Malaria Transmission: are we out of the woods?

Human activity is leading to deforestation in central Africa and bringing hunters and people in closer contact with our primate cousins and their collection of pathogens.  The Global Viral Forecasting Initiative is carefully studying these contacts between humans and other vertebrates in an effort to identify possible new viral epidemics. In the process they have also made observations about the origins of human malaria and the potential for transmission of primate plasmodium infections to people.

While transmission of malaria species between humans and other primates has been documented in South America and Africa, Southeast Asia has received the most attention.  In fact, the question is being raised as to whether P. knowlesi is becoming the fifth human malaria parasite. If new infections as well as new species are arising from the primate world, this has grave implications for efforts to eliminate malaria worldwide.

A recent study from Vietnam found that, “showed P. knowlesi infections in 32 (26%) persons with malaria (n = 125) and in 31 (43%) sporozoite-positive An. dirus mosquitoes (n = 73). The authors observed and warned that …

P. knowlesi–co-infected patients were largely asymptomatic and were concentrated among ethnic minority families who commonly spend nights in the forest. P. knowlesi carriers were significantly younger than those infected with other malaria parasite species. These results imply that even if human malaria could be eliminated, forests that harbor An. dirus mosquitoes and macaque monkeys will remain a reservoir for the zoonotic transmission of P. knowlesi.

In another study researchers confirmed P. knowlesi in humans in Cambodia. This is worrisome especially since Cambodia is one of the locations where resistance to artemisinin-based medicines is rising.

gametocytes-of-p-knowlesi-in-a-giemsa-stained-thin-blood-smear-from-a-patient-that-traveled-to-the-philippines.jpgLikewise, researchers in Malaysia reported that, “P. knowlesi is a major cause of severe and fatal malaria in Sabah. Artemisinin derivatives rapidly clear parasitemia and are efficacious in treating uncomplicated and severe knowlesi malaria.”  Gametocytes of P. knowlesi in a Giemsa-stained thin blood smear from a patient that traveled to the Philippines can be seen in the attached photo from CDC.

Simian reservoirs of malaria throughout Southeast Asia pose a major challenge for control efforts.  Tackling this problem in the forest habitats where people come into contact with monkeys will be daunting – we are not out of the woods yet for malaria elimination.

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