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It is useful to be acquainted with some of the basic concepts from botany and zoology. It is regrettable that in a number of medical curricula these subjects are missing or became less important. As a general training, an introduction to botany and zoology is highly recommended. Were this to be restricted to taxonomy and to overwhelming students with various life cycles, the benefit of such a foundation course might be questionable. The importance lies not so much in the details, but in the recognition of a framework of reference within which details may be organised. Not only do plants form the basis of most food chains and are thus the basis of all higher life but they also form an important part of the world economy. In addition to the discussion of the direct medical problems which some plants can cause, a number of subjects may be discussed during an introduction to evolutionary theory such as fertile hybrids, tetra- or polyploidy in most food plants, the consideration of chloroplasts and mitochondria as descendants of endosymbionts, etc. The same types of argument can be advanced for zoology. In order to be able to better understand some subjects it is useful to have some notions of comparative anatomy and basic concepts of (disease) ecology. Likewise a basic knowledge of zoology is useful for a good understanding of zoonoses, vectors (mosquitoes, fleas, flies, ticks, mites, snails, copepods, etc, …), scabies, various worm infections, venomous animals (snakes, scorpions, jellyfish, etc.) or traumatogenic animals (from processionary caterpillars to cat scratches). The discovery of specific fly larvae in a corpse can be important in forensic medicine. Medical problems related to xenotransplantations we shall simply leave out of consideration. Further removed is the possibility that in the future a plant disease might be transmitted to human hosts (one of the reasons why plant diseases are included in ProMed). Generally therefore, there is a very strong argument to keep botany and zoology in a medical curriculum. Regrettably enough, this falls outside the scope of tropical diseases, although occasionally reference will be made to it. The full impact of very rapid developments in the area of genetic engineering is still not clear. The work on transgenic vectors is still in its infancy. However, various arthropods can already be genetically manipulated in the laboratory to become resistant to infection by certain pathogenic organisms. Thus, it has been possible recently to make Aedes aegypti resistant to the dengue 2 virus. Studies of transgenic mosquitoes and yellow fever, transgenic tsetse flies and sleeping sickness, and transgenic bugs and South American trypanosomes are in full development.
There are several taxonomic methods:
Note concerning taxonomy, nomenclature and the concept of "species"
The only true entity is a specimen, an individual plant, mould, protist, bacterium or animal. Since many individuals resemble one another (a pheasant resembles another pheasant more than a quail), individuals are grouped into species. Much ink has flowed in the discussion of species as "true entities" (cf. the discussion about asexual organisms, races and fertile hybrids). There are more than a million different species of living organisms on our planet. Some of the most well-known or common have been given a popular name. These names, however, can be somewhat confusing, particularly in the exchange of information between people from different countries. For this reason formal Greek or Latin names are used. The reason for this is that Latin was previously the language of science. Sometimes strange linguistic hybrids are formed, e.g. "Australopithecus": "Auster": south (Latin) and pithecos = monkey (Greek).
The Swedish professor, Carl Linnaeus (1707-1778) proposed a simplification in the existing system of nomenclature. He attempted to classify all living organisms to uncover any underlying pattern in their creation (later identified with evolution). The tenth edition of his book, Systema Naturae, was published in 1758. This work represented a turning point in zoological terminology. After having first used long and complex names, in this edition, he simplified the system to a name with two parts: first the genus and then the species. E.g. Schistosoma mansoni, Escherichia coli, Aedes aegypti. If there are subspecies (races), a third word is added, e.g. Trypanosoma brucei gambiense. Thus, living organisms are divided into hierarchical groups according to the similarities in their structure. The successive groups are: Kingdom, Division (Phylum), Class, Order, Family, Genus and Species.
Sometimes additional levels are needed. Thus, the order of Hymenoptera (membrane-winged) is classified into two large suborders: the Apocrita (wasps, bees and ants) and the Symphyta (saw-flies, wood wasps). A suborder can then be further divided into a number of superfamilies. Thus, the suborder Apocrita are split into the superfamilies Apoidea and Vespoidea.
Sometimes a suborder is subdivided into individual infra-orders (these then end with the suffix -morpha). Superfamilies end in -oidea. Families of the animal kingdom always end in -idae (in the plant kingdom the family names always end in -aceae). [Note the difference between "-ea-" and "-ae-"]. Subfamilies always end in -inae. A subfamily can be subdivided into tribes which end in -ini. Sometimes a subgenus is given and is written between brackets, e.g. Aedes (Stegomyia) aegypti. When there are species complexes, as in Simulium damnosum, reference is often made to S. damnosum s.l. (sensu lato - in the broad sense, i.e. the species complex) or S. damnosum s.s. (sensu stricto - in the narrow sense). Different groups within a complex may exhibit very different patterns of behaviour. Thus, Anopheles gambiae sensu strictu is highly anthrophilic, while the sister species Anopheles quadriannulatus is totally zoophilic and has no medical significance. The presence of the latter in an environment, however, can cause confusion in a control programme.
Initially, this can appear to be too much of a good thing, but what is most important is to have a basic idea of how medically important arthropods are classified. According to the "International Code of Zoological Nomenclature", the genus name is always written with a capital letter and the species name always with a lower case letter (e.g. Glossina tachynoides). This applies even if the name is derived from a proper name, e.g. Culicoides grahamii. In scientific publications, genus and species name are italicised or underlined. Names also never contain an accent, apostrophe or umlaut (thus no Aëdes aegypti or Tipula o'neili). The name of the genus can be abbreviated, e.g. Anopheles funestus becomes A. funestus if this does not lead to confusion or potential mistakes in the text. Sometimes the generic name is abbreviated to two letters to prevent confusion. Suppose a text contains the mention of Culiseta and Culex. If both are abbreviated to C. then it is no longer possible to know to which this refers. If Culex is identified by Cx. then clarity is restored. Sometimes the name or the initials of the discoverer of the species are included (not italicised), possibly with the year of description: e.g. Enterobius vermicularis (Linnaeus, 1758). This mention of the name, however, is optional and does not form any part of the actual scientific name. Because Linnaeus described so many species, his name is sometimes only indicated with an L. The same applies to Fabricius, whose name is abbreviated to F. The principle of this notation system is internationally accepted. In view of the fact that knowledge and opinion are constantly changing, taxonomic classifications (certainly the "middle groups") sometimes differ from author to author and according to the time of publication. There is no such thing as "The One Final Correct Classification".
There are a number of difficulties, particularly associated with the concept of "species". The species is not a constant unit. It develops and often splits up into smaller units, which are known as subspecies or geographical races. Initially a species was defined on purely morphological grounds. However, as a result situations occurred where, for example, the male and female wild duck (which differ externally) were assigned to different species. Subsequently, reproduction became the focal point in terms of taxonomy. Conventionally, the species is defined as a population which can reproduce among itself and which is reproductively isolated from other populations. This appears clear when we talk for example of humans, horses, wild ducks or rattlesnakes, but with other organisms it is much less obvious. What is the situation with the taxonomy of extinct species? What about symbiotic organisms, from lichen to protozoa, which cannot live without their symbiont? Some organisms have no sexual reproduction (for example amoebae). If there are sterile hybrids (e.g. horse x donkey-> mule), then this is an answer. Sometimes however there are fertile hybrids (some animals, many plants). Sometimes only the number of sets of chromosomes in each cell differs. A number of plants reproduce by apomixis (= parthenogenesis), i.e. formation of seeds without prior fertilisation. Small variations in the offspring are immediately fixed and from then on transmitted to the following generations. Thus, extensive series of highly similar plants differing only in minor characteristics can occur. Parthenogenesis also occurs in animals, and likewise hermaphroditism. It all means that the concept of "species" starts to become somewhat blurred in certain cases. It becomes even more complex when we include the fact of the lateral transfer of genes in the discussion. Thus, it might be that the previous history of a particular gene in an individual organism differs totally from that of another gene in the same organism (cf. transposons, see e.g. mariner element). The problem of species definition is central in biology at present. This has practical implications for example for the better understanding of the variability of diseases such as amoebiasis, leishmaniasis or Chagas' disease. Better insights into vector populations also depend on good definitions (some morphologically identical mosquitoes appear genetically to consist of various complexes with, for example, differing biting or reproductive behaviour).
Identification and classification are two related, but differing concepts. Thus, we know for example of pentastomiasis (porocephalosis) caused by infection with Armillifer armillatus. The parasite is easy to identify, but there is considerable uncertainty in terms of its taxonomic status. Blastocystis hominis is another example. The organism is "incertae sedis" (Lat. "of uncertain location").
