Scientists Unveil the History of Zika Virus in Thailand

In 2016, the WHO responded to the South American Zika outbreak by declaring a "public health emergency of international concern", prompting global action to combat the disease. In Thailand, the government, in collaboration with the Institut Pasteur, the CNRS, US researchers and the Thai National Institute of Health, decided to investigate the history of the virus in Thailand. While the Aedes mosquito is found in both South-East Asia and South America, Thailand did not have widely reported cases of Zika. The nature of the virus furthered muddied the waters, as Zika rarely causes serious illness and, even when it does, can easily be confused with the flu or dengue. 

To collect data on cases of Zika, the government requested hospitals to collect urine, blood and plasma samples from individuals with fever or rash symptoms. Over two years, 368 individuals were found to be infected by Zika. Inidividuals were spread out across age groups and across regions. The latter observation was surprising to researchers, as long-term circulation had not occured in the South American outbreaks.

In response, the researchers examined the viral genome, using "the little errors that emerge during viral replication to piece together the historical spread". The scientists discovered significant genetic diversity among the patients and concluded that the ancestor virus could have appeared in Thailand as early as 2002. Researchers have suggested cross-immunity or changes prior to transmission could be responsible for this diversity. Nevertheless, the continued presence of the disease could remain a problem for Thai public health authorities.

For more information, see the article: https://www.pasteur.fr/en/press-area/press-documents/zika-silent-long-term-circulation-thailand or the corresponding study: https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(18)30718-7/fulltext.

-Ed