Now that the United States has several weeks' worth of Zika testing under its belt, the US Centers for Disease Control and Prevention (CDC) today released its first major report on the results, showing very few positives in asymptomatic people, including pregnant women.
In other Zika developments, a genetic study today shed more light on the family tree of the current outbreak and revealed some early clues on protein changes that might play a role in its explosive spread in the Americas.
Testing potentially exposed pregnant women
CDC researchers looked at results from 4,534 people tested for Zika virus from Jan 3 through Mar 5, publishing their findings in an early online edition of Morbidity and Mortality Weekly Report (MMWR).
On Feb 12, the CDC urged health providers to start testing asymptomatic pregnant women who had possible Zika exposure
Nearly three quarters (73.6%) of the people who were tested during the 2-month span were pregnant women. Of the entire group, 34% reported at least one Zika-specific symptom, 9.6% had at least one other sign or symptom, and 56.4% were asymptomatic.
Of those who had Zika symptoms, testing confirmed infections in 11.8%. A small percentage (2.7%) had unspecified other flavivirus infections. Among the asymptomatic people who were tested—of which nearly 95% were pregnant women—seven had confirmed Zika virus infections.
After the CDC expanded its recommendation to include asymptomatic pregnant women, 7 of 20 who tested positive had been asymptomatic. Five had lived in Zika transmission areas at some time during their pregnancies, and 2 were short-term travelers.
The authors said, given the potential for severe birth defect outcomes, health providers should continue to offer testing to asymptomatic pregnant women who might have been exposed to Zika virus.
They added the caveat that patterns so far apply to the current situation in the continental United States, where no local transmission is occurring. They also said that a traveler's risk could be higher based on different exposure variables, such as location and duration of travel.
As of Apr 13, the CDC has received reports of 358 Zika cases in travelers, 31 of them involving pregnant women and 7 sexually transmitted. US territories such as Puerto Rico have reported 471 locally acquired cases, 58 of them involving pregnant women.
Virus evolution, possible protein changes
In the gene study, comparisons of 41 Zika samples suggest significant changes in amino acid and nucleotide sequences over the past 50 years and divergence between the African and Asian strains, which may provide clues on the explosive spread in the Americas.
Researchers from the University of California, Los Angeles (UCLA), and China looked at 30 isolates from humans, 10 from mosquitoes, and 1 from monkeys. The team published its findings, which build on other recent genetic analyses, today in Cell Host & Microbe.
Among their main findings, the authors reported that sequences from contemporary Zika strains more closely resemble ones from the Malaysian 1966 strain than the 1968 Nigerian strain, a sign that the viruses implicated in recent human outbreaks evolved from the Asian lineage, which has been noted before.
Also, the most recent human strains are closer to the 2013 French Polynesian strain than the 2007 Micronesia strain, which supports the hypothesis that the variants evolved from a common ancestor.
When the researchers compared the Asian human and African mosquito subtypes, they found the premembrane precursor protein had the highest variability, with modeling methods suggesting that the differences contribute to a significant structural change.
Genhong Cheng, PhD, study coauthor and professor at the UCLA department of microbiology, immunology, and molecular genetics, said in a Cell Press media release today that the changes might shed light on the virus's capacity to spread exponentially in the Americas.
"These changes could enable the virus to replicate more efficiently, invade new tissues that provide protective niches for viral propagation, or evade the immune system, leading to viral persistence. Of course, all of these hypotheses will need to be tested in experimental models."
The next sequencing steps will likely focus on the strain fueling the outbreak in the Americas, which has been isolated in people, but not yet from a mosquito. Research groups, including Cheng's, will also work on uncovering the structure of the viral proteins, which would help guide drug and vaccine development.
Developments in the Americas
- In an epidemiologic update on Zika virus yesterday, the Pan American Health Organization (PAHO) said Belize is investigating possible local transmission based on a case reported by another country. PAHO also said that on Apr 12 Paraguay reported an unusual increase in Guillain-Barre syndrome in the first 3 months of the year. Also, the agency said Ecuador has detected evidence of Zika virus in tissue samples from a howler monkey, collected during an investigation into 39 monkey deaths in a national park in Manabi province.
- Colombia's health ministry yesterday confirmed two Zika-linked microcephaly cases, according to a Reuters report. The ministry said, however, that officials have seen no big surge in the birth defect. Recent updates from the World Health Organization have said Colombia was investigating 50 microcephaly cases that have been reported so far this year, including 7 with possible Zika links.
See also:
Apr 15 MMWR report
Apr 13 CDC Zika background
Apr 15 Cell Host Microbe report
Apr 15 Cell Press news release
