A total of 50 children with mean age of 6.6±3.5 years were included. There were more females than males (56.0% vs 44.0%). The average red blood cell count, hemoglobin, hematocrit, mean cell volume, mean cell hemoglobin, mean cell hemoglobin concentration, white blood cell count, platelet count and parasite density were 4.0±0.7 x106/µL, 10.5±1.6 g/dL, 30.8±4.6 %, 77.2±6.4 fL, 26.7±4.1 pg, 34.5±3.7 g/dL, 7.5±2.7 x103/µL, 101.0 (63.0-171.3) x103/µL and 35411.0 (15183.9-65280.7)/µL, respectively (Table 1).
Thirty-three different PfHRP2 amino acid sequences were identified among 50 PfHRP2 sequences obtained in this study. The size of PfHRP2 ranged from 225 to 304 amino acids among all isolates and 25 to 38 amino acid repeat types per isolate. The total number of repeats and the number of each repeat within PfHRP2 varied between isolates. Repeat types 2, 6, 7 and 8 were observed in 100% of the isolates. Repeat types 5 and 12 were observed in 98% whereas types 1, 3 and 10 were found in 92-96% of the samples. The repeat types 4 and 13 occurred in 26% and 10% of the isolates, respectively. None of the samples had repeat types 9 and 11 (Table 2).
Twelve rare PfHRP2 repeat types, two of which are previously unreported, were identified in this study; the two were APDAHHVAD and AHHAAAHDEAALI. Of the rare repeat types, types 2 (AHHAHHAAH) and 7 (AHHAAH) had the highest frequency (Table 3).
Predominant repeat types in this study were used to model the structural organization of PfHRP2 in Ghana. Although the structural organization of the PfHRP2 repeat types was variable, the repetitive regions found in most of our samples started with type 1 (94.0%) and all PfHRP2 sequences terminated with type 12 (100.0%) (Fig. 1a). Fifty-four percent of our isolates had a semi-conserved PfHRP2 repeat type motif composed of repeat types 2, 3, 5, 7, 8, 2 and 7. Partial amino acid repeat motif comprising types 7, 8, 2 and 7 was found in 34.0% of the isolates.
To explore the similarities between the modelled PfHRP2 sequence obtained from the Ghanaian isolates and those from other regions available at the NCBI, we performed BLASTP analysis of our amino acid sequence. Seven hits (Accession numbers: QBC65525.1, QBC65570.1, QBC65591.1, QBC65640.1, QBC65657.1 and QBC65674.1 from isolates in Kenya and AKO62989.1 from China-Myanmar border area) were obtained (Fig. 1b; Additional file 1: Table S2). Our modelled HRP2 sequence shared 85-94% similarity with Kenyan isolates and 94% similarity with the isolates from China-Myanmar border area. BLASTP of the sequences from each of the 50 samples revealed that 78.0% (39/50) have high similarities with isolates from Kenya, highlighting possible shared identity between PfHRP2 from Ghana and Kenya (Additional file 1: Table S3).
Although data on rapid diagnostic tests (RDTs) was unavailable, an obvious limitation of the study, we employed the Baker model [16] to determine the distribution on the basis of PfHRP2 diversity with respect to RDT sensitivity. Isolates were classified as Groups A, B, I and C if their Baker repeat (type 2 × type 7) was >100, 50-100, 44-49 and < 43, respectively. Group B was the highest occurring type (58.0%), followed by group C (22.0%) (Table 4).
Due to the relatively high percentage of group C isolates (22.0%), which have been reported to be associated with RDT non-sensitivity [30] obtained in this study, we explored the distribution of possible epitopes to be targeted by mAb RDTs based on the study by Lee et al. [29]. The predominant motif among the 50 isolates was AHHAADAHH which is recognized by the C1-13 mAb, followed by AHHAHHA, recognized by mAb 3A4. None of our isolates had the AYAHHAHHAAY motif, while the HAHHAHHAADAHH motif, recognized by C2-3, occurred at a lower frequency (Table 5).