Five years of post-validation surveillance of lymphatic filariasis in Thailand

doi:10.21203/rs.3.rs-3215928/v1

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Research Article

Five years of post-validation surveillance of lymphatic filariasis in Thailand

https://doi.org/10.21203/rs.3.rs-3215928/v1

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published 06 Dec, 2023

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Background: Eighteen countries and territory (Cambodia, The Cook Islands, Egypt, Kiribati, Maldives, Malawi, Marshall Islands, Niue, Palau, Sri Lanka, Thailand, Togo, Tonga, Vanuatu, Viet Nam, Wallis and Futuna,Yemen and recently Bangladesh are now acknowledged as achieving elimination of lymphatic filariasis (LF) as a public health problem (WHO web). Work on lymphatic filariasis (LF) in Thailand started in 1949 with the first survey to identify lymphoedema cases. The Division of Lymphatic Filariasis under the Department of Health, Ministry of Public Health (MoPH) Thailand was established in 1961. The National Programme for Elimination on of Lymphatic Filariasis (NPELF) was launched in 2001. As a result of intensive interventions with the NPELF over more than 15 years, WHO certified Thailand in September 2017 as having eliminated LF as a public health problem with recommendations that continued surveillance would be required and people affected with chronic filariasis would have full care and access to support systems for treatment and care. This article describes measures and progress made in Thailand with post elimination surveillance of LF from 2018 till 2022.

Methods: The implementing unit (IU) is a sub-village in 11 former LF endemic provinces. Human blood surveys are targeted in 10% of sub villages each year. In Wuchereria bancrofti areas, filaria antigen test kits (FTS) are used and in B. malayi areas, antibody test kits (FilariaDIAG RAPID) are used and positive cases confirmed by thick blood film (TBF) and PCR. Vector surveys are conducted in 1% of sub-villages where human blood surveys are conducted in that year. Selection is guided by sub villages where the current LF vectors are predominant and known environmental reservoirs (B. malayi areas). Three nights of mosquito trapping are done where mosquito species identification and Mf detection is done. Brugia spp. are sent for PCR confirmation. Blood surveys using FTS are conducted among migrants in five provinces that record the highest number of labour migrants registered in the previous year and target 300 - 500 samples per province among migrants who are non-registered. Additional migrant health screening data for registered migrants surveillance are through health check-up conducted by various agencies, including Provincial Health Office, the Division of International Disease Control Ports”, the Department of Medical Service of the Bangkok Metropolitan Authority (BMA) and the One Stop Service (OSS), Office of the Permanent Secretary of MOPH. In the four provinces previously endemic for B. malayi, active surveillance of cats are done in areas that previously recorded > 1.0% mf rate among cats, using thick blood film (TBF) and positive TBF are confirmed with PCR. LF chronic disease survey and management is done every 2 years and includes MMDP and health facility assessments in LF endemic areas where a chronic disease patients reside under the care of a sub-district health facility in each province. The register of chronic patients is updated every year by the province and DVBD. The primary tools are the MMDP kit for chronic patients and the WHO Direct Inspection Protocol version 1.1 for health facilities and includes trainings for health facility staff.

Results:

Of a total of 357 former endemic IUs in 11 provinces, over the period of 2018-2022, human blood surveys were conducted in145 IUs (41%) with an average population coverage of 81%. 24 Mf positive cases detected in 10 IUs of Narathiwat province only. An additional 2 cases were detected in one province through routine public health surveillance. Over 2018-2022, vector surveys were done in all 11 provinces in 47 IUs with Mf (all B. malayi) detected only in Narathiwat province in 2018 and 2019 in dissected Ma. annulata and Ma. bonneae species. There was a reduction in the cohort of LF patients with lymphoedema/elephantiasis from 114 in 2017 to 76 in 2022 with all patients under the care of 42 health facilities. Surveillance among unregistered migrants yielded 12 antigen positive cases in 6 provinces over 2018-2022. However no Mf in mosquitoes were detected in vector surveys in these same areas. Survey among domestic/peri-domestic cats in Narathiwat province yielded a decline in MPR from 1.9% in 2018 to 0.7% in 2022. All Brugia species are confirmed by PCR. In IUs where Mf positive cats were detected, new human Mf cases were also detected. MMDP assessments revealed gaps in health care provider’s management of chronic cases due to staff turnover.

Conclusions: Current WHO recommendations are for countries to continue with PVS for 10 years after validation of LF elimination. In 2022, after 5 years of PVS, Thailand has re-surveyed 41% of all its 357 previously endemic IUs in 11 provinces and demonstrating on-going transmission in only one province of Narathiwat where Mf prevalence was below the WHO transmission threshold of 1% for 2018-2022 except for 2019 where it was 1.44%. It is envisaged Thailand will recover from program setbacks in 2020-21 due to COVID-19 and achieve 100% coverage of its IUs by 2027. Passive surveillance nation wide, targeted migrant screening in specified provinces, MMDP refresher training especially for newly stationed health facility staff and interventions for possible zoonotic transmission in Narathiwat province needs to continue in the PVS phase.

Lymphatic filariasis

Elimination

Post validation

Surveillance

Neglected tropical diseases

Thailand

LF epidemiology in Thailand

Historically, Lymphatic filariasis (LF) had been endemic only in some parts of Thailand, with both Brugian and Bancroftian filariasis being reported [¹]. The first survey for LF was recorded as early as 1949 by the Department of Health and found that there was lymphatic filariasis, lymphoedema cases in six southern provinces; Chumphon, Suratthani, Nakhonsithammarat, Phatthalung, Pattani and Narathiwat with microfilarial positive rate averaging 21.0% (2.9 to 40.8%), all cases were Brugia malayi infection. The Elephantiasis rate was 5.2%. The vector identified were 4 species of Mansonia and 5 species of Anopheles infected with infective stage larvae of B. malayi [²]. The disease was recognized as being of public health importance in 1953. Between 1961 and 1988 numerous LF surveys were conducted in these provinces including three provinces along the Thailand-Myanmar border. Mf surveys were then expanded over 1994-1995 to cover 32 provinces [³].

A baseline epidemiological survey was initiated in 2001 identified 7 provinces endemic for W. bancrofti, transmitted by Aedes niveus mosquito species; and 4 provinces endemic for B. malayi, transmitted by Mansonia mosquito species. The W. bancrofti endemic provinces are located in north and central Thailand, B. malayi endemic provinces are in south Thailand. MDA with diethylcarbamazine citrate (DEC) and albendazole (ALB) was implemented in a total of 357 implementation units (IUs) in these 11 LF endemic provinces (total population of the 357 sub-villages in 2002 was 124,496) for a total of 5 rounds annually over 2002 – 2006. The IU was a sub-village. Although all 11 provinces are endemic, 4 of these provinces – Mae Hong Son, Tak, Kanchanaburi and Narathiwat, accounted for 336 of 357 (94%) of endemic sub-villages. In the same 11 provinces, a total of 283 villages were excluded from MDA, as the Mf and/or Ag prevalence was below threshold levels of 1.0% and 2.0% respectively. A total of five rounds of MDA annually were implemented over 2002–2006 in all IUs. Additional annual rounds of MDA were required in 87 IUs of Narathiwat province from 2007 to 2011 due to persistent infection. The annual national drug coverage with MDA over 2002–2012 was in the range of 68.0 to 95.4%. Stop-MDA surveys in 2006 in the 11 LF endemic provinces found nine mf positive cases in seven IUs in Narathiwat province with the highest prevalence of 0.8% (range: 0.1–0.8%). Three transmission assessment surveys (TAS-1, TAS-2 and TAS-3) conducted over 2012-2017, where all 357 IUs were surveyed. clearly indicated that transmission was completely interrupted in all five evaluation units (EUs). Contact tracing both all mf cases in all three TAS yielded no positive cases [⁴].

A 2001 survey of the chronic disease burden for lymphatic filariasis established a register of the number of people in endemic provinces with lymphedema/elephantiasis. The number of persons declined from 284 in 2001 to 99 patients followed-up under 34 health centers, of which, a total of 69 patients (70%) were under the care of 14 health centers in just one province of Nakhon Si Thammarat [3].

testing for bancroftian mf (mf challenge 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, in a number of provinces (average: 19, range: 13–25) where there were significant number of migrant workers registered, sentinel site surveillance for Mf was done annually between 1996 and 2001 with a

total of 204,108 persons tested with a blood film for Mf yielding an average positivity rate of 0.7% (range: 0.2–2.2%) over the same period [⁶]. With the commencement of the NPELF, the annual surveillance of migrants was focused on seven provinces over 2002–2017 with 23,477 persons tested for LF antigen using ICT test cards, showing a positivity rate of 0.75% (range: 0.14–2.75%) over the same period. Where antigen positivity was detected among migrants in these areas, the Thai populations residing in close proximity were also concurrently tested over the same period (average 2,616) with zero positivity rates [3,4].

In the province of Narathiwat, surveys and treatment with Ivermectin among domestic cats commenced in 1994 and was conducted annually in that province as a measure to prevent possible zoonotic transmission. In areas with >1.0% Mf rate among cats in Narathiwat province, annual ivermectin treatment resulted in a decline of Mf prevalence among cats from 8.07% in 1995 to as low as 0.84% in 2015 [3].

LF programme structure in Thailand

In the year 1961, the Division of Lymphatic Filariasis was established under the Department of Health with a primary strategy of using DEC to control LF in known endemic areas. Following the 1997 World Health Assembly Resolution calling for the elimination of LF as a public health problem and establishment of the Global Programme to Eliminate Lymphatic Filariasis (GPELF), the MoPH Thailand launched the National Programme to Eliminate LF (NPELF) in Thailand in 2001 [⁷]. The structure and organization of the programme is shown in figure 1. The Division (formerly Bureau) of Vector-Borne Diseases (DVBD) under the Department of Disease Control in the MOPH implements vector control and disease control programmes. Within the DVBD, the Cluster of Lymphatic Filariasis, headed by a programme manager coordinates the implementation of the NPELF.

At the province level, the respective provincial public health office, headed by the provincial chief medical officer coordinates the implementation of the programme (particularly during the MDA phase), within the endemic provinces through liaising with district health offices (DHOs). The provincial Vector Borne Disease Centre (VBDC) plays an important role in the monitoring and evaluation (M&E) and surveillance activities. The DHO interacts with sub-district and village level health workers and monitor the implementation of the programme, actively supported by the district level Vector Borne Disease Unit (VBDU) in M&E and surveillance activities. The staff of sub-district health facilities and village health volunteers continue to implement various activities of the programme such as MDA, M&E and surveillance and MMDP. The Royal Thai Government has also ensured that resources continue to be allocated for LF surveys, integrated vector control efforts and screening among at risk groups. With the establishment of the Universal Health Coverage (UHC) scheme in 2001 and subsequently migrant health insurance schemes, the provision of free morbidity management and disability prevention services were extended to the sub-district Tambon Health Promotion Hospital and for Thai, both registered and unregistered migrants. The Phikulthong Royal Development Study Center in Narathiwat province established the Thai Royal Filaria Project continues to provide all necessary support with infrastructure and required personnel for LF for post elimination efforts in Narathiwat province.

Lymphatic Filariasis post validation surveillance (PVS) in Thailand (2018-2027)

DVBD has developed national guidelines for PVS of LF over a 10-year post validation (2018-2027) period as currently recommended by WHO [⁸]. The objective is to maintain LF infection rate < 1%. Key strategies include human blood surveys in sub-villages and among migrant populations, vector surveillance, blood surveys in animal reservoir (cats) and morbidity management and disability prevention (MMDP). This is summarized in figure 2 and table 1.

Target areas for human blood and vector surveysare previous endemic areas (11 provinces, 357 IUs) that are now called sentinel sites and based on clinical case/mf infection, and vector reservoir. Human blood surveys are conducted in 10% of sub villages each year. The objective will be to achieve a population coverage of 100% in reported area study sites within 10 years of the PVS phase. In W. bancrofti areas, filaria antigen test kits (FTS) are used and in B. malayi areas, antibody test kits (FilariaDIAG RAPID) are used and positive cases confirmed by thick blood film (TBF) and PCR, as shown in figure 3. Targeted drug administration/Mass drug administration will be considered based on the case investigation of new case/s, for example where they are new indigenous case/s reported versus imported cases. Vector surveysare conducted in 1% of sub-village where human blood surveys are conducted in that year. Selection is guided by sub villages where the current LF vectors are predominant and known environmental reservoirs (B. malayi areas). Three nights of mosquito trapping are done where mosquito species identification and Mf detection is done. Brugia spp. are sent for PCR confirmation at DVBD.

Blood surveys among migrants in Thailand will be conducted using FTS in five provinces recording the highest number of migrants from LF endemic countries registered in the previous year (300 - 500 samples per province), who are non-registered. This is in addition to migrant health screening done for registered migrants (table 6a). There are 9 displaced population (DP) camps in 4 provinces along the Thailand-Myanmar border. If a case is confirmed during human blood surveys, a case investigation will follow, and treatment given regardless of nationality or legal status.

Cat surveys and treatment to prevent possible zoonotic transmission of LF. In the four provinces previously endemic for B. malayi, active surveillance of cats in areas that previously recorded > 1.0% mf rate among cats, using thick blood film (TBF), is done along with treatment of cats with ivermectin given subcutaneously for all cats during screening. Mf positive from TBF (figure 4) are confirmed with PCR at DVBD.

LF chronic disease survey and management includes MMDP and health facility assessments in LF endemic areas where a chronic disease patients reside under the care of a sub-district health facility in each province. The register of chronic patients is updated every year by the province and DVBD. The primary tools are the MMDP kit for chronic patients and the WHO Direct Inspection Protocol version 1.1 for health facilities. Home visits to patients and trainings for health facility staff are also included. These activities are conducted every 2 years. The MMDP strategy is patient centered with services given in a comprehensive care package closest to the patients home aligned to the WHO guidelines [⁹,¹⁰].

Health promotion. Health education on LF done simultaneously during all PVS activities. Regional offices also include LF education during general health education activities. DVBD, as part of health education for vector borne disease also transmits specific LF prevention messaging through online social media channels.

In 2022, DVBD initiated integrated vector borne diseases (VBD) training of trainers (TOT) training courses for regional offices – epidemiology, surveillance, sample collection, vector management.

Human blood surveys

Table 2a summarizes cumulative coverage of surveys in the targeted 11 provinces, over 2018-22. Table 2b details the results of human blood surveys in the 11 provinces, over 2018-2022. ‘Target population’ refers to population targeted in each sub village every year of survey. Over 2018-2022, coverage of IUs was a 100% in provinces of Chiang Mai, Krabi and Nakhon Si Thammarat where there were three or less IUs in each of these provinces. Although all IUs were surveyed, population coverage in the surveys varied, with 100% in Lamphun province and a median of 97.2% in Chiang Mai followed by 63.2% in Krabi and 50.1% in Nakhon Si Thammarat. No cases were detected in these provinces through FTS over 2018-2022. In the provinces that border to neighbouring Myanmar – Mae Hong Son (76 IUs), Tak (126 IUs), Kanchanaburi (49 IUs), Ratchaburi (4 IUs) and Ranong (2 IUs) the IU coverage over 2018-2022 was 31.6%, 32.5%, 38.3%, 75% and 100% respectively. The median population coverage was 74.2%, 86.9%, 89.8%, 92% and 62.5% respectively. No cases were detected through FTS over this period.

In the southern provinces of Surat Thani (6 IUs) and Narathiwat (86 IUs), the coverage of IUs in Surat Thani was 66.7% and 51.2% in Narathiwat. The median population coverage in Surat Thani was 100% and 88.7% in Narathiwat.

In Narathiwat province, a total of 44 out of a total of 86 IUs were surveyed over 2018-2022. FilariaDIAG RAPID are used and positive cases confirmed by TBF detected a total of 24 Mf positive cases in 4 subdistricts affecting 10 IUs, table 3. In all IUs in Narathiwat province the MPR was <1% during each survey except for one IU, Paye, in 2019 with 2 cases and APR of 3.0%.

Routine public health surveillance

In 2022, in Ratchaburi province, 2 cases were detected through passive/routine case surveillance in a new IU. One case was diagnosed as Mf+ and another case as Lymphoedema. In Narathiwat, 2 cases were detected through routine surveillance. Both cases were Mf+ diagnosed by FilariaDIAG RAPID and confirmed by TBF.

Vector surveys

Vector surveys were done in all the target 11 provinces in 47 IUs over 2018-2022, table 4a. Vector surveys are conducted in 1% of sub-village where human blood surveys are conducted in that year. Over the 5 years, Mf was detected only in Narathiwat province in one isolate in 2018 (Ma. annulata/L2 stage) and 5 isolates in 2019 (Ma. bonneae/L1, L2 stage; Ma. annulate/L1, L2 stage; Coquillettidia crassipes/L3stage). All species were B. malayi. In the vector surveys, three nights of mosquito trapping were done. Mosquito species identification and Mf detection was done by ODPC laboratory and confirmed by DVBD. All Brugia spp. were sent for PCR confirmation at DVBD. Table 4b lists mosquito species collected and dissected in 11 provinces during vector surveys, 2018-2022.

LF chronic disease survey and management

In 2022, there were 85 LF cases in Thailand, table 5, which as of 2022 were under the care of 42 health facilities in 8 provinces. The ODPC 11 Nakhon Si Thammarat monitors cases in the provinces of Chumphon, Surat Thani, Nakhon Si Thammarat, while ODPC 12 Songkhla monitors cases in the provinces of Songkhla, Patthalung, Pattani and Narathiwat. An MMDP assessment was carried out by the DVBD and ODPC in 2020-2021 in 7 provinces using the WHO Direct Inspection Protocol version 1.1 with 14 indicators with the addition of an indicator on Covid-19. The results of the assessment are shown in figure 5.

The assessment in 2021 as part of the PVS, showed the reducing number of LF patients from 114 (99 with lymphoedema/elephantiasis) in 2017 to a total of 85 (76 with lymphoedema/elephantiasis) in 2022. Of the total patients in 2022, 38 (44.7%) were under the care of 13 health facilities in just one province of Nakhon Si Thammarat. Of the thirty nine health staff surveyed, all new of COVID-19 measures to be taken for prevention, 84% on average knew signs of symptoms of LF, lymphoedema, how to manage a case of LF and report but only 67% on average were trained and aware of guidelines and relevant IEC material for the management of LF.

LF surveillance among migrants

Similar to LF surveillance among migrants prior to validation, in the PVS period, surveys are conducted in two ways. The first are annual LF surveys initiated by the ODPC, VBDC and VBDU in provinces where there are high number of registered migrant workers. These surveys are supervised by the DVBD. The PVS target is to screen 300-500 migrants who are unregistered in these areas. The second form of surveillance are for registered migrants through health check-up conducted by various agencies, including Provincial Health Office, the Division of International Disease Control Ports, the Department of Medical Service of the Bangkok Metropolitan Authority (BMA) and the One Stop Service (OSS), Office of the Permanent Secretary of MOPH. These are summarized in table 6a and results of surveys of unregistered migrants intable 6b. In the annual LF surveys among migrants conducted by DBVD and VBDU, FTS is used for screening and a total of 12 antigen positive cases among unregistered migrants were detected in 6 out of 10 provinces over 2018-2022. All positive cases were treated with DEC, followed up by local staff. No blood surveys were done in 2020 due to Covid-19 lockdowns and restrictions. In addition, in eight of these provinces where blood surveys were done, vector surveys were also conducted at the same time, in the same area table 6c. No vector surveys were done in 2020-2022 in migrant settlements due to Covid-19 lockdowns and restrictions. A total of 2,408 mosquitoes were collected over 2018-2022 and dissected by VBDC and ODPC and no microfilaria was detected.

Tak province borders to Kayin state in Myanmar. Two out of the five townships in Myanmar are endemic for LF. In these two townships, MDA for LF has been stopped and TAS-1 has been conducted with TAS-2 planned for 2023.

The provinces of Ratchaburi, Petchaburi, Chumporn, Prachuapkirikhan and Ranong border to Tanintharyi region in Myanmar. Three out of the four townships in this region are endemic for LF of which two townships have stopped MDA for LF, conducted TAS-1 and TAS-2 planned for 2023.

In the Kayah and Shan regions of Myanmar that border to Thailand, LF is not endemic.

There are 9 displaced population (DP) camps or temporary shelters located in 4 provinces in Thailand along the border with Myanmar which have been in existence between 10 to 20 years. These camps are under the responsibility of the ministry of interior and health services are managed by an international NGO. The current population is estimated at 84,133. Data on LF surveys and cases are not available prior to the start of the PVS. In 2019, LF surveys were conducted in all 9 camps. Screening was based on available financial, human resources and availability of FTS. A total of 2,947 (3%) persons were tested and 6 cases of Ag+ cases were detected and treated, table 7. In 2022, due to the COVID-19 pandemic and available resources, LF surveys was only done in one DP camp with 1,283 (55%) of the camp population tested by FTS and 5 Ag+ cases (AGR: 0.39) detected. Vector surveys for microfilaria are not done in DP camps. Cases detected in the DP camps are not included in the annual number of cases reported by the DVBD.

LF in cats

Post elimination surveillance in Narathiwat and three other B. malayi endemic provinces continued to include cat surveys and treatment to prevent possible zoonotic transmission of LF, table 8a. Survey is done in the same village as the human surveys for the year and are conducted by the VBDC and VBDU staff. In practice, during surveys, all cats surveyed (tested) are treated at the same time due to difficulty in following up. In 2018, the MPR among cats in Narathiwat province rose to 1.9% and in Surat Thani to 18.6%. It is unclear why there was a high rate for only 1 year in Surat Thani. However, the rates declined in subsequent years with no Mf positive cats detected in Surat Thani and to a MPR of 0.7% in Narathiwat. All Brugia species are confirmed by PCR, at DVBD and for 2018-2022 all were B. malayi. A register of household cats is maintained by ODPC 11 and the VBDC. For Narathiwat by the Phikulthong Royal Development Study Center in Narathiwat with the name/characteristics of the domestic cat and record of blood testing and ivermectin injection at a dose of 0.1cc/kg delivered subcutaneously. In IUs where Mf positive cats were detected, new human Mf cases were also detected (2019: 1; 2021: 1; 2022: 1), table 8b.

Human blood surveys. The human blood surveys over 2018-2022 showed that a total of 144 IUs (40.2%) were surveyed (total 357 IUs) with a median population coverage of 88.7%. The PVS strategy is 10% of total IUs per year for 10 years. Hence, at 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 are geographical differences, with provinces along the Myanmar border facing challenges due to the current political situation along the border. 100% of IUs in provinces of Chiang Mai, Ranong, Krabi and Nakhon Si Thamamarat where there were three or less IUs in each of these provinces WHO recommends 10 years PVS to integrate with other community efforts in these few IUs. 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.

Vector surveys. In Thailand and other countries in Southeast Asia, the principal vectors of B. malayi are Mansonia mosquitoes including Ma. uniformis, Ma. indiana, Ma. annulifera, and Ma. bonneae [¹¹,¹²]. With the current program protocol of conducting vector surveys in 1% of IUs in previously endemic provinces, only Narathiwat province has detected Mf in vectors. Mf found in vectors were not found in IUs (sub village) where Mf human cases were detected but were found in the same subdistricts. Only Paye IU in Narathiwat province has demonstrated autochonous transmission. As vector surveys is a labour intensive activity, and transmission of lymphatic filariasis is significantly influenced by vector density [¹³,¹⁴], the protocol may need revision. intervention programmes rolled out in endemic areas should be specific and targeted in each endemic foci where transmission is focal [¹⁵,¹⁶] while 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 existing low transmission levels in most endemic communities in Thailand, molecular based techniques, as shown in other countries, may be an effective tool for xenomonitoring [¹⁷,¹⁸].

LF chronic disease survey and management. The last assessment of the quality of lymphedema services in 2017, prior to validation of elimination, identified high staff turn-over that affected the knowledge of responsible officers and availability of MMDP materials. The assessment in 2021 also identified issues with retention of health staff who were previously trained either due being repositioned to other health offices and tasks or to retirement. Subsequently, their replacements were not trained on MMDP management. In most health facilities, the follow-up home visits of patient 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 include 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. There is also a need for producing more brochures/posters about filariasis diseases and ensuring these public health facilities have antifungal drugs and primary care equipment boxes. The next survey is planned for 2023.

LF surveillance among migrants. Although the number of migrants tested, method, target group through various agencies are reported, the figures are for general health checkup 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, prior to COVID-19, over 2018-2019, five non endemic 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 important to continue by the respective provinces with more than 5 provinces conducting these screening 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 of which 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 [¹⁹,²⁰], current data/information thus far is not sufficient to understand the vulnerabilities on how contagious is the parasite in such complex epidemiological settings as well as the receptivity of the vector in different ecological settings along the borders [9,10]. In vector surveys in migrant settlements over a total of 2,408 mosquitoes were collected and dissected over 2018-2019 and no microfilaria 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 challenge 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 [²¹]

Increased collaboration with the relevant agencies as shown in table 6a, are needed by DVBD to obtain testing and outcomes to better map distribution and monitor migrant patient follow up.

Better profiling migrant population [²²]and developing criteria for prioritized group/s for periodic surveillance could be used to promptly detect any LF cluster that may arise [²³,²⁴].

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 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). There are no records of MMDP being extended to the positive cases in the camp. Given the current unrest in Myanmar 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 influx of new displaced populations (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 are found, in addition to treatment, MMDP needs to be initiated by the NGO responsible.

LF in cats. 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 ivermection given subcutaneously as a strgategy to interrupt possible zoonotic transmission. PVS results (10% of previously reported B.m IUs) over 2018-22 showed positive Mf in cats in some of the 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.

LF and zoonotic transmission. The occurrence of other species of filarial parasites, such as B. pahangi and Setaria spp., have been documented in Thailand [9]. A recent study [²⁵] showed Armigeres 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 of the cases, proximity to B. pahangi-infected dogs or cats were documented. Microfilaria of Setaria spp. were also found in bullfighting cattle in the southern part of Thailand [²⁶]. Currently, B. pahangi can be observed in Ar. subalbatus, found in abundance in Rubber or oil palm plantations the natural vector for zoonotic B. pahangi and can also transmit the disease to human [²⁷] through reservoir animals such as cats and dogs [²⁸]. Although genetically B.p and B.m are closely related, the physiology, vector competence and transmission potential differ [29]. As the ecological landscape in Southern Thailand changes with peri-urban development, understanding the both exposure and receptivity of human-vector-animal interactions will be important [²⁹]. 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 STH and Leprosy control as well and could be an important institution to take this forward.

Sustained commitment by the government and dedicated health staff on the ground throughout the elimination phase, not only ensured the NPELF objectives were met finally in 2017 but also in ensuring that high quality of care be continued for chronic LF patients and adoption of a robust PVS program. The program will continue its LF surveillance efforts especially among migrant populations along provinces bordering Myanmar. WHO emphasizes that validation implies a potentially reversible state and countries should continue to conduct post-validation surveillance and ensure integration of MMDP and to health services. Current WHO recommendations are for countries to continue with PVS for 10 years after validation of LF elimination. In 2022, after 5 years of PVS, Thailand has re-surveyed 41% of all its 357 previously endemic IUs in 11 provinces and demonstrating on-going transmission in only one province of Narathiwat where Mf prevalence was below the WHO transmission threshold of 1%. It is envisaged Thailand will recover from program setbacks in 2020-21 due to COVID-19 and achieve 100% coverage of its IUs by 2027. Passive surveillance nation wide, targeted migrant screening in specified provinces, MMDP refresher training especially for newly stationed health facility staff and interventions for possible zoonotic transmission in Narathiwat province needs to continue in the PVS phase.

Ab: Antibody; Ag: Antigen/antigenaemia; AGR: antigen rate; ALB: Albendazole; DVBD: Division of Vector Borne Diseases; DDC: Department of Disease Control; EU: Evaluation Unit; FTS: filaria test strip; GPELF: Global Programme to Eliminate Lymphatic Filariasis; IU: Intervention/implementation Unit; LF: Lymphatic Filariasis; M&E: Monitoring and Evaluation; MDA: Mass Drug Administration; MMDP: Morbidity Management and Disability Prevention; Mf: Microfilaria; MoPH: Ministry of Public Health; NTD: Neglected Tropical Diseases: NPELF: National Programme to Eliminate LF; ODPC: regional office of disease control and prevention; PCD: passive case detection; PCR: polymerase chain reaction; PHO: Provincial Health Office; PVS: post-validation surveillance; SEARO: WHO South-east Asia Regional Office; STH: Soil-transmitted Helminthiases; TAS: Transmission Assessment Survey; VBDC: Vector Borne Disease Centre; VBDU: Vector Borne Disease Unit; WHO: World Health Organization.

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Availability of data and materials

The datasets are available from the corresponding author upon reasonable request.

Competing interests

The authors declare that they have no competing interests. DG is a staff member of the World Health Organization (WHO) at the time of writing of this paper and is alone responsible for the views expressed in this publication, which do not necessarily represent the decisions or policies of WHO.

Funding

WHO assisted with the procurement of FTS for the DVBD as well as supported LF surveys and training programmes for LF patients.

Authors’ contributions

RK, PM and TS participated in the data collection, analysis and interpretation of the data. RK, PM, TS and DG contributed to the data consolidation and interpretation. DG, RK, PM and TS drafted and DG, RK, PM and TS reviewed and revised the manuscript. All authors read and approved the final manuscript.

Acknowledgements

The authors of this report would like to thank field staff from the 11 endemic LF provinces, the village health volunteers and 5 regional offices for their commitment and hard work and who were instrumental in the implementation of the PVS strategies over the last 5 years. WHO assisted with the procurement of FTS for the DVBD as well as supported LF surveys and training programmes for LF patients. Authors are grateful for Ms Yuthika Chartmontree’s administrative support.

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Table 1: Thailand: LF post validation strategies 2018-2027

Activities	Target areas and population
	Routine surveillance		Health screening****
	Previous endemic area (357 Sub-villages target unit)	5 provinces with the highest number of registered migrant workers*** in the previous year	Migrant access through One Stop Service (OSS) health checks up for registered labour migrants	Unregistered migrants
Surveillance
Routine national disease surveillance reporting	+	+
Human blood survey, case investigation and treatment	+	+	+	+
Vector survey*	+	+
Reservoir survey (Bm. area)	+
Control measures
TDA/MDA**	+	+	+
Reservoir treatment (Ivermectin for cats)	+
Prevention
Health Promotion	+	+
Service provision and evaluation
Morbidity management and disability prevention (MMDP) (every 2 years)	+

* 1% of sub-village where human blood surveys are conducted

** Targeted drug administration/Mass drug administration - optional based on the case investigation of new case/s

*** Planned surveys conducted by provinces and DVBD

**** health checkup for registered and unregistered migrants through different channels (provincial health office,

immigration bureau, OSS, displaced persons shelters)

MDA: Mass drug administration

Bm.: Brugia malayi

Table 2a: Cumulative coverage of surveys in the targeted IUs in 11 provinces, over 2018-2022.

	Province	No. of PVS districts	No. of PVS sub-districts	No. of PVS villages	No. of PVS sub-villages (IU)	No. and (%) of PVS sub villages with blood surveys 2018-2022	% Population coverage median and (range)
1	Mae Hong Son	2	6	28	76	24 (31.6%)	72.5 (51.5-100.0)
2	Chiang Mai	1	2	2	2	2 (100%)	97.0 (94.4-100)
3	Tak	5	17	57	124	41 (32.5%)	81.3 (37.2-100.0)
4	Kanchanaburi	3	11	27	49	19 (38.8%)	89.6 (80.2-100.0)
5	Lamphun	1	1	3	3	1 (33.3%)	100.0 (100.0)
6	Ratchaburi	1	2	3	4	3 (75.0%)	86.3 (83.0-100.0)
7	Ranong	1	2	2	2	2 (100%)	54.0 (51.0-74.0)
8	Krabi	1	1	1	2	2 (100%)	74.7 (32.7-93.6)
9	Nakhon Si Thammarat	1	1	1	2	2 (100%)	55.3 (39.5-60.7)
10	Surat Thani	2	2	6	6	4 (66.7%)	94.4 (66.7-100.0)
11	Narathiwat	7	22	87	87	45 (51.7%)	85.3 (56.2-88.7)

Table 2b: Results of human blood surveys in 11 provinces: PVS 2018-2022

Province	Year	No. if IUS surveyed	Population surveyed	FTS	TBF	Ag+	Mf+	AGR%	APR%	Coverage (%)
Mae Hong Son	2018	0	0	0	0	0	0	0	0	0
	2019	8	1143	909	0	0	0	0	0	81.62
	2020	0	0	0	0	0	0	0	0	0
	2021	8	1895	1411	0	0	0	0	0	75.87
	2022	9	1763	1159	0	0	0	0	0	72.49

Chiang Mai	2018	0	0	0	0	0	0	0	0	0
	2019	0	0	0	0	0	0	0	0	0
	2020	0	0	0	0	0	0	0	0	0
	2021	1	250	236	0	0	0	0	0	94.40
	2022	2	211	211	0	0	0	0	0	100.00

Tak	2018	11	2587	0	1852	0	0	0	0	75.24
	2019	18	4878	4461	0	0	0	0	0	97.48
	2020	12	4144	3128	0	0	0	0	0	69.79
	2021	0	0	0	0	0	0	0	0	0
	2022	0	0	0	0	0	0	0	0	0

Kanchanaburi	2018	0	0	0	0	0	0	0	0	0
	2019	5	3411	3063	0	0	0	0	0	89.25
	2020	3	1395	1275	0	0	0	0	0	92.10
	2021	6	1409	1305	0	3	0	1.25	0	94.25
	2022	5	1931	1657	0	0	0	0	0	87.24

Lamphun	2018	0	0	0	0	0	0	0	0	0
	2019	0	0	0	0	0	0	0	0	0
	2020	0	0	0	0	0	0	0	0	0
	2021	0	0	0	0	0	0	0	0	0
	2022	1	238	238	0	0	0	0	0	100.00

Ratchaburi	2018	0	0	0	0	0	0	0	0	0
	2019	1	993	824	0	0	0	0	0	82.98
	2020	2	522	483	0	0	0	0	0	96.03
	2021	0	0	0	0	0	0	0	0	0
	2022 *	0	0	0	0	0	0	0	0	0

Ranong	2018	0	0	0	0	0	0	0	0	0
	2019	0	0	0	0	0	0	0	0	0
	2020	1	1692	510	0	0	0	0	0	30.14
	2021	1	1458	1080	0	0	0	0	0	74.07407
	2022	1	352	300	52	0	0	0	0	100

Krabi	2018	0	0	0	0	0	0	0	0	0
	2019	1	799	0	193	0	0	0	0	25.03
	2020	1	131	0	131	0	0	0	0	100.00
	2021	0	0	0	0	0	0	0	0	0
	2022	0	0	0	0	0	0	0	0	0

Nakhon Si Thammarat	2018	1	1388	0	530	0	0	0	0	38.18
	2019	0	0	0	0	0	0	0	0	0
	2020	2	917	0	372	0	0	0	0	40.62
	2021	1	132	0	132	0	0	0	0	100.00
	2022	0	0	0	0	0	0	0	0	0

Surat	2018	1	502	0	487	0	0	0	0	97.01
	2019	0	0	0	0	0	0	0	0	0
	2020	2	845	0	666	0	0	0	0	83.36
	2021	1	116	116	0	0	0	0	0	100.00
	2022	1	102	102	0	0	0	0	0	100.00

Narathiwat	2018	9	6115	0	4911	0	9	0	0.4 [0.08-0.77]	76.24
	2019	10	6847	0	6284	0	8	0	1.44 [0.45-3.03]	93.54
	2020	8	5200	0	3791	0	4	0	0.45	72.62
	2021	8	4819	0	3989	0	1	0	0.42	83.92
	2022	11	5292	0	4351	0	2	0	0.53	83.11

*in 2022, in Ratchaburi, 2 cases were detected through passive case surveillance in a new IU – 1 Mf+ and 1 Lymphoedema

Table 3: Mf positive cases in Narathiwat province: 2018-2022

Year	Province	District	Sub-district	Sub-Village	Target Population	TBF	Population Coverage (%)	Mf+	APR (%)
2018	Narathiwat	Mueang	Kaluwo	Koksila	656	440	67.1	1	0.2
2018	Narathiwat	Tak Bai	Bang Khun Thong	Bangkhunthong	1411	1252	88.7	1	0.1
2018	Narathiwat	Su-ngai Kolok	Puyo	Towo-og	960	775	80.7	4	0.5
2018	Narathiwat	Su-ngai Padi	Su-ngai Padi	Tasaenuea	462	389	84.2	3	0.8

2019	Narathiwat	Su-ngai Kolok	Puyo	Gubae-e-gae	595	595	100.0	5	0.8
2019	Narathiwat	Su-ngai Padi	Su-ngai Padi	Pawei	223	223	100.0	1	0.4
2019	Narathiwat	Su-ngai Padi	Su-ngai Padi	Paye	67	66	98.5	2	3.0

2020	Narathiwat	Tak Bai	Phron	Banyai	485	441	90.9	2	0.5
2020	Narathiwat	Su-ngai Kolok	Puyo	Gubae-e-gae*	595	595	100.0	2	0.3

2021	Narathiwat	Su-ngai Padi	Su-ngai Padi	Banta	248	239	96.4	1	0.4

2022	Narathiwat	Tak Bai	Bang Khun Thong	Kok-ngu	377	374	99.2	2	0.5

					6,079	4,796	90.6	24	0.7
	* in this sub-village population coverage was achieved in 2019 (100%). In 2020, a repeat survey was done for research purposes and detected 2 additional Mf+ cases.

Table 4a: Vector surveys: 2018-2022: 11 provinces: Number of IUs, mosquitoes dissected and positive with Mf

Province	Total No. of IUs surveyed	2018		2019		2020		2021		2022
Province	Total No. of IUs surveyed	No. of mosquitoes dissected	(No.) of mosquitoes with Mf	No. of mosquitoes dissected	(No.) of mosquitoes with Mf	No. of mosquitoes dissected	(No.) of mosquitoes with Mf	No. of mosquitoes dissected	(No.) of mosquitoes with Mf	No. of mosquitoes dissected	(No.) of mosquitoes with Mf
Chiang Mai	1	0	0	0	0	0	0	29	0	0	0
Lamphun	1	0	0	0	0	0	0	0	0	63	0
Kanchanaburi	19	0	0	361	0	162	0	34	0	975	0
Krabi	1	0	0	0	0	0	0	0	0	26	0
Mae Hong Son	6	272	0	60	0	0	0	35	0	90	0
Nakhon Si Thammarat	2	26	0	0	0	0	0	0	0	30	0
Narathiwat	9	1,132	1 *	2,344	5 **	493	0	1036	0	514	0
Ranong	1	0	0	71	0	0	0	0	0	0	0
Ratchaburi	1	0	0	55	0	0	0	0	0	0	0
Surat Thani	2	108	0	0	0	0	0	0	0	12	0
Tak	4	231	0	381	0	0	0	0	0	0	0

* Ma. Annulata/ stage L2 (1 specimen)

** Ma. bonneae, stage L1,L2 (2 specimens), Ma. annulata, stage L1,L2 (1 specimen), Cq. Crassipes, stage L3 (1 specimen), Ma. Annulata, stage L2 (1 specimen) L=Larva stage

Table 4b: List of mosquito species collected and dissected in 11 provinces during vector surveys, 2018-2022

Province	Mosquito species identified during vector surveys 2018-2022
Chiang Mai	Cx. pseudovisnui, Armigeres spp., An. tesselatus, An. philipinesis, An. spendidus
Lamphun	Cx. pseudovisnui, Armigeres spp., An. tesselatus, An. philipinesis
Kanchanaburi	Ae. desmotes, Ae. desmotes, Ae. albopictus , Ae. imitator, Ae. annandalei, Cx. quinquefasciatus, Ae. niveus
Krabi	Ma. uniformis, Ma. dives, Ma. boneae
Mae Hong Son	Ae. albopictus, Ar. subalbatus, An. barbirostris , Ar. flavas , Ae. annandalei, Cx. quinquefasciatus, An. minimus, An. maculatus, An. sawadwongporni, Ae. poicilius, Ae. vitatus
Nakhon Si Thammarat	Ma. boneae, Ma. dives, Ma. uniformis, Cx. gelidus
Narathiwat	Ma. bonneae, Ma. annulata, Ma. indiana, Ar. subalbatus, Ae. albopictus, Anopheles spp., Cq. nigrosinata, Cq. crassipes, Ma. uniformis, Cx. psuedosininsis, Ma. uniformis, Ma. annulifora, Cx. gelidus, Culex spp.
Ranong	Cx. quinquefasciatus, Cx. gelidus, Cx. sitiens, Ae. aegypti, Ae. albopictus
Ratchaburi	Ae. albopictus, Ae. subalbatus, Ma. indiana, An. barbirostris, Ae. vitatatus
Surat Thani	Ma. indiana, Ma. annulifera, An. karwari, An. letifer, Ae. albopictus, Culex spp., Ar. subalbatus, Ma. dives, Ma. bonneae, Ma. uniformis
Tak	Niveus spp., Ae. albopictus, Ae. imitator, Ar. subalbatus, Ar. leicesteria, Heizmannia sp., An. minimus, Ae. desmastes, Ae. annandalei, Ar. kesseli, Ae. poicilus , Cx. tritaeniorrynchus

Ma.= Mansonia, Cq.= Coquillettidia, An.= Anopheles, Cx.= Culex, Ae. = Aedes, Ar.= Armigeres

Table 5: LF patients on MMDP: 2018-2022: Number of Mf positive, Lymphodema and Elephantiasis

	2018				2019				2020				2021				2022
Provinces	Mf+	L	E	Total	Mf+	L	E	Total	Mf+	L	E	Total	Mf+	L	E	Total	Mf+	L	E	Total
Chumpon	0	0	2	2	0	0	2	2	0	0	2	2	0	0	1	1	0	0	1	1
Surat Thani	0	0	11	11	0	0	9	9	0	0	9	9	0	0	8	8	0	0	8	8
Nakhon Sri Thammarat	0	0	65	65	0	0	52	52	0	0	42	42	0	0	41	41	0	0	38	38
Songkhla	0	0	0	0	0	0	0	0	0	0	1	1	0	0	1	1	0	0	1	1
Pattalung	0	0	2	2	0	0	2	2	0	0	2	2	0	0	2	2	0	0	2	2
Pattani	0	0	9	9	0	0	8	8	0	0	8	8	0	0	8	8	0	0	6	6
Narathiwat	13	0	12	25	17	7	12	36	21	0	15	36	13	7	15	35	8	6	13	27
Ratchaburi																	1	1	0	2
Total	13	0	101	114	17	7	85	109	21	0	79	100	13	7	76	96	9	7	69	85

Mf+ = microfilaria positive; L = lymphodema; E* = elephantiasis

Table 6a: Surveillance of migrant populations in Thailand for LF through various agencies: Number of migrants tested, method and target group

Organization	target group screened	Method of screening	*Number of migrants screened by year**
Organization	target group screened		2018	2019	2020	2021	2022
Provincial office of Disease Control (VBDC, VBDU)	Non registered migrants	TBF**	24,485	27,079	6,436	4,970	618
Provincial Health Office	Registered labour migrant health check-up at designated hospitals by Department of Health Service Support	TBF/FTS	10,000	5,128	12,000	NA	NA
Division of International Disease Control in Ports	Non registered migrants	TBF	2,716	1,760	1,859	1,189	743
Department of Medical Service – Bangkok Metropolitan Authority (BMA)	Registered labour migrant health check-up	TBF	7,702	9,170	11,364	NA	2,559
One Stop Service (OSS), Office of the Permanent Secretary of MOPH	Registered labour migrant health check-up	TBF	683,959	NA	579,826	308,504	358,750
Total			728,862	16,058	25,223	1,189	-

*Figures are for general health checkup and inclusion of TBF is variable depending on local arrangements with hospitals and laboratories and available budget.

**TBF=thick blood film

Table 6b: Results of LF surveys among unregistered migrants: 2018-2022*

Province	2018				2019				2021				2022
	Screened	Ag+	AGR%		Screened	Ag+	AGR%		Screened	Ag+	AGR%		Screened	Ag+	AGR%
Bangkok	794	3	0.38
Tak	762	0	0		200	0	0						392	0	0
Samut Prakan	164	0	0		423	0	0
Chonburi	264	0	0		500	1	0.20
Ranong	498	0	0						339	0	0
Surat Thani	256	0	0		415	0	0		473	3	0.63		139	0	0
Chiang Mai					424	0	0
Nakhon Pathom					400	2	0.50
Phuket					484	2	0.41						167	0	0
Chumphorn					507	1	0.20		32	0	0
TOTAL	2,738	3		0	3,353	6		0	844	3		0	698	0	0

*2020 - No blood surveys conducted among migrants due to Covid-19 lockdowns and restrictions

AGR – antigen rate

Table 6c: Vector surveys in migrant settlements: 2018-2019*

Province	No. of districts	No. of sub-districts	No. of village	2018			2019
Province	No. of districts	No. of sub-districts	No. of village	Mosquito Species (number collected)	Total No. of mosquitoes collected	Mosquitoes Microfilaria Positive	Mosquito Species (number collected)	Total No. of mosquitoes collected	Mosquitoes Microfilaria Positive
Tak	1						Cx. quinquefasciatus (271), Cx. gelidus (1), Cx. vishnuis (1), Ma. uniformis (5), Ae. aegypti (1), Ar. subalbatus (17)	296	0

Samut Prakan				Cx. gelidus (30), Cx. quinquefasciatus (103), Cx. vishnuai (2), Ma. aunalifera (79), Ma. uniformis (15), Ma. bonnea (5), An. barbumbrosus (4), An. umbrosus (3)	241	0	Culex spp.(36)	36	0

Chonburi				An. barbirostris (2), Armigeres spp. (3), Cx. quinquefasciatus (86), Cx. fuscocephala (1), Cx. bitaeniorhychus (92), Cx. Gelidus (7), Ma. uniformis (14), Ma. aunulifera (2)	207	0	Culex spp.	33	0

Ranong	1	1	1	Cx. quinquefasciatus (350), Cx. selidus (4), Cx. sitiens (170), Ae. aegypti (17), Ae. albopictus (6)	547	0	Cx. quinquefastus (10), Cx. gelidus (6), Cx. sitiens (82), Ae. aegyti (6), Ae. albopictus (13), Ar. subalbatus (132)	249	0

Surat Thani	2	2	3	Ma. uniformis (8), Ma. indiana (13), Ae. albopictus (1), Cx. quinquefasciatus (2), Culex spp. (31), Ar. subalbatus (12)	67	0	Cx. quinquefasciatus (24), Cx. vishui (140), Cx. whitei (100), Ma. uniformis (11), Ar. subalbatus (8)	283	0

Chiang Mai	1	1	1				Cx. quinquefasciatus (160), Ar. subalbatus (46), Cx. pseudovishnui (30), Ma. indiana (16), Ma. uniformis (14), Ae. aegypti (2)	268	0

Phuket	2	2	2				Culex spp.(5), Ae. albopictus (8), Culex spp.(3), Ae. albopictus (2), Ae. aegypti (5)	23	0

Chumphon	1	1	1				Cx. quinquefasciatus (129), Cx. sitien (16), Ae. aegypti (9), An. epiroticus (4)	158	0

*2020-22 No vector survey were done in migrant settlements due to Covid-19 lockdowns and

Table 7: Results of LF human blood surveys in displaced population (DP) camps in Thailand: PVS 2019 and 2022

2019

2022

No.

Province

Number of DP camps

DP camp population in 2019

no. of persons tested

Ag+

AGR

DP camp population in 2022

no. of persons tested

Ag+

AGR

Tak

58,807

751

Mae Hong Son

31,711

1,480

0.25

Kanchanaburi

2,760

339

0.29

2,325

1,283

0.39

Ratchaburi

5,342

377

0.27

Total

98,620

2,947

2,325

1,283

0.39

DP=displaced population; Ag+=antigen positive; AGR= antigen positivity rate; NA=data not available

Table 8a: Number of cat survey, and microfilaria positive rate

Province	No. of cats tested_2018	Mf+	MPR%	No. of cats tested_2019	Mf+	MPR%	No. of cats tested_2020	Mf+	MPR%	No. of cats tested_2021	Mf+	MPR%	No. of cats tested_2022	Mf+	MPR%
Krabi	0	0	0	48	0	0	64	0	0	71	0	0
Nakhon Si Thammarat	0	0	0	46	0	0	55	0	0	67	0	0
Surat Thani	43	8	18.60	61	0	0	0	0	0	68	0	0
Narathiwat	211	4	1.90	818	9	1.1	451	3	0.67	601	2	0.00	573	4	0.70
Total	254	12	4.72	973	0	0	570	3	0.53	807	2	0.25	573	4	0.70

Mf + = microfilaria positive; MPR = microfilaria positive rate

Table 8b: IUs with both human and cat Mf+: Mf and MPR rates: 2018-2022

			Human blood survey				Cat blood survey
Year*	Total no. IUs with human blood survey	IUs with both human and cat Mf+	Target Population	TBF	Mf+	MPR%	Target Population	TBF	Mf+	MPR%
2019	10	3	885	884	8	0.9%	187	158	8	5.1%
2021	8	1	248	239	1	0.4%	34	29	1	3.4%
2022	11	1	377	374	2	0.5%	70	61	1	1.6%

TBF = thick blood film; Mf + = microfilaria positive; MPR = microfilaria positive rate

*there were no IUs recorded with both human and cat Mf+ in 2018 and 2020

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published 06 Dec, 2023

Read the published version in Infectious Diseases of Poverty →

Editorial decision: Major revision
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Five years of post-validation surveillance of lymphatic filariasis in Thailand

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Abstract

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Background

Results of PVS in Thailand 2018–2022

Discussion

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Abbreviations

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Tables

Status:

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