Chloroquine-resistant Plasmodium vivax in the Republic of Korea

Plasmodium vivax malaria, which was endemic on the Korean Peninsula for many centuries until the late 1970s, re-emerged in 1993 in the Republic of Korea.¹ The malaria-prevalent area has been confined to the area adjacent to the Demilitarized Zone (DMZ) from the early stage of the re-emergence, and malaria occurrence in the Republic of Korea has been directly influenced by the prevalence of malaria in the region of North Korea located near the DMZ.^1–3 The total number of malaria patients in the Republic of Korea and North Korea since the re-emergence likely approaches one million.^1–4 To cope with the situation, the Army of the Republic of Korea has performed chemoprophylaxis with hydroxychloroquine (HCQ) and presumptive anti-relapse therapy with primaquine since 1997.⁵ The cumulative number of the soldiers in the Army of the Republic of Korea given chemoprophylaxis exceeded 1.4 million by 2007. This extensive chemoprophylaxis campaign has helped prevent a rapid increase of malaria patients in the Army of the Republic of Korea. However, this success is tempered by the increased possibility of chloroquine (CQ)–resistant P. vivax strains.⁵

In this study, 484 patients from 6 hospitals in the Republic of Korea (5 in the malaria-prevalent region and 1 in Seoul) were enrolled during 2003–2007. Blood samples were collected from all patients before HCQ treatment and 24 hours after completion of treatment. Treatment responses were monitored by investigation of fever clearance time and parasite clearance time. Plasma concentrations of HCQ before and 24 hours after completion of treatment were measured by validated reversed-phase high-performance liquid chromatography⁶ with slight modifications.⁷ Additional examinations or blood collection were not performed. The study protocols were reviewed and approved by the institutional review board of each hospital. All patients enrolled in this study were admitted to the hospitals during HCQ treatment, and HCQ was taken under the physician supervision. There were no problems with HCQ treatment compliance.

Among 484 patients enrolled in the five-year study, HCQ treatment failed in two patients (Table 1). These two patients had not been in malaria-prevalent areas in other nations during the two years prior to their present hospitalization.

Patient A was a 26-year-old man (civilian) who had been discharged from the military in May 1998. Chemoprophylaxis was not performed during his military service. He was admitted to hospital I located in Goyang, a malaria-prevalent area in Kyonggi Province, on July 30, 2003. Plasmodium vivax malaria was confirmed and he was administered 2,000 mg of HCQ over a three-day period. More specifically, on day 0, he was given 800 mg of HCQ, with doses of 400 mg administered 6 hours and 24 hours later (day 1), and 48 hours later (day 2). Despite administration of the first cycle of HCQ treatment, fever did not subside until day 6 and P. vivax trophozoites were evident in a peripheral blood smear obtained on day 6. Parasite density on day 0 (before the treatment) and day 3 (24 h after completion of HCQ treatment) were 3,500/μL and 300/μL, respectively. Gene amplification by species-specific primers for small subunit ribosomal RNA⁸ showed that Plasmodia in the patient’s peripheral blood was P. vivax. The plasma concentration of HCQ 24 hours after the completion of HCQ treatment was 165 ng/mL. The patient was completely cured by administration of an additional cycle of HCQ treatment commencing on day 6.

Patient B was a 72-year-old woman. She was admitted to hospital II located in Seoul on July 24, 2007 (day 0), because of fever and chills. Plasmodium vivax malaria was diagnosed and HCQ was administered on July 25–27 (days 1–3). Treatment was unsuccessful in resolving the fever and severe headache, and parasites were evident both microscopically and by small subunit ribosomal RNA amplification until day 4. Parasite density on days 0 and 4 was 3,800/μL and 440/μL, respectively. The plasma concentration of HCQ 24 hours after the completion of HCQ treatment was 150 ng/mL. Salvage treatment with quinine sulfate and doxycycline was carried out for seven days beginning on day 4, followed by administration of primaquine. This regimen completely resolved the infection.

Chloroquine-resistant P. vivax strains have been reported from various areas^9–12 since its emergence in Papua New Guinea in 1989. ¹³ In the Republic of Korea, a large-scale chemoprophylaxis campaign has been performed since 1997. However, prophylaxis has consistently failed in many cases despite attainment of sufficiently high plasma concentrations of HCQ. Moreover, the length of time required for the elimination of P. vivax from patients’ blood by HCQ treatment has increased in the current decade. ¹⁴

Hydroxychloroquine has been reported to be as active as CQ against malaria parasites, ^15,16 and 400 mg of HCQ is the molar equivalent of 309.6 mg of HCQ base and 295.0 mg of CQ base. Therefore, a CQ concentration of 10 ng/mL in plasma, which is the minimum effective concentration against CQ-susceptible P. vivax, is equivalent to an HCQ concentration of 10.5 ng/mL of plasma. In this study, treatment with 2,000 mg of HCQ over a three-day period was not effective in 2 (0.4%) of 484 patients. For these two patients, plasma concentrations of HCQ 24 hours after completion of HCQ treatments were much higher than the minimum effective concentration of CQ against P. vivax.¹⁷ For the 482 patients with successful therapeutic outcomes, the mean and the standard deviation of plasma concentrations of HCQ 24 hours after completion of HCQ treatments were 220 ng/mL and 121 ng/mL, respectively, which were in not distinct from the two patients in whom HCQ treatment failed. This indicates that HCQ was absorbed and metabolized normally in the two patients, precluding the possibility that the treatment failure was caused by personal factors. In the two patients, parasitemias were reduced markedly, but not cleared, by HCQ administration. Patient A was cured by additional administration of HCQ; this success may have been the result of the infecting P. vivax being exposed to an increased trough concentration of HCQ for an extended period because of the cumulative dosage.

The present observations are the first report of CQ-resistant P. vivax from a temperate region of Asia. Surveillance activity should be strengthened to monitor the change of CQ susceptibility of P. vivax in the Republic of Korea.

Table 1

Demographic and clinical characteristics of two patients unsuccessfully treated with the conventional HCQ regimen, Republic of Korea*

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