Zika Virus Endemic In Thailand, Study Shows

Unlike in South America where the Zika virus disappears two or three years after outbreaks, Zika transmission in Thailand has persisted for at least 16 years.

AsianScientist (Mar. 14, 2019) – Scientists in Thailand, US and France have found that the Zika virus has been circulating in Thailand for more than 16 years. They published their findings in The Lancet Infectious Diseases.

The Aedes mosquito is found in most tropical and subtropical countries, including Southeast Asia, and is a vector for the transmission of flaviruses such as the Zika virus. While the spread of Zika virus is well documented in South America, information on Zika outbreaks in Southeast Asia is scarce.

To shed light on Zika circulation, an international team of scientists investigated the history of the Zika virus in Thailand. They made the surprising discovery that the Zika virus has been circulating in the country for at least 16 years and probably longer.

“It is a disease that is difficult to study, since most people infected by Zika do not fall ill or suffer only minor symptoms,” explained study co-author Dr. Henrik Salje, a statistician and epidemiologist at the Institut Pasteur, France. “Even when they do get sick, the symptoms can easily be confused with other diseases such as the flu or dengue.”

The scientists relied on the surveillance network set up by the Thai government in 2016 to arrive at their conclusions. A total of 1,717 samples were collected between 2016 and 2018, and 368 individuals were revealed to be infected by Zika, giving the scientists the material they needed to start their investigation.

“We noticed that from the very beginning of the surveillance campaign, Zika could be found all over Thailand,” Salje said. “This was not expected, since in South America the Zika virus largely disappeared after two or three years following the outbreaks; there does not appear to be widespread long-term circulation.”

This encouraged the teams to examine the viral genome. Because viruses do not have a genetic repair system, the researchers were able to use the little errors that emerge during viral replication to piece together the historical spread of viruses.

By producing a phylogenetic tree of the Zika virus in Thailand, the scientists discovered that the viral strains within Thailand were genetically very different from each other, and their ancestor seems to have appeared in the country in 2002.

“One thing [our study confirms] is that it is important to have surveillance systems in place that can monitor Zika on a long-term basis, even if there are no visible signs of its activity,” Salje concluded.

The article can be found at: Ruchusatsawat et al. (2019) Long-term Circulation of Zika Virus in Thailand: An Observational Study.


Source: Institut Pasteur. Photo: Institut Pasteur.
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