Insectborne Diseases

By adopting a more specific means of transmission, some parasitic organisms have become dependent on vectors for carriage to a new host. Several vectors may be used by some infecting organisms, such as arbo-viruses, but often a parasite is restricted to only one kind of vector. This may appear to reduce the chance of infection, but compared with the haphazard method of scattering large numbers of organisms into the environment, in the hope that one of them will find a new victim, using a vector can have a greater chance of success. The parasite is carried right to the new host and in many cases introduced directly into it. Often, a development stage takes place in the vector and the infective stage continues for the rest of the vector's life. However, transmission depends on the vector being able to find a new host, often within a limited period of time, at a vulnerable stage in the life cycle, where control methods are most likely to succeed.

Vector transmission is one of the commonest methods of spreading disease and many of the infections transmitted this way are of major importance, so large sections need to be devoted to them. Such is the importance that vector-transmitted diseases are discussed in two chapters - this chapter, which includes all the vectors that use flight, such as mosquitoes and tsetse flies, and the next chapter on ectoparasites, which attach to the host, such as fleas and lice. Since the vector is all-important in transmission, the diseases transmitted by them are grouped according to the vector.

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