A confluence of risk factors for NiV disease outbreaks occur in Thailand, notably the regular detection of NiV in Pteropus bats, presence of three NiV primary natural reservoirs species (P. lylei, P. hypomelanus, and P. vampyrus) which are found in communities and tourist areas in central and southern Thailand [2, 16], and pig farming in areas within flight range of Pteropus roosts. This study shows that P. lylei urine has been positive for NiV at the same location for 18 years, which indicates P. lylei is a natural reservoir for NiV in Thailand and the virus is shed annually. P. lylei inhabits the human-dominated areas in Thailand (53% of roost sites are located within Buddhist temples, while the number of roosts located on private, common, and state properties are 8 (27%), 3 (10%), and 3 (10%), respectively. There were few ecological limits of movement of P. lylei in the central plains of Thailand . Further, according to the DLD, a total of 12,228,255 pigs were recorded nationwide in 2020, an overall high density for a known intermediary host of NiV, . However, most breeding pig farms are raised in closed systems, especially in the Central region of Thailand, thus effectively reducing the risk of NiV transmission from bats to pigs. Additionally, a recent study predicted additional “likely reservoirs” of NiV using machine learning based on traits using several parameters . Thus, increased vigilance and collaboration between the human, animal, and wildlife sectors is imperative to staying ahead of a zoonotic disease outbreak.
In this study, we compiled findings from 20 years of proactive, multi-sectoral One Health surveillance, and research to facilitate preparedness for NiV prevention and control in Thailand. This included NiV serosurveillance in pigs since 2001, surveillance in bats since 2002, encephalitis patients from 2001 to 2012, and among healthy high-risk community volunteers in 2010, 2017 and 2018 (Table 1).
In the long-term surveillance of NiV in P. lylei bats in Thailand, NiV RNA has been detected in pooled urine samples every year since 2003 to 2020 (data from this study and published data [2, 24, 29, 39]) (139/2500, 5.56%; Table 2). This is a significantly higher rate than previous detections in Cambodia from 2012 to 2016 in the same bat species, which found 28 NiV-positive urine samples (from 3930; 0.7%) by PCR . Both NiV strains were found from P. lylei in Thailand however, NiV-BD was dominant (Fig. 2). The WGS of NiV RNA, sequenced directly from pooled bat urine specimens from Chonburi in 2017, shared 99.13% nucleotide identity to NiV from a Bangladeshi patient in 2004 (Genbank Accession no. AY988601.1). On the contrary, previous WGS of NiV from Cambodian (MK801755.1) bats were NiV-MY strain. Overall homology of naturally circulating NiV strains in Thailand with previously detected human infectious strains from Bangladesh highlights the outbreak potential of NiV from P. lylei to humans or pigs in the Southeast Asian region.
Routine NiV serosurveillance in pigs in Thailand has been conducted annually since 2001 by DLD. At least 4,000–5,000 pig sera are collected annually around the country for NiV antibody testing by ELISA at high-risk sites selected by DLD on the basis of bat NiV studies . To date, no NiV-seropositive pig has been identified from this DLD initiative . In this study, two targeted risk based NiV surveillances in pigs were conducted. The first study, from August 2011 to November 2012, was conducted by the researchers from the Faculty of Veterinary Science, Chulalongkorn University, where 246 and 233 specimens were negative for antibody testing in serum and NiV RNA testing in nasal swabs, respectively. The second study was conducted by DLD researchers from May 2017 to May 2018; all 1349 pig specimens tested negative for NiV RNA by PCR. The results of these two studies were similar to the studies conducted in Laos and Indonesia. In Laos during May 2008 to January 2009, 716 pig sera tested negative for NiV . In the Indonesian study, all 610 pig sera tested negative despite 19% of seropositive P. vampyrus to NiV antibodies . Additionally, the study by Kasloff et al. highlighted that pigs infected with NiV-BD did not show clinical signs, nor viremia, which emphasizes the need for laboratories and pig farm owners to remain vigilant . Together these findings suggest that bat to pig spillover of NiV is a rare event, especially given surveillance of healthy pigs.
In Malaysia, NiV serosurveillance among 177 indigenous, healthy volunteers from 4 communities, located 30–75 km from either previously confirmed NiV infections or locations where Pteropus bats were seropositive to NiV antibodies . It was found that 10.73% of the participants had antibodies against NiV nucleocapsid protein, suggesting possible exposure to NiV. To date there is no evidence of NiV infection in humans or pigs in Thailand. To identify potentially undetected spillovers of NiV from P. lylei to humans in the past, serosurveillance in community was conducted at the village where P. lylei bats were positive to NiV RNA and NiV antibodies [2, 24]. The first serosurveillance was conducted in November and December 2010, where 418 sera from people in the village tested negative by ELISA (using inactivated virus as the antigen). Oral swabs and urine samples from the villagers collected in May 2017 (n = 115) and May 2018 (n = 128) tested negative for NiV RNA. Serum samples of healthy villagers collected in 2018 (n = 128) further tested negative for NiV antibodies using Luminex assay. The timing of our human community-based surveillance (the month of May) corresponded to the highest prevalence of NiV RNA detected in bat pooled urine at this site (Wat Luang) . No evidence of NiV infection was found in the villagers at the study sites during this viral shedding period. Our findings on community serosurveillance are similar to a study in Cambodia where NiV has been reported in P. lylei, yet none of the 418 potentially exposed people were seropositive for NiV .
The low prevalence of NiV (less than 10% NiV RNA) and the seasonal prevalence pattern in P. lylei found in Thailand and Cambodia might reduce the risk of NiV transmission from bats to humans, in addition to the absence of key risk behaviors associated with viral transmission. From our assessment, drinking of raw date palm juice is not common in Thailand, and bat hunting is prohibited by law. A study showed NiV outbreak’s seasonal preference of cooler weather (below 17oC) , for example it has been shown that the virus thrives better in urine at lower temperatures than at higher temperatures . Apart from several days in January 2021, temperature in Chonburi does not often drop below 20oC, and often averages above 30oC which may help explain the lack of spillover to humans to date. However, this increases the risk of NiV outbreak as a result of the global climate change.
The risk of a NiV outbreak in Thailand is increasingly possible, as evidenced by the high identity shared between the NiV genome from Thai bats and the Bangladeshi patient, suggesting it is likely able to directly transmit to humans. The finding of NiV-infected bats trapped at the pig farms further raises concern of possible NiV transmission to pigs. Although there was no evidence of NiV infection in pigs in the same study, measures to prevent bats from coming into contact with pigs and avoidance of feeding pigs with partially eaten fruits must be actively promoted.
Study of community perceptions and knowledge of NiV transmission from bats in Bangladesh, underscores the need for educational interventions for targeted groups in the community . Public awareness and willingness to steer clear of risk behaviors are vital to sustainably protect people in high-risk areas from NiV outbreak, and an opportunity for policy makers to strengthen public health infrastructure. This may include highlighting the ecosystem services and conservation of bats, so as to improve people’s current knowledge and subsequent behaviors regarding the role of bats in ecology and the spreading of NiV. The One Health effort conducted in Thailand for 20 years is building preparedness on three fronts: laboratory, community and the government. Conducting long-term longitudinal studies, such as the one in Chonburi province, has enabled Thai scientists to provide evidence-based information to policy decision makers while strengthening laboratory capacity and disease surveillance systems. Communities are being engaged at the village level to raise awareness of both the risks and benefits in living with bats. In Wat Luang village, researchers from the DNP, Kasetsart University and TRC-EID-HSC utilize “Living Safely with Bats”, a USAID-developed teaching aid tailored to the non-scientist population and Health Promoting Hospitals at the community level (translated and distributed by TRC-EID-HSC) .