Monkey malaria in humans becoming more widespread

Malaysia finds itself first-in-line in the region to face this potential health challenge.

THE first strain of the malarial parasite Plasmodium knowlesi ever isolated came from a long-tailed macaque in 1932.

It wasn’t until the 1960s, however, that it was realised that humans could get infected with “monkey malaria”, and even then such cases were assumed rare.

In 2004, a study by scientists from Universiti Malaysia Sarawak (Unimas) sparked a public health revelation: not only were humans being infected with monkey malaria, P. knowlesi was also responsible for the majority of malarial infections in Kapit, Sarawak – and that was just the tip of the iceberg.

It turns out that years of misdiagnosis kept P. knowlesi, easily mistaken for human malarias – including the more benign P. malariae – under the radar.

Over a decade after the Unimas study, P. knowlesi has emerged as the most important malarial parasite in Malaysia.

A more standardised deployment of molecular screening techniques revealed that last year, 66% of all hospital admissions for malaria were for P. knowlesi.

Here, we take a fresh look at the subject, and the research being done to make sense of this emerging zoonosis.

New ground

In many ways, Malaysia’s efforts in tackling human malaria has simply meant that she has arrived at this predicament sooner than her South-East Asian counterparts that are still grappling with human malarias.

Molecular analysis of P. knowlesi derived from macaques and humans indicate it is an ancient parasite, and it is likely that humans have been acquiring infections ever since they ventured into forests where infected macaques were living.

What has changed in recent years – aside from the encroachment of urban and agricultural activities into the forest – is the vast decline in human malarias.

We know that logging and other such activities have led to a greater exposure toward infectious diseases lurking within the forest.

That could be only one part of the puzzle, because published papers indicate that P. knowlesi sets in when human malaria is reduced.

It is interesting to note that Sabah, the state which has experienced the most dramatic decrease in the incidence of malaria (49,192 cases in 1994 down to 2,032 cases in 2011 according to a World Health Organization report) has also seen one of the highest proportional increases of monkey malaria.

In Malaysia, the overall number of annual malaria hospital admissions has dropped from upwards of 300,000 during the post-World War II period, to mere thousands today.

Dr Reuben Sharma: More research is necessary to shed light on malaria parasite species interactions. Filepic

P. malariae, the parasite prone to microscopic misdiagnosis as P. knowlesi, constitutes a small proportion of these numbers.

According to Dr Reuben Sharma, who heads the parasitology lab at Universiti Putra Malaysia’s (UPM) Faculty of Veterinary Medicine, when it comes to infection by certain gastrointestinal parasites, some are dominant and don’t give a chance for colonisation by other gut parasites.

Could something similar be happening with co-infection by different species of malaria?

“We know so little of the biology of the parasite, it is possible that one species may be dominant and suppress infection by the other. But at this stage, it’s just speculative. More research is necessary to shed light on malaria parasite species interactions,” he says.

Speedier adaptation?

The question of whether or not our successful eradication of human malarias has opened up a new ecological niche for monkey malaria is interesting for two reasons.

One is rather contemplative in nature; though deforestation no doubt plays a significant role in bringing man and parasite together, perhaps the rise of P. knowlesi was more inevitable than we would like to think.

The second is the possibility that this new environment (for the parasite) could accelerate its adaptation for more efficient transmission in people.

Entomologist Dr Indra Vythilingam, who consults for Universiti Malaya’s Dept of Parasitology, points to a recent research presentation by Indonesian scientists at the 51st Annual Scientific Conference of the Malaysian Society of Parasitology and Tropical Medicine.

Out of thousands of malaria patient samples from South Kalimantan (where human malaria is still endemic) that were screened using molecular techniques, only two turned out to be P. knowlesi.

Similarly, where the east coast of Sabah has more human malaria, there are also fewer cases of P. knowlesi being documented.

Some feel it could be a general loss of immunity toward plasmodium species in populations where human malarias have largely been eradicated.

However, according to infectious disease consultant Dr Timothy William, who is based at Jesselton Medical Centre, Kota Kinabalu, it has already been determined that prior exposure to human malaria, namely P. falciparum and P. vivax, does not confer any protective effect on the risk of acquiring P.knowlesi, or protection from severe disease.

Currently, there is no evidence to suggest that absence of human malaria makes us more vulnerable to P. knowlesi infections.

However, Dr William says it is possible that certain aspects of the parasite’s biology could lead to competition between human and monkey malaria during co-infections; and therefore, the prevalence of human malaria in the past could have posed a barrier for P. knowlesi to take hold.

“Studies are still being conducted to see if this is the case or not,” he says. – The Star

Category: Health alert, Top Story