As the previous chapter has shown, plants are widely used for the treatment of malaria throughout the Tropics. The questions to be addressed here are which plant species are used, how widely they are used, which should be prioritised for future research, and which should be prioritised for conservation measures. To date, there has been no systematic overview of the literature to assess which plant species are the most important candidates for further research and conservation.
Natural selection probably favors the behaviour of self-medication with effective remedies. Malaria is a potentially fatal disease and has shaped human evolution through the selection of mutations such as the sickle-cell trait, G6PD deficiency, and thalassemia (Miller, 1999). These mutations in themselves are disadvantageous, and in homozygotes may prove fatal, but the protection they afford against malaria has been sufficient for them to spread in the human population. Therefore, it would not be surprising if evolution also favored behavioural patterns that included the use of effective antimalarial medicinal plants, even if these sometimes have unwanted adverse effects. Other primates have also been observed to self-medicate with antiparasitic plants (Huffman et al., 1996; Koshimizu et al., 1994).
It can further be hypothesised that the plant species in most common use today for the treatment of malaria are among the safest and most effective. Traditional healers constantly experiment with different plant remedies in an effort to find the safest and most effective preparations, and this knowledge is transmitted from generation to generation, either through the oral tradition (as in Africa) or through written manuscripts (as in traditional Chinese medicine and Ayurveda). For a remedy to be in common use, it needs to be not only safe and effective, but also widely available and easy to prepare. These are also important characteristics for a plant remedy to be used as part of a malaria control program.
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Bioprospectors, in search of new natural products, often screen plant extracts at random on a large scale (Mateo, 2000). However, important plants may be deemed inactive as a result of insufficient tests (see Chapter 16). Random screening yields few active extracts, compared to testing plants recommended by traditional healers (Brandao et al., 1985; Carvalho et al., 1991; Krettli et al., 2001). Leaman et al. (1995) have shown that plants widely used as antimalarials by traditional healers are significantly more active in vitro against Plasmodium falciparum than plants that are not widely used, or not used at all, for the treatment of malaria.
Paradoxically, effectiveness as a medicine may in the modern world cause a plant species to become endangered (Marshall, 1998). Plants have evolved over millennia to produce antiparasitic and other compounds for their own self-defense. Today, habitat destruction and overexploitation for whatever reason (including medicinal uses) threaten many plant species. There has been until now no published data on how many potentially antimalarial plants are under threat.
In this chapter, therefore, we set out systematically to examine the ethnobotanical literature, in order to identify the range and frequency of use of different plant species as antimalarials, and to discover which herbal remedies are the most widely used. We also investigated which of these species are threatened from a conservation viewpoint.
An attempt was made to identify as many primary ethnobotanical studies as possible from malaria-endemic countries. MEDLINE, EMBASE, CAB, and SOCIOFILE databases were searched systematically using the terms traditional medicine and malaria, and references to more studies were identified in these papers. Selected journals were searched by hand. Researchers and experts were also contacted to supply copies of unpublished work. Studies were included if they contained firsthand reports of the medicinal uses of plants. Literature reviews were consulted, but where possible the primary sources were found.
Using Microsoft Access, a database is being constructed of all remedies and plant species reportedly used for the treatment and prophylaxis of malaria, fevers (including febrile convulsions), and splenomegaly. For each remedy, data recorded included the plant species and part(s) used; method of preparation; route of administration; dose, frequency, and duration; contraindications; and traditional use(s). Conservation information was found by consulting the 2002 IUCN Red Data Book online (http://www.redlist.org).
Each species was assigned an IVmal (importance value for the treatment of malaria) according to how widely its use was reported. This scoring system was first developed for use at a local level by Leaman et al. (1995), with values 1 to 4. We have extended this scoring system to apply at an international level by adding values 5 to 8 (see definitions in Table 11.1).
Ninety-four original ethnobotanical publications have been included to date, concerning 33 tropical countries. Overall, 1277 species from 160 families have been reported for the treatment of malaria and fevers. This data set is still incomplete, as there are many more studies that have yet to be included in the database.
Eight hundred and forty-nine of the listed species were quoted by only a single study, which did not give enough information for an IVmal to be allocated — but it must be 3 or less. The number of species in each category of IVmal are given in Table 11.1. There were 11 species used as antimalarials or antipyretics in all three tropical continents (IVmal = 8; see Table 11.2), and 47 used in two continents (IVmal = 7; see Table 11.3 and Figures 11.1 and 11.2). A few of these have already been discussed in more detail in Part 2 of this book.
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Looked upon as a mysterious practice, reiki originated from Japan, around 1922. Started by a Japanese Buddhist, this practice of purported healing basically uses the palm of an individual to emit positive healing energy unto the patient. Sometimes reiki is referred to as oriental style treatment by professional medical bodies.