Malaria has been reported in Brazil since the 1500s when Plasmodium falciparum spread along the coast with the sugarcane industry. Migration patterns driven by later industry (gold panning, diamond mining, rubber tapping, agriculture, cattle ranching, etc) and colonization propagated malaria epidemics and the principal vector, Anopheles darlingi into the interior [1], [2], [3]. By the early 1940s, there were six million malaria cases per year in Brazil, which was then a seventh of the population [4]. After the introduction of DDT and chloroquine treatment, there were only 28,557 P. falciparum cases in 1970 among 92.3 million people and 73% of cases were reported in the Amazon region [4], [5], [6], [7], [8].

During the 1970s, immunological naïve people from the south and elsewhere migrated to the gold mining areas in the Amazon, particularly Rondônia [9], [10], [11]. The construction of roads, hydroelectric plants, livestock and agricultural projects, mines, and other infrastructural project also led to an increase in malaria transmission [6]. Plasmodium migrated between the various malarious regions in multiple directions [12]. Malaria cases tripled between 1970 and 1980 due to the removal of DDT, reduction of control efforts and integration with other public health services, deforestation of the Amazon, and drug resistance [6], [8], [13], [14], [15], [16]. By 1990, 99% of malaria in Brazil occurred in states around the Amazon basin: Acre, Amapá, Amazonas, Maranhão, Mato Grosso, Pará, Rondônia, Roraima and Tocantins [6].

Sulfadoxine pyrimethamine (SP) resistance developed relatively quickly. Pyrimathemine in combination with various sulphonamides was first used in Brazil during the early 1960s and low level pyrimethamine resistance was then reported at sites throughout the country [17], [18]. Resistance to SP was first reported in Goiás in 1972 and six years later in Maranhão [17], [19]. In 1982, RIII SP resistance was reported along the road between Manaus, Amazonas, and Porto Velho, Rondônia [19]. By the end of the 1980s, 90% of parasites were SP resistant.

Existing work on Brazilian P. falciparum suggests that there is limited genetic diversity as seen in other South American countries [20], [21], [22], [23], [24], [25], [26]. When combined with the history of Brazilian malaria, it appears that Brazilian P. falciparum may have gone through bottlenecks, multiple reintroductions due to human migration, and potentially clonal expansions. We previously showed that Peruvian P. falciparum populations were highly clonal [27] and here we used the same tools to examine Brazilian P. falciparum. Our expectation was that Brazilian parasites might have similar population structure. We examined isolates collected across Brazil through two decades from Amapá, Rondônia, and Pará state. By examining more microsatellites markers on more chromosomes than previous studies, we hoped to define the extent of low diversity, linkage disequilibrium, bottlenecks, population structure, and parasite migration within Brazil. There is little information regarding the prevalence of SP resistant dhfr and dhps alleles over time in Brazil and we used retrospective genotyping of samples from the 1980s and 1990s to explore their population genetics. The availability of historical samples from different sites in Brazil also provided the chance to test an existing hypothesis that the triple mutant dhfr mutations 50R/51I/108N and 51I/108N/164L developed in southern Amazon from a single origin of common or similar parasites [28].

The hypothesis that Brazilian P. falciparum has undergone bottlenecks followed by migration between sites is supported by its limited genetic diversity and the rejection of isolation by distance in our study, as seen in earlier papers [20], [21]. Unlike Peru, where parasite population structure appeared to be defined by recently reproductively isolated clonal lineages [40], there appeared to be gene flow across the Brazilian Amazon basin. We were unable to divide Brazilian parasite populations by clonal lineages and pairwise F_ST were lower than those reported for Peru (Table 4). Our AMOVA results agreed with earlier studies that found most parasite diversity was found within sites in the Brazilian Amazon [22]. We argue that the population structure of modern Brazilian Plasmodium falciparum appears to be based on ongoing admixture and reassortment of parasite lineages.

While the data suggested there was substantial migration between sites in the Brazilian Amazon Basin, there was no obvious structure to that migration (Figures 8 and 9). Interestingly, the two sites in Pará had lower F_ST values in comparison to Rondônia than to each other. The low reported F_ST for the comparison of Porto Velho, Rondônia and Marabá, Pará is also supported by the findings of a previous paper [20]. Rondônia might have acted as a source population multiple times for different sites in Pará or multiple different parasites populations from Pará may have populated Rondônia. Furthermore, parasites in Pará had gone through statistically significant bottlenecks, but those in Rondônia had not. This finding is in conflict with an earlier study that reported a potential bottleneck in Rondônia [22]. However, our data from Rondônia had a skewed allele frequency distribution (indicative of a bottlenecked population) and almost significant H_e excess, which suggest that limited sample size may have influenced our results. Although our findings are generally consistent with several previous studies, there was one study which did not find evidence of recent bottlenecks in Pará, Rondônia, Acre, or Amapá.[20]. In comparison to this study, we used more microsatellites on more chromosomes, as well as a different mutation model in Bottleneck.

It has been previously argued that highly resistant alleles of dhfr and dhps spread from the southern Amazon based on the understanding that high levels of SP resistance had first developed on the border of Bolivia and Brazil [19], [28]. Furthermore, it was suggested that the triple mutants arose from a single origin, with the 51I/108N/164L allele spread in a north-northwest direction across the Amazon, while the 50R/51I/108N allele spread north-northeast [28]. Our results suggest that there may have been at least two origins of double mutant 51I/108N dhfr alleles, which then led to the two separate triple mutant alleles. Furthermore, isolates from the 1980s in Amapá and Itaituba, Pará carried the 51I/108N/164L dhfr allele, though none were found in Rondônia (Figure 2). It is possible that highly resistant dhfr/dhps alleles were able to spread from the Bolivian border to the eastern coast of Brazil between 1982 and 1984, though somewhat unlikely.

Therefore these results challenge the hypothesis that the 51I/108N/164L alleles arose on the border with Bolivia, rather suggesting that it may have developed in the center of the Brazilian Amazon. Supporting this alternate hypothesis, the first reports of RIII resistance in 1982 occurred between Manaus and Porto Velho, a fair distance from the Bolivian border [19]. Additionally, triple mutant alleles had apparently spread throughout much of the Brazilian Amazon by the 1980s, which suggests that triple mutant alleles arose earlier than 1982. Indeed, it has been suggested that the double mutant dhfr and dhps mutations could have existed in Brazil as early as the 1960s based on clinical reports [18], [28]. RII pyrimethamine resistance was first reported in Goiás in 1968, which is in the center-south of the country, and resistance to the sulfadoxine-pyrimethamine drug combination was first reported in Brazil in that locality in 1972 [17], [18], [19]. These various historical reports suggest triple mutant dhfr alleles could have originated somewhere in the center of the Brazilian Amazon.

Whatever the origin of triple mutant dhfr and dhps alleles, our results suggest they did not develop in coastal Para, as this state appears to have only had parasites carrying double mutant dhfr (51I/108N) and dhps (437/581) parasites before 1986. In fact, that year may be when highly resistant alleles began to spread into this region.

By the 1990s, the frequency and distribution of the different dhfr and dhps alleles appears to have changed. In particular, the dhfr 50R/51I/108N allele had spread throughout the country and appeared to dominate populations. While the dhfr 51I/108N and 51I/108N/164L alleles were still present, they appeared along the eastern and western peripheries of the country. Meanwhile, the triple mutant dhps allele had disseminated throughout the country. The presence of both dhfr triple mutants in Rondonia during the 1990s, and the fixation of the triple mutant dhps allele, is verified by isolates sequenced by others [28], [45]. The wide distribution of dhfr 50R/51I/108N and 51I/108N/164L alleles is also supported by the analysis of isolates collected in Mato Grosso during the 1990s, but the study had inconsistent reporting for other regions [45]. Some of the distribution of dhfr alleles in Brazil during the 1990s may be the product of accidental migration. However, the preponderance of the 50R/51I/108N dhfr alleles after the wide use of SP suggest that it may have had higher fitness in the presence of SP in comparison to the reduced frequency of 51I/108N/164L. Assuming the two alleles have undergone similar selective pressure, the higher overall variation around the 50R/51I/108N dhfr allele (Figure 5) in comparison to the 51I/108N/164L allele (Figure 6) during the 1980s suggests 51I/108N/164L may have had more time to sexually recombine, implying it is the older allele. Furthermore, it appears that the reduced H_e seen around the 51I/108N/164L allele during the 1980s reduced over the next decade (Figure 6) due to sexual recombination with parasites carrying 50R/51I/164L, based on a qualitative examination of microsatellite haplotypes.

Though we had historical samples collected during the 1980s, we would have required samples from earlier years to understand how SP resistance spread across the basin or to fully describe the complex internal migration of Brazilian parasites after the colonization efforts of the past decades. However, it is clear that SP resistance, at both the double and triple mutant stages, developed twice and potentially in different regions of the Brazilian Amazon. Brazil might have had clonal structure similar to Peru during the 1960–1970s due to the reduction in malaria cases and genetic drift in isolated populations, but any such clonal lineages may have been masked over time due to extensive admixture and reassortment. Our recent paper from Peru indicates that the evidence for a bottleneck can rapidly disappear after parasite migration [40]. On the other hand, Brazilian malaria control efforts never completely suppressed malaria and therefore populations may have maintained high enough effective population sizes to avoid clonal expansions. Indeed, our bottleneck tests rejected this possibility generally, though they suggest that coastal Pará populations went through bottlenecks. From the standpoint of public health policy, the Brazilian Amazon basin may have sufficient internal migration for drug resistance reported in any particular region to rapidly spread to other parts of basin under similar drug pressure.