Monkey malaria - the next bird flu?

An expert at the NIH has highlighted the threat of an emergent highly virulent form of malaria, questioning whether the disease has made the jump from animal to man.

Writing in the April issue of Future Microbiology, Dr Thomas McCutchan of the National Institute of Allergy & Infectious Disease (NIAID) raises concerns that this disease may pose a huge threat to the populations of Southeast Asia and beyond if the disease spreads.

Malaria is a caused by protozoan parasites of the Plasmodium genus and is widespread in parts of the Americas, Asia and Africa. Malaria infections occur in more than 500 million people each year, with 1-3 million deaths. The parasite has two hosts in the course of its life cycle - a mosquito vector and vertebrate host. The parasites are transmitted to humans by female mosquitoes, multiplying within red corpuscles with consequential symptoms of anemia and fever.

The distinction between a disease that is occasionally transmitted to humans exposed in an area of natural occurrence and transmission (a zoonosis) and an actual human disease is one with serious biological implications. Although at least ten species of Plasmodium can infect humans, only four forms of specifically human malaria are believed to exist. In the case of these four established human malaria types, the parasite is transmissible from one human to another, and a stable transmission cycle is established in the absence of any other vertebrate host. Now Dr McCutchan has raised the question - has a monkey malaria made that switch and become the fifth human malaria?

The parasitic organism responsible, Plasmodium knowlesi, is an established cause of monkey malaria. However, recent studies report high levels of infection in human populations in Borneo. Furthermore, the findings of significant infection rates in humans have suggested not only the emergence of a virulent strain of P. knowlesi that readily infects people, but also that some other means of transmission is playing a role, such as direct blood transfers.

The problem seems, so far, largely to have been a silent one due to problems in accurate diagnosis. In Malaysia, malaria is most commonly caused by P. falciparum or P. vivax. If the involvement of these two parasites is eliminated, the default diagnosis is P. malariae. However, P. malariae and P. knowlesi are morphologically very similar and difficult to distinguish clinically by microscopy. The problem for physicians is that P. malariae does not demand immediate and aggressive treatment. In contrast, the P. knowlesi infections that are seen in Borneo do. It is likely, therefore, that underdiagnosis has both confounded effective treatment and misrepresented the prevalence of disease.

Dr McCutchan points out that we are now waiting, as in the case of avian flu, to discover whether or not P. knowlesi represents a pathogen that has taken the jump to a new host. “At this point,” he says, “the study of P. knowlesi is extremely significant regardless of whether it has entered humans or remains a zoonosis. In either case, we face a health problem of potentially widespread significance and one that will present new problems for malaria control.”