Toxoplasmosis and Pregnancy: Current Approaches for Favourable Fetal Outcome

Abstract

Toxoplasma gondii is one of the most widespread parasites worldwide. Although it causes asymptomatic infections in most cases, it can lead to severe disease in immunocompromised individuals. When acquired during pregnancy, it can negatively impact fetal development, resulting in unfavorable outcomes in neonates. The epidemiology of this parasite varies from country to country, leading to a lack of international consensus on screening, diagnosis, and management of congenital toxoplasmosis. This article aims to review aspects related to the modes of transmission, screening, fetal prognosis, management, and prevention of this infection to increase the chances of a favorable fetal outcome.

Introduction

Being one of the most successful infectious agents, Toxoplasma gondii is a protozoan parasite that infests most warm-blooded organisms, including humans [1]. Chronic toxoplasmosis is present in about 30% of the world population [2]. For the majority of patients, there are no manifestations of the disease. However, for immunocompromised patients and babies, the condition can have disastrous consequences [3]. Some of the effects of pregnancy-acquired toxoplasmosis are represented by postnatal disabilities and fetal demise [1]. In over 80% of cases, the disease causes no symptoms. However, when manifestations of the disease appear, they are uncharacteristic and non-severe and include an episode of elevated body temperature during 2-3 days, shivers, perspiration, cephalalgia, hepatosplenomegaly, muscle pain, pharyngitis, and a disseminate nonpruritic maculopapular skin eruption. The most frequent manifestation of the disease is represented by lymphadenopathy, which is present in 7% of pregnant women with acute toxoplasmosis. Although extensive lymphadenopathy occurs in a small number of cases, it usually lasts a few weeks, appears in the cervical region symmetrically, is unpainful and nonfluctuant, and the adenopathy size is under 3 cm. Another complication of the disease, which can lead to blindness or visual impairment, is represented by chorioretinitis [1]. Congenital toxoplasmosis can have severe consequences for the fetus. However, there are countries where toxoplasma screening is not recommended, even though screening and diagnosis techniques are available. Consequently, Toxoplasma infection in pregnancy is often neglected, and specific recommendations for patients at risk are not made, even though the implications of vertical transmission are of great importance [2].

Transmission Modalities

Toxoplasma gondii has a sophisticated reproductive cycle and has a definitive host represented by felines and an intermediate host. In definitive hosts, T. gondii expresses a sexual cycle, and in intermediate hosts, it undergoes an asexual process [6]. In intermediate hosts, the active form of Toxoplasma gondii, represented by tachyzoites, can transform into a dormant state represented by bradyzoites, usually located in organs like the brain and eyes. Consequently, this inactive form of Toxoplasma gondii is challenging to eliminate by the immune system or drugs that cannot pass the blood-brain barrier or the multiple layers of the ocular globe [7,8,9,10]. Ingestion of the bradyzoites by the definitive hosts is followed by several cycles of asexual reproduction and, finally, by sexual reproduction, which leads to the formation of unsporulated oocysts. Next, unsporulated oocysts are eliminated through feces and become infectious, which can contaminate the soil, fruits, vegetables, and water and can be ingested by intermediate hosts such as pigs, cows, and sheep [11,12]. The transmission modalities of Toxoplasma gondii to pregnant women described in the literature are represented by ingesting contaminated fruits, vegetables, water, seafood, unpasteurized milk, and undercooked meat [10,13,14,15,16].

During pregnancy, Toxoplasma gondii can be transmitted to the fetus if the mother has an acute infection with the passage of the parasite through the placenta. Studies showed that transmission probability varies with gestational age: 56% at 30, 23% at 18, and 2.2% at six weeks of gestation [17]. The transplacental transmission rate of maternal primary infection is 40% [18]. Another possible scenario is represented by maternal reinfection with a different strain. However, this probability is very low since only several cases are described in the literature [19,20]. In immunocompromised patients, such as HIV-infected individuals, there is a possibility of reactivation of an old maternal infection, although studies have demonstrated that this possibility is negligible or absent [21,22]. Concerning the vertical transmission of an infection acquired before pregnancy, the probability is very low, as only a small number of cases of transmission of an infection developed one or two months preconceptional have been described [23]. In Europe and the USA, the incidence of fetal infection with Toxoplasma gondii varies from 1 to 10 cases per 10,000 live births [24].

Effects on the Fetus

Congenital toxoplasmosis has a wide range of manifestations, the severity depending on the moment of acquisition of the infection. In the first trimester, it can lead to miscarriage, mental disability, hydrocephaly, or ocular lesions. In contrast, congenital toxoplasmosis is associated with a less severe outcome in the second and third trimesters, as the infection is often subclinical [12,25,26]. Reducing severity during advancing gestational age may be attributed to the maturation of the fetal immune system [26]. It is estimated that 90% of cases of congenital toxoplasmosis in France are asymptomatic. Two-thirds of the remaining cases present with moderate disease (peripheral chorioretinitis, brain calcifications), and one-third present with severe disease (generalized disease, severe ocular lesions, hydrocephalus) [27]. Other disease manifestations can include ophthalmic calcifications, thrombocytopenia, hepatomegaly, encephalomyelitis, central nervous system necrosis and calcification, low birth weight, jaundice, and meningoencephalitis [28].

Fetal impairment can be evaluated with ultrasound. There are some characteristic ultrasound signs that can be identified, such as intracranial calcification, advancing ventricular dilatation, and periventricular abscesses. Additionally, there are some less frequent ultrasound signs, such as shortened corpus callosum, periventricular echogenicity, cortical gyration lesions, lenticulostriate vessel vasculitis, cerebellar impairments, and subependymal modifications (cysts, echogenicities) that can be observed [29]. Intracranial ultrasound aspects can be isolated, or they can be associated with other intracranial factors or with extracranial elements such as being small for gestational age, hepatomegaly, ascites, hyperechogenic bowel, splenomegaly, and placentomegaly [29]. Since ultrasound anomalies can appear after an interval of a few months following maternal infection, there is a strong need for serial ultrasound evaluations. This tool is essential not only for predicting the fetal outcome but also for visualizing ultrasound signs associated with toxoplasmosis, which can raise awareness and indicate toxoplasmosis testing in countries where screening is not recommended [29].

Concerning the prognosis of these children, it is usually good, with no significant differences in development at 3-4 years compared to the general population [30]. Isolated intracranial calcifications are not associated with mental disability. However, they are incriminated as a risk factor for retinochoroiditis during childhood [31,32]. Isolated extracranial signs are not associated with a poor outcome [29]. In many cases, the classical toxoplasmosis triad of Sabin (hydrocephalus, intracranial calcification, chorioretinitis) is absent in neonates, although neural or eye lesions can develop later in life when the disease remains undiagnosed and untreated [33].

Screening Methods and Diagnostic Approach

In some parts of the world, such as the UK, USA, Canada, and some parts of Europe, Toxoplasma gondii screening is not recommended due to the low incidence of the disease and the high screening costs [34,35,36]. On the other hand, there are some European countries where Toxoplasma screening is included in prenatal routine tests and is recommended monthly, once every two or three months [4,37]. Due to more virulent strains and population risk factors in South America, Brazil is also encouraged by the authorities to include Toxoplasma screening of pregnant women in medical practice [38].

Screening for toxoplasmosis in pregnancy is based on serological testing since the infection is asymptomatic in the majority of cases [25]. The methods frequently used involve measuring IgG and IgM antibodies. IgM titers rise after 1 week of acquiring the infection and can remain at high levels for months and even years after the infection. Consequently, IgM titers cannot be used to diagnose a recent infection [1]. IgG titers reach a maximum level 6 to 8 weeks after infection, and antibody levels decrease over the next two years but remain positive [39]. The presence of IgG antibodies indicates that an infection has occurred, but this test does not provide any information about the moment when the mother acquired the parasite. In some clinical scenarios, additional tests might be needed, such as IgA and IgE antibodies. IgA antibodies appear faster than IgM antibodies and can last a few months. Due to the short duration of IgE antibodies, they could be more helpful in identifying a recent infection [1].

After serological testing, we can have four clinical scenarios:

• Nonreactive IgG and nonreactive IgM: The patients never had the infection. In this situation, the patient must be informed about the general prevention methods, and serological screening must be continued [12].
• Nonreactive IgG and reactive IgM: An acute infection is suggested. This situation requires the repetition of antibody testing after 2-3 weeks. The appearance of IgG antibodies confirms the diagnosis. In the persistent absence of IgG antibodies, high levels of IgM could signify a false-positive IgM result, which can occur in the context of antinuclear antibodies, rheumatoid factor, and nonspecific binding in vitro. This situation requires supplementary tests such as ELISA or Immunosorbent Agglutination Assay (ISAGA) [40].
• Reactive IgG and reactive IgM: An acute infection is suggested. This situation requires repeating antibody testing and IgG avidity testing after 2-3 weeks [25].
• Reactive IgG and nonreactive IgM: The infection occurred before pregnancy [12].

The IgG avidity test is used to differentiate between an acute or chronic infection [41]. During a Toxoplasma gondii infection, the avidity of parasitic antigens to different antibodies varies. For example, in a recent infection, the avidity is low; in an old infection, the avidity is high [25]. This test has limitations since low avidity can persist for several months after the infection. Consequently, while a high avidity result suggests an old infection, a low or intermediate avidity result cannot be used to diagnose a recent condition [25,42,43]. The long persistence of low avidity after infection can have several explanations. It has been suggested that antiparasitic treatment reduces the parasitic load and postpones the increase in the avidity index [44,45]. Significant inter-individual differences have been noted, and in some cases, a low avidity index could not be explained [44,45].

A high avidity result rules out a recent infection with a positive predictive value of almost 100% [46,47,48,49,50]. Depending on the laboratory, a past infection is defined as an infection older than 3, 4, or 5 months [25]. On the other hand, a low avidity result cannot be used to diagnose a recent infection, with the positive predictive value varying between 61.1% to 69.9% depending on the test used [46,49,50,51,52,53]. However, some authors suggest that a very low avidity result could strongly suggest a recent infection. For example, results lower than 15% with the Elecsys assay or lower than 17% with the Architect assay could make a reliable diagnosis for infections of less than 3 or 2 months, respectively [46,49]. Repeated serology testing after 2-3 weeks following a very low avidity result is obtained is required until additional studies are ruled out in order to confirm a recent infection [25].

A high avidity result in early pregnancy is indicative of an old infection. In this situation, Toxoplasma gondii screening can be discontinued if the antibody titers remain stable upon a second determination 3 weeks later. Special consideration must be given to a high avidity result at the end of the first trimester, as there can be a scarce situation of an infection acquired periconceptionally, and the result must be interpreted with caution [54,55]. Avidity testing results in the second and third trimesters are more complex than in the first. A high avidity index determined four months after conception is unreliable in diagnosing a past infection. In this situation, supplementary tests are needed to confirm a recent infection [12]. It has been suggested that the presence of IgA antibodies in combination with IgG and IgM antibodies is more suggestive of a recent infection than the presence of IgG and IgM antibodies in the absence of IgA antibodies.

Moreover, it was shown that the IgA titer decreases as the IgG avidity index increases [56]. This category of patients could receive amniocentesis, ultrasound follow-up, or medical treatment based on individual assessment [12]. In the case of an intermediate avidity index, the management becomes more difficult. When this type of result is obtained in the first trimester, it might be helpful to repeat the test using another testing method available on the market, hoping that it will yield a high avidity result. When the second determination of antibodies, conducted two to three weeks after the initial decision, shows stable titers, an intermediate avidity result is interpreted as a past infection. Otherwise, a recent infection cannot be excluded, and the management requires antiparasitic treatment and fetal testing [50,57,58]. In the case of a low avidity result, especially if it is a shallow result, a recent infection cannot be excluded and requires immediate action, including medical treatment and fetal testing [46,59].

When maternal infection is confirmed, fetal testing is required and can be performed through PCR from amniotic fluid or parasite isolation from the amniotic fluid [60]. Amniocentesis should not be completed before 18 weeks and within 4 weeks from the presumed Toxoplasma gondii infection. The probability of false negative results increases if amniocentesis is performed before this period due to the possibility of postponed passage of Toxoplasma gondii through the placenta [61]. A positive impact from amniocentesis alone is not an indication of the termination of pregnancy [12].

Management Strategies

At the moment, the most common therapeutic regimens include spiramycin, sulfadiazine with pyrimethamine, and sulfamethoxazole with trimethoprim. Alternative regimens include epiroprim and atovaquone [62]. The French Multidisciplinary Working Group recommends two types of medical treatment protocols. Type 1 protocol includes spiramycin 1 g three times per day [12]. Due to the fact that spiramycin accumulates at the placental level, it was suggested that if initiated early enough, it can reduce the chances of vertical transmission [63]. On the other hand, it has no effect on established parasitosis [64]. Type 2 protocol includes a combination of 50 mg of pyrimethamine once a day, 500 mg of sulfadiazine three times per day, and 25 mg of folinic acid twice per week. Alternatively, a combination of 2 capsules of Fandasir (500 mg of sulfadoxine and 25 mg of pyrimethamine) per week and 25 mg of folinic acid twice per week can be used [12].

In the first trimester, management requires spiramycin treatment as soon as possible. In the absence of amniocentesis, spiramycin must be continued until birth. Amniocentesis can be performed at 18 weeks, and a positive result requires switching to protocol 2, while a negative result requires the continuation of spiramycin In the second trimester, protocol 1 or 2 must be started immediately. After performing amniocentesis, a positive result requires protocol 2, and a negative effect requires protocol one until delivery. "In the third trimester, protocol two must be started as soon as possible and must be continued until birth in the absence of amniocentesis. If amniocentesis is performed, a negative result requires switching to spiramycin, and a positive impact implies the continuation of protocol two until delivery [12].

Prenatal monitoring must include monthly ultrasound follow-ups, with the frequency increasing to twice monthly in case of a positive result from amniocentesis. If there are challenges in interpreting ultrasound images, fetal MRI can be performed, although it is not used routinely [12].

Regarding obstetrical management, termination of pregnancy might be indicated in case of severe ultrasound lesions such as hydrocephaly. Intracranial isolated lesions are not associated with a negative outcome, and they don’t represent an indication for termination of pregnancy [31]. Deciding on immediate cesarean section or induction of birth is not justified since it is not proven that this management would reduce transplacental transmission of the parasite [65].

Prevention Strategies

There are several general rules that a pregnant woman can follow in order to protect herself from a Toxoplasma gondii infection. Drinking unfiltered water should be avoided as well as eating raw seafood [16]. Vegetables and fruits must be washed thoroughly. Activities such as gardening, where contact with contaminated soil is possible, require responsible hand washing to avoid accidental ingestion of soil [66,67,68]. Improperly prepared or raw meat is often a source of infection. Consequently, meat should be cooked at 66°C or higher. It has also been suggested that freezing the meat is the best way to eliminate the parasite, so the meat should be frozen at -12°C or lower for a minimum of 24 hours. Knives, cutting boards, counters, and the sink should be cleaned thoroughly after food preparation. When handling uncooked meat, pregnant women should avoid contact with mucous membranes. Pregnant individuals should avoid tasting meat while cooking [66,67,68,69,70,71]. Meat from animals raised in indoor conditions has a lower chance of contamination than meat from outdoor animals [66]. There is weak evidence suggesting that smoked or pickled meat might be unsafe. The risk increases in cases of specific local production methods involving the use of more than one type of meat, with insufficient drying and curing processes [66,72,73]. Pregnant women who own a cat are at low risk of toxoplasmosis since cats excrete infectious oocysts only for three weeks in their life. Also, fresh feces are not contagious, although it might be a good idea to ask others to clean the litter box [66,67,68]. For non-pregnant patients who have acquired toxoplasmosis, it is recommended to wait six months before planning a pregnancy. However, in an immunocompetent person, the chance of vertical transmission in case of infection one to three months before pregnancy is very low [74,75].

Without optimal medical treatment, vaccines against Toxoplasma gondii represent a crucial future perspective. Several vaccine types have been studied, including DNA, inactivated, and live attenuated vaccines [76]. At present, there is a single vaccine named Toxovax, which is used for breeding ewes at risk. Live attenuated vaccines represent a promising method for Toxoplasma gondii prevention in the future [77].

Conclusion

The complex life cycle of Toxoplasma gondii makes it one of the most well-adapted pathogens for parasitic survival. Its numerous transmission methods and multiple hosts that can be infected contribute to the global spread of this pathogen. Although the infection is generally asymptomatic, it can have disabling consequences for the developing fetus. Despite significant technological progress in recent years, diagnosing the disorder during pregnancy remains challenging in certain situations, and test results are difficult to interpret. A particular problem that draws attention is the rare reactivation of latent infection or reinfection with a different strain. In the case of diagnosing an old infection (reactive IgG, non-reactive IgM), recommendations involve discontinuing screening. Therefore, these rare situations mentioned above would go unnoticed. Literature suggests that an increase in IgG antibodies with two determinations performed three weeks apart using the same technique and at the same laboratory could indicate a recent infection. Hence, our approach involves continuing screening for patients with old infections by measuring IgG antibodies. Although it is a rare condition, obstetricians should be aware of toxoplasmosis with severe complications and, therefore, instruct patients on the general protective measures to be taken. Although still preclinical, vaccines against Toxoplasma gondii represent a promising future perspective.