Background
==========

Zika virus, an arbovirus belonging to the *Flaviviridae* family was first described in 1952 in the Zika Forest of Uganda \[[@b1-amjcaserep-20-723]\]. Infection with Zika virus is responsible for a mild exanthematic disease and most of the cases (about 80%) are asymptomatic. In 2007, a Zika virus outbreak in French Polynesia was associated with neurologic complications, especially Guillain-Barré syndrome \[[@b2-amjcaserep-20-723]\]. In 2015, the first cases of Zika virus were reported in Brazil. An unexpected rise in congenital microcephaly was noticed, and a link between these 2 occurrences was proposed \[[@b3-amjcaserep-20-723],[@b4-amjcaserep-20-723]\]. This association was subsequently confirmed, and the congenital Zika syndrome was described \[[@b5-amjcaserep-20-723]\]. Viral transmission occurred primarily through mosquito bites, although sexual passage through semen where the virus can persist for up to six months, vertical transmission through the placenta, and by transfusion of blood components was also confirmed \[[@b6-amjcaserep-20-723]--[@b8-amjcaserep-20-723]\]. Zika virus was also been reported in breast milk, although this mode of transmission to the newborn is still controversial \[[@b9-amjcaserep-20-723],[@b10-amjcaserep-20-723]\].

We now report a case of secondary microcephaly in a baby whose mother was positive for Zika virus only in her breast milk and who delivered a prior baby with severe microcephaly.

Case Report
===========

A 30-year-old female became pregnant again 19 months after delivery of a baby with severe microcephaly. The first baby, a girl, was positive for IgG antibody to Zika virus and to chikungunya virus although the mother was antibody-negative to both. The second pregnancy was uncomplicated with 5 normal gestational ultrasounds. A female baby was delivered by cesarean section at a gestational age by ultrasound of 39 weeks and by last menstrual period of 39 weeks and 6 days. Birth weight was 2650 g (z score=−1288); length 47 cm (z score=−1.2831) and head circumference 33 cm (z score=−0.379). All these parameters were considered normal by INTERGROWTH-21^st^ standards. Both mother and baby were negative by STORCH (acronym for disease group comprising syphilis, toxoplasmosis, other infections, rubella, cytomegalovirus infection, and herpes simplex). Blood, saliva, urine, and colostrum were collected for laboratory examinations. Analysis of samples by reverse transcriptase --polymerase chain reaction (RT-PCR) revealed the presence of Zika virus only in breast milk. Both mother and baby were Zika virus-negative by serology and RT-PCR.

The infant was exclusively breastfed since delivery. A re-evaluation at 23 days after birth revealed that the baby's head circumference remained at 33 cm (z score=−2.330). This lack of head growth was consistent with a diagnosis of secondary microcephaly \[[@b11-amjcaserep-20-723]\] as defined by the World Health Organization.

Other possible causes of microcephaly such as abnormal ventricles, craniosynostosis, congenital hypothyroidism, were excluded.

A final evaluation, at 7 months 29 days showed a weight of 8.1 kg; length 68.5 cm and head circumference 40.5 cm, with z scores of 0.20; −0.02; −2.10, respectively. This continued reduced head growth confirmed the diagnosis of secondary microcephaly. Additional neurological examinations were all normal.

Discussion
==========

After the 2015 Zika virus outbreak in Brazil, many cases associating microcephaly with Zika virus infection were reported. Subsequent studies were conducted to define the mechanism(s) of mother-fetal transmission and identify additional neurological consequences for the fetus. Considering that Zika virus can be transmitted vertically or intrapartum/perinatally, it is difficult to define when transmission occurs.

Colt et al. reported the presence of Zika virus RNA in breast milk from 3 infected mothers. Two of the babies had evidence of Zika virus exposure by serology and/or by its transient detection in urine. All 3 babies remained healthy and asymptomatic \[[@b12-amjcaserep-20-723]\]. In the present case, while there was no molecular or immunological evidence of Zika virus in the newborn. Her development of secondary microcephaly only after delivery, the exclusion of other apparent causes, and the development of Zika virus-related severe microcephaly in a previous baby from the same mother strongly suggests that the baby became infected with Zika virus that affected brain development after ingestion of breast milk. Reasons for the mother's failure to respond immunologically to Zika virus are unknown. She was not immunosuppressed and had normal levels of antibodies to other infectious agents. The mother's possible exposure to other flaviviruses might have masked or diminished her immune response to Zika virus reinfection prior to her second pregnancy and a possible mechanism for intra-familial transmission of Zika virus might have been Zika virus persistent infection in her partner's semen. It remains to be determined if Zika virus was present in her milk as a free virus or was associated with macrophages, lymphocytes, or other cells. Unfortunately, in the present case we were unable to evaluate these possibilities.

Differences in the severity of microcephaly in the 2 babies might have been due to the simultaneous present of a second flavivirus, chikungunya virus, in the first but not the second neonate. Concurrent infection by more than one flavivirus has been shown to alter immune responses \[[@b13-amjcaserep-20-723]\].

Conclusions
===========

Detection of Zika virus exclusively in breast milk after a second delivery, coupled with the child being born healthy but developing microencephaly only 1 month after delivery which persisted to date, was consistent with the possible transmission of Zika virus to the baby in breast milk and the initiation of neurological damage only after birth. The World Health Organization continues to support breastfeeding, considering the benefits of this practice, even for those mothers with suspected or confirmed Zika virus infection \[[@b14-amjcaserep-20-723]\]. Further studies are needed to better define the dynamics of Zika virus transmission via breast milk and its potential harm to newborns.

Ethical approval
----------------

Our data were obtained under the Cohort Zika Jundiaí Consortium, already approved on ethical committee (CAAE 53248616200005412); Research Ethics Review Committee (WHO ERC), World Health Organization (ZIKV100/2018).

The review of the study findings and suggested manuscript modifications by Dr. Ariana M. Witkin are gratefully acknowledged.

**Conflict of interest**

The authors declare no conflict of interest.

[^1]: Authors' Contribution:

[^2]: Study Design

[^3]: Data Collection

[^4]: Statistical Analysis

[^5]: Data Interpretation

[^6]: Manuscript Preparation

[^7]: Literature Search

[^8]: Funds Collection

[^9]: **Conflict of interest:** None declared

[^10]: **Source of support:** This study was funded by the Foundation for Research Support of the State of São Paulo -- FAPESP (SDP \#grant 2016/08578-0 and DV \#grant 2017/22007-8); London School of Hygiene and Tropical Medicine (SDV \#grant PC0002/16); and MoH Brazil/RENE Zika (SDP \#grant 861306/2017 and 861307/2017)
