Malaria due to infection with P. vivax is currently estimated at up to 390 million episodes per year worldwide, with substantial morbidity caused by the recurrent nature of its infection, associated anaemia and adverse effects on pregnancy, and ability to cause severe disease and death. The majority of malaria cases from all Plasmodium species in Malaysia are from the Eastern state of Sabah, where despite around a 36-fold reduction in incidence since the implementation of malaria eradication programs in 1961, minimum estimates based on unpublished microscopy data from Sabah State public health records still detailed 8685 malaria cases in 2009-2011. While P. vivax accounts for between 30-50% of these figures currently, studies have demonstrated as overall transmission rates in countries previously endemic for malaria decline the proportion of cases attributable to P. vivax increases. Mixed P. falciparum / P. vivax infections are also likely to be underestimated in areas of co-endemicity, with high rates of recurrence of P. vivax shown in studies following treatment for apparent P. falciparum mono-infection or mixed Plasmodium spp. infections.
Malaysian Ministry of Health guidelines currently recommends chloroquine and primaquine as first line treatment for the erythrocytic and hypnozoite life stages of uncomplicated P. vivax malaria respectively. While resistance to chloroquine has previously been documented in Sabah and also Peninsular Malaysia, the unstable transmission dynamics and recent reduction in P. vivax incidence due to public health measures mean the current risk of chloroquine-resistant P.vivax transmission is likely to be low. Despite this, due to documented increasing resistance in surrounding countries including Indonesia, Thailand, Vietnam, and PNG, transient populations of migrant workers, and recent concerns of the failing efficacy of hypnozoite eradication by primaquine, the need for ongoing therapeutic efficacy monitoring is recommended.
There is also growing support for artemisinin-based combination therapy (ACT) as a unified first line treatment choice in areas co-endemic for P. falciparum and P. vivax, as adopted by a small number of countries including PNG, Solomon Islands, Vanuatu and Papua, Indonesia, due to the regional increase of chloroquine-resistant P. vivax, and ongoing concerns over the microscopic misdiagnosis of Plasmodium species. This is also particularly relevant for P. knowlesi malaria, as it is frequently misidentified on microscopy as P. falciparum and P. malariae due to morphological similarities in the early trophozoite, and late trophozoite and schizont life stages respectively, with studies showing up to 80% of P. malariae and 7-12% of P. falciparum in this region are actually P. knowlesi when definitively evaluated with PCR. Misdiagnosis has concerning treatment implications, as although P. knowlesi has been shown to respond well to ACT and chloroquine, unlike P. malariae, knowlesi malaria has a rapid 24-hour replication rate and can cause hyperparasitaemia, severe complications and fatal outcomes, while the inadvertent use of chloroquine for widely chloroquine-resistant P. falciparum may also have fatal consequences.
Previous reluctance to use ACTs in presumed chloroquine-sensitive P.vivax areas were centered on concerns over efficacy and cost. The advent of generically produced ACTs is improving cost-related issues, while a recent Cochrane review of clinical trials looking at the use of ACT versus chloroquine for uncomplicated vivax malaria showed that ACTs are equivalent to chloroquine in preventing recurrent parasitaemia in the first 28 days (RR 1, 95% CI 0.30 to 3.39), and those with long half lives such as mefloquine or piperaquine are superior over a 6-8 week follow up, with fewer recurrent episodes after day 28 (RR 0.47, 95% CI 0.29 to 0.76). This post treatment prophylaxis benefit of longer acting agents extends to the reduction in gametocyte carriage, delay in relapse or re-infection and decreased risk of anaemia development, all of which contribute to decreased transmissibility and health care cost.
