetrate macrogametes to form a zygote [11]. Following fertilization an oocyst wall forms around the zygote. Infected cells rupture and release oocysts into the intestinal lumen to be excreted in the feces.

After an oocyst is excreted, it undergoes a process known as sporulation [11]. This results in the formation of sporozoite filled sporocysts. When ingested, sporozoites can differentiate into tachyzoites to invade host cells. Sporozoites along with tachyzoites and bradyzoites are the infectious forms of T. gondii. Interestingly, oocyst formation in felines is more likely to occur with the ingestion of tissue cysts rather than oocysts or tachyzoites [14]. This shows that the infection of an intermediate host improves the ability of the parasite to reproduce sexually.


Transmission,_wikipedia.jpgT. gondii can invade host cells when ingested [15]. Transmission may occur through:
  • Ingestion of undercooked meat containing tissue cysts, especially pork, lamb, or venison. This can also occur with hand to mouth contact after handling contaminated raw meat or contact with utensils or cutting boards used for meat preparation [2].
  • Ingestion of cat (or other feline) feces contaminated with oocysts. This may occur with hand to mouth contact following: handling cat litter, gardening, playing in a sandbox [2]. Oocysts can survive for 12-18 months in moist sand or soil [3].
Infected cats only shed oocysts in their feces for a few weeks following infection [13]. The amount of shedding depends on many factors, but the feces generally don’t become contagious until a few days after infection. Serological evidence shows that most infections result from consumption of tissue cysts in undercooked meat [1, 16]. Infection from direct contact with cats is not thought to be a primary risk due to their fastidious nature, the short term of oocyst shedding, and the passing of non-infective oocysts [2].

During the first exposure, T. gondii tachyzoites can travel through the bloodstream and come into contact with most organs including the placenta during pregnancy [17]. Following exposures result in a secondary immune response which reduces the probability of tachyzoites being able to reach the placenta. T. gondii can infect trophoblast cells in the placenta which are at the interface with the fetal compartment [18]. If this occurs, the infection could proceed into the fetus and be transmitted congenitally from mother to child [19].

Signs and symptoms

tachyzoite_penetraating_a_neutrophil.jpgAcute toxoplasmosis

Acute (primary) infections are caused by invasion by tachyzoites [12]. Common symptoms in individuals with healthy immune systems include: fatigue followed by fever, headache, malaise, and myalgia [20]. Some patients in Atlanta, Georgia reported anorexia. Swollen lymph nodes can occur and may persist or recur at different times during infection [21]. In young children and immunocompromised patients, acute infection leads to: brain lesions, encephalitis, hydrocephalus, and damage to the eyes (retinochoroiditis) [3, 12, 22]. Toxoplasmic encephalitis is most likely to affect the cerebral cortex, followed by the basal ganglia, cerebellum and brain stem [23].

Congenital toxoplasmosis

If acute infection occurs during pregnancy it could transmit to the fetus resulting in congenital toxoplasmosis [18]. Congenital infections can be more severe. Symptoms include: hydrocephalus, retinochoroiditis, intracerebral calcification, mental retardation, hearing loss, cholangitis, hepatosplenomegaly, pancytopenia, and fetal death [24]. Conversely, the congenital infection could become latent and asymptomatic; however, serious symptoms may manifest later in life.

Brain_cyst.jpgChronic toxoplasmosis

Patients with encysted bradyzoites are said to have chronic (latent) infections [12]. Healthy patients generally do not manifest symptoms; however, if the infection is congenital or if the patient becomes immunosuppressed (by contracting HIV/AIDS or induced artificially for organ transplant surgery) they could develop acute symptoms.

Ocular toxoplasmosis

Infection in young children or immunosuppressed individuals commonly results in damage to the eyes. Ocular symptoms include: strabismus, retinochoroiditis, inactive retinal scarring, panuveitis, uveitis, and neuroretinitis [25].

Behavioural changes

Infection with T. gondii has been shown to affect behaviour in rats; specifically, in an open field arena infected rats showed: significantly reduced climbing, rearing (standing on hind paws), and grooming activity; and a significantly longer time spent in the center of the arena [4]. Infected rats also showed reduced anxiety in a plus-maze (measured as increased social investigation and open arm exploration) [26]. Furthermore, infected rats are shown to be attracted to the odour of cat urine [27]. All of these behavioural changes are likely to increase the incidence of cat predation; therefore, increasing the transmission of T. gondii to its definitive host. This is consistent with the theory that T. gondii is capable of altering the behaviour of intermediate hosts in order to favour transmission.

Altered behaviour has also been observed in T. gondii infected humans. Statistical studies show an increased seroprevalence of toxoplasmosis in people involved in traffic incidents; furthermore, infected humans show prolonged reaction times [28]. Infected humans also show a stereotypically altered personality profile compared to uninfected controls reflected by: atypical behaviour in ordinary situations as well as in artificial situations such as behavioural experiments, change in clothing tidiness (decreased in men, increased in women), and different response patterns in experimental games (decreased altruism in men, increased in women) [29, 30]; a decreased novelty seeking score in the Cloninger's TCI test [31]; altered superego strength (decreased in men, increased in women), changes in suspiciousness (increased in men, decreased in women), and changed “warmth” (decreased or same in men, increased in women) in Cattell's 16PF questionnaire [32]; and a change in the perceived appeal of cat odours (increased in men, decreased in women) [33]. It should be noted that these studies either compare infected populations to uninfected populations, or they correlate seroprevalence of toxoplasmosis with the behaviour of certain populations; therefore, they cannot establish causality. A study that compares human behaviour before and after infection has yet to be done due to ethical issues.

Link to neuropsychiatric disorders

There have been a number of studies which link toxoplasmosis with schizophrenia [5]. A large number of studies found an increased incidence of toxoplasmosis seropositivity (presence of T. gondii in serum) among patients suffering from schizophrenia [34]. A meta-analysis of these studies shows a statistically significant correlation between toxoplasmosis and schizophrenia. Furthermore, antipsychotic medications have been shown to prevent T. gondii specific behavioural changes in rats [35]. T. gondii has also been shown to affect dopamine in the brain, which is a common pathological process shared with schizophrenia [36]. A causal relationship has yet to be established; however, treatment trails which show that treatment of toxoplasmosis improves schizophrenia symptoms are likely to elucidate this link [6]. Because of the association with dopamine signalling, toxoplasmosis has also been implicated in depression [5]. This is new research and requires further investigation.

Because toxoplasmic encephalitis commonly affects the basal ganglia, a shared pathology seen in obsessive compulsive disorder (OCD), studies have been done to investigate a possible link [23]. A recent study found an increased incidence of seropositivity for toxoplasmosis in OCD patients compared to healthy controls. Dopamine and the basal ganglia are also important in Parkinson’s disease (PD). PD patients also show an increased incidence of seropositivity for toxoplasmosis [37]. These data are preliminary and many more studies must be done to establish a definitive link to either OCD of PD.


ELISA_diagram.pngImmunological assays

Toxoplasmosis is ordinarily diagnosed using serological analysis [38]. T. gondii infection causes the body to produce antibodies against specific T. gondii antigens [3]. These antibodies can be detected in the blood using various immunoassays (e.g., VIDAS avidity assay [39], Dot-ELISA [40], etc…). Acute toxoplasmosis can be differentiated from chronic toxoplasmosis using flow-cytometry or VIDAS avidity assays to quantify the presence of different types of antibodies [38, 39]. Acute patients are positive for IgM antibodies but have a low IgG avidity index; whereas, chronic patients can be positive or negative for IgM antibodies but will have a high IgG avidity index. Differentiating between acute and chronic infections is important in pregnant women as acute infections can be transmitted to the fetus and cause complications in fetal development [19].

DNA assays

Toxoplasmosis can be detected in various tissues or bodily fluids using polymerase chain reaction (PCR) to probe for T. gondii specific DNA [7, 39]. PCR is used to diagnose fetal toxoplasmosis because in can successfully detect T. gondii in amniotic fluid [19]. A new faster method called loop-mediated isothermal amplification (LAMP) has been used to detect T. gondii DNA in mouse urine, and could be used in the future for routine diagnosis and therapeutic evaluation in humans [41].


gondii_lesions.pngToxoplasmosis can be diagnosed indirectly by imaging and treatment efficacy. Toxoplasmosis is a common treatable cause of brain lesions in immunocompromised patients. These lesions can be detected on an MRI, CT, or PET scans [7, 42]. When lesions are detected, it is common to immediately initiate treatment for acute toxoplasmosis. If symptoms improve, then toxoplasmosis is inferred have been the cause of the lesions [22]. While it is difficult to determine the source of lesions from imaging alone, a number of techniques such as (18)Fluorodeoxyglucose PET (FDG-PET) and dynamic contrast-enhanced MRI have demonstrated the ability to differentiate between cancerous and toxoplasmic lesions [42, 43].


Cyst and lesion formation

Frankel_pathophysiology_diagram.pngT. gondii tachyzoites or bradyzoites invade host cells by actively penetrating the plasmalemma or through phagocytosis [11]. These forms of T. gondii possess organelles known as rhoptries and dense granules which release their contents into host cells [7]. The released contents include proteins which can associate with the host cell membrane and aid with various parasitic processes such as the formation of a parasitophorous vacuole around the invading parasite (e.g., ROP2 is released and helps associate host mitochondria with the vacuole membrane) [44]. Once the vacuole is formed from both parasite and host tissue, the parasite begins replicating within the host cell to form a tissue cyst [11]. Likely due to parasite competition, the host cell will disintegrate when 8-32 tachyzoites are formed; however, if the tachyzoites differentiate into bradyzoites they can invade cells and replicate within intracellular cysts for a much longer period [12]. These intracellular cysts can evade the host immune system in chronic infection. Cysts can be as little as 5µm and contain as few as two bradyzoites [8]. In brain tissue, mature cysts are typically 50-70µm and contain 1000-2000 bradyzoites. CNS cysts have been reported in neurons, astrocytes, microglia, and retinal cells. Encysted T. gondii bradyzoites are capable of inhibiting cellular apoptosis, so they can persist in host cells for long periods of time [7]. As cysts grow, the host cell degenerates and may rupture thereby releasing bradyzoites which can differentiate into tachyzoites and invade surrounding cells [12].

In healthy individuals, the immune system effectively eliminates fast replicating tachyzoites, and bradyzoites replicate slowly enough that no serious damage results [12]. When cysts disintegrate, they lyse cells resulting in lesions to the infected tissue (primarily in the brain and eyes). If unchecked by the immune system, fast replicating tachyzoites invade and kill multiple cells resulting in large lesions. If these lesions occur in the brain, they can manifest behavioural symptoms by interfering with brain functions in the region surrounding the lesion via mass effects (e.g., hemineglect occurred in one patient with lesions to the cortex and basal ganglia) [22]. If untreated, the damage could lead to hydrocephalus, retinochoroiditis or fatal necrotic encephalitis [3, 8].

Behavioural changes and dopamine metabolism

Schem_Of_brain_changed.pngThe exact mechanisms by which behavioural changes occur are not well understood. Toxoplasmosis in rats has been shown to cause specific changes in brain activity [27]. Cat odours will activate sexual arousal pathways in the limbic system (posterodorsal medial amygdala, which reacts to opposite-sex mating stimuli) of infected rats; while in uninfected rats, cat odours elicit a defensive response (via the dorsomedial part of the ventromedial hypothalamus). This change in brain activity causes rats to approach cat odours rather than avoid them. There are a few hypotheses regarding the specific biochemical mechanisms underlying these neurological changes. One hypothesis predicts that brain cysts may perturb brain tissues via mass effects or paracrine secretions, hence altering function. Cysts should therefore cluster in brain areas relevant to the observed change in behaviour; however, a recent study shows no clear region dependent cyst distribution within the hippocampus or amygdala of mice [4]. This implies that cysts do not directly cause the observed behavioural changes by perturbation of the surrounding tissue.

A more promising hypothesis stems from a study which found that T. gondii infected mice show a 14% increase in dopamine, with no changes seen in other neurotransmitters [45]. A recent study found that dopamine is contained and released in high amounts from T. gondii infected brain tissue [36]. Furthermore, bradyzoites were shown to produce tyrosine hydroxylase (the rate limiting enzyme in dopamine biosynthesis). Antipsychotic medication used to treat schizophrenia (haloperidol and valproic acid) was shown to prevent T. gondii specific behavioural changes in infected rats [35]. These medications work as dopamine receptor antagonists. T. gondii cysts have been observed in functioning neurons [46]. Furthermore, tissue cysts have been observed in high concentrations in the amygdala and nucleus accumbens (dopamine containing limbic brain regions known to be important for: motor control (basal ganglia), motivation, pleasure, addiction, reward, and fear) [26]. This evidence suggests that T. gondii affects host behaviour by directly altering dopamine signalling in the brain [36]. Altered dopamine in these brain areas by encysted T. gondii could cause the observed behavioural changes in rats. This also provides a possible mechanism for behavioural changes in humans and the link to schizophrenia.


Drug therapy

Toxoplasmosis in healthy individuals generally goes untreated as tissue cysts are asymptomatic [12]. A number of drug therapies are available for treatment of acute toxoplasmosis; however, tissue cysts remain resistant to drugs. Treatment of toxoplasmosis in cats is important for the prevention of transmission to humans [2].
  • The recommended treatment for acute toxoplasmosis in humans is with a combination of the anti-malarial medication pyrimethamine and the antibiotic sulfadiazine (given with folinic acid to prevent a reduction in platelet count) [47].
  • Spiramycin has been used to prevent congenital transmission in pregnant women [48].
  • Cysts respond to treatments with atovaquone and clindamycin [49].
  • There is no recommended treatment in cats; however, sulphonamides, trimethoprim, pyrimethamine, and clindamycin, either alone or in combination, as well as ponazuril are being explored as possible treatments [2].
Treatments of latent cysts in immunosuppressed individuals with the chronic disease are being explored [50]. New treatments for tissue cysts in latent toxoplasmosis focus on differences in metabolism between t. gondii and host cells. For example, many biochemical pathways in T. gondii are plant-like and absent from the mammalian host. Drugs which target the isoprenoid pathway, dihydrofolate reductase, T. gondii histone deacetylase, or type II fatty acid biosynthesis show some promise as possible future treatments [7, 50].


Without an effective vaccine in humans, prevention of transmission may be the most effective way of combatting toxoplasmosis [2]. People should: practice good hygiene (e.g. washing hands after contact with soil, washing fruits and vegetables before eating them), freeze meat for 24 hours at −12 °C and/or cook meat to an internal temperature of 66 °C, and not drink untreated water [2, 47]. People at risk, including immunosuppressed individuals and pregnant women, should be educated about proper prevention. Also, it is recommended that cats should be kept indoors and cat litter should be changed daily. Dogs should be kept out of the cat litter and vaccinated for distemper (which shows a high comorbidity with toxoplasmosis infection) [47].

Computer simulations suggest that cats are primarily infected through ingesting mice while mice are primarily infected congenitally [15]. Therefore, reducing mice on farms can prevent transmission to livestock. Additionally, vaccinating cats against oocyst shedding is also being explored to prevent contamination of livestock [2].


In humans

Pappas_et_al._epidem..pngSerological studies show that toxoplasmosis has an extremely high prevalence in human populations all over the world [1]. Fetal death may result from congenital infection, this results in an estimated 2300 DALYs per annum in the Netherlands [51]. This makes toxoplasmosis the most important food born pathogen in terms of DALYs within the Netherlands. However, we cannot extrapolate these data across different populations as the prevalence and transmission of toxoplasmosis can vary drastically due to cultural differences.

A recent report shows toxoplasmosis seroprevalence in pregnant woman worldwide increases with age from 5.3% to 78% depending on the region [52]. In the Americas, prevalence ranges from 78% in Recife (Brazil) to 6.1% in Durango (Mexico). In Europe, seroprevalences range from 63% in Western Pomerania (Germany) to 8.2% in Lausanne and Geneva (Switzerland). The reported range in Asia was from to 64% in Babol (Iran) to 5.3% in Bangkok (Thailand). In Africa, reported ranges were 75% in Sao Tome and Principe to 25% in Burkina Faso. While there is limited information available for Canada, the seroprevalence in the pregnant women in the US is an estimated 10.2-11.8%.

In animals

T. gondii is capable of infecting most, if not all, warm blooded animals [2]. As toxoplasmosis is primarily spread through contaminated meat, prevalence in livestock is an important factor for infection of the human population [15]. Felines are the sole shedders of infective oocysts therefore are essential for the persistence of T. gondii in grazing animals (sheep, deer, cows, etc…) [2]. The estimated seroprevalence worldwide in domestic cats is 30-40% with estimates ranging from ~16-80% locally in the US.


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