America’s Zika Family Tree Originated in Brazil

Zika virus spreads at different times of year, depending on elevation and mosquito activity

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A couple of summers ago, the threat of mosquito-borne Zika virus disease in tropical areas of the Americas caused major concern, said the director of the National Institutes of Health (NIH).

The concern was driven by reports of Zika-infected women giving birth to babies with microcephaly, which includes small heads and incompletely developed brains. 

The Zika virus is just the latest in a long line of viral outbreaks quietly spreading around the globe.

“So, with summer vacation season now upon us, people are wondering what’s become of Zika,” said Francis S. Collins, M.D., Ph.D., Director of the NIH.

While pregnant women and couples planning on having kids should still take extra precautions when traveling to Zika impacted countries, the near-term risk has subsided for most Americans.

As of May 15, 2018, the US Zika Pregnancy & Infant Registry shows Pregnant Women with Any Lab Evidence of Zika Virus Infection:

  • US States and DC: 2,461
  • US Territories: 4,870

Many people living in Zika affected countries have already been exposed to the virus, which means they have developed protective immunity.

Puerto Rico is one of these areas previously impacted by the Zika virus.

Using data from the screening of blood donations collected in Puerto Rico during April –August 2016, and assuming a 9.9-day duration of viremia, the CDC estimated that 469,321 persons in Puerto Rico were infected during this period, for an estimated cumulative incidence of 12.9 percent. 

But, the Zika virus has by no means been eliminated.

Recently, an international research team used genomic tools to trace the spread of the Zika virus. This analysis suggests that the first Zika cases in Central America were reported about a year after the virus had actually arrived, and begun to spread.

This new work reveals that by the summer of 2013, the Zika virus had already hopped unnoticed to Honduras.

From there, it spread rapidly to Guatemala and Nicaragua, then northward to southern Mexico, likely by late 2014 and into 2015. Genomic epidemiology reconstructs the introduction and spread of Zika virus in Central America and Mexico.

Because Zika is detectable in people only for a few days and usually at low levels, collecting enough of the virus’ genetic material to track its spread from country to country has been a real challenge.

The team used a newly developed strategy designed to sequence all DNA and RNA present in a sample, while specifically enriching for genetic material from the Zika virus.

The Zika family tree shows, with a few exceptions, the viruses that infected people in Central America or Mexico descended from viruses collected from Brazilians.

In fact, based on the analysis, the vast majority of Zika infections in Central America and Mexico traced back to a single importation event from Brazil.

The researchers next used the family trees to estimate when and where particular Zika strains diverged from one another.

When the first cases of Zika were reported in Brazil in May 2015, the virus was already widely circulating in Central America and Mexico.

While the first outbreak in Mexico was thought to have reached its peak in the middle of 2016, the new evidence suggests an earlier outbreak had gone undetected early in 2015.

The evidence also shows that many locations in Central America and Mexico experienced two waves of Zika infection per year, not just one.

To better understand that pattern of infection, peaking approximately every 6 months, the researchers analyzed the environmental suitability in each country for the Aedes aegypti mosquitoes that primarily transmit Zika virus to people.

Their findings suggest that conditions ripe for the spread of Zika virus occur at different times of the year, depending on elevation and other factors.

While more study is needed, this spatial and temporal variability will be an important consideration for programs designed to control mosquitoes, and for predicting how the virus is most likely to spread.

In the meantime, there’s more encouraging news on the Zika virus vaccine development. 

  • Phase 1 clinical trials of an experimental DNA vaccine developed by scientists at the NIH’s National Institute of Allergy and Infectious Diseases showed that the vaccine is safe and induced an immune response in healthy adults.
  • Separately, a phase 2 trial designed to test the vaccine’s ability to protect against Zika virus launched in April 2017 and is expected to reach completion within a couple of years.

This article includes excerpts from a blog by Francis S. Collins, M.D., Ph.D., Director of the NIH.