Suzanne Leech, PhD
Herpes Simplex Virus
During pregnancy, numerous pathogens present significant risks to the fetus, the number and severity of which vary depending on geographical location. In middle to lower income countries, 50 percent of the deaths of children under one year old are due to infection1.
The core set of pathogens known as TORCH present the most significant risks to the unborn child. TORCH comprises Toxoplasma gondii, others (including syphilis, Treponema pallidum, listeria, varicella, HIV, and parvovirus B19), rubella virus, cytomegalovirus (CMV), and herpes simplex virus (HSV). These pathogens can cause miscarriage, premature labor, retarded growth, and various other developmental abnormalities. The risks are amplified by the fact that pregnant mothers and their unborn children have weakened and undeveloped immune systems, respectively, increasing their vulnerability to infection. To minimize harm to the fetus, infections should be diagnosed as early as possible, preferably in the first trimester.
Broad-spectrum TORCH screening
TORCH pathogens include a variety of viruses, bacteria, and protozoal parasites, and an “others” category comprising a set of ill-defined pathogens that tend to vary over time and between countries. Many of the pathogens present as only mild symptoms in the mother; therefore, awareness of the risks and the ability to recognize the symptoms are important aspects of prenatal care. Most TORCH infections can be screened by detecting pathogen-specific antibodies, typically in the maternal blood. In addition, there are research projects developing simple microfluidic tests that cover the TRCH pathogens in one serological test.2 However, these do not cover the myriad of pathogens included in the “others” category. The value of general TORCH screening has been called into question by physicians who say that only individual tests need be applied when there is reason to suspect infection.3
In some cases, ultrasound investigation will reveal signs of a prenatal infection, such as plural effusions. For example, many of the fetal abnormalities associated with syphilis can be identified with high-resolution ultrasound. However, any abnormalities must be
verified as being due to infection using more definitive serological or amniotic fluid tests (amniocentesis).
TORCH antibody serological assays are available for the detection of pathogen IgG and IgM antibodies for Toxoplasma gondii, rubella, CMV, and HSV, which can be used in a variety of assay formats including ELISA, rapid assays, and bead-based assays. Reactivity for the IgM, but not IgG, usually indicates a current infection, while IgG without IgM suggests a past infection. However, for some pathogens, including toxoplasmosis and CMV, IgG can indicate a primary infection and the use of IgM is not considered reliable. Paired-serological tests are probably the most useful technique for analyzing the mother’s blood; a blood sample is taken during symptoms of the illness and the test is repeated four weeks later to determine any changes in the IgM/IgG antibody titers and avidities, which are used for diagnosis. In rapid IgM capture assays, which facilitate sensitive diagnosis of early-stage infections, an anti-IgM antibody fixed to a solid phase is used to capture pathogen IgM in patient samples. Immunofluorescent antibody assays (IFA) can also be used to definitively diagnose certain TORCH agents and may be necessary in some cases to distinguish between strains, e.g., HSV-1 and HSV-2. Immunological serological tests can provide information on the mother’s likelihood of infection; however, the pathogens may not cross to the fetus. To confirm congenital infection, molecular analysis of amniotic fluid is usually recommended.
Real time PCR kits are available for the diagnosis of most TORCH pathogens, and PCR analysis of amniotic fluid samples is considered the gold standard for many congenital infections. A meta-review concluded that PCR for toxoplasmosis performed on amniotic fluid sampled up to five weeks after maternal diagnosis has a sensitivity of 87 percent and specificity of 99 percent.4 However, there was considerable heterogeneity between studies because of the lack of test standardization. The reviewers expressed hope that use of quantitative PCR could lead to better test standardization. In addition, false negatives may occur because of the small amount of pathogen DNA in the amniotic fluid, particularly in early stages of infection.5 Current research has shown that multiplex nested PCR could provide the simultaneous and highly sensitive testing of seven pathogens.1 The nested PCR technique can be used to amplify very low copy number sequences, decreasing the incidence of false negative results.
Examples of prenatal infection diagnosis
The protozoan Toxoplasma gondii is a prolific parasite of humans, infecting approximately 30 percent of the global population. Severe infection can lead to fetal death and miscarriage, pre-term birth, and neurological or ocular abnormalities. In the US, pregnant women are only tested for toxoplasmosis if there are abnormal signs on ultrasound; in suspected cases, the maternal blood is checked using IgM and IgG serology, and congenital infection is confirmed with PCR analysis of amniotic fluid.
An HIV-positive pregnant woman will pass the virus to her unborn child in around one out of three cases if treatment is not provided.6 Along with hepatitis B and syphilis, the HIV test is part of the standard prenatal screening recommended by the CDC. The American College of Obstetricians and Gynecologists recommends antibody–antigen combination screening tests for HIV as early as possible in pregnancy.7 Early use of combined anti-retroviral treatment can reduce the risk of vertical transmission to one to two percent.
Commonly known as chickenpox or shingles virus, the varicella-zoster virus (VZV) can cause a particularly severe form of pneumonia in pregnant women. Furthermore, VZV infection of the fetus can result in serious abnormalities and even mortality. Because of extensive vaccination programs, the virus is not a common problem in places like Europe and the US; however, the vaccination rates are much lower in less developed countries. The virus may be diagnosed using PCR for viral DNA in amniotic fluid, at least one month after maternal infection to minimize false negatives. However, there is evidence that viral DNA in amniotic fluid, without infectious virus, leads to false positive results. In addition, serology has been shown to have poor sensitivity and specificity, even for umbilical cord and fetal blood samples. Therefore, there is no gold standard test for this pathogen,8 although molecular methods are considered to be the most reliable of the available tests.
Advances in vaccination, prenatal care, clinical hygiene, anti-infective and anti-viral drugs, prophylaxis, and birthing methods have greatly decreased the risk and severity of many pathogens in utero. However, obstetricians are still commonly challenged with unknown or potentially serious cases of prenatal infection. In most cases, early diagnosis is the key to minimizing the risk to the unborn child. A combination of ultrasound examination, maternal blood serology, amniocentesis, and PCR analysis is usually the most effective strategy for the diagnosis of congenital infections.
1. Yamamoto, Lidia, et al. "Performance of a multiplex nested polymerase chain reaction in detecting 7 pathogens containing DNA in their genomes associated with congenital infections." Archives of Pathology & Laboratory Medicine (2020).
2. Qiu, Xianbo, et al. "A bead-based microfluidic system for joint detection in TORCH screening at point-of-care testing." Microsystem Technologies 24.4 (2018): 2007-2015.
3. Greenough, Anne. "The TORCH screen and intrauterine infections." Archives of Disease In Childhood Fetal and Neonatal Edition 70.3 (1994): F163.
4. de Oliveira Azevedo, Christianne Terra, et al. "Performance of polymerase chain reaction analysis of the amniotic fluid of pregnant women for diagnosis of congenital toxoplasmosis: a systematic review and meta-analysis." PLoS One 11.4 (2016).
5. Boudaouara, Yosr, et al. "Congenital toxoplasmosis in Tunisia: prenatal and neonatal diagnosis and postnatal follow-up of 35 cases." The American Journal of Tropical Medicine and Hygiene 98.6 (2018): 1722-1726.
6. Soilleux, Elizabeth J., and Nicholas Coleman. "Transplacental transmission of HIV: a potential role for HIV binding lectins." The International Journal of Biochemistry & Cell Biology 35.3 (2003): 283-287.
7. Laperche, S., M. Maniez-Montreuil, and A. M. Couroucé. "Screening tests combined with p24 antigen and anti-HIV antibodies in early detection of HIV-1." Transfusion Clinique et Biologique: Journal de la Societe Francaise de Transfusion Sanguine 7 (2000): 18s-24s.
8. Mandelbrot, Laurent. "Fetal varicella–diagnosis, management, and outcome." Prenatal Diagnosis 32.6 (2012): 511-518.