The majority of malaria hospitalizations in Malaysia are now caused by a dangerous and potentially deadly monkey-borne parasite once rarely seen in humans, researchers say, and deforestation is the potential culprit in a growing number of infections that could allow this virulent malaria strain to jump from macaque monkeys to human hosts.
An analysis of malaria patients in Malaysian Borneo in 2013 showed that 68 percent had been sickened by Plasmodium knowlesi, says Dr. Balbir Singh, director of the Malaria Research Center at the University of Malaysia in Sarawak, who presented his findings this week at the American Society of Tropical Medicine and Hygiene (ASTMH) Annual Meeting in New Orleans.
The parasite is increasingly associated with malaria deaths and is three times more frequent as a cause of severe malaria in Borneo than the more common P. falciparum parasite, which is currently considered the world’s most deadly form of the disease, says Dr. Singh.
After transmission to the human body, malaria parasites multiply in the liver, and then infect red blood cells. Symptoms of malaria include fever, headache, and vomiting, and usually appear between 10 and 15 days after the mosquito bite. If not treated, malaria can quickly become life-threatening by disrupting the blood supply to vital organs.
The World Health Organization estimates that 500 million people are infected each year. It is prevalent in Latin America, Africa and Southeast Asia. Malaria kills about one and one-half million people per year, most of them pregnant woman or very young children. In many parts of the world, the parasites have developed resistance to a number of malaria medicines.
Humans may be new host of knowlesi malaria
The main host of knowlesi malaria has been the long-tailed and pig-tailed macaques found in the tropical forests of Malaysia and elsewhere in Southeast Asia. The rapidly increasing rates of human infections are concentrated in areas of Malaysia where, over the last decade, massive loss of native forest to timber and palm oil production has led to substantially increased human interactions with macaques.
That puts knowlesi malaria in the company of a growing list of dangerous emerging and re-emerging diseases—including Ebola and AIDS—that are being passed from animals to humans as development peels back more and more layers of tropical forest previously uninhabited by humans.
“This is a form of malaria that was once rarely seen in people, but today, in some remote areas of the country, all of the indigenous malaria cases we are seeing are caused by the P. knowlesi parasite,” said Dr. Singh. “If the number of cases continue to increase, human-to-human transmission by mosquitoes becomes possible. In fact, this may already have happened, which would allow P. knowlesi malaria to spread more easily throughout Southeast Asia.”
Person-to-person transmission via mosquito bite seems likely
Evidence to date has strongly suggested that victims of P. knowlesi malaria have been bitten by mosquitoes that had first bitten an infected macaque, making humans a dead-end host for the parasite. Of concern, however, is recent research that the parasite could change so that it can jump from person to person via mosquito bites, without requiring a monkey as part of its life cycle.
Laboratory tests in the 1960s indicated that a mosquito variety in Malaysian Borneo that carries the two most common human malaria parasites—P. falciparum and P. vivax—also can spread the knowlesi parasite. Moreover, P. knowlesi was recently found in Vietnam in mosquitoes that transmit falciparum and vivax malaria, raising the possibility that human-to-human transmission is already occurring.
P. knowlesi is the fifth species of malaria known to infect humans in nature. The parasite causes only mild malaria in macaques, says Dr. Singh, but in humans it is the fastest replicating malaria parasite, multiplying every 24 hours in the blood.
Deforestation driving disease outbreaks
The majority of the macaques carrying the parasite once lived in remote forested regions that saw little human activity or settlements. This has changed over the last ten years as a result of significant deforestation in Malaysia. According to a 2013 study in the journal Science, Malaysia lost about 47,000 square kilometers of forest between 2000 and 2012, or about 14 percent of its total land area, which environmentalists attribute to logging and conversion of native forests to palm oil plantations.
Researchers have been warning for decades that more frequent human incursions into undeveloped tropical forests will significantly increase the threat from diseases that could spread far beyond the forest canopy. The current Ebola outbreak is linked to a growing number of people living and hunting in forested areas and consuming “bush meat” from infected animals, chiefly chimpanzees.
Meanwhile, illegal mining operations in tropical forests have been linked to the recent resurgence of malaria in Venezuela and may have intensified the rise of drug resistant malaria in Thailand. Several studies have shown a connection between In one recent study, scientists found that a loss of just four percent of forest cover was associated with a staggering 50 percent increase in Malaria cases in western Brazil, where selective logging along with road construction and forest fires are also driving spikes in malaria.
These interactions are prompting a growing interest in research that probes the threat of disease from multiple vantage points—including economical, biological, and anthropological—an approach known as One Health.
Unprecedented challenge to malaria control efforts
Dr. Singh said that P. knowlesi malaria is currently a major public health problem in Malaysia, as it is causing illness serious enough to require medical treatment in about two thousand people a year. “But the P. knowlesi strain of malaria should stay within Southeast Asia as there are no mosquitoes outside the region capable of carrying these parasites,” he said.
Dr. Singh also pointed out that, in terms of overall burden of disease, knowlesi malaria still ranks far behind dengue fever. Infections and deaths with that mosquito-borne disease have more than tripled in Malaysia in just the last year.
The rising threat of the P. knowlesi parasite, however, which is carried by mosquitoes that prey on humans when they are outdoors, presents a new challenge for the broader effort to control and eliminate malaria in Southeast Asia—a fight that has been focused on using bed nets and indoor spraying to prevent malaria infections caused mainly by mosquitoes that attack indoors and at night. Malaria control campaigns also have not faced a malaria strain that is entrenched in a large animal population.
“Controlling a zoonotic—meaning an animal-to-human infection—carried by outdoor feeding mosquitoes is almost impossible with currently used methods,” said Dr. Singh.