“It feels almost like P. knowlesi follows
deforestation,” Fornace says. Several years after a
forest is cut back, nearby communities “get a peak
of P. knowlesi.”
Today, the hill where the American surveyor
camped in 1965 is a small island in a sea of oil palm
estates. From 2000 to 2012, Malaysia cleared a total
amount of forest equaling 14. 4 percent of its land
area, more than any other country, according to a
study published in 2013 in Science. A study in 2013
in PLOS ONE used satellite images to show that in
2009, only one-fifth of Malaysian Borneo was intact
forest. Almost one-fourth of all forest there had
been logged, regrown and logged many times over.
Since 2008, oil palm acreage in Malaysian
Borneo has increased from 2.08 million hectares
to 3. 1 million, according to the Malaysian Palm
Oil Board. In Malaysia, the four states hit hardest by deforestation — Sabah, Sarawak, Kelantan
and Pahang — report 95 percent of the country’s
P. knowlesi cases.
Fornace thinks deforestation and the ecological
changes that come with it are the main drivers of
monkey malaria’s rise in Malaysia. She has seen
long-tailed macaques spend more time in farms
and near houses after their home forests were
being logged. Macaques thrive near human communities where food is abundant and predators
stay out. Parasite-carrying mosquitoes breed in
puddles made by farming and logging vehicles.
Where monkeys go, mosquitoes follow. Indra
Vythilingam, a parasitologist at University
of Malaya in Kuala Lumpur, studied human
malaria in indigenous communities in the early
1990s. Back then, she rarely found A. cracens, the
mosquito species that carries monkey malaria in
Peninsular Malaysia. But in 2007, that species
made up over 60 percent of mosquitoes collected
at forest edges and in orchards, she reported in
2012 in Malaria Journal. “It’s so much easier to
find them” now, she says.
As Fornace points out, “P. knowlesi is a really
good example of how a disease can emerge and
change” as land use changes. She recommends
that when big projects are evaluated for their
impact on the economy and the environment,
human health should be considered as well.
What to expect
While P. knowlesi cases are climbing in Malaysia,
scientists have found no evidence that P. knowlesi
transmits directly from human to mosquito to
human (though many suspect it happens, albeit
Following a review by experts in 2017, WHO
continues to exclude P. knowlesi from its malaria
elimination efforts. Rabindra Abeyasinghe,
a tropical medicine specialist who coordinates
WHO malaria control in the western Pacific
region, says the agency will reconsider P. knowlesi
as human malaria if there is new evidence to
show that the parasite transmits within human
In Malaysia last year, only one person died
from human malaria, but P. knowlesi killed 11.
“We don’t want that to happen, which is why
[P. knowlesi] is our priority even though it is not in
the elimination program,” says Rose Nani Mudin
from the country’s Ministry of Health.
Unable to do much with the monkeys in the
trees, Malaysian health officers focus on the people most likely to be infected with P. knowlesi.
Programs raise awareness of monkey malaria and
aim to reduce mosquitoes around houses. New
mosquito-control methods are needed, however,
because conventional methods like insecticide-treated bed nets do not work for monkey malaria
mosquitoes that bite outdoors around dusk.
Fighting malaria is like playing chess against
an opponent that counters every good move we
make, says Culleton in Japan. Malaria parasites
can mutate quickly and “go away and hide in
places and come out again.” Against malaria, he
says, “we can never let our guard down.” s
s Bridget E. Barber et al. “World Malaria Report:
time to acknowledge Plasmodium knowlesi
malaria.” Malaria Journal. March 31, 2017.
Yao-Hua Law is a freelance science writer based in
Kuala Lumpur, Malaysia.
Kimberly Fornace (left)
and collaborators at
Sabah collect blood
samples from people
in northern Sabah. The
group checks blood for
evidence of current and
past malaria infections.