Modelling the drivers of the spread of Plasmodium falciparum hrp2 gene deletions in sub-Saharan Africa

  1. Oliver J Watson  Is a corresponding author
  2. Hannah C Slater
  3. Robert Verity
  4. Jonathan B Parr
  5. Melchior K Mwandagalirwa
  6. Antoinette Tshefu
  7. Steven R Meshnick
  8. Azra C Ghani
  1. Imperial College London, United Kingdom
  2. University of North Carolina, United States
  3. Université de Kinshasa, Democratic Republic of the Congo
5 figures, 1 video, 3 tables and 2 additional files

Figures

Figure 1 with 5 supplements
Predicted increase in pfhrp2-deletion upon RDT introduction after 10 years.

Graphs show the time course of pfhrp2-deletion emergence under (a) different transmission intensities (10%, 25% and 60% PfPR) and 8% starting frequency of pfhrp2-deletion prior to RDT introduction and under (b) different assumed starting frequencies of pfhrp2-deletion prior to RDT introduction (2%, 8% and 12% starting frequency) and 25% PfPR. Five years after RDT introduction, the proportion of strains that are pfhrp2-deleted (c), and the proportion of the population that are infected with only pfhrp2-deleted mutants (d) is recorded. The dark grey dots denote individual simulation runs with a LOESS regression fit shown in blue. Source data for Figure 1 is provided within Figure 1—source data 1.

https://doi.org/10.7554/eLife.25008.004
Figure 1—source data 1

Effect of transmission intensity and pfhrp2-deletion starting upon pfhrp2-deletion emergence.

https://doi.org/10.7554/eLife.25008.010
Figure 1—figure supplement 1
Impact of increase pfhrp2-deletion upon malaria prevalence.

Graphs show the increase in malaria prevalence over time as a result of increasing pfhrp2-deletion upon using only HRP2-based RDTs to guide treatment decisions, with the greatest increase in prevalence observed at the lowest starting prevalence.

https://doi.org/10.7554/eLife.25008.005
Figure 1—figure supplement 2
Impact of pfhrp2-deletion fitness cost.

Graphs show the mean time course of pfhrp2-deletion emergence under different assumptions concerning the negative impact of pfhrp2-deletion. The fitness cost is incorporated by comparatively reducing the contribution to the human infectious reservoir made by the deletion strains in order to represent an assumed decrease in parasitaemia. The fitness cost is only implemented at the time of RDT introduction to illustrate the sum effect of the opposing selection pressures.

https://doi.org/10.7554/eLife.25008.006
Figure 1—figure supplement 3
Impact of microscopy use and non-adherence to RDT results.

Graphs show the mean time course of pfhrp2-deletion emergence under different assumptions concerning the use of microscopy as an additional diagnostic and the impact of non-adherence to RDT test results, that is an individual receiving treatment despite yielding a negative RDT result. Microscopy use and nonadherence to RDT results are only implemented at the time of RDT introduction to illustrate the sum effect of the opposing selection pressures.

https://doi.org/10.7554/eLife.25008.007
Figure 1—figure supplement 4
Impact of non-malarial fever.

Graphs show the mean time course of pfhrp2-deletion emergence under different assumptions concerning the rate of non-malarial fever (NMF). The introduction of non-malarial fevers increases the selection pressure in favour of pfhrp2-deletion, with 125% the observed rate of non-malarial fever yielding the quickest emergence of pfhrp2-deletion. Non-malarial fever is only implemented at the time of RDT introduction to illustrate the sum effect of the opposing selection pressures.

https://doi.org/10.7554/eLife.25008.008
Figure 1—figure supplement 5
Combined impact of model assumptions.

Graphs show the mean time course of pfhrp2-deletion emergence under different assumptions concerning any negative fitness cost associated with pfhrp2-deletion, use of microscopy-based diagnosis, non-adherence to RDT results and non-malarial fever (NMF). These factors were explored at three different relative rates and compared to the method used within the main investigation (red line). An increase in pfhrp2-deletion is observed in all cases, with the method used within the main investigation exhibiting an increase in pfhrp2-deletion slightly slower than exhibited by the intermediate level of model assumptions (blue line).

https://doi.org/10.7554/eLife.25008.009
Figure 2 with 1 supplement
The predicted rate at which the population is only infected with pfhrp2-deleted mutants.

The graphs show the time in years after RDT introduction at which 20% of the population are only infected with pfhrp2-deleted mutants up to a maximum follow-up time of 20 years post RDT introduction. PfHRP3 epitopes were assumed to cause a positive RDT result in (a) 0% or (b) 25% of individuals only infected with pfhrp2-deleted mutants. The plotted years represent the mean time grouped in each prevalence and treatment setting, with black dots representing where 20% was reached in less than five years. Each simulation had a starting pfhrp2-deletion frequency of 8% before RDT introduction. Source data for Figure 2 is provided within Figure 2—source data 1.

https://doi.org/10.7554/eLife.25008.011
Figure 2—source data 1

Years after RDT introduction at which 20% of the population are only infected with pfhrp2-deleted parasites, with an assumed PfHRP3 epitope effect equal to 0% and 0.25%.

https://doi.org/10.7554/eLife.25008.013
Figure 2—figure supplement 1
Frequency of pfhrp2-deletion after 20 years.

The graphs show the frequency of pfhrp2-deletion within the population 20 years after RDT introduction. PfHRP3 epitopes were assumed to cause a positive RDT result in (a) 0% or (b) 25% of individuals only infected with pfhrp2-deleted mutants. Each simulation had a starting pfhrp2 deletion frequency of 8% before RDT introduction.

https://doi.org/10.7554/eLife.25008.012
Figure 3 with 1 supplement
Simulated province level burden of pfhrp2-deleted mutants within the DRC, with an assumed probability of a clinical case seeking treatment, who is only infected with pfhrp2-deleted mutants, producing a positive RDT result (ε) equal to 0.25.

In (a) the mean simulated proportion of children aged 6–59 months who are infected with only pfhrp2-deleted mutants is shown in red. Each region had an assumed starting frequency of 6% pfhrp2-deletion prior to RDT introduction in 2010 (2007 in North- and South-Kivu). The results in grey represent the recorded burden from the DHS survey (Figure 3—source data 1), with both datasets fitted with a LOESS regression. Error bars show the 95% confidence interval. In (b) the same simulation conditions were used as in (a) however it is assumed that no selection pressure is exerted by the introduction RDTs, i.e. ε = 1. Source data for Figure 3 is provided within Figure 3—source data 1.

https://doi.org/10.7554/eLife.25008.014
Figure 3—source data 1

Estimates of the proportion of pfhrp2-deleted mutants from a national study in DRC.

Sourced from Parr JB, Verity R, Doctor SM, Janko M, Carey-Ewend K, Turman BJ, Keeler C, Slater HC, Whitesell AN, Mwandagalirwa K, Ghani AC, Likwela JL, Tshefu AK, Emch M, Juliano JJ, Meshnick SR. 2016. Pfhrp2-deleted Plasmodium falciparum parasites in the Democratic Republic of Congo: A national cross-sectional survey. J Infect Dis: 1–34. doi: 10.1093/infdis/jiw538. Data is provided additionally in an importable format for plotting (Figure 3.csv).

https://doi.org/10.7554/eLife.25008.016
Figure 3—source data 2

Simulated proportion of children aged 6–59 months who are only infected with pfhrp2-deleted parasites within the Democratic Republic of Congo, with an assumed PfHRP3 epitope effect equal to 0.25% and 1%, that is under no selection pressure.

https://doi.org/10.7554/eLife.25008.017
Figure 3—figure supplement 1
Simulated province level burden of pfhrp2-deleted mutants within DRC, with an assumed probability of a clinical case seeking treatment, who is only infected with pfhrp2-deleted mutants, producing a positive RDT result (ε) equal to 0.

In (a) the mean simulated proportion of children aged 6–59 months who are infected with only pfhrp2-deleted mutants is shown in red. Each region had an assumed starting frequency of 4.5% pfhrp2-deletion prior to RDT introduction in 2010 (2007 in North- and South-Kivu). The results in grey represent the recorded burden from the DHS survey, with both datasets fitted with a LOESS regression. Error bars show the 95% confidence interval. In (b) the graph shows the plot of the residuals when comparing the simulation predicted proportion of children aged 6–59 months only infected with pfhrp2-deleted mutants to the recorded DHS data.

https://doi.org/10.7554/eLife.25008.015
Figure 4 with 5 supplements
Predicted areas of HRP2 concern in comparison to recorded prevalence and treatment seeking rate, with an assumed probability of a clinical case seeking treatment, who is only infected with pfhrp2-deleted mutants, producing a positive RDT result (ε) equal to 0.25.

The graphs show (a) the recorded malaria prevalence in children aged 2–10 by microscopy in 2010, (b) the frequency of people seeking treatment in 2010 and (c) the predicted concern for the impact of pfhrp2-deleted mutants. In (c), high, moderate and slight risk represent >20% infection due to only pfhrp2-deleted mutants by 2016, 2022 and 2030 respectively, and marginal risk represents <20% by 2030. In 2010, each region was assumed to have a starting frequency of 6% pfhrp2-deletion. Source data for Figure 4 is provided within Figure 4—source data 1.

https://doi.org/10.7554/eLife.25008.018
Figure 4—source data 1

Recorded malaria prevalence in children aged 2–10 by microscopy in 2010 (sourced from the Malaria Atlas mapping project [see Metadata - Datasets]), the frequency of people seeking treatment in 2010 (sourced from Cohen et al., 2012 [see Metadata – Datasets]) and the simulated predicted concern for the impact of pfhrp2-deleted mutants, with an assumed PfHRP3 epitope effect equal to 0.25%.

High, moderate and slight risk represent >20% infection due to only pfhrp2-deleted mutants by 2016, 2022 and 2030 respectively, and marginal risk represents <20% by 2030.

https://doi.org/10.7554/eLife.25008.024
Figure 4—figure supplement 1
Model malaria prevalence output against Malaria Atlas Project prevalence 2010 (Bhatt et al., 2015).

The maps show the reported microscopy prevalence in children aged 2–10 from (a) the Malaria Atlas Project and (b) the presented model outputs at the first-administrative unit.

https://doi.org/10.7554/eLife.25008.019
Figure 4—figure supplement 2
HRP2 Concern heat maps.

The graphs show the time after RDT introduction at which 20% of the population are only infected with pfhrp2-deleted mutants with an assumed probability of a clinical case seeking treatment, who is only infected with pfhrp2-deleted mutants, producing a positive RDT result (ε) equal to (a) 0.25 and (b) 0. Areas in grey represent simulation space in which, after 20 years, the proportion of 2–10 year olds only infected with pfhrp2-deleted mutants was less than 20%.

https://doi.org/10.7554/eLife.25008.020
Figure 4—figure supplement 3
Predicted areas of HRP2 concern in comparison to recorded prevalence and treatment coverage with an assumed probability of a clinical case seeking treatment, who is only infected with pfhrp2-deleted mutants, producing a positive RDT result (ε) equal to 0.

The graphs show (a) the recorded malaria prevalence in children aged 2–10 in 2010, (b) the frequency of people seeking treatment in 2010 and (c) the predicted concern for the impact of pfhrp2-deleted mutants. In (c), high, moderate and slight risk represent >20% infection due to only pfhrp2-deleted mutants by 2016, 2022 and 2030 respectively, and marginal risk represents <20% by 2030. In 2010 each region was assumed to have a starting frequency of 4.5% pfhrp2-deletion.

https://doi.org/10.7554/eLife.25008.021
Figure 4—figure supplement 4
Impact of different assumptions about starting frequency of pfhrp2-deletion upon the geographical pattern of selection-driven increase in pfhp2-deletion.

Three different starting frequencies of pfhrp2-deletion were explored, with an assumed probability of a clinical case seeking treatment, who is only infected with pfhrp2-deleted mutants, producing a positive RDT result (ε) equal to 0.25. The frequency of pfhrp2-deletion after 20 years was recorded and admin regions ranked accordingly, with the first rank representing the highest frequency of pfhrp2-deletion, and tied ranks being represented with the same colour.

https://doi.org/10.7554/eLife.25008.022
Figure 4—figure supplement 5
Years in which RDTs were used at community level in Sub-Saharan Africa.

The map shows the year that RDTs were reported to be available at the community level within WHO malaria country profiles in 2012 (World Health Organization, 2012b). Countries in grey were not reported to use RDTs at the community level, or there was insufficient data.

https://doi.org/10.7554/eLife.25008.023
Transmission Model.

Flow diagram for the human component of the transmission model, with dashed arrows indicating superinfection. S, susceptible; T, treated clinical disease; D, untreated clinical disease; P, prophylaxis; A, asymptomatic patent infection; U, asymptomatic sub-patent infection. All parameters are described within Table 2.

https://doi.org/10.7554/eLife.25008.026

Videos

Video 1
The projected increase in individuals who are only infected with pfhrp2-deleted parasites, from 2010 to 2030, with an assumed starting frequency of 6% pfhrp2-deletion, and an assumed PfHRP3 epitope effect equal to 0.25%.

The video relates directly to Figure 4.

https://doi.org/10.7554/eLife.25008.025

Tables

Table 1

Published studies reporting P. falciparum in Africa with deletions or no deletions of the pfhrp2 gene (Cheng et al., 2014).

https://doi.org/10.7554/eLife.25008.003
OriginSource of samples*Initial evidenceGene deletion analysis by PCRAntigen analysisRefPrevalence (no. of samples, year of collection)
CountryAreaMicroscopyQuality RDTSpecies PCRpfhrp2 (exon 1 and 2)No. single copy genesFlanking genesHRP ELISA2nd quality RDT
MaliBamakoA/SDNDDD1NDNDND(Koita et al., 2012)2% (480, 1996)
DRC,Gambia, Kenya, Mozambique, Rwanda, Tanzania, UgandaSDNDDExon 2 onlyNDNDDND(Ramutton et al., 2012)0% (77, 2–19 per country, 2005–2010)
SenegalDakarSDNDDD1NDNDND(Wurtz et al., 2013)2.4% (136, 2009–2012)
GhanaAccra and Cape CoastADDDExon 2 only2NDNDND(Amoah et al., 2016)29.5% (315, 2015)
ZambiaChoma, South ZambiaA/SDDDD1NDNDND(Laban et al., 2015)20% (61, 2009–2012)†
DRCCountry-wideADDDD3DNDND(Parr et al., 2016)6.4% (783, 2013–2014)
RwandaBusogo, Musanze, KayonzaSDDDExon 2 only1NDNDD(Kozycki et al., 2017)23% (140, 2014–2015)
EritreaAnseba, Debub, Gash-Barka, Northern Red-SeaSDDDND1NDNDD(Berhane et al., 2017)80% (51, 2015)
  1. *Source of samples: S = Symptomatic case, A = Asymptomatic case, U = not specified, D = done; ND = not done.

    Authors suggested that failure to detect pfhrp gene in some samples was more likely to be the result of low parasite density rather than deletion

  2. Note: Quality RDT indicates RDTs that meet the WHO RDT recommended procurement criteria based on WHO Malaria RDT Product Testing.

Table 2
Parameters used within the human transmission and mosquito population models.
https://doi.org/10.7554/eLife.25008.028
ParameterSymbolEstimate
Human infection duration (days)
 Latent period

dE

12
 Patent infection

dA

200
 Clinical disease (treated)

dT

5
 Clinical disease (untreated)

dD

5
 Sub-patent infection

dU

110
 Prophylaxis following treatment

dP

25
Treatment Parameters
 Probability of seeking treatment if clinically diseased

fT

Variable
 Probability of a clinical case seeking treatment, who is only infected with pfhrp2-deleted mutants, producing a positive RDT result.

ε

0 or 0.25
Infectiousness to mosquitoes
 Lag from parasites to infectious gametocytes

dg

12 days
 Untreated disease

cD

0.0680 day−1
 Treated disease

cT

0.0219 day−1
 Sub-patent infection

cU

0.000620 day−1
 Parameter for infectiousness of state A

γ1

1.824
Age and heterogeneity
 Age-dependent biting parameter

ρ

0.85
 Age-dependent biting parameter

a0

8 years
 Daily mortality rate of humans

μ

0.000130
 Variance of the log heterogeneity in biting rates

σ2

1.67
Immunity reducing probability of infection
 Maximum probability due to no immunity

b0

0.590
 Maximum relative reduction due to immunity

b1

0.5
 Inverse of decay rate

dB

10 years
 Scale parameter

IB0

43.879
 Shape parameter

κB

2.155
 Duration in which immunity is not boosted

uB

7.199
Immunity reducing probability of clinical disease
 Maximum probability due to no immunityϕ00.791
 Maximum relative reduction due to immunityϕ10.000737
 Inverse of decay rate

dCA

30 years
 Scale parameter

IC0

18.0237
 Shape parameter

κC

2.370
 Duration in which immunity is not boosted

uC

6.0635
 New-born immunity relative to mother’s

PM

0.774
 Inverse of decay rate of maternal immunity

dM

67.695
Immunity reducing probability of detection
 Minimum probability due to maximum immunity

d1

0.161
 Inverse of decay rate

dID

10 years
 Scale parameter

ID0

1.578
 Shape parameter

κD

0.477
 Duration in which immunity is not boosted

uD

9.445
 Scale parameter relating age to immunity

aD

21.9 years
 Time-scale at which immunity changes with age

fD0

0.00706
 Shape parameter relating age to immunity

γD

4.818
 PCR detection probability parameters state A

αA

0.757
 PCR detection probability parameters state U

αU

0.186
Mosquito Population Model
 Daily mortality of adults

μM

0.132
 Daily biting rate

αk

0.307
 Extrinsic incubation period

dEM

10 days
Table 3
HRP2 classifiers used in sub-Saharan Africa mapping assuming RDT introduction in 2010.
https://doi.org/10.7554/eLife.25008.030
Proportion of population only infected with pfhrp2-deleted mutantsConcern classifier
 >20% by 2016High
 >20% by 2022Moderate
 >20% by 2030Slight
 <20% by 2030Marginal

Additional files

Supplementary file 1

Simulation model pseudocode.

Mathematical style pseudocode description of the simulation model.

https://doi.org/10.7554/eLife.25008.029
Transparent reporting form
https://doi.org/10.7554/eLife.25008.031

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  1. Oliver J Watson
  2. Hannah C Slater
  3. Robert Verity
  4. Jonathan B Parr
  5. Melchior K Mwandagalirwa
  6. Antoinette Tshefu
  7. Steven R Meshnick
  8. Azra C Ghani
(2017)
Modelling the drivers of the spread of Plasmodium falciparum hrp2 gene deletions in sub-Saharan Africa
eLife 6:e25008.
https://doi.org/10.7554/eLife.25008