By Aileen Marshall
What should you know about the Zika virus? It’s been around for over 50 years, but it’s only recently that it’s spread has increased around the world, especially in South America. The Zika virus is spread by mosquitoes, but for most people it only causes a mild infection. However, an infection in pregnant women can cause a birth defect called microcephaly, in which the skull and brain don’t fully develop. At this point, there’s limited diagnostic tests and no cure, so labs are scrambling to develop these products.
The Zika virus was discovered in 1947 in the Zika Forest of Uganda. It was isolated from the blood of a rhesus monkey there, as part of a Yellow Fever monitoring program. It was then found in an Aedes africanus mosquito from the same area, a year later. The first human infected was found in 1952 in Uganda and Tanzania. A study in India that year found a significant number of Indians who had antibodies to Zika, an indication that it had been prevalent in that population. There were sporadic outbreaks of Zika over the later years in equatorial areas of Africa and Asia. Then in 2007, an outbreak of what initially appeared to be dengue or chikungunya occurred in the French Polynesian island of Yap. It was later confirmed to be Zika, the first outbreak outside of Africa or Asia. By 2013 it had spread to other South Pacific islands with some patients who also had neurological effects and there were some cases of microcephaly. In March of 2015, health officials in Brazil noted an increase in Zika-like symptoms and rash in the northeast part of the country. By that summer, there was a great increase in the number of children born with microcephaly, especially in that same area. By later that year, there were confirmed cases of Zika infections in other South and Central American countries, and the Caribbean. On February 1 of this year, the World Health Organization declared it a public health emergency of international concern.
The Zika virus belongs to the same family, Flaviviridae, as dengue, chikungunya, yellow fever and West Nile viruses, which is why the antibodies often cross-react in diagnostic tests. It has a single strand positive sense RNA genome, which means it replicates in one step. The strain in this recent outbreak has been sequenced and it has found to be the same strain from the South Pacific outbreak.
It is transmitted by a couple of species of mosquitoes under the Aedes genus of mosquitoes. These tend to be relatively aggressive biters who bite during the day and like to stay indoors. If a mosquito bites someone with an active Zika infection, the insect can then pass it on to the next person it bites. Evidence of the virus has been found in blood, semen, saliva and urine. There have been some cases of person-to-person transmission by blood and semen. It is not known whether it can be transmitted by a person’s saliva, or kissing. The mechanism of maternal to fetal transmission is also not known. According to Claudia Dos Santos of the Instituto Carlos Chagas/Fiocruz in Brazil, it is found in Hofbauer cells, a type of white blood cell found in the placenta. “It’s possible that Zika virus can cross the placenta and infect the brains of fetuses” says Melody Li, of our own Rice lab.