Category Archives: Zoonoses

The Monkey on the Back of Malaria Elimination

Concerning malaria elimination, “WHO grants this certification when a country has proven, beyond reasonable doubt, that the chain of local transmission of all human (emphasis added) malaria parasites has been interrupted nationwide for at least the past 3 consecutive years.” This target is challenging enough, but becomes more complicated when we consider that zoonotic transmission of malaria among monkeys and humans has been documented in Brazil and Southeast Asia. We cannot expect monkeys to sleep under bednets, so creative and realistic solutions are needed.

The Malaria Eradication Research Agenda (malERA) recognizes this problem. Plasmodium knowlesi, originally found in macaque monkeys in Southeast Asia has been dubbed the fifth human malaria due to its spread to people as deforestation has disturbed the habitat of the monkeys. In particular malERA addresses the challenge of understanding the upward trend of this malaria infection in that region and the need for better understanding of transmission dynamics and proper diagnosis.

The danger of P. knowlesi is heightened by difficulties in diagnosing it and distinguishing it from other malaria species. “Recently, the prevalence of human infection with a simian malaria parasite, P. knowlesi, has become an important issue in a wide area of Southeast Asia. The identification of this parasite by microscopy is very difficult because it resembles the P. malariae parasite. However, the symptoms caused by P. malariae and P. knowlesi are very different, with only P. knowlesi causing severe and life-threatening malaria” (Komaki-Yasuda et al.)

Reports from Brazil highlight another ‘simian hotspot.’ While P. Knowlesi represents monkey infections reaching humans, the opposite may have happened to establish a reservoir in the New World. “P. vivax lineages appearing to originate from Melanesia that were putatively carried by the Australasian peoples who contributed genes to Native Americans. Importantly, mitochondrial lineages of the P. vivax-like species P. simium are shared by platyrrhine monkeys and humans in the Atlantic Forest ecosystem, but not across the Amazon, which most likely resulted from one or a few recent human-to-monkey transfers.”  But looking even further back in natural history, Escalante and colleagues found, “compelling evidence that P. vivax is derived from a species that inhabited macaques in Southeast Asia.”

A recent study in this area found the worrying results that, “The low incidence of cases and the low frequency of asymptomatic malaria carriers investigated make it unlikely that the transmission chain in the region is based solely on human hosts, as cases are isolated one from another by hundreds of kilometers and frequently by long periods of time, reinforcing instead the hypothesis of zoonotic transmission.”

In Africa, Linda Duval and co-researchers, who found P. falciparum in blood samples from two chimpanzees belonging to two different subspecies, warn that, “If malignant malaria were eradicated from human populations, chimpanzees, in addition to gorillas, might serve as a reservoir for P. falciparum,”

It appears that the dynamics between monkeys, malaria and humans has a long history. Even once certified malaria-free countries face the threat of imported malaria from people crossing borders. Now we must recognize that the threat may already live within borders. So since existing malaria interventions to protect humans from malaria cannot be applied to monkeys, accelerated research on the genetics of the parasite and the mosquito is needed to prevent both primate groups from getting malaria.

Population Health: Malaria, Monkeys and Mosquitoes

On World Population Day (July 11) one often thinks of family planning. A wider view was proposed by resolution 45/216 of December 1990, of the United Nations General Assembly which encouraged observance of “World Population Day to enhance awareness of population issues, including their relations to the environment and development.”

A relationship still exists between family planning and malaria via preventing pregnancies in malaria endemic areas where the disease leads to anemia, death, low birth weight and stillbirth. Other population issues such as migration/mobility, border movement, and conflict/displacement influence exposure of populations to malaria, NTDs and their risks. Environmental concerns such as land/forest degradation, occupational exposure, population expansion (even into areas where populations of monkeys, bats or other sources of zoonotic disease transmission live), and climate warming in areas without prior malaria transmission expose more populations to mosquitoes and malaria.

Ultimately the goal of eliminating malaria needs a population based focus. The recent WHO malaria elimination strategic guidance encourages examination of factors in defined population units that influence transmission or control.

Today public health advocates are using the term population health more. The University of Wisconsin Department of Population Health Sciences in its blog explained that “Population health is defined as the health outcomes of a group of individuals, including the distribution of such outcomes within the group.” World Population Day is a good time to consider how the transmission or prevention of malaria, or even neglected tropical diseases, is distributed in our countries, and which groups and communities within that population are most vulnerable.

World Population Day has room to consider many issues related to the health of populations whether it be reproductive health, communicable diseases or chronic diseases as well as the services to address these concerns.

Earth Day, Climate, Environment and Malaria

The Earth Day website notes that, “Our planet is currently losing over 15 billion trees each year—that’s 56 acres of forest every minute. We’re working hard to reverse that trend by supporting global reforestation projects. Earth Day Network’s Reforestation Campaign benefits local communities, increases habitat for species, and combats climate change.”

This habitat change if often conducive to the spread of malaria in areas and among populations that may not have been affected before. Specifically, “More risks associated with El Niño are: flooding and landslides in the Americas, drought in Southeast Asia and Australia, scrambled fisheries, and malaria, cholera, and dengue outbreaks.”

Terry Devitt reported that the incidence of malaria jumps when Amazon forests are cut, establishing a firm link between environmental change and human disease. The report, which combines detailed information on the incidence of malaria in 54 Brazilian health districts and high-resolution satellite imagery of the extent of logging in the Amazon forest, shows that clearing tropical forest landscapes boosts the incidence of malaria by nearly 50 percent (according to Olson and colleagues).

Moyes et al. Predicted the geographical distributions of the macaque hosts and mosquito vectors of Plasmodium knowlesi malaria in forested and non-forested areas of Southeast Asia.  When urbanization and deforestation bring people into habitats they never lived in, zoonotic transmission of malaria results. Fornace et al. similarly observed that, “Marked spatial heterogeneity in P. knowlesi incidence was observed, and village-level numbers of P. knowlesi cases were positively associated with forest cover and historical forest loss in surrounding areas. These results suggest the likelihood that deforestation and associated environmental changes are key drivers in P. knowlesi transmission in these areas” of Malaysia.

Back to Brazil, de Alvarenga  and co-researchers reported in the transmission of Plasmodium simian malaria in the Brazilian Atlantic forest as a natural infection of capuchin monkeys (Cebinae subfamily). Because of human movement into forest areas, cases among people have now been documented.

The zoonotic transmission of malaria to humans due to changes in climate, environment and habitat pose another unwanted challenge to global efforts to eliminate malaria. On Earth Day it is imperative for malaria control and elimination workers to collaborate closely with colleagues in environmental health and protection.