Zika virus: A new vector-borne threat in the Americas
Serap Aksoy, PhD.
Professor of Epidemiology and Public Health, Yale School of Public Health
Ebola virus infection commanded most of the infectious and tropical disease headlines of 2014 and 2015. Now a very different disease, Zika virus or Zika fever, has become a global news item. The World Health Organization (WHO) has declared Zika virus infection a global emergency and is estimating about 3-4 million cases by the end of 2016.
What is Zika infection: Zika virus infection is caused by a mosquito-transmitted flavivirus first isolated in Uganda in 1947. For many decades, the disease caused only occasional small outbreaks in Africa and in Southeast Asia, with only a handful of human cases. It is believed to have been introduced into Latin America in 2014, with speculation that the disease agent was brought from the French Polynesia into Brazil during the 2014 FIFA World Cup. Since May 2015 there has been hundreds of documented cases in Brazil and in neighboring countries in south and central America and the Caribbean with many imported cases now being reported in the USA.
How is Zika infection transmitted: Zika virus is transmitted to humans through the bite of an infected mosquito. The most important mosquito species involved in transmission is Aedes aegypti. However, another mosquito species, Ae. albopictus (also known as the Asian Tiger mosquito), has also been implicated in transmission of Zika in Africa and in laboratory investigations. Given that both species of Aedes mosquitoes are present in the Americas, both are important for this outbreak. Transmission of the virus through other modes is also being investigated. The presence of the virus has been detected in saliva and urine of patients, and in several cases a mode of sexual transmission has also been reported.
What are Zika infection symptoms: Like many flaviviruses including dengue virus, zika virus typically causes fever, rash, headaches and a non-purulent conjunctivitis, but some patients remain asymptomatic. An important difference of the zika virus, however, is that it also appears to be highly neurotropic, and has been linked to Guillain-Barré syndrome, an auto-immune disorder that leads to muscle weakness, paralysis and potentially death if breathing is sufficiently impaired. In the recent Brazilian outbreak, the main worry is over the virus’s possible link to microcephaly, a condition that causes babies to be born with unusually small heads and, in the vast majority of cases, damaged brains. The causal link between zika and microcephaly is based on detection of viral DNA in the blood and tissues of babies with microcephaly, and detection of the virus in the amniotic fluid of pregnant women whose fetuses were diagnosed through ultrasonography with microcephaly.
Control of Zika infection: The availability of a zika vaccine could be a game-changer, but a vaccine is several years off (at best). Diagnosis of the infection has also been challenging due to immunological cross-reactivity between zika and other flaviviruses, including dengue endemic in the region. At the present time, mosquito control remains the only way to control the virus. The traditional vector control strategies include insecticide treatment of breeding sites as well as indoor spaces, and practicing personal protection to prevent mosquito bites. There are however novel methods that scientists are developing to reduce mosquito populations or to prevent virus transmission in mosquitoes. Coincidentally testing of some of these methods in the field have been taking center-stage in Brazil. The British/American Company Oxitec has been releasing genetically modified reproductively sterile Ae. aegypti mosquitoes in Brazil for several years to suppress mosquito populations. An international consortium of scientists have also been testing the impact of a bacterial endosymbiont (Wolbachia), which when introduced into mosquitoes makes them resistant to infection and transmission of pathogens. The EliminateDengue program has initiated field releases of Wolbachia-infected mosquitoes for dengue virus control in 5 countries, including Brazil (Eliminatedengue.org). While these efforts have been designed to reduce dengue virus transmission, the strategies will also limit zika virus transmission.
Controlling this new epidemic will rely on a rapid public health response in affected countries, as well as proactive measures taken in countries likely to see virus introductions. In the meantime, the US Centers for Disease Control (CDC) has recommended that pregnant women avoid travelling to Zika-affected regions, and several countries in the Americas has made the recommendation that women avoid getting pregnant until 2018. Brazil is preparing to host the Olympics this summer, and a global ban on travel, even just for pregnant women, would deliver a serious blow to the Brazilian government.
Vector borne disease threats in Turkey: Due to increased globalization, Ae. aegypti is now one of the most widespread mosquito species, present in Africa, the surrounding tropics and subtropics, south eastern US, the Middle East, South East Asia, Pacific and Indian Islands and Northern Australia. Its presence has recently been reported from the north-eastern Black Sea Coast as evidence of its adaptability to varying ecological conditions. Aedes aegypti is a known vector of yellow fever virus, dengue virus, chikungunya virus and now zika virus. Aedes albopictus has also rapidly spread from its native tropical forests of Southeast Asia across the world and is now found in Europe and Turkey. The dramatic global expansion of this aggressive mosquito has increased public health concern due to its ability to transmit numerous arboviruses, including chikungunya as seen in the recent epidemics in Italy. The frequent introductions of various arboviruses in Europe by infected travellers further indicate an increase in the risk of outbreaks. The risk of outbreaks also exists for Turkey, particularly at a time when there is increased movement of people coming from disease endemic areas. Beyond mosquito transmitted diseases, sand fly transmitted leishmaniasis poses a real risk for Southern regions of Turkey, as the sandly vectors are endemic in these regions and the disease is highly prevalent in regions of the Middle East where many migrants are currently coming from. Scientists and the public health system have to be vigilant in Turkey so that costly vector borne epidemics can be prevented.