Toxoplasmosis

Etiology

T gondii is an obligate intracellular parasite that can infect all warm-blooded animals, including farm animals, birds, and humans. T gondii has a complex life cycle, requiring a definitive host and an intermediate host to complete sexual and asexual cycles, respectively. The family Felidae members, including the domestic cat, are the only known definitive hosts for this organism. Birds and terrestrial and aquatic mammals are the intermediate hosts.[6] Domesticated and wild cats can become infected by ingesting infected oocysts or tissue cysts from intermediate hosts or the environment. Parasite transmission can occur by ingesting T gondii oocysts shed from felids, consuming infected tissue from intermediate hosts, and congenital infection (vertical transmission via the placenta).[7][8]

There are 3 infectious stages of forms within the T gondii life cycle: the tachyzoites, the bradyzoites, and the sporozoites.[9] When improperly cooked meat or food contaminated with cat feces is ingested, the outer cyst wall surrounding sporozoites and bradyzoites is proteolyzed by gastric juice in the alimentary canal.[6] These uncoated sporozoites and bradyzoites enter the gut epithelium and differentiate into tachyzoites. Tachyzoites are rapidly multiplying forms of T gondii.[6][9] Tachyzoites can penetrate any nucleated cell, including dendritic cells, monocytes, and neutrophils, resulting in dissemination. With the onset of the host immune response, these tachyzoites are repressed and eventually converted into slow-replicating forms termed bradyzoites. Bradyzoites form a thick cyst wall around them, forming a tissue cyst enclosing thousands of bradyzoites. These cysts remain in a dormant form in the immunocompetent host. However, they can get reactivated when the host immune system is compromised.[6][9] 

The definite host, the felids, can become infected by ingesting sporulated oocysts or ingesting infected mammals or birds.[6][10] The intermediate hosts, mammals, including humans, can become infected with T gondii via various pathways, including ingesting infectious oocysts from the environment, eg, food, water, or plants contaminated with feline feces, or from eating tissue cysts or tachyzoites from undercooked, raw meat, or animal viscera, from blood products or organ or tissue transplantation, or ingestion of unpasteurized milk, from organs may contain cysts or tachyzoites, and congenitally through the maternal placenta.[6][10]

Infected cats shed millions of unsporulated oocysts in the feces for about 1 to 3 weeks after initial infection.[6] Oocysts take 1 to 5 days to sporulate and transform into infectious oocysts (sporozoites), which can remain infective in the environment for up to a year. When intermediate hosts, such as birds or rodents, ingest these oocysts from plants, soil, or water, the oocysts become tachyzoites (rapid multiplication usually leading to acute infection), which embed in neural or muscle tissue and are then called bradyzoites.[6] Bradyzoites are slowly multiplying and are characteristic of chronic infection.[6]

Epidemiology

Toxoplasmosis is one of the most prevalent parasitic diseases that can infect terrestrial mammals, including humans. Toxoplasmosis is a true zoonotic disease since humans can acquire it from domestic, wild, companion, or farm animals. This disease exemplifies the One Health Principle, which is present in all aspects of the environment, including humans, soil, plants, water, and animals. Epidemiological data reveal that humans acquire infections primarily by exposure to oocysts via ingestion of contaminated soil, water, or undercooked meat.[11]. In the United States (US), toxoplasmosis was ranked second out of 5 foodborne pathogens with the highest public health burden, considering the high cost of illness and quality-adjusted life year.[12]

Although infection is usually asymptomatic in immunocompetent people, it can still cause nonspecific symptoms, including fever, lymphadenopathy, and ocular disease, usually posterior uveitis.[3] Toxoplasmosis can rarely lead to myocarditis, hepatitis, and disseminated disease in a small number of immunocompetent people. In pregnant individuals, primary infection leads to vertical transmission, transmission to the fetus, and congenital toxoplasmosis.

T gondii infection or reactivation in patients with profound immunosuppression can be life-threatening. These are patients with HIV/AIDS, solid organ transplants (SOT), hematopoietic stem cell transplantation (HSCT), and those receiving chemotherapy who develop toxoplasmosis either through primary infection via contaminated food or a transplanted organ containing latent cysts.[4] In patients with HIV/AIDS, risk factors affecting the prevalence of toxoplasmosis in these patients are similar to those of the general population. Still, they are at risk for developing opportunistic infection, more frequently cerebral toxoplasmosis, due to the reactivation of latent T gondii, notably when the cluster determinant 4 (CD4)

lymphocyte count drops below 100 cells/μL.[13]

Patients with SOT and HSCT who have reactivated disease or primary disease can develop central nervous system disease, chorioretinitis, and disseminated infection.[5][14][15] Study results have shown that the seroprevalence for toxoplasmosis in patients with HSCT is variable and can reach up to 70%. Still, it is similar to age-matched healthy individuals in the same country.[4][5] In these patients, serological screening for immunoglobulin G, or use of the polymerase chain reaction to detect T gondii, can be undertaken to assess previous infection status and make decisions regarding prophylactic treatment. Lack of prophylaxis and profound immunosuppression in these patients can lead to poor outcomes.[16] 

Population studies have shown 3 predominant clonal lineages globally—types I, II, and III— found in specific geographical regions worldwide. T gondii clonal lineage type II strains are predominantly found in humans and agricultural animals in Western Europe and North America.[11] Complete prevalence estimates that reflect the actual global burden of seropositivity of T gondii are lacking. There are differences in surveillance and reporting between countries, but very importantly, toxoplasmosis may be underdiagnosed. Underdiagnosis may occur not only in those who are asymptomatic (either due to the presence of nonspecific symptoms, lack of resources, or low awareness of the disease and low index of suspicion by clinicians) but also in more severe cases of opportunistic infections—again due to lack of awareness or lack of resources. Thus, the disease is underdiagnosed and underreported. 

According to a US Centers for Disease Control and Prevention survey from 2011, more than 11% of the US population aged 6 or older were infected and had a positive immunoglobulin G for T gondii.[17][18] Some risk factors for seropositivity for T gondii included male sex, older age, persons without a high school diploma living in crowded conditions, and those living below the poverty level.[18] Among women of reproductive age, a decrease in seroprevalence from 9.1% to 7.5% was seen from 2011 to 2014, but this was without statistical significance.[18] 

T gondii infection is estimated to have an average global seroprevalence rate of 25.7% (95% confidence interval 25.6%-25.8%). The overall range of seroprevalence was determined to be 0.5% to 87.7%. African countries had the highest average seroprevalence rate of 61.4%, followed by Oceania at 38.5%, South America at 31.2%, Europe at 29.6%, the US and Canada at 17.5%, and Asia at 16.4%.[19] The prevalence of infection varies both between countries and locally within a country. Differences in prevalence can be due to many factors. Some factors that increase the prevalence in certain areas include the higher seroprevalence of T gondii in warm and humid areas since oocysts survive better in these environments and also that areas with more livestock can be a direct source of infection and a possible reservoir for T gondii.[20] 

Pathophysiology

T gondii can overcome the host defenses and disseminate through the body to the liver and spleen, eluding the immune system and reaching the central nervous system by crossing the highly regulated blood-brain barrier and infecting the brain, where the parasite resides. Although most infections with T gondii are subclinical, a small percentage of immunocompetent people can develop acute toxoplasmosis that can manifest with nonspecific, constitutional symptoms, including fever, malaise, and lymphadenopathy, as well as central nervous system pathology and ocular disease—usually posterior uveitis.[3] After the initial infection, T gondii becomes latent and persists for life; the latent tissue cysts can reactivate in profoundly immunosuppressed people, and reactivation can also occur during transplantation from the donor organ.[4] 

Intracellular growth of tachyzoites results in direct cytopathic effects, cellular inflammation, and necrosis. Type 1 cell-mediated immunity is required to control acute and chronic infection of T gondii. Thus, any defects in cell-mediated immunity predispose the host to severe manifestations of toxoplasmosis.[21] In response to the damage caused by tachyzoite entry, the gastrointestinal epithelial cells produce chemokines that act as chemical messengers, resulting in the recruitment of dendritic cells, macrophages, and neutrophils to the site of damage. The entry of tachyzoites into these inflammatory cells stimulates the production of interleukin-12 that induces interferon-γ (IFN-γ) synthesis by natural killer cells and T lymphocytes.[21]

Synthesis of IFN-γ is required to control acute and chronic infection. Low CD4 lymphocyte counts in patients with AIDS lead to reduced IFN-γ levels, allowing unchecked multiplication of tachyzoites in acute infection and reactivation of bradyzoites in latent infection. This results in severe disease, manifesting as cerebral and extracerebral toxoplasmosis. Though cell-mediated immunity plays a crucial role in controlling the T gondii infection, humoral immunity also contributes by synthesizing antibodies, modulating CD4 and CD8 T-cell responses, and amplifying IFN-γ production.[22]

Histopathology

Patients with toxoplasmosis often present with cervical and generalized lymphadenopathy, cerebral encephalitis, and ocular toxoplasmosis. T gondii can be visualized during histopathological examination as either tachyzoites or cysts. Tachyzoites look like crescents and are pathognomonic for acute disease. When cysts are visualized, they usually represent reactivation disease or latent infection.[23][24] Usual stains such as hematoxylin and eosin, Wright, or Giemsa can be applied to tissue samples from blood, brain, sputum, centrifuged cerebrospinal fluid, or amniotic fluid. Hematoxylin and eosin tissue staining demonstrate T gondii parasites (either in the form of tachyzoites or tissue cysts) in healthy and necrotic areas. If these findings are observed near blood vessels, this may indicate hematogenous dissemination of T gondii.[25] Histological staining for diagnosing T gondii is not well-established, but it is suggested that this staining should be used in combination with other methodologies for diagnostic reasons. 

The histological findings of biopsies of commonly involved organs are:

• Lymph node biopsy: Parasites are rarely seen on the tissue specimen, but the triad of follicular hyperplasia, microgranulomas, and monocytoid B-cell hyperplasia are highly specific to T gondii infection.[23][26]
• Brain biopsy: Histology usually reveals necrotic areas with patchy diffuse encephalitis, cyst-containing lesions, microglial nodules, granulomas, and lymphocytic vasculitis.[27]
• Ocular biopsy: This is characterized by necrosis in segmental panophthalmitis with associated tissue cysts and tachyzoites.[25]

History and Physical

Obtaining a complete history of the present illness and exposure history is paramount in all patients because exposure histories are essential to increasing the clinician's index of suspicion and leading to the correct diagnosis. Most immunocompetent individuals infected with T gondii are asymptomatic; the T gondii then remains in the body and becomes latent. However, some immunocompetent individuals with primary, acute infection may complain of nonspecific constitutional symptoms, including fevers, chills, headaches, pharyngitis, myalgias, rash, and nontender cervical lymphadenopathy. More rarely, those who are immunocompetent have been reported to have hepatitis and myocarditis.[3][28][29][30]

A complete exposure history is essential; inquiries about exposure to cat feces are mandatory. However, since T gondii is also present in the environment and can be acquired from environmental and food sources, narrowing exposure and infection down to only cats could miss certain cases. Always considering toxoplasmosis in the differential diagnosis of any patient who presents with fevers and cervical lymphadenopathy, whether they have had an exposure history that is compatible with toxoplasmosis is important.[31] 

In immunosuppressed individuals, such as patients with HIV/AIDS and those who have received a solid organ or hematopoietic cell transplantation, toxoplasmosis can reactivate and present as an opportunistic infection. Patients can develop symptoms and signs of encephalitis, myocarditis, hepatosplenomegaly, retinochoroiditis, and pneumonitis.[32] In patients with HIV/AIDS, the parasite can reactivate and cause disease, usually when the CD4 lymphocyte count falls below 100 cells/μL; the risk of reactivation in these individuals can be as high as 30% if they are seropositive for T gondii and are not taking prophylactic treatment.[33]

The most common presentation of T gondii reactivation disease in patients with HIV/AIDS with CD4 lymphocyte counts less than 100 cells/μL is signs and symptoms of central nervous system disease from toxoplasmic encephalitis. Cerebral toxoplasmosis usually presents with neurological symptoms based on the region of the brain involved and the number of lesions. Symptoms may include fevers, seizures, headaches, changes in vision, altered mental status, focal neurological deficits, cognitive dysfunction, ataxia, involuntary movements, and stupor and coma.[13] Cerebrospinal fluid analysis may demonstrate nonspecific findings, such as mild protein elevation and monocytosis. Extracerebral disease can also occur, typically as pneumonitis and chorioretinitis, although cardiac, gastrointestinal, genitourinary, musculoskeletal, and disseminated diseases can also occur. Pneumonitis can present with fever, cough, and shortness of breath. Chorioretinitis commonly presents with visual changes, floaters, or ocular pain.[34][35]

Evaluation

The diagnosis of toxoplasmosis may be challenging in immunocompetent and immunosuppressed individuals. The diagnosis may be missed in immunocompetent people with few nonspecific symptoms due to a lack of clinical suspicion. The diagnosis can be made indirectly with serological testing for IgM and IgG. Serologic testing alone is not diagnostic in immunosuppressed people with central nervous system lesions. First, immunosuppressed individuals may not mount an immune response to make antibodies, and anti-toxoplasmosis IgG is prevalent in the general population. Hence, a more definitive diagnostic approach is necessary.

Biopsy of the affected organ provides a definite diagnosis, but this may not always be possible. Those who are immunosuppressed with central nervous system disease will usually need to have more than a single finding and also have symptoms compatible with the disease, computed tomography (CT) or magnetic resonance imaging (MRI) evidence of multiple, hypodense, ring-enhancing focal lesions in the cortex and periventricular areas, primarily in the basal ganglia, thalami, and corticomedullary junction, and detection of toxoplasma deoxyribonucleic acid (DNA) in cerebrospinal fluid or the presence of the organism in a biopsy specimen.[15]

Although a biopsy of the affected organ provides a definitive diagnosis, toxoplasmosis can also be diagnosed using noninvasive testing, including labs and imaging.

• Serological testing: This is the primary diagnostic method for determining infection with T gondii by identifying IgM and IgG antibodies using enzyme-linked immunosorbent assays. IgM antibodies are usually detectable from day 5 following infection, reaching maximum levels in 1 to 2 months. IgG antibodies are detectable after 1 to 2 weeks of infection, reaching maximum levels in 3 to 6 months. T gondii IgM antibody testing lacks specificity. Immunosuppressed individuals may not be able to mount an appropriate immune response, and therefore, serologies may not be helpful in the diagnosis of these patients.[15]
• Molecular testing: T gondii DNA can be detected by polymerase chain reaction (PCR) from blood and other bodily fluids to diagnose primary, reactivation, or latent infection, especially in those who are severely immunosuppressed. Although PCR is not routinely performed on immunocompetent individuals with fever and lymphadenopathy, it can be helpful for the diagnosis of T gondii in those who are immunocompromised. There is no standardized PCR for toxoplasmosis; the test remains helpful for diagnosing immunocompromised hosts with suspected central nervous system, pulmonary, or disseminated disease.[36]
• Radiological diagnosis: CT or MRI of the brain will typically reveal multiple, hypodense, ring-enhancing focal lesions in the cortex and periventricular areas, primarily in the basal ganglia, thalami, and corticomedullary junction.[15] An “eccentric target sign” may be present, consistent with an eccentric nodule along the rim of an enhancing lesion. This is considered pathognomonic.[15] Minimal inflammatory changes seen during the early stages may not be appreciated well on CT; MRI is preferred for diagnosing brain lesions in cerebral toxoplasmosis. Single-photon emission CT or positron emission tomography scans have high specificity to rule out differential diagnoses.
• Biopsy: Although this provides a definitive diagnosis by demonstrating tachyzoites and tissue cysts of T gondii, it is not usually performed. A biopsy is indicated to rule out other differential diagnoses in individuals failing to show clinical or radiological improvement of symptoms within 14 days of starting the therapy.

Treatment / Management

The main goal of treatment is to limit parasite multiplication during active infection. Prophylaxis with trimethoprim-sulfamethoxazole can prevent acute infection in those who are immunosuppressed and should be considered in patients who need it according to local or international guidelines.[16][37][38] Treatment is indicated in immunocompetent individuals with severe or prolonged symptoms and all those who are immunocompromised.(A1)

In immunosuppressed individuals, it is essential to ascertain which patients need prophylactic medication to avoid reactivation of toxoplasmosis. Reactivation of latent cysts is the primary mechanism of toxoplasmosis following HSCT and SOT and when patients with HIV/AIDS have a CD4 lymphocyte count of less than 100 cells/μL. The preferred regimen for prophylaxis in these patients is trimethoprim-sulfamethoxazole.[16][38] Early diagnosis of toxoplasmosis and initiation of empiric treatment, as well as antiretroviral therapy in patients with HIV/AIDS, improves outcomes in immunosuppressed individuals.[38] 

In patients who have received HSCT and SOT, toxoplasmosis is associated with high mortality.[4] Patients who are most likely to develop reactivation of toxoplasmosis are those who were seropositive before transplantation, especially those who do not receive chemoprophylaxis with trimethoprim-sulfamethoxazole.[16] Targeted chemoprophylaxis improved outcomes for SOT recipients.[4] Prophylactic therapy with trimethoprim-sulfamethoxazole may be indicated depending on patients' immune status and serological or PCR positivity. Decisions for initiating prophylactic regiments in patients undergoing HSCT and SOT who are seropositive for T gondii and who may not be able to tolerate prophylactic regimens may be screened by using quantitative PCR to determine whether there is active infection early as a pre-emptive strategy to initiate treatment.[16]

In cases of suspected T gondii infection, empirical therapy based on presumptive diagnosis is preferred rather than waiting for test results. The combination of pyrimethamine (200 mg loading dose followed by 50 mg daily for patients weighing less than 60 kg and 75 mg daily for patients who weigh more than 60 kg) and sulfadiazine (1000 mg 4 times a day for patients who weigh less than 60 kg and 1500 mg 4 times a day for patients who weigh more than 60 kg) is the preferred regimen for treatment. An alternative treatment can be trimethoprim-sulfamethoxazole. Initial therapy should be continued for 6 weeks and be followed by chronic maintenance therapy.[39] Folic acid is usually added to the treatment regimen to prevent folic acid deficiency because sulfadiazine inhibits folic acid biosynthesis. In central nervous system and ocular toxoplasmosis, steroids can be added to the standard regimen to treat cerebral edema and ocular toxoplasmosis.[40][41] (A1)

Differential Diagnosis

The differential diagnosis for toxoplasmosis may include the following:

• Central nervous system lymphoma
• Metastatic brain lesions
• Progressive multifocal encephalopathy
• Cerebral tuberculosis
• Bacterial or fungal brain abscess 
• Cytomegalovirus
• Acute HIV infection
• Herpes simplex encephalitis

Prognosis

The prognosis and outcomes of toxoplasmosis in patients vary and depend on factors such as their immune competence and the location of the reactivation. Acute toxoplasmosis is typically self-limited in immunocompetent individuals. The prognosis is excellent without any long-term effects of infection. Early diagnosis of toxoplasmosis and initiation of empiric treatment, as well as antiretroviral therapy in patients with HIV/AIDS, improves outcomes in immunosuppressed individuals.

Prophylactic treatment is indicated in all patients with HIV/AIDS with CD4 lymphocyte counts less than 100 cells/μL with trimethoprim-sulfamethoxazole. Toxoplasmosis is associated with high mortality in patients who have received HSCT and SOT.[4] Targeted chemoprophylaxis improved outcomes for SOT recipients.[4] Further, depending on patients' immune status and serological or PCR positivity, prophylactic therapy with trimethoprim-sulfamethoxazole may be indicated.

Complications

Toxoplasmosis can cause severe life-threatening complications in immunosuppressed individuals, manifesting as toxoplasmic encephalitis or extracerebral toxoplasmosis. Initiation of treatment can be complicated by immune reconstitution inflammatory syndrome, resulting in a paradoxical worsening of symptoms. Treatment involves continuing anti-infective therapy, antiretroviral therapy, and starting steroids (or increasing the dose if patients are already on steroids).