Trends in Parasitology
ReviewFollowing the path of most resistance: dhps K540E dispersal in African Plasmodium falciparum
Section snippets
Drug resistance in African Plasmodium falciparum
Artemisinin combination therapies (ACT) such as artemether–lumefantrine, artesunate–mefloquine, artesunate-SP or artesunate–amodiaquine are now the recommended first-line treatment for falciparum malaria throughout the world. Unfortunately the future efficacy of ACTs has been overshadowed by reports of the emergence of artemisinin resistant P. falciparum in Pailin, western Cambodia, near the border with Thailand [1]. Although the mechanism of resistance has not yet been elucidated, there is a
Chloroquine resistance
The first confirmed clinical reports of chloroquine resistance (CQR) occurred in Pailin, the same area of western Cambodia where artemisinin tolerant parasites are now emerging 1, 8. A global review of CQR reports was compiled by Payne (D. Payne, Ph.D. thesis, University of London, 1989) [9]. Two initial foci of confirmed resistance in Asia and South America in 1960 were described as well as their subsequent dispersal in all epidemiologically feasible directions 8, 9. The first appearance of
Pyrimethamine resistance
Pyrimethamine monotherapy was used widely in the 1950s and 1960s for treatment and for mass prophylaxis [14]. In contrast with CQR, the pyrimethamine resistance trait emerged almost immediately in a multifocal manner in both East and West Africa. Early reports showed rapid emergence of pyrimethamine resistant malaria in Kenya (1954), Tanzania (1954), Sudan (1955), Cameroon (1958), Burkina Faso (1958), Nigeria (1960), Ghana (1962), Gambia (1963), Senegal (1966), and Liberia (1980) (Figure 2) [14]
Inferring dispersal from reports of resistance
The pan-African distributions of Asian derived pfcrt and dhfr lineages today gives no clue to their original site(s) of introduction or their routes of dispersal. The passage of pfcrt can be inferred from the geospatial pattern of chloroquine treatment failure reports (Figure 1) [10], but an equivalent trail of pyrimethamine resistance is not recorded because the successful combination of SP masked the presence of pyrimethamine resistant parasites. It was only when sulphadoxine resistance
Sulphadoxine resistance
Resistance to sulfadoxine is acquired by the progressive accumulation of mutations in the dhps gene. The emergence of resistant dhps began in 1993 and heralded the arrival of SP treatment failure in Africa. In particular, the dhps double mutant (A437G + K540E) combined with the dhfr triple mutant (i.e. quintuple mutant or full house) is predictive of early SP treatment failure 31, 32, 33. The A437G mutation can occur alone or in combination with K540E, but the K540E mutation is almost invariably
Molecular surveillance coverage
In the review of the published literature (Box 1 and Table 1), we identified 212 surveys in 148 unique geographic localities of 37 countries Africa, where a total of 21 362 P. falciparum isolates were tested for the K540E mutation. The K540E mutation was present in 40% of the isolates. The sites of these survey data points are presented in Figure 3. The 106 references which describe them are listed in the Supplementary Data S1, and a Google map of their exact location is available at //www.drugresistancemaps.org/maps/dhps540/
Molecular surveillance methodology
In all the studies identified, PCR was used to amplify a dhps sequence fragment, generally from small quantities of parasite DNA, collected by finger-prick blood sampling. A variety of molecular methods were used to detect resistance mutations in the amplified sequence. Of the 104 articles reporting K540E prevalence, 50% used PCR-RFLP, 23% used direct sequencing methods, whereas probe hybridization (11%), primer extension (4%) and mutant specific PCR amplification (10%) accounted for the
Crossing the East–West divide
The time sequence of reports confirming the presence or absence of the K540E mutation between 1993 and 2008 is illustrated in Figure 4. The data spans 15 years and is subdivided into 5 year blocks. In the first 5 years between 1993 and 1998, the mutation was confirmed in East Africa and absent in West Africa. The earliest records of K540E came from East Africa. In Kenya, it was found in samples collected from 1993 to 1995 [35], in Tanzania during 1995 [36] and in Malawi from1995 to1996 [37].
Independent origins in northeast and southeast African populations
There are two major lineages of K540E, as well as the first lineage, designated SGE1, which has the wide geographical range described above, there is a second independently evolved lineage, designated SGE2. This emerged over the same time period as SGE1 but has been found exclusively in parasites from northeast Africa [30]. In northeastern Sudan, K540E was absent in 1993 but had appeared by 1998 [58]. In neighbouring countries, the K540E mutation has been confirmed; samples collected in
Macro-geographic distribution and local prevalence of K540E
The K540E was clearly abundant in the southeast African region from 1995 onwards, and it is clear that an overriding determinant of the presence or absence of K540E mutations in African parasite populations is their place in the continent-wide distribution of the mutant lineages. Pearce et al.[30] described five regional parasite gene pools defined on the basis of dhps resistance allele sharing. Within these five regional clusters, the same mutant dhps lineages were common to all survey sites.
Parasite populations are linked through networks of human circulation
Once a resistance mutation is established at a site in one of the major regional gene pools it can be disseminated rapidly to other populations in the same regional gene pool. Circulation of infected people between the major regions and the mainland and island populations is less frequent and consequently exchange of resistance genes is more stochastic.
Rates of dispersal on the African mainland
CQR and K540E arrived in the same east African region independently and perhaps via the same route (see below). Having arrived, they each spread
Predicting resistance dispersal in the future?
Will artemisinin resistance follow the same path as CQR and the dhps mutation K540E? Since Pailin was the crucible for both CQR and artemisinin resistance, it now perhaps seems probable that the same patterns of dispersal will play out all over again. There are a number of significant factors that have changed or are changing: (i) patterns of human circulation change according to the dictates of war, trade and transport infrastructure. New trade partnerships have been established between Africa
Conclusions
Although local resistance levels are moderated by local drug use, it is now clear that the availability of resistance genes to a local parasite population is largely determined by its geographical location. This is because a mutation's distribution is governed by pathways of parasite circulation which describe large, natural populations and their regional boundaries. With this insight, can molecular surveillance be made more efficient? The molecular surveillance coverage of Africa illustrated
References (64)
Mutations in the P. falciparum digestive vacuole transmembrane protein pfcrt and evidence for their role in chloroquine resistance
Mol. Cell
(2000)Spread of chloroquine resistance in Plasmodium falciparum
Parasitol. Today
(1987)Antifolate antimalarial resistance in southeast Africa: a population-based analysis
Lancet
(2003)Spread and evolution of Plasmodium falciparum drug resistance
Parasitol. Int.
(2009)High level of resistance of Plasmodium falciparum to sulfadoxine-pyrimethamine in children in Tanzania
Trans. R. Soc. Trop. Med. Hyg.
(1996)Resistance to pyrimethamine/sulfadoxine in Plasmodium falciparum in 12 villages in north east Tanzania and a test of chlorproguanil/dapsone
Acta Trop.
(1997)Can pretreatment screening for dhps and dhfr point mutations in Plasmodium falciparum infections be used to predict sulfadoxine-pyrimethamine treatment failure?
Trans. R. Soc. Trop. Med. Hyg.
(2001)Polymorphisms in Plasmodium falciparum dhfr and dhps genes and age related in vivo sulfadoxine-pyrimethamine resistance in malaria-infected patients from Nigeria
Acta Trop.
(2005)High Plasmodium falciparum resistance to chloroquine and sulfadoxine-pyrimethamine in Harper, Liberia: results in vivo and analysis of point mutations
Trans. R. Soc. Trop. Med. Hyg.
(2002)Averting a malaria disaster
Lancet
(1999)