Prevalence and patterns of antifolate and chloroquine drug resistance markers in Plasmodium vivax across Pakistan

Malaria imposes a significant public health burden in Pakistan, causing a reported 1.6 million cases per year according to the World Health Organization (WHO) [1]. Plasmodium vivax is the most prevalent species of human malaria in the country, accounting for 200,000 (67%) of all cases in 2011 [2], with Plasmodium falciparum malaria accounting for the remaining one-third. Chloroquine plus primaquine is the first-line treatment for P. vivax in most of the world, including Pakistan. After the rise of chloroquine resistance in P. falciparum, sulphadoxine-pyrimethamine (SP) treatment became widespread throughout Asia and has since been replaced by artesunate plus SP for treatment of falciparum malaria in Pakistan. Despite its prevalence and public health importance, P. vivax has not been well-characterized with respect to drug resistance to chloroquine or to SP (to which it may be exposed during presumptive treatment of P. falciparum) in Pakistan [3].

Chloroquine resistance in P. vivax has been observed in the Pacific [4], Latin America [5], and parts of Asia [6, 7] but has not yet been reported in Pakistan and Afghanistan [8, 9], and chloroquine remains effective against P. vivax in India [10, 11]. Chloroquine resistance in P. vivax is thought to be mediated by single nucleotide polymorphisms (SNPs) in the pvmdr1 gene of P. vivax[12‐14]. It has been suggested that point mutations at codons Y976F and F1076L of the pvmdr1 gene may be involved in resistance to 4-aminoquinolones, chloroquine and amodiaquine [12, 15]. In vitro data show that Indonesian P. vivax isolates with a prevalence of 96% for mutation Y976F had a significantly higher IC₅₀ for chloroquine compared to Thai isolates, which had a 25% prevalence of Y976F [16]. Whereas an association of mutation F1076L with chloroquine resistance has not yet been found [16, 17], the most common pvmdr1 haplotypes found in nature are F1076L [18‐20], and Y976F plus F1076L [21], with the single mutantY976F reported only rarely [16]. This observation has given rise to the theory that a two-step mutational trajectory, with mutations at codon F1076L followed by Y976F, may lead to chloroquine resistance [12, 22]; however, in vivo substantiation of this theory is required. Polymorphisms in copy number of the pvmdr1 gene may also modulate drug resistance in P. vivax, as shown by a link between gene amplification and decreased in vitro susceptibility to mefloquine, artesunate, and amodiaquine but increased susceptibility to chloroquine [23].

Although SP has not been indicated for treatment of P. vivax, there is evidence that intensive SP use for treatment of P. falciparum may also select for resistant P. vivax in regions where both species coexist [24‐26]. An efficacy study of SP, chlorproguanil-dapsone and chloroquine carried out from 2004 to 2006 suggests that antifolates were effective against P. vivax in Afghanistan and Pakistan at that time [9]. In the absence of more recent data, molecular markers may be used to monitor the effects of SP pressure on P. vivax.

Pyrimethamine and sulphadoxine target the dhfr and dhps enzymes in P. falciparum and point mutations in these genes confer resistance to each drug [27‐34]. Mutations in codons I13L, P33L, F57L/I, S58R, T61M, S117N/T and I173L/F of the P. vivax dhfr enzyme have been proposed as conferring similar resistance to pyrimethamine in P. vivax[19, 35]. In vitro studies of pvdhfr expression in Escherichia coli, P. falciparum and yeast have shown that pvdhfr double and triple mutants S58R + S117N, F57L + S58R, and F57L + S58R + S117T confer significant (up to several hundred-fold) reductions in susceptibility to antifolates [36‐39] and are associated with reduced drug affinity to the target enzyme [38]. In P. vivax dhps, mutations at codons S382A/C, A383G, K512M/T/E, and A553G/C are hypothesized to confer resistance to sulphadoxine [25, 40].

In Pakistan, published data are not available on the prevalence of point mutations in pvmdr1, and limited information is available regarding the prevalence of mutations in pvdhfr and pvdhps. In 2009, a study conducted in Bannu district of Pakistan reported a prevalence of 1.7% of pvdhfr F57L, 19% for S58R and 93.5% for S117N [3]. In pvdhps, a study conducted in 2010 in the Federally Administered Tribal Areas of Khyber Pakhtunkhwa province reported that 15% of samples had A383G and 54% had A553G/C [41]. Because up to date information on clinical efficacy and molecular markers of anti-malarial resistance in Pakistan is not available, this study was designed to evaluate the current prevalence of point mutations in pvdhfr, pvdhps and pvmdr1 genes of P. vivax in 14 sites around the country. The results presented here provide indications of whether efficacy of current anti-malarials is being maintained against P. vivax in Pakistan or requires further assessment.

Of 801 malaria-positive isolates by microscopy, PCR analysis revealed that 536 (76%) isolates were positive for P. vivax, 128 (18%) were P. falciparum, and 43 (4%) were mixed infections of both P. falciparum plus P. vivax. None of the samples was positive for P. malariae or P. ovale and 94 samples were PCR-negative for Plasmodium DNA. Results discordant with microscopic diagnosis were re-tested by PCR for confirmation, and a further comparison of microscopy and PCR results is presented in a separate manuscript [42]. Age among the 801 subjects ranged from two months to 75 years, with a median of 24 years. 64% of samples were collected from males and 36% from females. The gender distribution was identical and age distribution nearly identical in the subset of 372 of 579 P. vivax-positive samples by PCR that were selected for sequencing (age range = two months to 70 years; median = 25 years). Of these, sequencing was successful in 90% (336/372) for pvdhfr, 91% (339/372) for pvdhps and 92% (342/372) for pvmdr1. A subset of 120 PCR-positive P. vivax isolates was randomly selected for the quantification of pvmdr1 copy number variation, and the assay was successful in 108 (90%) of isolates.

Sequenced regions of each gene were monoclonal with no evidence of mixtures of SNPs among any of the samples, allowing the investigators to identify haplotypes. For codons I13L, P33L, F57L/I, S58R, T61M, S117N/T and I173L/F of pvdhfr, 151 (45%) of the isolates had the wild-type haplotype IPFSTSI. One hundred isolates (30%) had the S117N single mutation, 76 (23%) harboured S58R plus S117N double mutants, five isolates had the F57L plus S58R double mutant and one sample had the S58I plus S117N double mutant. The two mutations responsible for conferring pyrimethamine resistance in vitro, S58R and S117N, [46], were found in 84 (25%) and 177 (53%) of isolates, respectively. The pvdhfr S58R plus S117N double mutant haplotype was most prevalent among isolates in Sindh (6.8%) and Khyber Pakhtunkhwa province (6.5%) (Table 2). Although the pvdhfr double mutant was more common in infected patients over the age of 12 years than in children aged 12 and under prevalence of 24% versus 15%, the difference was not statistically significant (chi-square = 1.87; p = 0.17). Similarly, the double mutant was present in 28% of infections in females and 20% of infections in males but the difference was not significant (chi-square = 2.67; p = 0.10). Eight additional non-synonymous SNPs were detected, corresponding to amino acids N50I, S57R, S58R, S58I, S93R, S93H, S117N and N123K. Two of these, S93R and K123N, have not been reported previously. Non-synonymous and synonymous mutations are given in tabulated form in Additional file 1.

Based on the use of P. vivax strain ARI/ Pakistan (GenBank accession no X98123) as reference for pvdhfr, three indels were detected in the study, all 18 base pairs (bp) in size (Table 3). Seven isolates had an 18 bp repeat insertion between amino acid positions 91 and 92, and 21 samples had an 18 bp repeat insertion between codons 103 and 104. Ten isolates were discovered with an 18-bp deletion between amino acids 92 and 97.

The majority of samples were wild-type for pvdhps point mutations associated with sulphadoxine resistance, with the exception of seven samples carrying the A383G mutation, one carrying the A553G mutation, and one carrying the A383G plus A553G double mutation (Table 1). Four of the samples carrying the A383G mutation were from Khyber Pakhtunkhwa while the other three were found in each of three provinces, and the A383G plus A553G double mutant was found in Punjab (Table 2). Six non-synonymous mutations were detected (Additional file 1), four of which have not been previously reported.

The three most prevalent pvdhfr plus pvdhps haplotypes across all sites were wild-type (44%), single pvdhfr mutantI₁₃P₃₃F₅₇S₅₈T₆₁N₁₁₇I₁₇₃/ S₃₈₂A₃₈₃K₅₁₂A₅₅₃V₅₈₅ (30%) and double pvdhfr mutant I₁₃P₃₃F₅₇R₅₈T₆₁N₁₁₇I₁₇₃/ S₃₈₂A₃₈₃K₅₁₂A₅₅₃V₅₈₅ (21%).

All isolates were wild type at pvmdr1 codon Y976F while 335 (98%) isolates carried the point mutation at codon F1076L (Table 1). One hundred and ten, or 32% of all isolates with pvmdr1 F1076L, were found in Khyber Pakhtunkhwa and 84 (25%) were found in Punjab province. Three other non-synonymous SNPs (Y963C, N1010S and S1071G) not previously reported and a single synonymous SNP were also found (Additional file 1). All 108 isolates tested carried a single copy of the pvmdr1 gene.

Pakistan adopted artesunate plus SP combination therapy as a first line treatment against uncomplicated P. falciparum in 2008 [47]. Chloroquine is no longer indicated for treatment of P. falciparum but is recommended in combination with primaquine for treatment of P. vivax. Although the nature of anti-malarial use in Pakistan is not well-characterized, the availability of SP and other drugs as monotherapy has been documented [48] and, along with misdiagnosis of mixed infections [49] and presumptive treatment [50], likely results in P. vivax infections being frequently treated with AS + SP or SP alone. This study sought to characterize the current distribution of chloroquine resistance-associated polymorphisms in pvmdr1 and SP resistance-associated point mutations in pvdhfr and pvdhps in Pakistan.

Resistance of P. falciparum to chloroquine is nearly fixed in Pakistan [3, 51], and the emergence of chloroquine-resistant P. vivax in neighbouring India [52‐54] along with established resistance in Thailand [55, 56], Indonesia [57, 58] and Vietnam [59] suggests that the treatment of P. vivax with chloroquine in Pakistan could be in danger of being compromised. Clinical trials conducted during 2004-2006 indicate that chloroquine was still efficacious against P. vivax in Pakistan [9]. As a more recent follow-on to clinical evidence, this study presents the first report of pvmdr1 gene polymorphisms in Pakistan. Nearly all isolates were wild type at pvmdr1 Y796F, suggesting that chloroquine resistance in P. vivax has not emerged, but many carried the F1076L mutant. These results are consistent with previous studies showing the rise of pvmdr1 F1076L prior to Y976F [12, 23, 60]. If the hypothesis that the two-step trajectory of mutations at codon F1076L followed by Y976F may lead to chloroquine resistance [12] is correct, then pvmdr1 F1076L may provide an early signal of emerging chloroquine resistance in Pakistan prior to the appearance of the drug resistance phenotype in the population [22], particularly in Khyber Pakhtunkhwa where its prevalence is highest. Polymorphisms in pvmdr1 should continue to be monitored in case the mutation at position Y976F appears. Amplification of the pvmdr1 gene was not detected in this study, reflecting the absence of mefloquine drug pressure in Pakistan. Regions with a history of mefloquine use for falciparum malaria have shown amplification of both pfmdr1 in P. falciparum and pvmdr1 in P. vivax, likely caused by inadvertent drug exposure [17, 23].

Plasmodium vivax is likely exposed to SP pressure (either alone or in combination with artesunate) in a bystander effect, particularly because it is found alongside P. falciparum in regions with mixed-species infections such as Balochistan (10%), Khyber Pakhtunkhwa (8%), and Punjab (5%) [42], that are vulnerable to misdiagnosis of Plasmodium species [3, 41, 49, 51]. Exposure of P. vivax parasites to SP pressure, with likely subtherapeutic levels in some patients, could favour the emergence of SP resistance [61].

In this study, the pvdhfr double mutant S58R and S117N was found in 20% of isolates. The double mutant has been reported previously from one site in Pakistan [3] as well as in neighbouring India [62] and Iran [63], and is associated with in vitro pyrimethamine resistance [64]. In contrast, point mutations in pvdhps A383G and A553G were detected in a very small number of samples, confirming similar results from a previous report in Bannu district in 2007 [3]. These mutations have also been reported as rare in Iran [63] and Afghanistan [65], suggesting that markers associated with sulphadoxine resistance in P. vivax remain rare in this region. Because SP treatment failure is closely associated with infections carrying four or more mutations in pvdhfr[35] and additional pvdhps mutations [66], it appears that SP likely remains efficacious against P. vivax in Pakistan as previously reported [9].

The prevalence of pvdhfr S117N single and S58R plus S117N double mutants was highest in Sindh and Khyber Pakhtunkhwa provinces. Khyber Pakhtunkhwa had the second highest prevalence of mixed species and P. falciparum infections in this survey [42], indicating that perhaps regions with P. falciparum experience higher SP use (with or without artesunate) that also exerts pressure on co-occurring P. vivax infections. Khyber Pakhtunkhwa province also had the highest prevalence of pvmdr1 F1076L mutants, and may be especially vulnerable to the influx or emergence of drug-resistant parasites because of the large-scale migration of Afghan refugees and internally displaced people (IDPs) from conflict areas to this region, along with conditions of underdevelopment that may prevent adequate diagnosis and treatment of malaria [67].

This is the first comprehensive report on molecular patterns of drug resistance in P. vivax in Pakistan. The major chloroquine resistance-mediating mutation was not detected in this study; however continued molecular monitoring of polymorphisms in pvmdr1 is crucial to detect the potential emergence of chloroquine-resistant P. vivax in Pakistan. Although some evidence of pyrimethamine selective pressure was observed, polymorphisms associated with sulphadoxine resistance were rare, and markers of clinically relevant SP resistance in P. vivax were not present. Thus, the data on molecular markers presented here suggest that efficacy of both chloroquine and SP against P. vivax is currently high, and can serve as a baseline reference for regional studies of P. vivax drug sensitivity in Pakistan.