Thai-Myanmar Borders Research Articles

2283. COVID-19 among Febrile Patients Presenting without Respiratory Symptoms in Six Hospitals along the Thai-Lao and Thai-Myanmar Borders, 2021-2011

Abstract Background Thailand’s national surveillance system to detect COVID-19 infection typically relies on a combination of fever, respiratory symptoms, and risk factors. We describe the frequency, epidemiologic characteristics, and risk factors for COVID-19 in patients without respiratory symptoms and the utility of saliva and nasal swabs (NS) to detect SARS-CoV-2. Methods During June 2021-December 2022, a period of high COVID-19 incidence in Thailand, we enrolled patients aged >2 years presenting with a temperature ≥37.5 °C or a history of fever ≤14 days and no cough, dyspnea, or sputum production at 6 hospitals in Nakhon Phanom and Tak provinces, Thailand. Saliva, NS, and nasopharyngeal/oropharyngeal swabs (NP/OP) were collected to detect SARS-CoV-2 using real-time RT-PCR. Multivariable analysis was done to assess factors independently associated with COVID-19 infection. Cases were defined as persons with SARS-CoV-2-positive RT-PCR specimens. Results Cases were detected in 358/1,648 (22%) persons. The median age among cases was 31 (IQR 21-45) years; 210 (59%) were female. Compared to non-cases, cases were more likely to present with malaise (52% vs. 40%, p< 0.001), hyposmia (10% vs. 1.4%, p< 0.001), dysgeusia (11% vs. 4.8%, p< 0.001), and muscle pain (53% vs. 47%, p=0.04); cases were less likely to have chills (41% vs. 50%, p=0.002), low appetite (29% vs. 41%, p< 0.001), nausea/vomiting (17% vs. 29%, p< 0.001) and abdominal pain (4.7% vs. 16%, p< 0.001). Cases were more likely to have been in contact with persons with COVID-19 infection (adjusted odds ratio [95% confidence interval]: 12 [8.1-16]), febrile household members (2.6 [1.8-3.8]), and persons with respiratory symptoms (2.2 [1.4-3.7]), have a history of travel (1.7 [1.0-2.7]), current alcohol consumption (1.7 [1.2-2.4]) and obesity (1.5 [1.1-1.9]). Among cases, SARS-CoV-2 was detected in 95% of saliva, 92% of NS, and 99% of NP/OP specimens; RT-PCR was positive in all 3 specimen types from 319 (89%) cases. Compared to NP/OP, the sensitivity of testing saliva and NS were 96% and 93%. Conclusion SARS-CoV-2 was detected in more than one-fifth of febrile patients without respiratory symptoms. A broader national surveillance case definition would increase case detection. Saliva and NS demonstrated high sensitivity for SARS-CoV-2 detection. Disclosures All Authors: No reported disclosures

Surveillance of drug resistance molecular markers in Plasmodium vivax before and after introduction of dihydroartemisinin and piperaquine in Thailand: 2009-2019.

Resistance to antimalarial drugs is a serious issue around the world. Widespread Plasmodium vivax and P. falciparum coinfections are commonly found in Thailand. Dihydroartemisinin and piperaquine (DHA-PPQ) have been used as first-line treatments for P. falciparum since 2015, and chloroquine (CQ) and primaquine (PQ) have remained first-line drugs for P. vivax for more than 60 years. Coinfections may lead parasites to evolve with regard to genetics under selective drug pressure. This study is aimed at investigating genes linked to antimalarial resistance in P. vivax before and after introduction of DHA-PPQ as a new drug regimen in Thailand. A total of 400 P. vivax isolates were collected from samples along the Thai-Myanmar and Thai-Malaysian borders before (2009-2015) and after (2016-2019) introduction of DHA-PPQ. Genomic DNA of P. vivax was obtained and subjected to analysis of five drug resistance-associated genes (Pvdhfr, Pvdhps, Pvmdr1, Pvcrt-o, and PvK12) by nested polymerase chain reaction (PCR), restriction fragment length polymorphism (RFLP), and nucleotide sequencing. A high prevalence of Pvdhfr was found in both endemic areas over the period. The quadruple (57I/58R/61M/117T) Pvdhfr haplotype was predominant in both periods in both endemic areas. Although the wild-type haplotype of Pvdhps was predominant in Thai-Malaysian isolates in both periods, a single mutant haplotype (383G) was dominant in Thai-Myanmar isolates during both periods. A low prevalence of the Pvmdr1 976F mutation was found in both periods among Thai-Myanmar isolates. A significant decrease in Pvmdr1 976F was identified in Thai-Malaysian isolates from the second period (p < 0.01). Only one nonsynonymous mutation of Pvcrt-o (193E) and one synonymous mutation of PvK12 (R584) were detected in four isolates (4.7%) and one isolate (0.5%) in the first period among Thai-Myanmar isolates, respectively. Thus, with limited clinical efficacy data, the low prevalence of drug-resistance markers may suggest that there is a low prevalence of P. vivax-resistant strains and that the current drug regimen for P. vivax is still effective for treating this P. vivax parasite population. Continued surveillance of antimalarial drug resistance markers and monitoring of clinical drug efficacy should be conducted for epidemiological and policy implications.

Cellular mechanisms of action and resistance of Plasmodium falciparum to artemisinin.

The recent reports of high failure rates and decline in in vitro sensitivity of Plasmodium falciparum to artemisinin-based combination therapies (ACTs) suggest the possibility of clinical artemisinin resistance along the Thai-Cambodian and Thai-Myanmar borders. The study investigated cellular mechanisms of action and resistance of P. falciparum to artesunate (stage specific activity, interaction with hemozoin, and anti-oxidant levels) in the two paired P. falciparum isolates (MSF046 and MSF060) collected before treatment with a 3-day artesunate-mefloquine and at the time of recrudescence. In addition, the link of these cellular mechanisms to the polymorphisms of the candidate artemisinin-resistant genes (pfatp6, pfcrt, pfmdr1, pfmrp1, and K13 propeller) was also investigated. Morphological change was observed in both pairs of the primary and recrudesced P. falciparum isolates during 12-48h of exposure to artesunate (at IC90). A marked decrease in parasite viability was found in the recrudesced isolates of both MSF046 and MSD060. The extent of the reduction (% change of baseline) in total glutathione concentrations was significantly lower in recrudesced (32.1 and 1.7%) compared with primary (45.5 and 53.7%) isolates of both MSF046 and MSF060. The extent of reduction of hemozoin content in MSF046 was significantly higher in the recrudesced (76.8%) isolate compared with the primary isolate (99.5%). For MSF060 on the other hand, increase in hemozoin content was found in the recrudesced isolate and the extent of such increase was significantly higher in recrudesced (93.1%) than the primary isolate (87.5%). Polymorphism of K13 (N458Y) together with pfmdr1 copy number correlated well with sensitivity of both isolates to artesunate. Results of this preliminary study suggests possible role of glutathione-dependent detoxification system as well as heme degradation as cellular mechanisms of action and resistance of artemisinins.

Health as a bridge to peace and trust in Myanmar: The 21st Century Panglong Conference

The twenty-first Century Panglong Conference, proposed by Aung San Suu Kyi and her National League for Democracy, has concluded with unsatisfactory results. This five-day conference attracted global attention and wide endorsements from the United Nations and many other key stakeholders. The broad framework of the peace dialogue included various social and economic issues. However, the implication of the conference on the health system strengthening efforts in ethnic areas was largely unknown. Although a “convergence model” was proposed by organizations in Thai-Myanmar borders as a roadmap for integrations of the national and ethnic health systems years ago, the genuine bottlenecks beneath have not been addressed. This commentary discussed the Panglong Conference and its implication for the health systems in ethnic regions, as well as the bottlenecks of a “peace process” in health sector. It outlined a few key steps to achieve health system convergence between national and ethnic health systems, the outcome of which will not only improve the health status of the ethnic regions, but also help strengthen mutual trust and understanding among peoples, as a powerful bridge for peace.

Selection and spread of artemisinin-resistant alleles in Thailand prior to the global artemisinin resistance containment campaign.

The recent emergence of artemisinin resistance in the Greater Mekong Subregion poses a major threat to the global effort to control malaria. Tracking the spread and evolution of artemisinin-resistant parasites is critical in aiding efforts to contain the spread of resistance. A total of 417 patient samples from the year 2007, collected during malaria surveillance studies across ten provinces in Thailand, were genotyped for the candidate Plasmodium falciparum molecular marker of artemisinin resistance K13. Parasite genotypes were examined for K13 propeller mutations associated with artemisinin resistance, signatures of positive selection, and for evidence of whether artemisinin-resistant alleles arose independently across Thailand. A total of seven K13 mutant alleles were found (N458Y, R539T, E556D, P574L, R575K, C580Y, S621F). Notably, the R575K and S621F mutations have previously not been reported in Thailand. The most prevalent artemisinin resistance-associated K13 mutation, C580Y, carried two distinct haplotype profiles that were separated based on geography, along the Thai-Cambodia and Thai-Myanmar borders. It appears these two haplotypes may have independent evolutionary origins. In summary, parasites with K13 propeller mutations associated with artemisinin resistance were widely present along the Thai-Cambodia and Thai-Myanmar borders prior to the implementation of the artemisinin resistance containment project in the region.

Mutations in Plasmodium falciparum K13 propeller gene from Bangladesh (2009-2013).

BackgroundBangladesh is a malaria hypo-endemic country sharing borders with India and Myanmar. Artemisinin combination therapy (ACT) remains successful in Bangladesh. An increase of artemisinin-resistant malaria parasites on the Thai-Cambodia and Thai-Myanmar borders is worrisome. K13 propeller gene (PF3D7_1343700 or PF13_0238) mutations have been linked to both in vitro artemisinin resistance and in vivo slow parasite clearance rates. This group undertook to evaluate if mutations seen in Cambodia have emerged in Bangladesh where ACT use is now standard for a decade.MethodsSamples were obtained from Plasmodium falciparum-infected malaria patients from Upazila health complexes (UHC) between 2009 and 2013 in seven endemic districts of Bangladesh. These districts included Khagrachari (Matiranga UHC), Rangamati (Rajasthali UHC), Cox’s Bazar (Ramu and Ukhia UHC), Bandarban (Lama UHC), Mymensingh (Haluaghat UHC), Netrokona (Durgapur and Kalmakanda UHC), and Moulvibazar (Sreemangal and Kamalganj UHC).ResultsOut of 296 microscopically positive P. falciparum samples, 271 (91.6%) were confirmed as mono-infections by both real-time PCR and nested PCR. The K13 propeller gene from 253 (93.4%) samples was sequenced bi-directionally. One non-synonymous mutation (A578S) was found in Bangladeshi clinical isolates. The A578S mutation was confirmed and lies adjacent to the C580Y mutation, the major mutation causing delayed parasite clearance in Cambodia. Based on computational modeling A578S should have a significant effect on tertiary structure of the protein.ConclusionThe data suggest that P. falciparum in Bangladesh remains free of the C580Y mutation linked to delayed parasite clearance. However, the mutation A578S is present and based on structural analysis could affect K13 gene function. Further in vivo clinical studies are required to validate the effect of this mutation.

Genetic diversity of the msp-1, msp-2, and glurp genes of P lasmodium falciparum isolates along the Thai-Myanmar borders

To study the genetic diversity at the msp-1, msp-2, and glurp genes of Plasmodium falciparum (P. falciparum) isolates from 3 endemic areas in Thailand: Tak, Kanchanaburi and Ranong provinces. A total of 144 P. falciparum isolates collected prior to treatment during January, 2012 to June, 2013 were genotyped. DNA was extracted; allele frequency and diversity of msp-1, msp-2, and glurp genes were investigated by nested polymerase chain reaction. P. falciparum isolates in this study had high rate of multiple genotypes infection (96.5%) with an overall mean multiplicity of infection of 3.21. The distribution of allelic families of msp-1 was significantly different among isolates from Tak, Kanchanaburi, and Ranong but not for the msp-2. K1 and MAD20 were the predominant allelic families at the msp-1 gene, whereas alleles belonging to 3D7 were more frequent at the msp-2 gene. The glurp gene had the least diverse alleles. Population structure of P. falciparum isolates from Tak and Ranong was quite similar as revealed by the presence of similar proportions of MAD20 and K1 alleles at msp-1 loci, 3D7 and FC27 alleles at msp-2 loci as well as comparable mean MOI. Isolates from Kanchanaburi had different structures; the most prevalent alleles were K1 and RO33. The present study shows that P. falciparum isolates from Tak and Ranong provinces had similar allelic pattern of msp-1 and msp-2 and diversity but different from Kanchanaburi isolates. These allelic variant profiles are valuable baseline data for future epidemiological study of malaria transmission and for continued monitoring of polymorphisms associated with antimalarial drug resistance in these areas.

Distribution of pfmdr1 polymorphisms in Plasmodium falciparum isolated from Southern Thailand

BackgroundDrug resistance in Plasmodium falciparum is a major problem in malaria control especially along the Thai-Myanmar and Thai-Cambodia borders. To date, a few molecular markers have been identified for anti-malarial resistance in P. falciparum, including the P. falciparum chloroquine resistance transporter (pfcrt) and the P. falciparum multidrug resistance 1 (pfmdr1). However no information is available regarding the distribution pattern of these gene polymorphisms in the parasites from the Thai-Malaysia border. This study was conducted to compare the distribution pattern of the pfcrt and pfmdr1 polymorphisms in the parasites from the lower southern provinces, Thai-Malaysia border and the upper southern provinces, Thai-Myanmar border. In addition, in vitro sensitivities of anti-malarial drugs including chloroquine, mefloquine, quinine, and artesunate were determined.MethodsIn all, 492 P. falciparum-positive blood samples were collected from the lower southern provinces: Songkhla, Yala and Narathiwas. From the upper southern part of Thailand, Ranong and Chumphon, 66 samples were also collected. Polymorphisms of the pfcrt and the pfmdr1 gene were determined using PCR techniques. In vitro sensitivities of anti-malarial drugs were determined using radioisotopic method.ResultsAll parasites from both areas contained the pfcrt 76 T allele. The pfmdr1 86Y allele was significantly more common in the parasites isolated from the lower southern areas. In contrast, the pfmdr1 184F allele was predominant among the parasites from the upper southern areas especially Ranong. In addition, the parasites from Ranong contained higher copy numbers than the parasites from other provinces. All adapted parasite isolates exhibited CQ-resistant phenotype. Neither QN nor MQ resistance was detected in these isolates.ConclusionThe parasites from Thai-Malaysia border exhibited different resistant patterns compared to other areas along the international border of Thailand. This information will be useful for anti-malarial drug policy in Thailand.

Four years’ monitoring of in vitro sensitivity and candidate molecular markers of resistance of Plasmodium falciparum to artesunate-mefloquine combination in the Thai-Myanmar border

BackgroundThe decline in efficacy of artesunate (AS) and mefloquine (MQ) is now the major concern in areas along the Thai-Cambodian and Thai-Myanmar borders.MethodsThe correlation between polymorphisms of pfatp6, pfcrt, pfmdr1 and pfmrp1 genes and in vitro sensitivity of Plasmodium falciparum isolates to the artemisinin-based combination therapy (ACT) components AS and MQ, including the previously used first-line anti-malarial drugs chloroquine (CQ) and quinine (QN) were investigated in a total of 119 P. falciparum isolates collected from patients with uncomplicated P. falciparum infection during 2006–2009.ResultsReduced in vitro parasite sensitivity to AS [median (95% CI) IC50 3.4 (3.1-3.7) nM] was found in 42% of the isolates, whereas resistance to MQ [median (95% CI) IC50 54.1 (46.8-61.4) nM] accounted for 58% of the isolates. Amplification of pfmdr1 gene was strongly associated with a decline in susceptibility of P. falciparum isolates to AS, MQ and QN. Significant difference in IC50 values of AS, MQ and QN was observed among isolates carrying one, two, three, and ≥ four gene copies [median (95% CI) AS IC50: 1.6 (1.3-1.9), 1.8 (1.1-2.5), 2.9 (2.1-3.7) and 3.1 (2.5-3.7) nM, respectively; MQ IC50: 19.2 (15.8-22.6), 37.8 (10.7-64.8), 55.3 (47.7-62.9) and 63.6 (49.2-78.0) nM, respectively; and QN IC50: 183.0 (139.9-226.4), 256.4 (83.7-249.1), 329.5 (206.6-425.5) and 420.0 (475.2-475.6) nM, respectively]. The prevalence of isolates which were resistant to QN was reduced from 21.4% during the period 2006–2007 to 6.3% during the period 2008–2009. Pfmdr1 86Y was found to be associated with increased susceptibility of the parasite to MQ and QN. Pfmdr1 1034C was associated with decreased susceptibility to QN. Pfmrp1 191Y and 1390I were associated with increased susceptibility to CQ and QN, respectively.ConclusionHigh prevalence of CQ and MQ-resistant P. falciparum isolates was observed during the four-year observation period (2006–2009). AS sensitivity was declined, while QN sensitivity was improved. Pfmdr1 and pfmrp1 appear to be the key genes that modulate multidrug resistance in P. falciparum.

Distribution of dihydrofolate reductase (dhfr) and dihydropteroate synthase (dhps) mutant alleles in Plasmodium vivax isolates from Thailand

The analysis of prevalence and distribution of pvdhfr and pvdhps mutations were performed in 169 samples collected from patients with Plasmodium vivax infection who attended the malaria clinics in the provinces along the three international borders of Thailand (Thai-Myanmar, Thai-Cambodian, and Thai-Malaysian borders). SNP-haplotypes of the pvdhfr at amino acid positions 13, 33, 57, 58, 61, 117, and 173 and of the pvdhps at positions 383 and 553 were examined by nested PCR-RFLP. Significant differences in the prevalence and distribution of pvdhfr and pvdhps combination alleles were observed in P. vivax isolates collected from all the three border areas. The most prevalent combination alleles were triple mutant pvdhfr 57L/58R/117T alleles/double wild-type pvdhps alleles (n=18), double mutant pvdhfr 58R/117N alleles/double wild-type pvdhps alleles (n=10), and triple mutant pvdhfr 58R/61M/117N alleles/double wild-type pvdhps alleles (n=52) or with single mutant pvdhps 383G allele (n=28), respectively. These information on prevalence and patterns of pvdhfr and pvdhps polymorphisms obtained from the present study suggest the presence of SP pressure on P. vivax isolates in Thailand which could be linked to the introduction of malaria from neighboring countries. Results did not support the application of SP for P. vivax control program in Thailand as well as the neighboring countries.

Editorial Commentary: Single-Dose Primaquine as Gametocytocidal Treatment in Patients With Uncomplicated Falciparum Malaria

Editorial Commentary: Single-Dose Primaquine as Gametocytocidal Treatment in Patients With Uncomplicated Falciparum Malaria

Polymorphisms of the pfmdr1 but not the pfnhe-1 gene is associated with in vitro quinine sensitivity in Thai isolates of Plasmodium falciparum

BackgroundThe emergence of Plasmodium falciparum resistance to most currently used anti-malarial drugs is a major problem in malaria control along the Thai-Myanmar and Thai-Cambodia borders. Quinine (QN) with tetracycline/doxycycline has been used as the second-line treatment for uncomplicated falciparum malaria. In addition, QN monotherapy has been the first-line treatment for falciparum malaria in pregnant women. However, reduced in vitro and in vivo responses to QN have been reported. To date, a few genetic markers for QN resistance have been proposed including Plasmodium falciparum chloroquine resistance transporter (pfcrt), P. falciparum multidrug resistance 1 (pfmdr1), and P. falciparum Na+/H+ exchanger (pfnhe-1). This study was to investigate the role of the pfmdr1 and pfnhe-1 gene on in vitro QN sensitivity in Thai isolates of P. falciparum.MethodsEighty-five Thai isolates of P. falciparum from the Thai-Myanmar and Thai-Cambodia borders from 2003-2008 were determined for in vitro QN sensitivity using radioisotopic assay. Polymorphisms of the pfmdr1 and pfnhe-1 gene were determined by PCR-RFLP and sequence analysis. Associations between the in vitro QN sensitivity and the polymorphisms of the pfmdr1 and pfnhe-1 gene were evaluated.ResultsThe mean QN IC50 was 202.8 nM (range 25.7-654.4 nM). Only four isolates were QN resistant when the IC50 of >500 nM was used as the cut-off point. Significant associations were found between the pfmdr1 mutations at codons N86Y and N1042D and in vitro QN sensitivity. However, no associations with the number of DNNND, DDNNNDNHNDD, and NHNDNHNNDDD repeats in the microsatellite ms4760 of the pfnhe-1 gene were identified.ConclusionData from the present study put doubt regarding the pfnhe-1 gene as to whether it could be used as the suitable marker for QN resistance in Thailand. In contrast, it confirms the influence of the pfmdr1 gene on in vitro QN sensitivity.

Genetic diversity and population structure of Plasmodium falciparum in Thailand, a low transmission country

BackgroundThe population structure of the causative agents of human malaria, Plasmodium sp., including the most serious agent Plasmodium falciparum, depends on the local epidemiological and demographic situations, such as the incidence of infected people, the vector transmission intensity and migration of inhabitants (i.e. exchange between sites). Analysing the structure of P. falciparum populations at a large scale, such as continents, or with markers that are subject to non-neutral selection, can lead to a masking and misunderstanding of the effective process of transmission. Thus, knowledge of the genetic structure and organization of P. falciparum populations in a particular area with neutral genetic markers is needed to understand which epidemiological factors should be targeted for disease control. Limited reports are available on the population genetic diversity and structure of P. falciparum in Thailand, and this is of particular concern at the Thai-Myanmar and Thai-Cambodian borders, where there is a reported high resistance to anti-malarial drugs, for example mefloquine, with little understanding of its potential gene flow.MethodsThe diversity and genetic differentiation of P. falciparum populations were analysed using 12 polymorphic apparently neutral microsatellite loci distributed on eight of the 14 different chromosomes. Samples were collected from seven provinces in the western, eastern and southern parts of Thailand.ResultsA strong difference in the nuclear genetic structure was observed between most of the assayed populations. The genetic diversity was comparable to the intermediate level observed in low P. falciparum transmission areas (average HS = 0.65 ± 0.17), where the lowest is observed in South America and the highest in Africa. However, uniquely the Yala province, had only a single multilocus genotype present in all samples, leading to a strong geographic differentiation when compared to the other Thai populations during this study. Comparison of the genetic structure of P. falciparum populations in Thailand with those in the French Guyana, Congo and Cameroon revealed a significant genetic differentiation between all of them, except the two African countries, whilst the genetic variability of P. falciparum amongst countries showed overlapping distributions.ConclusionPlasmodium falciparum shows genetically structured populations across local areas of Thailand. Although Thailand is considered to be a low transmission area, a relatively high level of genetic diversity and no linkage disequilibrium was found in five of the studied areas, the exception being the Yala province (Southern peninsular Thailand), where a clonal population structure was revealed and in Kanchanaburi province (Western Thailand). This finding is particularly relevant in the context of malaria control, because it could help in understanding the special dynamics of parasite populations in areas with different histories of, and exposure to, drug regimens.

In vitro atovaquone/proguanil susceptibility and characterization of the cytochrome b gene of Plasmodium falciparum from different endemic regions of Thailand

BackgroundThe emergence of Plasmodium falciparum resistant to most currently used antimalarial drugs is the major problem in malaria control along the Thai-Myanmar and Thai-Cambodia borders. Although artemisinin-based combination therapy has been recommended for the treatment of multidrug-resistant falciparum malaria, these combinations are not available for some people, such as travelers from North America. A fixed-dose combination of atovaquone and proguanil (Malarone) has been proved to be effective for the treatment and prophylaxis of malaria which is already approved by countries in North America and Europe. Determination of the phenotypes and genotypes related to atovaquone/proguanil response in Thai isolates of P. falciparum will be useful for rationale drug use. The main purpose of this study was to explore the in vitro atovaquone/proguanil susceptibility of recently adapted Thai isolates of P. falciparum. Genotypic characterization of the cytb gene of these isolates was also determined since it has been reported that point mutations, particularly codon 268 in the cytochrome b gene (cytb) have been linked to atovaquone/proguanil treatment failure.MethodsEighty three P. falciparum isolates collected during 1998 to 2005 from four different multidrug resistance areas of Thailand were determined for the in vitro atovaquone/proguanil susceptibilities using radioisotopic assay. Mutations in the cytb gene were determined by PCR-RFLP and sequence analysis.ResultsThe mean atovaquone and proguanil IC50 was 3.4 nM and 36.5 μM, respectively. All 83 Thai isolates were atovaquone sensitive. None of the 83 isolates contained the mutations at codon 268 of the cytb gene. DNA sequencing of the cytb gene of 20 parasite isolates showed no other mutations.ConclusionIn agreement with a recent efficacy study of atovaquone/proguanil, the present information indicates that atovaquone/proguanil can be one of the drugs of choice for the treatment and prophylaxis of multidrug-resistant falciparum malaria in Thailand.

In vitro antimalarial drug susceptibility in Thai border areas from 1998–2003

BackgroundThe Thai-Myanmar and Thai-Cambodia borders have been historically linked with the emergence and spread of Plasmodium falciparum parasites resistant to antimalarial drugs. Indeed, the areas are often described as harbouring multi-drug resistant parasites. These areas of Thailand have experienced significant changes in antimalarial drug exposure patterns over the past decade. This study describes the in vitro antimalarial susceptibility patterns of 95 laboratory-adapted P. falciparum isolates, collected between 1998 and 2003,.MethodsNinety five P. falciparum isolates were collected from five sites in Thailand between 1998 and 2003. After laboratory adaptation to in vitro culture, the susceptibility of these parasites to a range of established antimalarial drugs (chloroquine [CQ], mefloquine [MQ], quinine [QN] and dihydroartemisinin [DHA]) was determined by the isotopic microtest.ResultsMefloquine (MQ) sensitivity remained poorest in areas previously described as MQ-resistant areas. Sensitivity to MQ of parasites from this area was significantly lower than those from areas reported to harbour moderate (p = 0.002) of low level MQ resistance (p = 000001). Importantly for all drugs tested, there was a considerable range in absolute parasite sensitivities. There was a weak, but statistically positive correlation between parasite sensitivity to CQ and sensitivity to both QN and MQ and a positive correlation between MQ and QN. In terms of geographical distribution, parasites from the Thai-Cambodia were tended to be less sensitive to all drugs tested compared to the Thai-Myanmar border. Parasite sensitivity to all drugs was stable over the 6-year collection period with the exception of QN.ConclusionThis study highlights the high degree of variability in parasite drug sensitivity in Thailand. There were geographical differences in the pattern of resistance which might reflect differences in drug usage in each area. In contrast to many other studies there were weak, but statistically significant positive, correlations between sensitivity to CQ and sensitivity to MQ and QN. Over the six years of sample collection, parasite sensitivity appears to have stabilized to these drugs in these sites.