Zika Virus Pandemic
Arbovirus includes hundreds of predominantly RNA viruses that are transmitted by arthropods such as mosquitoes and ticks. Arboviruses often have complex life cycles involving mammals, birds and blood-feeding insect vectors. Examples of arboviruses that cause human disease include yellow fever virus, Saint Louis encephalitis virus, West Nile, and Chikungunya virus.
Zika virus was incidentally discovered in Uganda in 1947 during mosquito and primate surveillance. It was named for the forest in Uganda where it was first discovered. Until recently Zika virus remained confined to a narrow equatorial belt running across Africa and into Asia and circulated predominantly in wild primates and Aedes africanus mosquitos. Human infections were rare.
More recently, Zika virus has spread from Africa into several countries in Latin America along with the Aedes aegypti mosquito. Zika virus transmission has been reported in 14 Western Hemisphere countries and territories including Brazil, Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Puerto Rico, Paraguay, Suriname and Venezuela. Brazilian officials registered at least 2,782 cases in 2015, compared with just 147 in 2014 and 167 the year before. A few isolated cases have been detected in the United States, including one in Texas in January 2016.
Infection with Zika virus is characterized by a mild disease course including fever, muscle aches, eye pain, prostration, and maculopapular rash. Zika has not been associated with hemorrhagic fever or death. Zika virus infection has been associated with Guillain-Barre' syndrome. Infection of women during the first trimester of pregnancy has been associated with microcephaly. Zika virus has been isolated from Brazilian infants who died from microcephaly. No other arbovirus has been shown to have teratogenic effects. Public health officials are trying to decide if they should issue a warning for pregnant women to avoid travel to Brazil and other Latin American and Caribbean countries.
In a pure Zika epidemic, a diagnosis can be made reliably on clinical grounds. Unfortunately, the fact that dengue and chikungunya, which result in similar clinical pictures, have both been epidemic in the Americas confounds clinical diagnoses. Specific tests for dengue and chikungunya are not always available, and commercial tests for Zika have not yet been developed. Because Zika is closely related to dengue, serologic samples may cross-react in tests for either virus. Gene-detection tests such as the polymerase-chain-reaction assay can reliably distinguish the three viruses, but Zika-specific tests are not yet widely available.
Many decades ago, African researchers noted that aedes-transmitted Zika epizootics inexplicably tended to follow aedes-transmitted chikungunya epizootics and epidemics. An analogous pattern began in 2013, when chikungunya spread pandemically from west to east, and Zika later followed. Zika has now circled the globe, arriving not only in the Americas but also, in September, in the country of Cape Verde in West Africa, near its presumed ancient ancestral home.
Arboviruses continually evolve and adapt within ecologic niches that are increasingly being perturbed by humans. In our human-dominated world, urban crowding, constant international travel, climate change and ecological perturbations are causing seemingly innocuous infectious agents to emerge unexpectedly. Climate change may play an important role. Increasing temperatures allow mosquitos to thrive in new regions. Increased precipitation increases the number of breeding grounds. Drought can cause people to collect water in outdoor containers that serve as mosquito habitats.
The explosive pandemic of Zika virus infection occurring throughout South America, Central America, and the Caribbean is the most recent of four unexpected arrivals of important arthropod-borne viral diseases in the Western Hemisphere over the past 20 years. The other viruses include dengue, West Nile virus and Chikungunya.
Zika virus forces us to confront a potential new disease-emergence phenomenon: pandemic expansion of multiple, heretofore relatively unimportant arboviruses previously restricted to remote ecologic niches.
Fauci AS and Morens DM, Zika Virus in the Americas-Yet another arbovirus threat. NEJM Jan 13, 2016; DOI: 10.1056/NEJMp1600297.
