Characterization of Placental Infection by Zika Virus in Humans: A Review of the Literature

Objective  The aim of the current review is to present a systematic evaluation of reported human placental findings in cases of zika virus (ZIKV) infection. Data sources  We reviewed the EMBASE, PUBMED, and SCIELO databases until June 2019, without language restrictions. Selection of studies  The search terms placenta AND zika virus were used. The inclusion criteria of the studies were studies that reported placental findings in humans. Experimental studies, reviews, notes or editorials were excluded. A total of 436 studies were retrieved; after duplicate exclusion, 243 articles had their titles screened, and 128 had their abstract read; of those, 32 were included in the final analysis (18 case reports, 10 case series, and 4 cohorts) Data collection  We collected data concerning the author, year of publication, study design, number of participants, number of placental samples, onset of symptoms, perinatal outcomes, and main findings on histological analysis. Data synthesis  The placental pathologic findings were described as mild and nonspecific, similar to those of other placental infections, including chronic placentitis, chronic villitis, increased Hofbauer cells, irregular fibrin deposits, increased mononuclear cells in the villus stroma, villous immaturity, edema, hypervascularization, stromal fibrosis, calcification, and focal necrosis of syncytiotrophoblasts. Conclusion  Zika infection presents unspecific placental findings, similar to other infections in the toxoplasmosis, other agents, rubella, cytomegalovirus, and herpes (TORCH)group. Characterizing and standardizing placental findings after zika virus infection is key to understanding the mechanisms of congenital diseases.


Introduction
Zika virus (ZIKV) is a flavivirus much similar to other arboviruses of relevance, such as dengue, West Nile, yellow fever, and Japanese encephalitis viruses. It is transmitted mostly by Aedes aegypti mosquitoes, and was first recognized in humans in Uganda in 1952, with two main previous outbreaks, in Yap, Micronesia, in 2007 and in the French Polynesia, in 2013. 1,2 The ZIKV may also be transmitted to humans according to other routes non-vector reliant, such as blood transfusion, sexual transmission, or maternal-fetal transmission. 3 Brazil had the most significant and recent outbreak of ZIKV, with major relevance not only due to the total number of cases reported (over 200 thousand), but also because of its severity and association to fetal malformations. 4 The fetal consequences were further defined as Congenital Zika Syndrome (CZS), which includes a spectrum of congenital defects (not only microcephaly). 5 These conditions are similar of those caused by "TORCH" pathogens. The TORCH acronym stands for: Toxoplasma gondii infection, Other (Treponema pallidum, Listeria monocytogenes, parvovirus B-19, and human immunodeficiency virus (HIV), amongst others), Rubella, Cytomegalovirus (CMV), and Herpesviruses (HSV) 1 and 2. After the Brazilian zika outbreak, some authors have suggested the inclusion of ZIKV among the group "others" in the acronym or even a more direct inclusion such as TORCHZ. 6 The precise mechanisms of placental infection and maternal-fetal transmission during pregnancy, not only in ZIKV but in the other TORCH infections as well, remains unclear. Described routes include: ascending infection, direct crossing or infection of syncytiotrophoblasts (SYN), infection of extravil-lous trophoblasts through maternal microvasculature, and trafficking of and/or signaling from maternal immune cells. 6 The SYN layer is the outer layer of the placental villus, of multinucleated, terminally-differentiated cells in direct contact with the maternal blood. The extravillous trophoblasts (EVTs) anchor cells to the uterine wall. Both of these are differentiated from the cytotrophoblast layer (CTB) throughout pregnancy. 7 Hofbauer cells (HCs) are placental macrophages of fetal origin, existent in the chorionic villus throughout the entire gestation. 8 Hofbauer cells have been associated to ZIKV infection, with description of hyperplasia of such cells in the placenta. 9 The study of placentas of suspected cases of ZIKV is recommended, as part of optimum healthcare for these women and newborn. Histopathologic examination of the placenta, with ZIKV ribonucleic acid (RNA) testing (via reverse transcription-polymerase chain reaction [rRT-PCR]), may confirm fetal infection, since viral detection in the serum is time-sensitive and the window for ZIKV detection might be missed. 4 The aim of the present review is to present an integrative evaluation of reported placental findings in human studies on ZIKV infection during pregnancy.

Palavras-chave
Hofbauer ► TORCH used the following Medical Subject Heading (MeSH) search terms: placenta AND zika virus. The inclusion criterion of the studies was reporting of placental findings in humans, while studies that did not report placental findings, experimental studies, reviews, notes or editorials were excluded. The current study followed all recommendations of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. In the first step of this review, two independent reviewers performed a title screening of all studies identified in the database search; in the second step, the remaining studies were evaluated considering their abstracts by two independent reviewers and further full text, for inclusion. Discordances between the primary reviewers were solved by a third senior reviewer. After the final selection of the studies that were included in this review, each study was evaluated, and the following charac-teristics for each study were obtained: author, year of publication, study design, number of participants, number of placental samples, onset of ZKV infection symptoms, perinatal outcomes, and main findings on histological analysis. Those results were stored in a Microsoft Excel spreadsheet (Microsoft Corp., Redmond, WA, USA) and further organized in a table with detailed description of data.

Results
A total of 436 articles were retrieved in the databases search (PubMed ¼ 164; EMBASE ¼ 270 and SCIELO ¼ 2); of those, 193 were duplicated articles, so 243 had their title screened. One hundred and fifteen articles were excluded after title screening, and the remaining 128 studies had their abstract read. After that, 96 studies were excluded (27 reviews, 45    14 Case report 1/1 1 -----1 -High viral load found in placenta, fetal membranes and umbilical cord by RT-PCR. ZIKV RNA in Amniotic fluid, fetal brain, liver, lung and spleen. Martines et al (2016) 11 Case report 4 / 4 2 -2 -2 --2 Placenta with fibrosis, calcification, and deposits of fibrin. Material consistent with third trimester gestation ¼ RT-PCR negative. Two abortions, one had dense and heterogeneous chorionic villitis with calcification, sclerosis, edema, increased perivillous fibrin deposition, and patchy lympho histiocytic intervillositis and the other had minute fragments of placental. Both placental RT-PCR positive for ZIKV.

Discussion
The current review evaluated studies that reported placental findings among women with ZIKV infection during pregnancy. Placental pathological findings are mostly mild and nonspecific, suggesting an important role for HCs within the villi. These findings are consistent with the effects of other viruses in the placenta, such as human CMV, 43,44 leading to proinflammatory responses, impaired remodeling of spiral arteries in the decidua, and cell death; ultimately compromising suitable utero-placental blood-flow. 45 The amount of placental inflammation is associated to the severity of fetal findings. 46 The present review points toward an important role of HCs, which are of fetal origin, monocytic derived, and part of the normal component of the stroma of the chorionic villi, shown to appear very early in gestation. Hofbauer cells have been described as alternatively activated macrophages 9,47 responsible for the phagocytosis of fluids and apoptotic materials, antigen presentation, and perhaps an angiogenic role in early placental vasculogenesis, placental water balance, and endocrine function. Hyperplasia of the HCs has been previously reported in other maternal-fetal infections, such as those in the TORCH group and its proliferation within the chorionic villous stroma is also confirmed. 9,48,49 The placenta is an important virus reservoir, that can confirm the diagnosis when infection was not confirmed during the acute phase, due to limitations on adequate and timely sample collection, which is a serious concern in ZIKV infection. 4 There is a worldwide variation regarding antenatal screening availability and follow-up for women with fetal congenital abnormalities. In Latin America, many countries, including Brazil, consider abortion or termination of pregnancy due to fetal congenital abnormalities illegal or highly restricted. 50 Both factors help explain the sparsity of tissue samples from earlier gestational ages reported in the literature. A possible bias from our results is that the placental tissues evaluated were from late-pregnancy infection or infections in apparently unaffected neonates. 50 Another important point our review highlights is that there is no standardized description of placental findings related to ZIKV. A common global pattern of description of those findings would be helpful to gather results from different groups, settings and countries, allowing researchers to empower results and provide more robust conclusions. It would also help clinicians to justify the importance of histological analysis of placental tissue in suspect or confirmed cases of ZIKV during pregnancy.

Conclusion
Characterizing placental infection is key for understanding the severity of the disease and fetal malformations. The ZIKV presents similar features to other TORCH infections, with a significant role of HCs. Missed opportunities of such evaluation are evident when considering the limited number of studies included in the present review. However, it is very important to address the need for adequate sampling and evaluation of placental findings during an outbreak, among suspected and confirmed cases of ZIKV infection. For that, specific evaluation on different placental layers, combined with studies on RNA detection and standardization of results presentation is fundamental.

Contributions
Venceslau E. M. and Guida J. P. S. contributed by collecting data and writing the first draft; Amaral E. and Modena J. L. P. reviewed the first draft; and Costa M. L. had the original idea for the present study and coordinated it, supervised the data collection, and reviewed the first draft.

Conflicts of Interest
The authors have no conflicts of interest to declare.