Three reports have been published that together make a compelling case that Zika virus is causing microcephaly in Brazil.
An epidemic of Zika virus infection began in Brazil in April 2015, and by the end of the year the virus had spread through 19 states, many in the northeastern part of the country. Six months after the start of the outbreak, there was a surge in the number of infants born with microcephaly. It was not known if most of the mothers had been infected with Zika virus, as results of serological tests, virus isolation, or PCR were not available.
An initial report of 35 Brazilian infants with microcephaly born to women who either resided in or traveled to areas where Zika virus is circulating revealed that 74% of mothers had a rash (one sign of Zika virus infection) in the first or second trimester. At the time of this study no laboratory confirmation of Zika infection was available, but the infants did not have other infections associated with birth defects, including syphilis, toxoplasmosis, rubella, cytomegalovirus or herpes simplex virus.
Yesterday the CDC reported on the analysis of tissues from two infants with microcephaly who died within 20 hours of birth, and two miscarriages, all from the state of Rio Grande do Norte in Brazil. The mothers all had rashes typical of Zika virus infection in the first trimester of pregnancy, but were not tested for infection.
All four specimens were positive for Zika virus RNA by polymerase chain reaction (PCR) done with primers from two different regions of the viral RNA. Staining of tissues with anti-viral antibodies revealed the presence of viral antigens in two of the four samples, in the brain of one newborn and in the placenta from one of the miscarriages.
A second report from the University of Sao Paulo documents ocular abnormalities in Brazilian infants (from the state of Bahia) with microcephaly and presumed Zika virus infection. The mothers of 23 of 29 infants (79.3%) with microcephaly reported signs of Zika virus infection (rash, fever, joint pain, headache, itch, malaise). Of these, 18 (78.3%) had symptoms during the first trimester of pregnancy, 4 (17.4%) during the second trimester, and 1 (4.3%) during the third trimester.
No laboratory results were available to confirm Zika virus infections, but toxoplasmosis, rubella, cytomegalovirus, herpes simplex virus, syphilis, and HIV were ruled out.
Abnormalities of the eye were found in 10 of 29 (34.5%) of infants with microcephaly. These included focal pigment mottling, chorioretinal atrophy, optic nerve abnormalities, displacement of the lens, or a hole in the iris.
These observations suggest that Zika virus infection may also cause lesions of the eye, although confirmation of infection needs to be done to prove causation. This uncertainty is reflected in the title of the article: “Ocular findings in infants with microcephaly associated with presumed Zika virus congenital infection in Salvador, Brazil” (italics mine).
The final paper is, in my opinion, the blockbuster. In this single case report, a 25 year old European woman working in Natal, Brazil, became pregnant in February 2015. In the 13th week of gestation she had fever, muscle and eye pain, and rash. Ultrasound in Slovenia at 14 and 20 weeks revealed a normal fetus.
At 28 weeks of gestation fetal abnormalities were detected, including microcephaly, and the pregnancy was aborted. Autopsy revealed severe brain defects, and 42 to 54 nm virus particles were detected in the brain by electron microscopy.
Infection with a variety of microbes was ruled out, but Zika virus RNA was subsequently detected in brain tissue by PCR.
Here is the clincher – the entire Zika virus genome was identified in brain tissue by next-generation sequencing! Analysis of the sequence revealed 99.7% nucleotide identity with a Zika virus strain isolated from a patient from French Polynesia in 2013, and a strain from Sao Paulo from 2015. These findings agree with the hypothesis that the current Brazilian outbreak was triggered by a virus from Asia.
Up to now there have been few data that strongly link Zika virus infection to congenital birth defects. Of these three new studies, the recovery of a full length Zika virus genome from an infant with microcephaly is the most convincing. Given the rapidity by which new data are emerging, it seems likely that additional evidence demonstrating that Zika virus can cause microcephaly will soon be forthcoming.
I’m amazed that a flavivirus can cause birth defects – when no flavivirus has done so before*. This is a virus spread by mosquitoes, and to which most of the world is not immune. The Zika virus outbreak will surely test our ability to respond rapidly with substantial mosquito control, diagnostics, antivirals, and a vaccine.
Update 2/11/16: A second paper has been published documenting ocular abnormalities in ten infants born to mothers in Brazil who had symptoms consistent with Zika virus infection.
Update 2/12/16: *Japanese encephalitis virus and West Nile virus have been shown to cross the placenta and infect the fetus. Such events must be rare because a larger association with birth defects has not been reported.