Researchers in Malaysia have found that an emerging new form of malaria, thought previously only to infect monkeys, that can be easily confused with a less serious type, is widespread among humans in the region and is potentially fatal if not diagnosed and treated early.

The Wellcome Trust funded study about the discovery is published in the 15 September 2009 issue of Clinical Infectious Diseases, and was led by Professors Balbir Singh and Janet CoxSingh of the University Malaysia Sarawak, Kapit Hospital (also in Sarawak, Malaysia) and the University of Western Australia.

The researchers conducted a prospective study to identify key laboratory and clinical features of the new form of malarial infection, which is caused by the mosquito parasite Plasmodium knowlesi, previously thought to infect only monkeys, particularly the longtailed and pigtailed macaques that live in the rainforests of Southeast Asia.

Singh and CoxSingh and colleagues recently showed that the parasite was also widespread among humans in Malaysia, which with further reports from neighbouring countries have led experts to recognize P. knowlesi as the fifth cause of malaria in humans.

Around one million people die from malaria every year. The illness is caused by a mosquito parasite that gets into the bloodstream of humans via bites from infected mosquitoes.

There are many species of malaria parasite, four of which commonly cause disease in humans. The most deadly to humans is P. falciparum, found mostly in African countries. Another species is P. malariae, which usually causes milder symptoms and is found in tropical and subtropical regions around the world.

Singh told the press that

“P. knowlesi malaria can easily be confused with P. malariae since these two parasites look similar by microscopy, but the latter causes a benign form of malaria.”

He explained that infection by P. knowlesi is potentially fatal because the parasites reproduce every 24 hours in the blood, making early diagnosis and treatment essential.

“Understanding the most common features of the disease will be important in helping make this diagnosis and in planning appropriate clinical management,” he added.

The researchers recruited over 150 patients who were hospitalized in Kapit Hospital in Sarawak, Malaysian Borneo, between July 2006 and January 2008. Tests using blood film slides showed that all patients were infected with Plasmodium, and molecular detection revealed that P. knowlesi was present in over two thirds of the patients.

Malaria parasites cause in a wide spectrum of diseases in humans, and P. knowles is no different. Among the study participants, most infections were uncomplicated and responded to the two commonly used antimalaria drugs chloroquine and primaquine.

However, about 1 in 10 patients had developed complications and two of the patients died. The complications included breathing and kidney problems, including kidney failure in a small number of cases (this is also common in severe cases of P. falciparum infection).

The researchers calculated that the fatality rate of P. knowlesi was just under 2 per cent, which is about the same as P. falciparum malaria. However, they stressed that there werent enough participants in the sample to be sure about this figure.

An unexpected characteristic of P. knowlesi infection was that they found all the patients infected with this parasite, including those who had not complications, had a low blood platelet count. Low blood platelet counts usually occur in fewer than 80 per cent of other types of human malaria cases.

The researchers also found that the blood platelet counts were much lower than for other types of malaria and said this feature could be used to help diagnose infection by P. knowlesi.

But, although blood platelets are important for blood to be able to clot, the researchers said they saw no cases of excessive problems with clotting or bleeding among the participants.

There have been reports in recent years of Europeans who after travelling in Malaysia, and also one American who travelled in the Philippines, having to be hospitalized with malaria when they got back home and who were then found to be infected with the knowlesi species of Plasmodium.

Doctors in Western countries should be aware of the possibility that patients who present with malaria symptoms and who have recently returned from an area where P. knowlesi is spreading may need urgent diagnosis and treatment.

“The increase in tourism in Southeast Asia may mean that more cases are detected in the future, including in Western countries,” said Singh.

“Clinicians assessing a patient who has visited an area with known or possible transmission should be aware of the diagnosis, clinical manifestations, and rapid and potentially serious course of P. knowlesi malaria,” he added.

“Clinical and Laboratory Features of Human Plasmodium knowlesi Infection.”
Cyrus Daneshvar, Timothy M. E. Davis, Janet CoxSingh, Mohammad Zakri Rafaee, Siti Khatijah Zakaria, Paul C. S. Divis, and Balbir Singh.
Clinical Infectious Diseases 2009; Vol 49, No 6, pp 852860.
DOI 10.1086/605439

Sources Wellcome Trust.

Written by Catharine Paddock, PhD