Plasmodium malariae has reshaped malaria transmission in Khanh Hoa Province
A sustained Plasmodium malariae outbreak has altered the malaria profile of Khanh Hoa Province, Vietnam, and the pattern has not simply faded back to baseline. Over 3 years, malaria moved from rare sporadic detection to a co-endemic situation in which three parasite species circulated at the same time, then narrowed again without disappearing. The epidemiology now centers on Khanh Vinh district, where the heaviest burden accumulated and where transmission remained concentrated in a small number of communes.
The province recorded 493 symptomatic malaria cases from January 2023 through December 2025, all confirmed by microscopy and molecular testing targeting parasite 18S rRNA. Before that surge, malaria was uncommon in the area, with only 3 cases in 2021 and 12 in 2022. Then the curve steepened abruptly in 2023, peaking in July, when P. malariae first emerged as a major force in the local transmission network. The annual counts then settled at 209 cases in 2023, 199 in 2024, and 85 in 2025, showing gradual decline but not eradication.
The species mix changed in ways that matter for surveillance and control. Across the 3-year period, P. malariae accounted for 44.8% of cases, P. falciparum for 36.9%, and P. vivax for 17.9%; mixed infections were rare, at 0.4%. Three Plasmodium species were co-endemic in 2023 and 2024, and by 2025 only P. malariae and P. falciparum remained prevalent. In practical terms, the dominant parasite species in the province was no longer the familiar pairing of falciparum and vivax alone. The arrival and persistence of malariae changed both the species balance and the geographic focus of control efforts.
Transmission remained concentrated in Khanh Vinh district and its communes
Khanh Vinh district carried the epidemic. Of the 8 districts and cities in Khanh Hoa Province, Khanh Vinh accounted for 93.1% of cumulative malaria cases, while the other areas each had fewer than 15 cases. That concentration matters biologically and operationally: a localized transmission niche can sustain an uncommon species if case detection, vector contact, and human movement all line up in the same environment. Within Khanh Vinh, the spatial pattern shifted from commune to commune over time, but the district stayed the center of gravity.
In 2023, P. malariae was the most common species in Khanh Vinh, with 104 cases, followed by P. vivax with 62 and P. falciparum with 29. Geographic overlap of the three species appeared in six communes, including Khanh Thuong and Lien Sang, while several other communes carried only one identified species. During 2024, falciparum expanded in some communes, especially Khanh Dong and Khanh Phu, while malariae dropped to 72 cases. Vivax fell sharply to 22 cases and narrowed geographically. By 2025, total malaria cases in the district had declined to 77, no vivax cases were detected, and malariae still overlapped with falciparum in five communes. The local epidemic contracted, but the species mix remained unstable.
One detail stands out: every P. malariae case in Khanh Hoa Province was a monoinfection. That finding pushes against the assumption that malariae appears mostly as a secondary passenger inside mixed-species infections with falciparum. In this setting, malariae was not just present; it was independently established.
Forest exposure, adult men, and Raglai communities carried most of the burden
The demographic pattern was as consistent as the species pattern was volatile. Among patients in Khanh Vinh district, 338 were male and 121 were female, a 2.8:1 ratio. Most patients, 344 in total, were older than 15 years. Forest-goers made up the largest occupational group by far, with 314 cases, followed by rangers and a smaller number of government or factory workers.
Ethnicity tracked risk as well. The Raglai community accounted for 304 cases, or 66.2% of the district total, followed by Kinh and Co Ho patients. Heavy dependence on forest work and forest farming places the Raglai population at higher exposure to mosquito-borne infection, and the case distribution here fits that known vulnerability. The age and occupation pattern points away from classic childhood malaria transmission and toward adult exposure during labor, travel, and time spent in forested areas where vectors remain active.
No meaningful association emerged between demographic characteristics and the specific Plasmodium species infecting a patient. That absence suggests the species shift was driven more by ecological and transmission dynamics than by any simple demographic sorting of parasites into different host groups.
Why malariae may have gained ground as falciparum and vivax control improved
The cause of the outbreak remains uncertain, but several explanations are plausible. Interventions directed at P. falciparum and P. vivax may have opened ecological space for P. malariae, a species that can linger quietly and evade routine attention. Persistent asymptomatic infection is another likely reservoir. A pilot survey in Khanh Vinh found asymptomatic and submicroscopic falciparum and malariae infections in residual blood samples, which supports the idea that hidden carriage can seed continued transmission even when symptomatic case counts decline.
Genetic analysis also suggests that malariae in the province is not a static population. The parasite showed notable variation in major polymorphic markers, including apical membrane antigen 1 and circumsporozoite surface protein, despite the short study period and limited geographic range. No confirmed molecular markers of antimalarial drug resistance have been established for P. malariae, and parallel analyses of orthologous resistance genes in falciparum and vivax from the same province did not show remarkable mutations associated with resistance. No clinical drug failure has been identified in the region to date.
The control picture is therefore complicated but not opaque. The outbreak appears contained, yet the province now sits in an unstable state of co-endemicity, with malariae persisting alongside falciparum after vivax disappeared from the 2025 district profile. Continued surveillance will need to do more than count cases. It will have to distinguish species accurately, sample asymptomatic reservoirs, and follow transmission in forest-linked communities where a quiet parasite can remain entrenched long after the main epidemic crest has passed.
The remaining question is not whether malaria has declined in Khanh Hoa Province. It has. The harder issue is whether the surveillance system can detect a low-level, species-shifting reservoir before P. malariae expands again into a broader epidemic pattern.