The human blood surveys over 2018–2022 showed that 144 IUs (40.2%) were surveyed (a total of 357 IUs), with a median population coverage of 88.7% among the surveyed areas. The Thailand LF PVS strategy for human blood surveys is to study 10% of total IUs per year for 10 years. Hence, at the 5-year mark (5 years: 2018–2022), the achievement is slightly below the targeted 50% total IU coverage but a significant achievement as most of the eleven provinces continued to conduct human blood surveys in designated IUs over 2020–2022 despite the COVID-19 pandemic. There were geographical differences in coverage, mainly due to COVID-19 border restrictions on travel, access to villages under lockdown measures, and COVID-19 varying case burden and transmission patterns. Over 2020–2022, different provinces were affected by the sequential waves of the COVID-19 pandemic. In provinces along the border with Myanmar, these affected both in-outbound migrants and also for health staff access to communities, in particular in 2022 due to the political situation in the neighboring country. In provinces of Chiang Mai, Ranong, Krabi, and Nakhon Si Thamamarat, where there are three or fewer IUs in each of these provinces, PVS human blood surveys could be integrated with other community health efforts and programs as a more cost-effective strategy. The role of PCD through routine notifiable disease surveillance is essential in the PVS phase in both endemic provinces and in provinces where there is a high risk of importation, especially among migrants, as recorded in 2022 in the province of Ratchaburi. The reasons for ongoing transmission in Narathiwat could be attributed to the persistence of vectors in and around a protected peat swamp forest of 66,000 acres, which spreads over 3 districts of Tak Bai, Sungai Kolok, and Sungai Padi, where the majority of the LF cases in Narathiwat are reported from. The persistence of non-modifiable factors (where source reduction is not possible) favourable for vector breeding, in this case, protected peat swamp forests, and a few IUs with zoonotic B.malayi, will result in incident cases of LF occurring over the PVS phase. Aiming for zero cases will be extremely difficult, even if MDA were to be resumed. Other preventive measures, ie prevention of mosquito bites, could be considered, although the 3 districts are currently free of malaria and dengue. More research is needed in these IUs for the effectiveness and acceptability of insecticide-treated bed nets, and mosquito repellents taking into account the biting behaviour and exposure to the main vector, Mansonia spp. In addition, in IUs that document local infection with Mf rate > 1%, triple therapy or IDA (ivermectin, DEC, and albendazole) could be considered as part of Thailand’s PVS strategy to address focal persistence of LF [11, 12].

In Thailand and other Southeast Asian countries, the principal vectors of B. malayi are Mansonia mosquitoes, including Ma. uniformis, Ma. indiana, Ma.annulifera, and Ma. onneae [13, 14]. With the current program protocol of conducting vector surveys in 1% of IUs in previously endemic provinces, only Narathiwat Province has detected filarial larvae in vectors. These larvae found in vectors were not found in IUs (sub-village) where MF human cases were detected but in the same subdistricts. Only Paye IU in Narathiwat province has demonstrated possible autochthonous transmission. As vector surveys and mosquito dissection is a labor-intensive activity, and transmission of lymphatic filariasis is significantly influenced by vector density [15, 16], the current protocol may need revision in light of the low Mf/filarial larvae detection rate. Intervention programs rolled out in endemic areas should be specific and targeted in each endemic foci where transmission is focal [17, 18]. At the same time, the integration of vector surveys with other diseases, for example malaria or dengue, could be considered where both human and financial resources are limited. In addition, given the low transmission levels in most endemic communities in Thailand, molecular-based techniques, as shown in other countries, may be an effective tool for xenomonitoring [19, 20].

The last assessment of the quality of lymphedema services in 2017, before validation of elimination, identified high staff turnover that affected the knowledge of responsible officers and the availability of MMDP materials. The assessment in 2020 also identified issues with the retention of health staff who were previously trained, either due to being repositioned to other health offices and tasks or retirement. Subsequently, their replacements were not trained in MMDP management. In most health facilities, the follow-up home visits of patients were done by the village health volunteers who integrated with other home visit health activities. The assessment also highlighted improvements needed for the MMDP. These included integrating MMDP care as part of the long-term care of the elderly, providing MMDP guidelines in digital media format, which can consist of knowledge on LF and MMDP in two languages, Thai and Melayu/Jawi languages as well as for staff training purposes for example, demonstrations of caring for patients with filariasis symptoms. In 2022, DVBD initiated integrated VBD TOT training courses for regional offices—epidemiology, surveillance, sample collection and vector management. The DVBD is developing online training modules to ensure regular training of health staff involved in MMDP. There is also a need for producing more brochures/posters about filariasis diseases and that these public health facilities have antifungal drugs and primary care equipment boxes. The following survey is planned for 2023.

Although the number of migrants tested, method, and target group through various agencies are reported, the figures are for general health checkups, and inclusion of TBF is variable depending on local arrangements with hospitals and laboratories and available budget. In addition, the number of LF positives from these migrant health screenings are not captured in the reporting. However, before COVID-19, over 2018–2019, five nonendemic provinces including Bangkok, reported twelve cases in the LF migrant screening program (provincial AGR: 0.2–0.5%), where all cases were among Myanmar nationals. In 2021, three cases among migrants were reported in the endemic province of Surat Thani (AGR 0.63%). Although these screening programs showed low yield over the 2018–2022 PVS period, these were perceived as essential to continue by the respective provinces, with more than 5 provinces conducting these screenings annually over 2018–2019. Due to the COVID-19 restrictions, this activity was not implemented in 2020 but resumed in 2021 and 2022. Due to budgetary constraints, only 4 provinces conducted this in 2021 and 2022, respectively, against the PVS target of five provinces per year. Both measures, the MDAs conducted in Myanmar [vide supra] and the screening and treatment of migrants in Thailand, were probable contributors to the decline in the number of LF cases detected in Thailand among Myanmar migrants. Among Thailand’s border with its neighbours, only Myanmar has several provinces that are endemic for LF caused by W. bancrofti and transmitted mainly by Culex quinquefasciatus. Although there has been some debate on human-vector combinations on the risk of W. bancrofti transmission across the Thai-Myanmar borders [21, 22], current data/information thus far is not sufficient to understand the vulnerabilities on how contagious the parasite is in such complex epidemiological settings as well as the receptivity of the vector in different ecological settings along the borders [3, 24]. In vector surveys in migrant settlements a total of 2408 mosquitoes were collected and dissected over 2018–2019, and no microfilaria or larva was detected, likely suggesting that the positive MF cases among migrants were importation and not indigenous transmission.

Since 2001, the Thai MoPH set up the migrant health insurance scheme for all migrants (documented and undocumented) who are not covered by social health insurance, allowing mandatory health screening (during the first entry and subsequent yearly renewal of the residence permit) which includes testing for bancroftian Mf (Mf provocation test with DEC) which is done at all district hospitals and for which a full course of treatment (single dose of DEC + ALB) is offered if found to be positive. In addition, the local health facilities are encouraged to treat the immigrant population regardless of legal status. Barriers to receiving DEC were lack of official documents, unemployed status, daily employment, short-term immigrant status, and living in a fishery area for immigrants [23].

DVBD needs increased collaboration with the appropriate agencies (vide supra) to obtain testing and outcomes to map distribution better and monitor migrant patient follow-up. Better profiling migrant populations [24] and developing criteria for prioritized group/s for periodic surveillance could be used to detect any LF cluster that may arise promptly [25, 26].

In the 9 displaced population (DP) camps located in 4 provinces in Thailand along the border with Myanmar, human blood surveys for LF are dependent on the availability of resources annually both at DVBD and in the PHO. Although surveys conducted in 2019 and 2022 have been limited in coverage of testing and yield of positive cases, a prior survey in one camp in Tak province in 2018 tested 2,634 persons using TBF and found 20 positive cases (microfilaria positive rate: 0.76%) who were treated after the survey in addition to MDA in the camp (personal communication, SR). Since there have been no records of lymphoedema or elephantiasis cases in DP camps, MMDP has not been initiated in these camps. Should there be a need, health clinics in the DP camps managed by an international NGO will be engaged to provide MMDP services. Given the current unrest in Myanmar and the increased movement of population across the border into Thailand, the current PVS strategy will require additional resources to extend both human blood and vector surveys in selected camps where previous cases have been recorded and that report an influx of new DPs into the camps. An appropriate sampling methodology will need to be developed for human blood surveys in addition to MDA protocols. Where positive LF is found, in addition to treatment, MMDP needs to be initiated by the NGO responsible.

As early as the late 1980s, cat surveys documented B. malayi and B. pahangi infection among domestic cats in all four B. malayi endemic provinces of Surat Thani, Nakhon Si Thamarat, Krabi, and Narathiwat. [5, 13]. Beginning in 2003, active surveillance of cats in areas with > 1.0% Mf rate among cats was done along with mass treatment of cats with ivermectin given subcutaneously as a strategy to interrupt possible zoonotic transmission. PVS results (10% of previously reported B. malayi IUs) over 2018–2022 showed positive Mf in cats in some IU where human Mf cases were found. More definitive studies are needed, although current cat surveys and treatment could be justified in targeted IUs in Narathiwat, where new human cases are found.

The occurrence of other species of filarial parasites, such as B. pahangi and Setaria spp., have been documented in Thailand [27] and demonstrated Ar. subalbatus to be a vector of zoonotic B. pahangi in Suratthani, Southern Thailand, where Thai children have been infected with zoonotic B. pahangi. Four cases were documented over 2012–-2020 in children less than 2 years of age living in rubber and oil palm plantations with varying manifestations of fever with or without lymphatic pathology. In two cases, proximity to B. pahangi nfected dogs or cats was documented. Microfilaria of Setaria spp. were also found in bullfighting cattle in the southern part of Thailand [28]. Currently, B. pahangi can be observed in Ar. Subalbatus, found in abundance in rubber or oil palm plantations, is the natural vector for zoonotic B. pahangi and can also transmit the disease to humans [29] through reservoir animals such as cats and dogs [30]. Although genetically, B. pahangi and B.malayi are closely related, their physiology, vector competence, and transmission potential differ [27]. As the ecological landscape in southern Thailand changes with peri-urban development, understanding both the exposure and receptivity of human-vector-animal interactions will be necessary [31]. This will require strengthening the capacity in diagnosis and surveillance for zoonotic infections through a One Health approach. The Phikulthong Royal Development Study Center in Narathiwat continues monitoring for zoonotic LF transmission while focusing on soil-transmitted helminthiasis (STH) and leprosy control as well. It could be an essential institution to take this forward.

Our study also has limitations. Only testing data on migrant worker routine health screening through various agencies was available to the DVBD. The yield from these screenings was not available to the authors. Further analysis is required to determine the magnitude of imported cases of LF. Our study could not assess the impact of health education activities and tools for LF prevention and control in the community. This would be an important area for further research for better targeting of communities at risk given the low prevalence even in persisting endemic IUs.