Tropical Diseases

TROPICAL DISEASES, diseases characteristic of or exclusively found in regions with a hot climate, or occurring with particular frequency there. Ruge classifies as strictly tropical diseases only those conditions that do not have an endemic distribution outside of hot climate countries. He permits an exception only for malaria, because this disease is so widespread in all tropical countries and plays such a large role in overall morbidity there that it is the most specific to the tropics. Tropical diseases are contrasted with cosmopolitan diseases, which are distributed throughout the entire globe. It should be noted, however, that certain diseases of the temperate zone, such as scarlet fever, have only a minor distribution in the tropical zone. Some authors include tropical diseases in the group of exotic diseases, which comprises all conditions predominantly distributed outside of central and northern Europe. Tropical diseases are also identified in a number of cases with "colonial" diseases, which are contrasted with diseases prevalent in metropolitan countries. In some countries, the demarcation of tropical diseases from cosmopolitan ones is of more than just theoretical interest. Thus, in Germany, which encourages the activities of its citizens in tropical countries and former colonies, tropical diseases are included in the list of conditions entitling one to accident insurance (Unfallversicherung), whereas among cosmopolitan diseases, the right to compensation is granted only for typhus or scurvy. Among the countries of the tropical zone, those most affected by all sorts of diseases are regions that, alongside high temperatures, have high humidity, mainly lowland areas and river valleys. These regions, such as the valleys of the Ganges and Indus, the Nile, and the Amazon, are distinguished by great fertility. Accordingly, they have attracted man since ancient times, and population density in them reaches very high figures in places. These same regions are also endemic foci of many severe diseases—such as cholera, plague, tropical malaria, African sleeping sickness, kala-azar, and others—that regularly claim a huge number of victims. Some of these diseases from time to time produce colossal epidemic outbreaks spreading far beyond the boundaries of endemic areas. The peculiarities of countries with a hot climate that medically determine the endemic distribution of a number of specific diseases (tropical diseases) are by no means directly caused by climatic data. Man easily adapts to the most diverse climatic conditions, and for the inhabitants of tropical countries, a hot climate represents just as normal conditions of existence as a temperate climate does for the inhabitants of the corresponding zone. Thus, the climatic features of the tropical zone do not in themselves play the role of direct sources of disease except upon violation of certain hygienic rules or with respect to newly arrived subjects who have not had time to adapt to unusual conditions. At the same time, the indirect influence of the tropical climate on the spread of a number of diseases is very great. Of greatest significance for the nosogeography of tropical countries is the fact that the tropical zone is the sole or primary territory for the distribution of a number of pathogens of infectious and invasive diseases, their vectors and intermediate hosts, and a number of poisonous animals and plants absent in the temperate zone. Just as the hot zone possesses a richer macrofauna and macroflora compared to temperate countries, similarly the number and diversity of pathogens of all kinds of diseases and their vectors and intermediate hosts in the countries of the hot zone are much higher than in regions with a temperate climate. The abundance of blood-sucking arthropods (mosquitoes—numerous species of Anopheles, Aedes, and others; sandflies—Phlebotomus; tsetse flies—Glossina; bugs—Triatoma; fleas; ticks—Ornithodorus, Dermacentor, and many others) determines the endemic distribution of disease agents, partly transmitted mechanically through a bite, but for the most part undergoing a complex developmental path in the body of these insects or ticks. The endemic zones of many invasive diseases peculiar to the tropics are also limited by the range of distribution of a number of aquatic organisms in which the pathogens undergo their specific cycles and stages—mollusks, crabs, crustaceans, and others. High temperatures, by accelerating the development period of insect disease vectors, lead to their appearance in huge quantities, and by shortening the development period of the parasite within them, lead to the rapid mass accumulation of virus in nature, to mass infections that determine a severe course of infection and the frequent appearance of clinical forms that are extremely rare in the same disease in the temperate zone (e.g., comatose forms of malaria). High temperatures also cause the rapid development of harmful microorganisms in food products and waste, and can thus contribute to the preservation of the viability of pathogenic microorganisms outside the human body. The peculiarities of the tropical climate can also affect the spread of tropical diseases through its influence on human daily life and habits. For example, the same high temperature, which leads to a reduction in clothing size, simultaneously makes large surfaces of the body accessible to the bites of insects and poisonous animals. Walking barefoot creates conditions for the spread of ancylostomiasis. The nature of water usage in the tropics and certain types of agricultural crops create their own conditions for the spread of a number of diseases (the role of water carriers in the spread of dracunculiasis, the role of rice fields in the spread of malaria). Finally, the main role in maintaining the endemicity of many diseases in the tropics is played by general socio-economic conditions—the colossal exploitation of natives by colonizers, the inhuman conditions of existence to which huge masses of humans are subjected, crowded into dirty barracks, poorly fed, and exhausted by unbearable slave labor. Strictly tropical diseases can be divided into groups according to their etiological moment. Diseases associated with the direct impact of climatic factors are of the least significance. These include solar erythema, or sunburn, which is no different from burns that can occur in the temperate zone from exposure of unprepared skin to sunlight; sunstroke and heat stroke; and tropical lichen (lichen tropicus), the scourge of the humid tropics—a dermatitis accompanied by severe itching caused by skin erosion resulting from heavy sweating and sweat stagnation. Among tropical diseases caused by protozoa, the most important are malaria and hemoglobinuric fever, African sleeping sickness, amoebic dysentery, kala-azar, Chagas disease, cutaneous leishmaniasis, and mucosal leishmaniasis. Tropical diseases caused by spirochetes include Central Asian or Persian relapsing fever, African relapsing fever, American relapsing fever, yaws, and leptospiroses: Japanese seven-day fever, Spirochaetosis febrilis of the Dutch Indies, which is similar to swamp fever (see). Bacteria are the causative agents of the following tropical diseases: plague, cholera, melioidosis (B. whitmorii). A number of tropical diseases are caused by rickettsiae. This includes the group of typhus fevers: Brazilian typhus, Manchurian typhus, Mexican typhus (tabardillo), varieties of typhus described in India and the Sunda Islands; kedani (tsutsugamushi disease)—Japanese river fever. Bartonellae cause Oroya fever in Peru and its primary manifestation—verruga peruana (verruca peruviana). Fungi are the causative agents of a number of tropical mycoses: tinea circinata, tinea imbricata, pinta, piedra, Madura foot, dermatitis verrucosa, and others. Among the tropical diseases caused by filtering viruses is a series of febrile forms peculiar to the tropics and transmitted by blood-sucking dipterans: yellow fever, dengue, phlebotomus fever. A huge role is played by tropical diseases caused by parasitic worms. Among infections caused by nematodes, the most important are ancylostomiasis (Ankylostoma, Necator), filariasis (Wuchereria bancrofti, Loa loa, Onchocerca volvulus, Onchocerca caecutiens, and others), and strongyloidiasis. Trematodes are the causative agents of urogenital and intestinal schistosomiasis, as well as Japanese schistosomiasis. A significant pathogenic role is played by liver parasites Clonorchis and Fasciolopsis, the lung parasite Paragonimus, the widely distributed intestinal parasite Metagonimus yokagawai, and others. Among cestodes characteristic of the tropics, parasites of the genus Sparganum should be noted. Among tropical diseases caused by arthropods, note should be made of myiasis (fly larvae of Cochliomyia, Calliphora, Lucilia, Cordylobia, Auchmeromyia; botfly larvae of Oestridae—Dermatobia, Hypoderma, Gastrophilus); tick paralysis caused by bites of ticks Dermacentor venustus, Haemaphysalis cinnabarina, and Ixodes holocyclus (Australia), Ixodes pilosus (South Africa); and infestation by the chigger flea (Sarcopsylla penetrans). Also widespread in the tropics are a number of poisonous animals causing diseases, often fatal, either through their bites (snakes, spiders, fish) or by injecting venom with the aid of special appendages (scorpions, mollusks, fish).

Some species of edible animals specific to the tropics turn out to be passively poisonous, such as the fish Tetrodon and sea urchins. In the tropics there are also many plants with poisonous fruits or leaves. For example, the leaves of the Chinese plant Atriplex littoralis are poisonous; eating them causes atriplicism, a disease similar to Raynaud's disease. The unripe fruits of Blighia sapida on the island of Jamaica cause a fatal vomiting disease. The juices of a number of plants supply the natives of the tropics with potent arrow poisons. In many tropical countries the natives use plant extracts both as effective therapeutic agents and as poisons for criminal purposes (in India from Nerium odorum, in Brazil from Paullinia pinnata, in the Sahara from Hyoscyamus fahez-lez). Among avitaminoses, tropical diseases include primarily beriberi, associated with a monotonous diet of polished rice. Close to avitaminoses is sprue (aphthae tropicae). Tropical diseases also include a number of skin and bone lesions of diverse, partly unclear etiology: tropical ulcer, mainly on the shins, caused by Spirochaeta Schaudinni in association with the fusiform bacillus; mossy foot, a skin lesion of the feet common in South and Central America; goundou, a tumor-like lesion of the nasal bones in Central Africa, China, Brazil, and other countries; ainhum, a lesion of the phalanges of the little toes in Africa, Brazil, Asia; Nodositas juxta-articularis, mostly symmetrical bone tumors under the knee and elbow joints in the Dutch East Indies, Africa, and elsewhere; gangosa or rhinopharyngitis mutilans. The latter two forms are apparently tertiary manifestations of yaws or syphilis. Among tropical diseases of direct practical importance for the USSR are plague, malaria, hemoglobinuric fever, visceral and cutaneous leishmaniasis, sprue, Central Asian relapsing fever, pappataci fever, and among helminthic infestations, ankylostomiasis, Strongyloides stercoralis, guinea worm (Dracunculus medinensis) in Bukhara (now almost completely eradicated), in the Far East Clonorchis, Metagonimus, and the recently described Nanophyetus schichobalowi (an intestinal parasite). Along with these diseases found on the territory of the USSR, the study of tropical diseases prevalent in states adjacent to the USSR is of great importance for the USSR: Turkey, Iran, Afghanistan, India, China, and Japan. Familiarity with tropical diseases of more distant countries is necessary for doctors accompanying ships on relevant voyages and for doctors of diplomatic missions in tropical countries. The study of tropical diseases and the resolution of the practical tasks of combating them will require a significant broadening of the outlook and working methods of medical workers. The example of tropical diseases has most vividly highlighted the close connection of medicine with other fields of human knowledge and the necessity of building the fight against the spread of diseases on the basis of a thorough study of parasitological, zoological, meteorological, social, and a number of other factors. The study of tropical diseases has led to the discovery of a whole series of facts that have played a huge role for all of medicine as a whole, enriching parasitology and related disciplines. Suffice it to point to Ross's discovery in India of the transmission of malaria via the mosquito, to Looss's established migration of hookworm larvae in the vertebrate body, which subsequently led to the establishment of analogous processes also in infestation with Ascaris and some other nematodes, to the establishment of the etiology of avitaminoses (using the example of beriberi), to the significance of tropical diseases for the development of chemotherapy and comparative pathology. Theobald Smith, Manson, Ross, Laveran, Koch, Reed, Eijkman—these are only a few names of researchers of tropical diseases who have made a huge contribution to the treasury of all medicine as a whole, parasitology, microbiology, etc. The study of tropical diseases, in addition to institutions located directly in the tropics, is pursued by a number of institutes and societies operating in Europe and America in large university centers. There are societies of tropical (or colonial) medicine and hygiene, and exotic diseases in France (2), Belgium, Germany, Italy, the USA, and other countries. Institutes and courses on tropical diseases function in London, Liverpool, Cambridge, Edinburgh, Paris, Rome, Modena, Brussels (moved to Antwerp), Hamburg, Amsterdam, Leiden, Lisbon, San Francisco, and other cities. In the USSR, the first tropical institute (Institute of Protozoan Diseases and Chemotherapy) was founded in 1920 by Prof. E. I. Martsinovsky in Moscow; currently there are tropical institutes in Moscow, Kharkov (protozoan institute), Samarkand, Stalinabad, Ashkhabad, Tiflis, Sukhum, Batum, Baku, Erivan, Rostov-on-Don, and Makhachkala. Unlike Western European institutes, which are only centers of scientific research, the tropical institutes of the USSR, along with versatile research work, also conduct organizational work to combat the spread of malaria and other protozoan diseases, as well as helminthiases and leprosy, and direct the work of a network of malaria and tropical stations on parasitology (protozoology, arachno-entomology, helminthology), the clinic and therapy of tropical diseases, and experimental chemotherapy. At the tropical institute in Moscow and in other union institutes, courses for doctors on malaria and diseases of hot countries are held annually.