From 2018 to 2020, the Chikungunya virus (CHIKV) outbreak re-emerged in Thailand with a record of more than 10,000 cases up until the end of 2020. Here, we studied acute CHIKV-infected patients who had presented to the Bangkok Hospital for Tropical Diseases from 2019 to 2020 by assessing the relationship between viral load, clinical features, and serological profile. The results from our study showed that viral load was significantly high in patients with fever, headache, and arthritis. We also determined the neutralizing antibody titer in response to the viral load in patients, and our data support the evidence that an effective neutralizing antibody response against the virus is important for control of the viral load. Moreover, the phylogenetic analysis revealed that the CHIKV strains we studied belonged to the East, Central, and Southern African (ECSA) genotype, of the Indian ocean lineage (IOL), and possessed E1-K211E and E1-I317V mutations. Thus, this study provides insight for a better understanding of CHIKV pathogenesis in acute infection, along with the genomic diversity of the current CHIKV strains circulating in Thailand.

Japanese encephalitis virus (JEV) is a member of the Flaviviridae family and one of Asia’s most common causes of encephalitis. JEV is a zoonotic virus that is transmitted to humans through the bite of infected mosquitoes of the Culex species. While humans are dead-end hosts for the virus, domestic animals such as pigs and birds are amplification hosts. Although JEV naturally infected monkeys have been reported in Asia, the role of non-human primates (NHPs) in the JEV transmission cycle has not been intensively investigated. In this study, we demonstrated neutralizing antibodies against JEV in NHPs (Macaca fascicularis) and humans living in proximity in two provinces located in western and eastern Thailand by using Plaque Reduction Neutralization Test (PRNT). We found a 14.7% and 5.6% seropositive rate in monkeys and 43.7% and 45.2% seropositive rate in humans living in west and east Thailand, respectively. This study observed a higher seropositivity rate in the older age group in humans. The presence of JEV neutralizing antibodies in NHPs that live in proximity to humans shows the occurrence of natural JEV infection, suggesting the endemic transmission of this virus in NHPs. According to the One Health concept, regular serological studies should be conducted especially at the animal–human interface.