High incidence of asymptomatic cases during an outbreak of Plasmodium malariae in a remote village of Malaysian Borneo

An outbreak of Plasmodium malariae occurred in Sonsogon Paliu village in the remote area of Ulu Bengkoka sub-district of Kota Marudu, Northern Sabah, Malaysian Borneo from July through August 2019. This was the first outbreak of malaria in this village since 2014. On 11(th) July 2019 the Kota Kinabalu Public Health Laboratory notified the Kota Marudu District Health Office of a Polymerase Chain Reaction (PCR) positive case of P. malariae. This index case was a male from Sulawesi, Indonesia working for a logging company operating in Sonsogon Paliu. During the resulting outbreak, a total of 14 symptomatic cases were detected. All of these cases were positive by thick and thin blood smear examination, and also by PCR. During the outbreak, a mass blood survey screening was performed by light-microscopy and PCR. A total of 94 asymptomatic villagers 31 (33.0%) were PCR positive but thick and thin blood smear negative for P. malariae. Both symptomatic and asymptomatic cases received treatment at the district hospital. When symptomatic and asymptomatic cases were considered together, males (29/45. 64.5%) were infected more than females (16/45, 35.6%), the male:female ratio being 1.8:1. Adults were the predominant age group infected (22/45, 48.9%) followed by adolescents (19/45, 42.2%) and children under five years of age (4/45, 8.9%). This report illustrates that symptomatic and submicroscopic cases pose a challenge during P. malariae outbreaks and that PCR is a valuable tool for their identification. The rapid identification and control of imported malaria is crucial for the continued control of malaria in Malaysia. Author summary In Malaysia, malaria caused by the human-only malaria parasites has been all but eliminated, and the major obstacles to a malaria-free status are now emerging malaria caused by zoonotic transmission of Plasmodium knowlesi parasites and outbreaks caused by imported parasites. One such outbreak occurred in 2019 in a village in northern Sabah. This outbreak was unusual, as it was found to be caused by P. malariae, an often-benign human malaria parasite rarely seen in Malaysia. We investigated this outbreak and found that the index case could be traced to an Indonesian migrant worker employed in the logging industry. An additional 13 people in the village developed malaria symptoms and were subsequently found to be positive for P. malariae. A further 94 inhabitants of the village without symptoms were then tested by a sensitive PCR technique, and 31 of them were also found to be positive for the parasite. These results illustrate the threat posed by asymptomatic carriage of malaria parasites, and the importance of PCR testing for outbreaks of residual malaria in elimination settings.

Automated summary

In 2019, a rare outbreak of Plasmodium malariae occurred in a village in northern Sabah, Malaysia, with the index case being a male from Indonesia. A total of 14 symptomatic cases and 94 asymptomatic cases were detected during the outbreak. Males were more infected than females, with adults being the predominant age group affected.