With news that the Zika virus is “spreading explosively” throughout the Americas, much of the medical community, including the World Health Organization, remains on high alert. Below, Peter C. Doherty, author of Pandemics: What Everyone Needs to Know, discusses the nature, characteristics, and scope of the Zika virus.
First isolated in Uganda in 1947, this normally mild, non-fatal mosquito-born flavivirus infection is characterized by transient fever, joint pain, and malaise. The current explosive Zika virus epidemic in the Americas is, however, causing great concern because of what looks to be a sudden, dramatic increase in the incidence of microcephaly (small brain/head size) in newborns. Flaviviruses are not known to cause this type of problem and the Zika link is yet to be definitively established, but indications are that the fetus is at greatest risk in the first trimester when, following a bite from a carrier Aedes spp mosquito, virus crosses the placenta to cause brain damage at this vulnerable stage of development. Early, high level viremia (virus in blood) may also explain the one known case of venereal transmission when an American traveller returning from Africa had unprotected sex with his wife. Clinically normal at the time, he soon developed symptoms, while she became ill some time later.
It’s generally thought that vertebrate hosts—in this case humans and some monkeys—have to circulate a lot of virus to infect a biting mosquito. In a healthy adult human, the developing adaptive or specific immune response will normally control the infection within seven to 14 days of initial exposure. Then, at least with other flaviviruses like West Nile virus, live Yellow Fever virus vaccine, the continued presence of neutralizing antibodies in the blood will protect in the very long term. One flavivirus, the hepatitis C virus, does cause continuing viremia, but this is a unique pathogen and, apart from some limited evidence of low level persistence for the related tick-borne encephalitis viruses, the general view is that this class of infections is completely cleared from the human body.
What we are seeing in the Americas is a classic “virgin soil” epidemic. Enormous numbers of people and mosquitoes are being infected, the virus is transmitting at a very high level, and there may be as many as 4×106 cases. Apart from affected neonates, all will likely recover, with increasing “background” immunity progressively limiting the number of new infections in subsequent years. The current molecular technology is such that making a protective vaccine should be technically straightforward, but the process of safety testing and evaluation could take several years.
All cases so far in the United States, Europe, and Australia have been in travelers returning from infected areas. Though the related West Nile virus (WNV) is now endemic in North America, the difference is that Zika virus—like Yellow Fever virus—is maintained in a mosquito/primate life cycle, while WNV is primarily an infection of birds, especially corvids (crows, magpies) which it kills in large numbers. WNV can, at times, also kill us.
In the meantime, though Zika infection may spread north, any impact in the United States is likely to be limited by mosquito control measures that are particularly well established in Southern communities. The big risk will be at times of extreme summer rainfall when spraying insecticide may be less effective. But, unlike the situation for WNV, we will not see outbreaks associated with drought, as occurred recently in Texas, where birds and mosquitoes come together at scarce water sources.
The long-term prospect with Zika virus is that we will live reasonably comfortably with it, especially if there is a vaccine to protect women of reproductive age. The principal decision for responsible authorities, like National Governments in endemic areas and the WHO, is whether there is a case for fast-tracking, then funding, a vaccine to protect all young women. For the present, pregnant women are advised not to travel to these countries and, for those where this in not an issue, insect repellant also offers some protection against much nastier viruses like dengue.
Image Credit: “Equipe de combate ao mosquito Aedes aegypti vai ao Lago Sul” by Agência Brasília. CC BY 2.0 via Flickr.