"To wrest from nature the secrets which have perplexed philosophers in all ages, to track to their sources the causes of disease, to correlate the vast stores of knowledge, that they may be quickly available for the prevention and cure of disease - these are our ambitions".
Sir William Osler.
The great majority of diseases in tropical regions are cosmopolitan which means they are found throughout the world: pneumonia, burns, fractures, diarrhea, asthma, diabetes, hypertension and schizophrenia. Some disorders were also previously found in Europe, but here they have largely disappeared: leprosy, vivax malaria, plague. Only a few diseases occur exclusively in tropical regions, e.g. African sleeping sickness. A number of diseases have disappeared in the West as a result of the improvement of living conditions. The classic, predominantly parasitic tropical diseases are for the most part not the main cause of disease in developing regions, except in certain localized areas where there is a high prevalence. The main medical problems in Third World countries at present continue to be respiratory tract infections, diarrhea, tuberculosis, malaria, AIDS, measles, accidents, anemia and pregnancy-related problems. Hepatitis B and C, pelvic inflammatory disease and epidemic meningococcal meningitis are also frequent problems.
As economies develop, other diseases previously first seen in Western countries will become more common, such as cancer, caries, cardiovascular diseases and multiresistant micro-organisms. Problems typical of large cities will become more important in the near future as urbanization increases in less developed countries. The poor neighborhoods and slums of conurbations such as Cairo, Lagos and Kinshasa in Africa, Sao Paulo, Rio, Lima and Bogota in South America, Dhaka, Calcutta, Bombay, Delhi, Karachi and Manila in Asia pose their own problems, but also offer opportunities for improvement.
Some concepts recur constantly in these lecture notes and are explained below.
Parasite: a parasite is an organism that lives in or on another organism and draws its nourishment from it (from the Greek "para-sitos": beside food). Strictly speaking, it has no connotation of harmfulness or otherwise. Usually, however, the meaning is taken in a narrower sense and the term is used to refer to various worms, protista and arthropods which have another organism as their habitat. Parasites often have a complicated life-cycle with well-defined hosts and a specific mode of transmission.
Protista: unicellular organisms that contain a cell nucleus surrounded by a nuclear membrane: eukaryotes (as opposed to prokaryotes - bacteria). There are specific treatments for each disease. e.g. Sleeping sickness, malaria, amoebiasis, leishmaniasis, giardiasis, toxoplasmosis.
Metazoa: multicellular eukaryotic organisms, diverging considerably in size and taxonomic relationship. E.g. whip worms, schistosomiasis, scabies, lung flukes.
Paratenic host: a host in which a parasite lives and survives, but does not develop further.
Vector: an intermediate host, which transports a parasite from the previous host to the subsequent one. E.g.: the tsetse fly is the vector of African sleeping sickness.
Arthropod: invertebrate animal with articulated legs. In medical practice the main arthropods belong to the group of insects and arachnids (including ticks and mites). Copepods are also arthropods and are vectors for a number of organisms.
Epidemic: infection which fairly suddenly affects a large number of people at the same time. E.g. the plague epidemics in the Middle Ages in Europe, the meningitis epidemics in the Sahel.
Pandemic: epidemic which spreads around the whole world. E.g.: Flu (influenza), AIDS pandemic
Endemic: a disease is endemic if it is chronically present in a particular region. E.g.: in Africa there are foci of endemic malaria.
Transmission: transport of an organism can occur in various ways.
Mechanical transmission, comparable to sharing a dirty needle. This can occur in rapid repetitive blood meals of mobile insects on different hosts, e.g. the host reacts to the pain caused by the bite and interrupts the insect's feeding. The hungry insect will soon try to bite a second host and infect him via the blood of the first host which is still sticking to its mouthparts. This sort of transmission, however, is rare, e.g. tularemia spread by tabanid flies.
Biological transmission, in which the pathogenic organism either (1) reproduces in the vector (e.g. plague, arboviruses), (2) undergoes maturation before it becomes infectious (e.g. river blindness), (3) both reproduces and undergoes maturation (e.g. malaria, sleeping sickness).
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