Toxoplasmosis

 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], [|retinochoroiditis]or [|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.

**__ Contents: __**
> 1.1 Life cycle > 1.2 Transmission > 2.1 Acute toxoplasmosis > 2.2 Congenital toxoplasmosis > 2.3 Chronic toxoplasmosis > 2.4 Ocular toxoplasmosis > 2.5 Behavioural changes > 2.6 Link to neuropsychiatric disorders > 3.1 Immunological assays > 3.2 DNA assays > 3.3 Imaging > 4.1 Cyst and lesion formation > 4.2 Behavioural changes and dopamine metabolism > 5.1 Drug therapy > 5.2 Prevention > 6.1 In humans > 6.2 In animals  = Cause: //Toxoplasma gondii// =
 * 1. Cause: Toxoplasma gondii**
 * 2. Signs and symptoms**
 * 3. Diagnosis**
 * 4. Pathophysiology**
 * 5. Treatment**
 * 6. Epidemiology**
 * 7. References**

Life cycle
The 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
//T. gondii// can invade host cells when ingested 15]. Transmission may occur through: 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].
 * 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].

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 =

[[image:tachyzoite_penetraating_a_neutrophil.jpg width="216" height="144" align="right" link="neurosciencefigures2012hmb300/Tachyzoite penetrating a neutrophil"]]Acute toxoplasmosis
<span style="font-family: Arial,sans-serif; font-size: 10pt;">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
<span style="font-family: Arial,sans-serif; font-size: 10pt;">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. 

[[image:Brain_cyst.jpg width="112" height="158" align="right" link="neurosciencefigures2012hmb300/Mouse brain cyst"]]Chronic toxoplasmosis
<span style="font-family: Arial,sans-serif; font-size: 10pt;">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
<span style="font-family: Arial,sans-serif; font-size: 10pt;">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
<span style="font-family: Arial,sans-serif; font-size: 10pt;">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.

<span style="font-family: Arial,sans-serif; font-size: 10pt;">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
<span style="font-family: Arial,sans-serif; font-size: 10pt;">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.

<span style="font-family: Arial,sans-serif; font-size: 10pt;">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. = Diagnosis =

[[image:ELISA_diagram.png width="74" height="185" align="right" link="neurosciencefigures2012hmb300/Example ELISA assay"]]Immunological assays
<span style="font-family: Arial,sans-serif; font-size: 10pt;">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
<span style="font-family: Arial,sans-serif; font-size: 10pt;">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]. 

Imaging
<span style="font-family: Arial,sans-serif; font-size: 10pt;">Toxoplasmosis 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].

 = Pathophysiology =

Cyst and lesion formation
<span style="font-family: Arial,sans-serif; font-size: 10pt;">T. 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].

<span style="font-family: Arial,sans-serif; font-size: 10pt;">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
<span style="font-family: Arial,sans-serif; font-size: 10pt;">The 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 ( <span style="color: #212121; font-family: Arial,sans-serif; font-size: 10pt;">posterodorsal medial [|amygdala], which reacts to opposite-sex mating stimuli) <span style="font-family: Arial,sans-serif; font-size: 10pt;"> 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.

<span style="font-family: Arial,sans-serif; font-size: 10pt;">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 //<span style="font-family: Arial,sans-serif; font-size: 10pt;">T. gondii //<span style="font-family: Arial,sans-serif; font-size: 10pt;"> 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 //<span style="font-family: Arial,sans-serif; font-size: 10pt;">T. gondii //<span style="font-family: Arial,sans-serif; font-size: 10pt;"> affects host behaviour by directly altering dopamine signalling in the brain 36]. Altered dopamine in these brain areas by encysted //<span style="font-family: Arial,sans-serif; font-size: 10pt;">T. gondii //<span style="font-family: Arial,sans-serif; font-size: 10pt;"> could cause the observed behavioural changes in rats. This also provides a possible mechanism for behavioural changes in humans and the link to schizophrenia.  = Treatment =

Drug therapy
<span style="font-family: Arial,sans-serif; font-size: 10pt;">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]. <span style="font-family: Arial,sans-serif; font-size: 10pt;">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]. 
 * <span style="font-family: Arial,sans-serif; font-size: 10pt;">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].
 * <span style="font-family: Arial,sans-serif; font-size: 10pt;">[|Spiramycin]has been used to prevent congenital transmission in pregnant women 48].
 * <span style="font-family: Arial,sans-serif; font-size: 10pt;">Cysts respond to treatments with [|atovaquone]and [|clindamycin] 49].
 * <span style="font-family: Arial,sans-serif; font-size: 10pt;"> 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].

Prevention
<span style="font-family: Arial,sans-serif; font-size: 10pt;">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].

<span style="font-family: Arial,sans-serif; font-size: 10pt;">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].  = Epidemiology =

In humans
<span style="font-family: Arial,sans-serif; font-size: 10pt;">Serological 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.

<span style="font-family: Arial,sans-serif; font-size: 10pt;">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
//<span style="font-family: Arial,sans-serif; font-size: 10pt;">T. gondii //<span style="font-family: Arial,sans-serif; font-size: 10pt;"> 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. == = References =

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