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Zika spread in Americas long before detection
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Researchers said Zika virus was spreading in parts of the Americas for months before the first locally transmitted cases were confirmed in Brazil in 2015. Similarly, local transmission of the virus was likely to have started in the spring of 2016 in Florida — several months before it was initially detected there.
In addition, genome sequencing offered insight into the epidemiology of Zika virus (ZIKV) as it spread from South America and the Caribbean to the continental United States, they concluded in two studies published in the journal Nature.
“We found that ZIKV circulated undetected in multiple regions for many months before the first locally transmitted cases were confirmed, highlighting the importance of surveillance of viral infections,” the researchers wrote. “We identified mutations with possible functional implications for ZIKV biology and pathogenesis, as well as those that might be relevant to the effectiveness of diagnostic tests.”
In one of the studies, the researchers analyzed 110 ZIKV genomes — the largest number collected so far — in samples gathered throughout the epidemic in the Americas. They estimated that the most recent common ancestor to the samples arrived in Brazil in early 2014 (95% CI, August 2013-July 2014). A phylogenic tree illustrated the progression from the common ancestor.
“The shape of the tree near the root remains uncertain ... because there are too few mutations to clearly distinguish the branches,” the researchers said. “This pattern suggests rapid early spread of the outbreak, consistent with the introduction of a new virus to an immunologically naive population.”
They further found that ZIKV genomes from several areas clustered in distinct clades, each of which is a group with a common ancestor, according to where they circulated. They included 10 genomes from Columbia, 18 from Honduras and three from Puerto Rico.
A fourth clade came from patients who had been infected in one of three Caribbean countries — the Dominican Republic, Haiti and Jamaica — or the continental U.S. That clade included 30 of 32 genomes from the Dominican Republic and 19 of 20 from the U.S.
The researchers estimated that the Caribbean–U.S. ZIKV clade arrived 9 months before the first confirmed case in that group of countries, 5.5 months earlier in Columbia, 8 months earlier in Honduras and 4.5 months earlier in Puerto Rico.
Pointing out that their study revealed 1,030 mutations in all, they stressed that genome analysis can help officials in all affected countries better plan their control and prevention efforts.
In another study, researchers focused on ZIKV’s arrival in Florida, where the first confirmed case was reported in July 2016. The Florida outbreak caused 256 confirmed infections, 241 of which were reported in Miami-Dade County.
The researchers gathered 24,351 mosquitoes in Miami from June to November 2016. Nearly all of them — 99.8%— were Aedes aegypti. After testing the mosquitoes for ZIKV, the researchers estimated that one in every 1,600 A. aegypti mosquitoes (.061%; 95% CI, .028%-.115%) was infected.
They sequenced 39 genomes, which they included in phylogenic trees with 65 previously published genomes from other regions. The researchers found that the ZIKV genomes from Florida fell into four different lineages, three of which were in the same clade.
Two of the lineages, which the researchers called F1 and F2, made up 92% of the Florida genomes. Assuming the linages came in separate introductions, they estimated that F1 and F2 arrived in March or April 2016 (95% highest posterior density [HPD], January-May 2016).
The researchers said that lineages F1 and F3 clustered with genomes from the Dominican Republic and Guadeloupe, both Caribbean islands.
They estimated that, in the first half of 2016, 60% to 70% of people with travel-related ZIKV had come from the Caribbean. In addition, 67% of people with travel-associated ZIKV in Florida reported visiting that region.
Although their analysis showed four separate introductions of ZIKV to Florida, the researchers suspected that as many as 40 contributed to the outbreak there.
“The high volume of traffic entering Florida from ZIKV–affected regions, especially the Caribbean, is likely to have provided a substantial supply of ZIKV–infected individuals,” the researchers wrote. “Because Florida is unlikely to sustain long-term ZIKV transmission, the potential for future ZIKV outbreaks in this region is dependent upon activity elsewhere.” – by Joe Green
References:
Grubaugh ND, et al. Nature. 2017;doi:10.1038/nature22400.
Metsky HC, et al. Nature. 2017;doi:10.1038/nature22402.
Disclosure: The researchers report no relevant financial disclosures.