Zika Virus Update

Last week I summarized the most recent information about the Zika virus pandemic. Since then, two important articles were published by the Centers for Disease Control in the January 19 and January 22 issues of MMWR. These articles demonstrated that the pandemic is continuing to spread and included information about laboratory testing. CDC has also recommended that pregnant women consider postponing travel to endemic areas. Today's blog updates the last issue with this new information.

Zika virus is a mosquito-borne flavivirus transmitted primarily by Aedes aegypti mosquitoes. 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 few countries in 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. Brazilian officials estimate that between 440,000 and 1,300,000 cases have occurred in 2015. In 2016, Zika virus transmission has been reported in 19 other countries or territories in the Americas. Western Hemisphere countries and territories include Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Puerto Rico, Paraguay, Suriname and Venezuela. A few isolated cases have been detected in travelers returning to the United States. These imported cases might result in local human-to-mosquito-to-human spread of the virus in limited areas of the continental United States that have Aedes aegypti and Aedes albopictus mosquitoes.
An estimated 80% of persons infected with Zika virus are asymptomatic. If symptoms develop they are usually mild and include fever, maculopapular rash, arthralgia, or nonpurulent conjunctivitis. Symptoms usually last from several days to 1 week. Severe disease requiring hospitalization is uncommon, and fatalities are rare. Guillain-Barré syndrome has been reported in patients following suspected Zika virus infection. Zika has not been associated with hemorrhagic fever or death.
Maternal-fetal transmission of Zika virus has been documented throughout pregnancy resulting in congenital infection. Pregnant women can also become infected by sexual transmission and blood transfusion. Infection of women during the first trimester of pregnancy has been associated with microcephaly. Zika virus infections have been confirmed in infants with microcephaly. No other arbovirus has been shown to have teratogenic effects.

Because there is neither a vaccine nor prophylactic medications available to prevent Zika virus infection, CDC recommends that all pregnant women consider postponing travel to areas where Zika virus transmission is occurring. If a pregnant woman travels to an area with Zika virus transmission, she should be instructed to avoid mosquito bites. Women who traveled to an area with ongoing Zika virus transmission during pregnancy should be evaluated for Zika virus infection and tested in accordance with CDC Interim Guidance.

In a pure Zika epidemic, a diagnosis can be made reliably on clinical grounds. Unfortunately, dengue and chikungunya viral infections have the same geographic distribution and cause similar symptoms, confounding clinical diagnosis. Pregnant women who develop a clinically compatible illness during or within 2 weeks of returning from an area with Zika virus transmission should be tested for Zika virus infection. Fetuses and infants of women infected with Zika virus during pregnancy should be evaluated for possible congenital infection.

No commercial test for Zika virus is available. In the United States, testing is performed at the Centers for Disease Control and a few state health department laboratories. Reverse transcription-polymerase chain reaction (RT-PCR) testing is recommended for serum specimens collected from patients during the first week of illness. Immunoglobulin M (IgM) and neutralizing antibody testing should be performed on specimens collected ≥4 days after onset of symptoms. Because Zika is closely related to dengue and yellow fever virus, serologic samples may cross-react with other flaviviruses. Results must be interpreted cautiously. Zika virus RT-PCR testing can be performed on amniotic fluid. A positive RT-PCR result on amniotic fluid is suggestive of intrauterine infection.

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.