QUESTION

I recently read an ISOS world malaria day poster saying 5 species of plasmodium cause malaria. I think that is confusing as we always talked about 4, ovale, vivax, falciparum and malaria….are they referring to the way we now split ovale into 2 sub species? or is this a typo on their part?

ANSWER

That is a really interesting question, and a good observation on your part! I imagine the fifth species they are referring to is Plasmodium knowlesi, which is found in parts of south-east Asia, with the majority of cases being reported from Borneo. Originally known only from macaque monkeys, it appears to be occurring more frequently in humans. However, it is not known whether this is a new host switch, or whether it is simply a matter of better detection methods—the morphology of P. knowlesi closely resembles that of P. falciparum in its early trophozoite stages, and P. malariae in later trophozoite and other life stage forms. Moreover, some molecular-based tests for P. knowlesi cross-react with other forms of malaria, such as P. vivax, leading to greater diagnostic confusion.

There is also a hypothesis that changes in land use in tropical forests may be resulting in greater human exposure to the vectors which carry P. knowlesi, which accounts for its increased recent prevalence in humans. P. knowlesi is the only known malaria in humans (and indeed, in all primates) with a 24-hour reproductive cycle, which means that without treatment, high levels of parasitaemia can accumulate rapidly in the blood, and lead to severe clinical symptoms. This makes its apparent emergence of great public health concern in south-east Asia. Luckily, at this point, P. knowlesi is completely susceptible to chloroquine treatment and other medications, and so is easily controlled once diagnosed.

One of our contributors, Christina Faust, wrote a blog post last year on P. knowlesi entitled Of Macaques and Men. More information on recent research about P. knowlesi can be found in the article, Monkeys Provide Malaria Reservoir for Human Disease in South-East Asia.