Limits...
Drug coverage in treatment of malaria and the consequences for resistance evolution--evidence from the use of sulphadoxine/pyrimethamine.

Malisa AL, Pearce RJ, Abdulla S, Mshinda H, Kachur PS, Bloland P, Roper C - Malar. J. (2010)

Bottom Line: The influence of policy on the parasite reservoir was profound.The national policy change brought about a shift in treatment practice and the resulting increase in coverage had a substantial impact on drug pressure.The selection applied by first-line use is strong enough to overcome recombination pressure and create significant linkage disequilibrium between the unlinked genetic determinants of pyrimethamine and sulphadoxine resistance, showing that recombination is no barrier to the emergence of resistance to combination treatments when they are used as the first-line malaria therapy.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biological Sciences, Faculty of Science, Sokoine University of Agriculture, SUA, PO Box 3038, Morogoro, Tanzania.

ABSTRACT

Background: It is argued that, the efficacy of anti-malarials could be prolonged through policy-mediated reductions in drug pressure, but gathering evidence of the relationship between policy, treatment practice, drug pressure and the evolution of resistance in the field is challenging. Mathematical models indicate that drug coverage is the primary determinant of drug pressure and the driving force behind the evolution of drug resistance. These models show that where the basis of resistance is multigenic, the effects of selection can be moderated by high recombination rates, which disrupt the associations between co-selected resistance genes.

Methods: To test these predictions, dhfr and dhps frequency changes were measured during 2000-2001 while SP was the second-line treatment and contrasted these with changes during 2001-2002 when SP was used for first-line therapy. Annual cross sectional community surveys carried out before, during and after the policy switch in 2001 were used to collect samples. Genetic analysis of SP resistance genes was carried out on 4,950 Plasmodium falciparum infections and the selection pressure under the two policies compared.

Results: The influence of policy on the parasite reservoir was profound. The frequency of dhfr and dhps resistance alleles did not change significantly while SP was the recommended second-line treatment, but highly significant changes occurred during the subsequent year after the switch to first line SP. The frequency of the triple mutant dhfr (N51I,C59R,S108N) allele (conferring pyrimethamine resistance) increased by 37% - 63% and the frequency of the double A437G, K540E mutant dhps allele (conferring sulphadoxine resistance) increased 200%-300%. A strong association between these unlinked alleles also emerged, confirming that they are co-selected by SP.

Conclusion: The national policy change brought about a shift in treatment practice and the resulting increase in coverage had a substantial impact on drug pressure. The selection applied by first-line use is strong enough to overcome recombination pressure and create significant linkage disequilibrium between the unlinked genetic determinants of pyrimethamine and sulphadoxine resistance, showing that recombination is no barrier to the emergence of resistance to combination treatments when they are used as the first-line malaria therapy.

Show MeSH

Related in: MedlinePlus

Allele frequency changes at dhps in Rufiji (A), and Kilombero/Ulanga (B). The sensitive allele (dotted line with diamonds), the double mutant A437G + K540E allele (solid line with triangles), and single S436A mutant (dashed line with squares).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC2908640&req=5

Figure 3: Allele frequency changes at dhps in Rufiji (A), and Kilombero/Ulanga (B). The sensitive allele (dotted line with diamonds), the double mutant A437G + K540E allele (solid line with triangles), and single S436A mutant (dashed line with squares).

Mentions: As well as the highly resistant dhps A437G K540E double mutant, a number of single 436 mutant alleles were recorded. Among these, by far the most common was the S436A, which was consistently found at frequencies of 10%-20% at all time points in both districts. Figure 3 shows the frequencies of the sensitive S436A single mutant and the A437G K540E double mutant alleles through time in both districts. From these data it is clear that the rising frequency of the double mutant allele displaced the sensitive allele, which decreased significantly during 2001-2002 in both Kilombero/Ulanga (0.7-0.6) (p ≤ 0.001, 95% CI) and Rufiji (0.77-0.60) (p ≤ 0.0001, 95% CI). Interestingly, the frequency of the A436A allele remained static at all time points in both sites.


Drug coverage in treatment of malaria and the consequences for resistance evolution--evidence from the use of sulphadoxine/pyrimethamine.

Malisa AL, Pearce RJ, Abdulla S, Mshinda H, Kachur PS, Bloland P, Roper C - Malar. J. (2010)

Allele frequency changes at dhps in Rufiji (A), and Kilombero/Ulanga (B). The sensitive allele (dotted line with diamonds), the double mutant A437G + K540E allele (solid line with triangles), and single S436A mutant (dashed line with squares).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2908640&req=5

Figure 3: Allele frequency changes at dhps in Rufiji (A), and Kilombero/Ulanga (B). The sensitive allele (dotted line with diamonds), the double mutant A437G + K540E allele (solid line with triangles), and single S436A mutant (dashed line with squares).
Mentions: As well as the highly resistant dhps A437G K540E double mutant, a number of single 436 mutant alleles were recorded. Among these, by far the most common was the S436A, which was consistently found at frequencies of 10%-20% at all time points in both districts. Figure 3 shows the frequencies of the sensitive S436A single mutant and the A437G K540E double mutant alleles through time in both districts. From these data it is clear that the rising frequency of the double mutant allele displaced the sensitive allele, which decreased significantly during 2001-2002 in both Kilombero/Ulanga (0.7-0.6) (p ≤ 0.001, 95% CI) and Rufiji (0.77-0.60) (p ≤ 0.0001, 95% CI). Interestingly, the frequency of the A436A allele remained static at all time points in both sites.

Bottom Line: The influence of policy on the parasite reservoir was profound.The national policy change brought about a shift in treatment practice and the resulting increase in coverage had a substantial impact on drug pressure.The selection applied by first-line use is strong enough to overcome recombination pressure and create significant linkage disequilibrium between the unlinked genetic determinants of pyrimethamine and sulphadoxine resistance, showing that recombination is no barrier to the emergence of resistance to combination treatments when they are used as the first-line malaria therapy.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biological Sciences, Faculty of Science, Sokoine University of Agriculture, SUA, PO Box 3038, Morogoro, Tanzania.

ABSTRACT

Background: It is argued that, the efficacy of anti-malarials could be prolonged through policy-mediated reductions in drug pressure, but gathering evidence of the relationship between policy, treatment practice, drug pressure and the evolution of resistance in the field is challenging. Mathematical models indicate that drug coverage is the primary determinant of drug pressure and the driving force behind the evolution of drug resistance. These models show that where the basis of resistance is multigenic, the effects of selection can be moderated by high recombination rates, which disrupt the associations between co-selected resistance genes.

Methods: To test these predictions, dhfr and dhps frequency changes were measured during 2000-2001 while SP was the second-line treatment and contrasted these with changes during 2001-2002 when SP was used for first-line therapy. Annual cross sectional community surveys carried out before, during and after the policy switch in 2001 were used to collect samples. Genetic analysis of SP resistance genes was carried out on 4,950 Plasmodium falciparum infections and the selection pressure under the two policies compared.

Results: The influence of policy on the parasite reservoir was profound. The frequency of dhfr and dhps resistance alleles did not change significantly while SP was the recommended second-line treatment, but highly significant changes occurred during the subsequent year after the switch to first line SP. The frequency of the triple mutant dhfr (N51I,C59R,S108N) allele (conferring pyrimethamine resistance) increased by 37% - 63% and the frequency of the double A437G, K540E mutant dhps allele (conferring sulphadoxine resistance) increased 200%-300%. A strong association between these unlinked alleles also emerged, confirming that they are co-selected by SP.

Conclusion: The national policy change brought about a shift in treatment practice and the resulting increase in coverage had a substantial impact on drug pressure. The selection applied by first-line use is strong enough to overcome recombination pressure and create significant linkage disequilibrium between the unlinked genetic determinants of pyrimethamine and sulphadoxine resistance, showing that recombination is no barrier to the emergence of resistance to combination treatments when they are used as the first-line malaria therapy.

Show MeSH
Related in: MedlinePlus