Zika virus Epidemiology

The very first known case of Zika fever was in a sentinel rhesus monkey stationed on a tree platform in the Zika Forest in Uganda in 1947. Population surveys at the time in Uganda found a 6.1% prevalence. The first human cases were reported in Nigeria in 1954. A few outbreaks have been reported in tropical Africa and in some areas in Southeast Asia.

The first major outbreak, with 185 confirmed cases, was reported in 2007 in the Yap Islands of the Federated States of Micronesia. A total of 108 cases were confirmed by PCR or serology and 72 additional cases were suspected. The most common symptoms were rash, fever, arthralgia and conjunctivitis, and no deaths were reported. The mosquito Aedes hensilli, which was the predominant species identified in Yap during the outbreak, was probably the main vector of transmission. While the way of introduction of the virus on Yap Island remains uncertain, it is likely to have happened through introduction of infected mosquitoes or a viraemic human with a strain related to those in Southeast Asia. This was also the first time Zika fever had been reported outside Africa and Asia.

In 2013 another large outbreak was reported in French Polynesia that was thought to be from an independent introduction of the virus from Asia than the Yap Island outbreak.

Figure 1: Global distribution of countries that have past or current evidence of Zika virus transmission (as of 18 April 2016). Adapted from CDC . The CDC has a comprehensive site describing countries and areas with active Zika virus transmission.

In May 2015, the Pan American Health Organization (PAHO) issued an alert regarding the first confirmed Zika virus infections in Brazil. According to the Brazilian Health Ministry, as of November 2015 there was no official count of the number of people infected with the virus in Brazil, since the disease is not subject to compulsory notification. Even so, cases were reported in 14 states of the country. Mosquito-borne Zika virus is suspected to be the cause of 2,400 cases of microcephaly and 29 infant deaths in Brazil in 2015.

The emergence of Zika virus in South America led to a rapid spread throughout South and Central America, reaching Mexico in November 2015. It has appeared sporadically in travellers to the United States and Europe but has not established person to person spread in those areas. Because of the "growing evidence of a link between Zika and microcephaly" the CDC issued a travel alert on January 15, 2016 advising pregnant women to consider postponing travel to the following countries and territories: Brazil, Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Suriname, Venezuela, and the Commonwealth of Puerto Rico. The agency also suggested that women thinking about becoming pregnant should consult with their physicians before traveling.

Video 1. WHO: Zika virus spreading to almost all the Americas.