Knowlesi malaria: Emergent strain poses threat to global containment efforts

Macaques have been blamed for spreading malaria

April 24, 2012
By Katherine Rowland
Financial Times

A malaria parasite emerging in south-east Asia could pose a setback to global elimination goals.

Since 2004, plasmodium knowlesi malaria, long known to affect macaque monkeys, has been identified in hundreds of people in Malaysia. There have also been scattered cases in Thailand, Cambodia, Vietnam and the Philippines.

While knowlesi is unlikely to reach pandemic proportions, health authorities are concerned it could harm eradication initiatives. Kevin Palmer, former director of the western Pacific regional office of the World Health Organisation, says health systems are ill-prepared to respond to a zoonotic form of malaria, a strain of the disease that transmits from animals to humans.

A big problem is that knowlesi malaria is often misdiagnosed. Under microscopy, the most common diagnostic tool, it is indistinguishable from a benign strain, malariae malaria. Even though knowlesi responds to the artemisinin-based therapies used to cure most malarias, treatments are infrequently administered. “Clinicians don’t really know what they’re dealing with,” says Dr Palmer.

The consequences of not receiving proper care can be severe, says Janet Cox-Singh of the St George’s University of London Centre for Infection. Routine misdiagnosis has resulted in prolonged sickness, treatment complications, fatalities and at least one instance in which a patient was operated on for the wrong condition.

Researchers think the spread of knowlesi in humans is being fuelled by deforestation. In Malaysian Borneo, where the first cases appeared, the combination of population growth and forest loss has brought record numbers of people into jungle tracts. As a result, humans, monkeys and mosquitoes have come into close contact.

Unlike the malaria strains common to South America and Africa, knowlesi is transmitted from monkeys to humans by a particular mosquito that is confined to the forest.

However, John Barnwell, chief of the Malaria Branch Laboratory at the US Centers for Disease Control and Prevention, says: “If other types of mosquito acquire the parasite, knowlesi malaria could become a much wider problem.”

Dr Cox-Singh, who is also a senior fellow at the Malaria Research Centre in Malaysia Sarawak, says that vivax malaria – the second-most common type in humans – emerged from the same monkey population some 30,000 to 40,000 years ago.

“The question is whether we’re creating environmental conditions for knowlesi malaria to do the same. Is it going to make the switch from animals to humans?”

Anu Kantele, of the Helsinki University Central Hospital, who was the first clinician to diagnose knowlesi in a western traveller, says the growing numbers of cases indicate the disease is beginning to make the switch.

But other researchers are uncertain whether humans will become a reservoir for the parasite.

Dr Palmer says deforestation will outpace the ability of the disease to survive. “We’ll see a decrease in incidence as forest losses deplete the habitats for the monkeys carrying the disease.”

In February 2011, the WHO convened a meeting with south-east Asian regional experts to discuss the public health threat of zoonotic malaria. The group recommended dedicated diagnostic labs, clinician awareness campaigns and studies to assess the true scope of the disease, which is assumed to be vastly under-reported.

A lack of resources presents a barrier to implementing surveillance or treatment plans.

“There’s no funding for knowlesi,” says Dr Barnwell. “The problem isn’t just misdiagnosis. People who don’t receive proper treatment get really sick. This has the potential to be much more severe.

“It’s something we have to keep an eye on.”