pLDH is one of the target antigens that is widely used in developing the monoclonal antibodies that are part of the RDT that comprises the non-microscopic immunochromatographic assay. Interestingly, the level of pLDH has been shown to decline in parallel with the clearance of asexual parasitaemia; therefore, it has been suggested that the disappearance of the parasite-specific enzyme pLDH after anti-malarial drugs may be useful in predicting treatment failure [
23]. These characteristics of pLDH led us to investigate the sequence variability of pLDH in Korean isolates. PvKtype19, which was the predominant form of pLDH in Korean isolates, exhibited higher identity with
P. knowlesi (96.8%, JF958130) than with
P. falciparum Ori-1 (JN547218) (Figures
4 and
5). However, PvKtype19 showed 97.8-100% identity with other subspecies of
P. vivax (Figures
6 and
7). Only one synonymous SNP was found in 20 Korean isolates, at base pair 456 (n = 1) (Figures
2 and
3).
Plasmodium vivax has presumably been prevalent in Korea for a long time. However, as a result of a national malaria eradication programme and with help from the World Health Organization (WHO), the incidence of vivax malaria has rapidly decreased [
24,
25]. After the latest report of two malaria patients in 1985 [
26], there were no additional reported cases until one case was reported in 1993 [
27] and two indigenous cases were reported in 1994 [
28]. Malaria cases then rapidly increased until approximately 2000 [
29,
30]. After that, the reported malaria cases decreased for several years due to efforts to limit the incidence of malaria. However, malaria has not been thoroughly eradicated in the Korean peninsula because 2 to 3% of patients experience failed drug treatment every year, and many travellers and workers come from malaria-prevalent areas, including North Korea [
31]. For these reasons, serological diagnostic tools are needed to support both traditional microscopic diagnosis and antibody testing on a population level, to get a proxy about exposure to malaria in Korea. Currently, IFAT (Immunofluorescence antibody test) is used as the standard serological diagnostic method due to its high sensitivity in this context. However, the sensitivity might be affected by the training and ability of examiners. Therefore, a new antigen is needed for serodiagnosis. Several recombinant proteins cloned from Korean isolates of
P. vivax have been tested for use as antigens for serodiagnosis, including circumsporozoite protein (CSP), subtypes Pv210 [
32] and Pv247 [
33], merozoite surface protein (MSP) [
34], and CSP and MSP chimeric proteins [
35,
36]. None of these antigens were capable of replacing the IFAT method because their sensitivity was less than that of IFAT. Therefore, it was decided to focus on pLDH. Monoclonal antibodies against pLDH have been used in several RDTs and exhibit a relatively high sensitivity for the detection of malaria parasites. However, the ELISA detected only 85.0% (34/40) of microscopic positive samples, even though it was cloned from a Korean vivax malaria strain (pVKtype19, Figure
5). Therefore, antibody detection using the pLDH recombinant protein is not sufficient to compensate the disadvantage of antigen detection using its monoclonal antibody. However, it should be investigated whether pLDH recombinant protein can detect asymptomatic patients or symptomatic patients who have low parasitaemia (under 50/μl) using by antibody detection methods, for example, ELISA or Western blot. Therefore, when using the RDT in the field, it is likely better to use both antigen and antibody detection RDTs to compensate for their individual limitation.