Several weeks ago I addressed the issue of asymptomatic malaria and had also addressed it a year ago. I am not convinced that asymptomatic malaria is an illness. Most malaria researchers with their parasite model will argue that they are ‘cases’. However, I would love to see how many cases a method such as that described in Malaria World this week would find in a ‘malaria free’ country. But don’t expect that type of control experiment any time soon.
What surprised me most about ‘Asymptomatic malaria reservoirs are the last challenge in the elimination in Cambodia’ by Doum et al was how low the case numbers were. And they used PCR (polymerase chain reaction) as the detection technique. We remember that PCR was used extensively during the COVID19 pandemic and was criticised because, if a large number of cycles were used, it had a high false positive level. Many of the ‘cases’ detected were asymptomatic.
The paper summary states that all qPCR-diagnosed cases were asymptomatic. Malaria cross-sectional surveys were conducted in high-risk populations (forest dwellers, forest goers and forest rangers) at three different time (T0, T1, T2) from October 2022 to February 2023, overlapping the rainy, malaria transmission season and into the dry season. In Mondulkiri, the prevalence of Plasmodium falciparum was 0.63% at T0, increasing to 0.81% at T1, and decreasing to 0.18% at T2. Plasmodium vivax decreased from 4.80% at T0 to 1.97% at T1 and 1.65% at T2. In Kampong Speu, overall prevalence was 7.06% at T0, declining to 5.19% at T1 and 4.59% at T2. Plasmodium falciparum prevalence was 0.30% at T0, decreasing to 0.09% at T1 and rising slightly to 0.10% at T2. The forest goers showed a prevalence increase to 1.95% at T1 and decrease to 1.46% by T2, while forest dwellers decreased to 3.25% at T1 and further to 3.13% at T2.
Just like with COVID RDTs (rapid diagnostic tests) were less likely to be positive. Passively reported malaria cases showed 1.09% of cases in Mondulkiri and 0.21% of cases in Kampong Speu were rapid diagnostic test (RDT) positive.
This study was carried out on high-risk populations and all cases were asymptomatic. RDTs had 1% positivity or less. With PCR higher numbers were detected but all were still <10%. It would seem to me that a case could be made that malaria has now been eliminated from Cambodia, if this is as bad as it gets. I repeat it would be interesting to see how many positive cases there would be in a similar study in a malaria free country.