LOOKING at the many vector-borne diseases, particularly viral, one finds that there exists a complex host-parasite relationship among various animals, their arthropod vectors and infective organisms. The results of human entrance into the infectious chain are deleterious, since man becomes an integral part of the host-parasite relation which changes his environment. Therefore, before thinking of controlling the diseases, efforts must be made to understand vector-transmitted infections in man in relationship to his environment.
Parasitic diseases of humans and animals are obviously a part of the broad evolutionary development. The application of the theory of insect-carriers led to a better understanding of diseases such as sleeping sickness, malaria, yellow fever, bubonic plague and typhus. While the role of animals, vectors and man in the natural cycle of disease transmission was established about two centuries ago, not much importance has been given to understanding the role of the environment and the necessity of an ecological approach to study this. The lack of accurate knowledge concerning the ecology of wild reservoir hosts, vectors and the human victims in nature has been responsible for a poor understanding of the epidemiology of many diseases, particularly arthropod-borne viral diseases.
The Russian parasitologist and geographer Eugene N. Pavlovsky’s extensive researches in the middle of the last century gave us a greater understanding of the evolution of natural adaptations of infectious diseases. The concept means that wild enzootic foci of many diseases exist in nature independently of man and domestic animals. These foci present well-defined ecological peculiarities wherein pathogens and natural hosts are associated, often through an intermediate vector.
The environmental factors determining these associations are climate, soil, vegetation and topographical features (landscape epidemiology). These serve as reliable indicators of the existence of certain diseases. Areas at the edge of deserts, with burrowing rodents, may harbour Cutaneous leishmaniasis (a skin infection, as in Rajasthan); areas at the junction of mountains, forests and agricultural fields or grasslands (interfaces) may harbour many vector-borne diseases. These natural foci, which may be called “silent zones of diseases”, may remain undetected until susceptible human beings come into contact with them directly or indirectly and become infected. With accurate ecological knowledge, similar foci in other areas may be detected before human disease can be predicted.
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