The first formal description of Zika virus (“Zika”) was published in 1952 but interest in the virus was limited for many years. In fact, it was not until 2007 that the first Zika outbreak in humans was recorded. Over the past decade, Zika has begun to spread outside of tropical regions of Africa and Asia with serious resulting complications. Between January 2014 and February 2016, 33 countries reported circulation of the Zika virus. Concomitant with the spread of the virus, there have been increasing reports of neurological disorders including microcephaly and Guillain-Barrė syndrome (“GBS”). On February 1, 2016, following an outbreak in the Americas, the World Health Organization (“WHO”) declared Zika a Public Health Emergency of International Concern.
Zika is a member of the virus family Flaviviridae and the genus Flavivirus. The Flavivirus genus is comprised of enveloped, single-stranded RNA viruses and includes more than 70 types of viruses, such as the Yellow Fever virus, the dengue virus, and the West Nile virus.
Zika virus is primarily transmitted through the bite of an infected mosquito from the Aedes genus, mainly aedes aegypti in tropical regions. The Aedes mosquito is also known to transmit other viruses from the Flavivirus genus. Critically however, unlike the dengue virus and other common viruses in the Flavivirus family, Zika may also be spread through sexual contact, potentially increasing the risk of transmission.
Symptoms associated with Zika infection are variable. It is asymptomatic in up to 80% of cases, and when symptoms do occur, they are typically mild and nonspecific. These symptoms include mild fever, skin rashes, conjunctivitis (pink eye), muscle and joint pain, malaise, and headache. The incubation period (e.g. time from exposure to symptoms) of Zika is not clearly defined but is likely to be a few days. Symptoms can last from two to seven days.
While the acute manifestations of Zika infection are typically mild, the disease has been associated with a number of neurological complications. There is scientific consensus that Zika can cause congenital microcephaly, a condition where a child is born with a smaller than expected head due to abnormal brain development. In adults, Zika can also lead to Guillain-Barrė syndrome, a disorder which causes the body’s immune system to attack the nerves, leading to muscle weakness or, in severe cases, paralysis. Intense efforts are underway to investigate the link between Zika and a range of other neurological disorders including brain ischemia, myelitis (inflammation of the spinal cord), and meningoencephalitis.
Zika was first isolated in a rhesus monkey in the Zika forest of Uganda in 1947. In 1952, the first human cases of Zika were detected in Uganda and the United Republic of Tanzania. Further immunological and viral studies over the years found that Zika extended throughout various regions in Africa and Asia, but before 2007, only 14 cases of natural infection of human beings were reported.
In 2007, the first large Zika outbreak in humans occurred in the Pacific Island of Yap, in the Federated States of Micronesia. This was also the first time the virus was detected outside of Africa and Asia. An estimated 73% of Yap residents were infected with Zika, however the reported symptoms were mild.
In October 2013, another large outbreak with 10,000 registered cases took place in French Polynesia. This was also the first time that the virus was associated with an increase in the incidence of neurological complications. During the outbreak, there was a 20-fold increase in the incidence of Guillain-Barrė syndrome. A case-control study of a large series of patients confirmed the link between Zika and GBS.
Between February and June of 2014, a new Zika outbreak started on Easter Island, Chile. It is believed French Polynesians attending a cultural festival introduced the virus to the island. Within a year, the virus had spread to the northeastern states of Brazil, likely during the 2014 World Cup. Zika continued to spread throughout Latin America and, as of February 2016, was reportedly circulating in 33 countries.
With the virus’ most recent outbreak, there has been a profound increase in the number of reports of congenital microcephaly and other neurological disorders. For example, in Brazil, an average of 163 cases of microcephaly were recorded annually between 2001 and 2014; however, in the year up to January 30, 2016, 4,783 microcephaly and/or neurological malformation cases have been recorded, a nearly 30-fold increase in the number of reports. Brazil, Colombia, El Salvador, and Suriname have all also reported sharp increases in GBS cases during the course of 2015 up to the present, concurrent with the Zika outbreak.
WHO and CDC Response
On January 22, 2016, the U.S. Centers for Disease Control (“CDC”) Emergency Operations Center (“EOC”) was activated for Zika. The EOC will monitor and coordinate the emergency response to Zika, bringing together top scientists to better understand the virus and its impact on reproductive health, birth defects, developmental disabilities, and travel health. The group will also monitor and report cases of Zika, study its spread, and provide surveillance of the virus in the U.S. and U.S. territories.
On February 8, 2016, President Obama announced a request for $1.8 billion in emergency funds for several agencies to accelerate research into a vaccine and to educate populations at risk for Zika infection.
The WHO and its partners set out an updated strategic response to Zika on June 17, 2016, which places a greater focus on preventing and managing medical complications caused by Zika infection, with increased support for women and girls of childbearing age.
According to the WHO’s Strategic Response Plan to Zika, “vigilance needs to remain high”. The virus has continued to spread globally and can reach all countries where aedes aegypti (the mosquito known to be the main virus carriers) is found. Other Aedes mosquitos can also be competent carriers and have a much greater geographical reach. The outbreak will likely have a more negative impact on poor and marginalized communities, due to poor living conditions, insufficient infrastructure, inadequate access to information, and limited resources for prevention and care.