The emerging of the fifth malaria parasite (Plasmodium knowlesi): a public health concern?

After examining the most recent scientific evidences, which assessed the role of some malaria plasmodia that have monkeys as natural reservoirs, the authors focus their attention on Plasmodium knowlesi. The infective foci attributable to this last Plasmodium species have been identified during the last decade in Malaysia, in particular in the states of Sarawak and Sabah (Malaysian Borneo), and in the Pahang region (peninsular Malaysia). The significant relevance of molecular biology assays (polymerase chain reaction, or PCR, performed with specific primers for P. knowlesi), is underlined, since the traditional microscopic examination does not offer distinguishing features, especially when the differential diagnosis with Plasmodium malariae is of concern. Furthermore, Plasmodium knowlesi disease may be responsible of fatal cases, since its clinical presentation and course is more severe compared with those caused by P. malariae, paralleling a more elevated parasitemia. The most effective mosquito vector is represented by Anopheles latens; this mosquito is a parasite of both humans and monkeys. Among primates, the natural hosts are Macaca fascicularis, M. nemestina, M. inus, and Saimiri scirea. When remarking the possible severe evolution of P. knowlesi malaria, we underline the importance of an early recognition and a timely management, especially in patients who have their first onset in Western Hospitals, after journeys in Southeast Asian countries, and eventually participated in trekking excursions in the tropical forest. When malaria-like signs and symptoms are present, a timely diagnosis and treatment become crucial. In the light of its emerging epidemiological features, P. knowlesi may be added to the reknown human malaria parasites, whith includes P. vivax, P. ovale, P. malariae, and P. falciparum, as the fifth potential ethiologic agent of human malaria. Over the next few years, it will be mandatory to support an adequate surveillance and epidemiological network. In parallel with epidemiological and health care policy studies, also an accurate appraisal of the clinical features of P. knowlesi-affected patients will be strongly needed, since some preliminary experiences seem to show an increased disease severity, associated with increased parasitemia, in parallel with the progressive increase of inter-human infectious passages of this emerging Plasmodium.

Malaria infection and the anthropological evolution / Infecção por Malária e evolução antropológica

During the evolution of the genus Homo, with regard to species habilis, erectus and sapiens, malaria infection played a key biological role, influencing the anthropological development too. Plasmodia causing malaria developed two kinds of evolution, according to a biological and philogenetical point of view. In particular, Plasmodium vivax, Plasmodium malariae, and Plasmodium ovale, would have either coevolved with human mankind (coevolution), or reached human species during the most ancient phases of genus Homo evolution. On the other hand, Plasmodium falciparum has been transmitted to humans by monkeys in a more recent period, probably between the end of Mesolithic and the beginning of Neolithic age. The authors show both direct and indirect biomolecular evidences of malaria infection, detected in buried subjects, dating to the Ancient World, and brought to light in the course of archeological excavations in some relevant Mediterranean sites. In this literature review the Authors organize present scientific evidences: these confirm the malarial role in affecting the evolution of populations in Mediterranean countries. The people living in several different regions on the Mediterranean Sea sides, the cradle of western civilization, have been progressively influenced by malaria, in the course of the spread of this endemic disease during the last millennia. In addition, populations affected by endemic malaria developed cultural, dietary and behaviour adaptations, contributing to decrease the risk of disease. These habits were not probably fully conscious. Nevertheless it may be thought that both these customs and biological modifications, caused by malarial plasmodia, favoured the emergence of groups of people with a greater resistance against malaria. All these considered factors decreased demographical impact, influencing in a favourable way the general development and growth of civilization.

Durante a evolução do gênero Homo a infecção por malária exerceu papel biológico chave, influenciando até o desenvolvimento antropológico. Os Plasmódios causadores da malária desenvolveram dois tipos de evolução, segundo um ponto de vista biológico, e filogenético. Em particular, os Plasmódios vivax, malariae, e ovale poderiam ter coevoluído com a espécie humana ou ter atingido esta durante as fases mais antigas da evolução do gênero HOMO. Por outro lado, o Plasmódio falciparum teria sido transmitido aos humanos por macacos num período mais recente, provavelmente entre o fim da Idade Mesolítica e o início da Neolítica. Os Autores mostram evidências biomoleculares, tanto diretas como indiretas, de infecção por malária, observada em indivíduos enterrados, reportando ao Mundo Antigo, e trazidas à luz no decorrer de escavações arqueológicas em alguns sítios relevantes Mediterrâneos. Nesta revisão da literatura, os autores organizam evidências modernas. Confirmam o papel da malária ao afetar a evolução das populações no Mediterrâneo. As pessoas que moram nas várias regiões do Mediterrâneo foram progressivamente influenciadas pela malária, ao longo do avanço desta doença. Além disso, as populações afetadas desenvolveram adaptações culturais, alimentares, e de comportamento, contribuindo para uma redução no risco da doença. Entretanto, pode ser que ambas modificações (as biológicas e as dos hábitos) usadas pelos Plasmódios da malária, favoreceram a emergência de grupos de pessoas com maior resistência contra a malária. Todos estes fatores considerados reduziram o impacto demográfico, influenciando de uma maneira favorável o desenvolvimento em geral, e o crescimento da civilização.