Mosquito-borne pathogens such as West Nile virus can replicate in fetal tissue like Zika virus. | Yu-Chan Chen/ Flickr
Emerging viruses already circulating in the Western Hemisphere could infect fetal tissue and might have the capacity to cause birth defects, according to preclinical findings published January 31 in Science Translational Medicine.
Both West Nile virus, a mosquito-borne virus that is has been detected across the continental United States, and Powassan virus, a tick-transmitted disease, most commonly reported in the Great Lakes region, crossed the placenta and caused fetal death during infections in pregnant mice. The pathogens also replicated in samples of both maternal and fetal tissues obtained from healthy pregnant human donors in their second trimesters.
"Zika virus does not appear uniquely capable of replicating in the human placenta or causing congenital infection," said Jonathan Miner, a professor of microbiology at Washington University in St. Louis and the study's senior author.
Because both West Nile virus and Powassan virus are spread by insects, Americans can protect themselves by limiting their exposure to the viral vectors.
"The most important thing is avoiding mosquito bites and tick bites," said Miner.
Most individuals infected with West Nile virus experience only mild symptoms, but the virus does occasionally invade the nervous system to cause dangerous inflammation of the brain or its surrounding membranes. As of January 9, 2018, 47 states and the District of Columbia have reported cases of West Nile virus to the United States Centers for Disease Control and Prevention.
Powassan virus has a more limited range, although more and more deer have been testing positive for the pathogen in recent years. Roughly 10% of Powassan virus infections are fatal, and almost half of survivors develop life-long neurological symptoms.
Ticks are also a vector of Zika-family viruses. | U.S. Department of Agriculture/ Flickr
Neither pathogen has previously been definitively implicated as a potential cause for birth defects.
The ability of Zika virus to induce birth defects did not come to light until the massive 2015 South American epidemic, which sickened more than 1.5 million people. Some studies suggest that the virus recently acquired the ability to infect the placenta and cause microcephaly, yet the precise events that led to the pathogen becoming a global public health concern have been difficult to pin down.
"It might be that Zika virus has always caused birth defects, but it just hadn't been noticed until 2015," said Miner. "The outbreak in the Americas was so widespread that it generated a large number of simultaneous cases, which probably made it easier for epidemiologists to identify the association between birth defects and Zika virus."
Speculating that other pathogens might have similar potential to cause harm, Derek Platt and colleagues in Miner's lab at Washington University in St. Louis tested whether several members of the flavivirus family — a group of insect-transmitted viruses that includes Zika virus — could infect placentas and fetuses in pregnant mice.
"The strength of our study is in that these viruses were analyzed in parallel," said Miner.
Of the four viruses the scientists tested, only West Nile virus and Powassan virus established lethal congenital infections. The other two pathogens in the panel, Chikungunya virus and Mayaro virus, replicated inside fetuses without killing the developing mice.
"We still need additional studies to more clearly define whether these Zika virus-related flaviviruses have similar effects in humans," said Miner
Miner's lab is currently working to understand how the fetal immune system fights infections, which he says might eventually lead to novel therapies to prevent viruses from crossing the placenta and causing birth defects.