These data show that the anti-malarial drug resistance situation in Thailand (and most likely the GMS) is dynamic with considerable fluctuation over time. Surveillance systems in the GMS are therefore crucial for malaria control efforts, especially with regard to the use of anti-malarials. Since drug resistant strains appear to have arisen in this area and subsequently spread to other parts of the world in the past, surveillance in this area is important not only for the immediate region but also for the entire world. This is perhaps especially the case with the potential for ART-resistant strains coming out of the GMS. If ART-resistant strains were to spread to Sub-Saharan Africa, the results could be catastrophic.
CQ was widely used globally following its introduction after World War II. In Thailand, as in other parts of the world, resistance quickly developed. By the late 1950s, Thailand already experienced CQ-resistant parasites, even before CQ was designated the official anti-malarial. While CQ use for P. falciparum infections was discontinued in Thailand during the 1970s, other parts of the world, especially Africa, continued to use the anti-malarial for most of the 20th century. Resistance to the drug was widespread anywhere it was in use. Tests for in vitro and in vivo sensitivity to CQ have subsequently shown increased sensitivity shortly after removal of drug pressure . Furthermore, mutations that have been associated with CQ resistance (in PfCRT and Pfmdr1) have been shown to decrease in frequency after cessation of CQ use [16–18]. These data indicate that in Thailand CQ resistance has decreased in eastern provinces while potentially rising in western Provinces (such as Tak and Kanchanaburi). This pattern may in part be explained by the continued use of CQ for treating Plasmodium vivax infections. Furthermore, some research has suggested that mixed infections are relatively widespread and under-diagnosed in Thailand . If patients infected with both species of malaria are diagnosed with P. vivax and subsequently treated with CQ, this will inevitably retain the CQ pressure and therefore CQ resistance in P. falciparum.
Previous research has noted abnormally high MQ IC50 values along the Thai-Myanmar border in the mid-1990s . Other studies in the north-western region of Thailand showed a strong correlation between high MQ IC50 values and clinical failure rates . Decreased parasite sensitivity to MQ corresponded to increased P. falciparum gametocytaemia in the study population . However, as ACT was introduced in the area around 1995, it appeared that parasite sensitivity to MQ increased. This increase in parasite sensitivity to MQ has largely been attributed to the introduction of ART and its derivatives for P. falciparum infections [20, 21]. However, these data indicate that after a decrease in IC50 values around the turn of the century, MQ IC50 values have again begun to rise in the north-western areas of Thailand. There is some evidence that ART resistance may be rising in Tak Province. In addition, at both Thai-Cambodian and Thai-Myanmar border areas, clinical resistance to ATS-MQ has been on the rise [3, 6, 22]. The fact that sensitivities to MQ and ART family drugs are often correlated [23, 24] and ATS-MQ is the most commonly used ACT formulation in this region suggest that MQ sensitivity needs to be continuously monitored.
These results also have broad relevance for other regions where combination therapy is being implemented in order to combat parasite resistance to monotherapies. These data suggest that while combination therapies are likely to work in the short term, parasites may develop resistance to multiple, non-structurally related anti-malarials over time. With regards to malaria control efforts, this indicates a need for careful monitoring, proactive policies, and a multi-focused approach that focuses on vectors, the human host, and environmental factors in malaria transmission along with continued efforts to find other anti-malarials.
Parasites that are resistant to QN have been reported in Thailand since the 1980s . QN was used in combination with tetracycline, and remains to be used as a second line treatment for falciparum infection and as a monotherapy for infections in pregnant patients . These data indicate general sensitivity in parasites with regards to QN, which is expected given its infrequent use in the region. Some studies have indicated shared resistance mechanisms between the amino-alcoholic drugs and QN [23, 25]. As such, the extensive use of MQ both alone or in ACT suggests that continued monitoring of QN resistance is strongly warranted.
ART has been used for several decades in Cambodia, Vietnam and China, however its use is relatively new in other parts of the GMS. In the last several years there have been reports of potential reduced sensitivity in P. falciparum with regards to ART drugs in Thailand. The first reports concerned reduced in vivo sensitivity along the Thai-Cambodia border and more recently, tests of in vitro sensitivity along the Thai-Myanmar border have shown reduced sensitivity to ART and ART derivatives [5–7, 22]. This research further supports these previous findings. While most of the sentinel provinces exhibit low ART IC50 values, Tak (on the Myanmar border) and Trat (along the Cambodian border) both show high, fluctuating values (Figure 3). It is curious to note that these data indicate high ART and MQ IC50 values in Tak and Trat Provinces around the same time that ATS-MQ became the standard therapy for falciparum malaria in the same locations. It is possible that sensitivity to ART was present because the drug was already in use by populations that frequently move across the borders to Myanmar (bordering Tak) and Cambodia (bordering Trat) where ART drugs has been in use for a longer period of time. However, since testing was done by different laboratory technicians, places, and times, and since there are no detailed data on individual assays it is possible that the high ART values in Tak and Trat represent variations in laboratory conditions. The data also indicate that DHA IC50 values are high in north-western Thailand and perhaps especially in Tak Province (Figure 4). These data are more reliable than the microtest III data, since they include results from individual assays and have error bars for comparison across regions. Given the importance of ART and its derivatives as the last line of defence against malaria, these findings are alarming. Careful monitoring and use of ART family drugs is crucial for global malaria control and eradication efforts.
There are several limitations to this study, which need to be considered for improvement in the future. The sampling method was not systematic, taken opportunistically from patients seeking treatment from malaria clinics and having high parasitaemia, and thus the samples may not represent the overall parasite population. Direct comparison of IC50 values across sentinel provinces may be problematic because of varying laboratory assay conditions and levels of technical training. Mixed infections, which are likely to be much more common in the GMS than was previously thought , can lead to inaccuracy in drug sensitivity testing . In addition, while several studies have shown a rough correlation between in vitro tests and clinical outcomes, the potential remains for discrepancies between the two . Nevertheless, given the limited resources (especially funding and skilled labour), in vitro tests remain an important proxy measure for clinical outcomes (e.g., parasitaemia quantification) in malaria infections. The HRP2 method is less labour intensive than the WHO microtest, meaning that it can be used in a more systematic way. If resources permit, it is ideal to test the same drugs, at least each year, continuously over time. Furthermore, quality control measures including keeping detailed records concerning variance in assays, changing laboratory personnel, and other laboratory conditions, should be implemented and preserved. Finally, combination of this test with more accurate in vitro tests of culture-adapted parasites and clinical efficacy (e.g., in vivo parasitaemia over time, after the administration of anti-malarials) data will significantly strengthen the drug surveillance data.
Furthermore, surveillance efforts in the GMS and Thailand must adapt to changing malaria epidemiology. Over the last several decades, P. falciparum prevalence has decreased markedly in Thailand, while the proportion of P. vivax cases has increased. In general, vivax malaria has largely been ignored in epidemiology, but researchers have come to the realization that the so-called ‘benign tertian malaria’ poses a credible threat to public health [28–30]. Yet, drug resistance surveillance for this parasite has remained scant . Although CQ remained relative effective in treating P. vivax malaria in some regions of the GMS such as Thailand and China [31, 32], cases of vivax resistance have been recorded the GMS [33–37]. In 2010 Cambodia began using a combination therapy (DHA-piperaquine and primaquine) as the first line treatment for P. vivax malaria. This points towards a demand for more close monitoring of P. vivax resistance to commonly used anti-malarials especially CQ and potentially the need for a combination therapy for P. vivax cases .
In the face of potential parasite resistance to ART and its derivatives, several actions should be taken. Since the border regions surrounding Thailand have historically been the site of persistent drug resistance (which may have subsequently spread to other parts of the world), surveillance of drug resistance in this region is crucial for global health and drug policy concerns. Currently there are in vivo studies of drug resistance, focusing on parasite density over time within patients that have been administered anti-malarials, in all nations of the GMS. However, such testing is troubled by the difficulty of following some patients over time and not all subregions or areas within the GMS have the manpower to conduct lengthy testing for drug resistance. The geographical range of both in vitro and in vivo surveillance should expand into areas of the GMS that aren’t covered by current surveillance efforts. A more holistic, co-ordinated approach would ensure better surveillance across the GMS, revealing a more comprehensive picture of the drug resistance situation. There should be a move towards the careful regulation of anti-malarials and combination therapy should be used throughout the region. Several studies have shown that fake and substandard pharmaceuticals are widespread throughout the GMS; therefore surveillance efforts should also be put into place to investigate this crucial problem. Finally, while anti-malarials are undoubtedly important for malaria control efforts, other modes of control should continue to be pursued. For example studies into behavioural components, environment and disease interactions, as well as both human and vector ecology have been and will continue to be important for malaria control efforts.