But as countries reduce human malaria, they
will eventually have to deal with monkey malaria,
Espino says, echoing an opinion widely shared by
monkey malaria scientists.
“Something nasty” could emerge from the pool
of malaria parasites in monkeys, says malariologist Richard Culleton of Nagasaki University in
Japan. Culleton studies the genetics of human
and monkey malaria. Malaria parasites can
mutate quickly — possibly into new types that can
more easily infect humans (SN: 9/6/14, p. 9). To
Culleton, the monkey malaria reservoir “is like a
black box. Things come flying out of it occasionally
and you don’t know what’s coming next.”
Malaysia is very close to reaching the WHO target of human malaria elimination. In 2017, only
85 people there were infected with human malaria.
But that success feels hollow as monkey malaria
gains a foothold. And while monkey malaria has
swelled into a public health threat only in Malaysia,
the same could happen in other parts of Southeast Asia and beyond. Even in southeastern Brazil,
where human malaria was eliminated 50 years
ago, the P. simium malaria parasite that resides in
howler monkeys caused outbreaks in humans in
2015 and 2016.
From tool to threat
In the late 1800s, scientists discovered the
Plasmodium parasite and its Anopheles mosquito
carriers. Humans retaliated by draining marshes
to stop mosquito breeding and spraying insec-
ticides over whole communities. Governments
and militaries pursued antimalarial drugs as the
disease claimed countless soldiers during the two
Scientists soon found malaria parasites in birds,
rodents, apes and monkeys. To the researchers,
the parasites found in monkeys were a tool for
testing antimalarial drugs, not a threat. An accident, however, showed otherwise.
In 1960, biologist Don Eyles had been studying
the monkey malaria P. cynomolgi at a National
Institutes of Health lab in Memphis, Tenn.,
when he fell ill with malarial fevers. He had
been infected with the parasites found in his
research monkeys. His team quickly confirmed
that the malaria parasites in his monkeys could
be carried by mosquitoes to humans. Suddenly,
monkey malaria was not just a tool; it was an animal disease that could naturally infect humans.
The news shook WHO, Mc Wilson Warren said
in a 2005 interview recorded by the Office of
NIH History. Warren, a parasitologist, had been
Eyles’ colleague. Five years before Eyles became
infected, WHO had launched the Global Malaria
Eradication Programme. Banking on insecticides
and antimalarial drugs, the agency had aimed to
end all malaria transmissions outside of Africa. A
monkey malaria that easily infects humans would
sink the program because there would be no way
to treat all the monkeys.
A team of American scientists, including Eyles
and Warren, traveled to Malaysia — then the
Federation of Malaya — where the P. cynomolgi
parasites that infected Eyles came from. Funded
by NIH, the scientists worked with colleagues
from the Institute of Medical Research in Kuala
Lumpur, established in 1900 by the British to
study tropical diseases.
From 1961 to 1965, the researchers discovered
five new species of monkey malaria parasites and
about two dozen mosquito species that carry the
parasites. But the researchers did not find any
human infections. Then, in 1965, an American
surveyor became infected with P. knowlesi after
spending several nights camping on a hill about
160 kilometers inland from Kuala Lumpur.
Warren surveyed the forested area where the
infected American had camped. The hill sat beside
a meandering river. Monkeys and gibbons, a type
of ape, lived on the hill and in adjacent forests.
The closest house was about two kilometers away.
Warren sampled the blood of four monkeys and
more than 1,100 villagers around the hill; he collected mosquitoes too.
He found P. knowlesi parasites in the monkeys,
but none among the villagers. Only one mosquito
After becoming infected
in 1960 with the malaria
parasite found in his
biologist Don Eyles took
a team from the National
Institutes of Health to
Southeast Asia to study
the parasites in nature.
He is shown here with
his pet gibbon, a type of
ape, in Kuala Lumpur.