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The inclusion of a chapter on ectoparasite control in a work otherwise devoted to vertebrates has a great deal of justification; the ecologies of vertebrates and their invertebrate parasites are inseparable, thus, the vertebrate control specialist is brought into intimate contact with ectoparasites and ectoparasite problems. In many cases, the need for vertebrate and ectoparasite control problems is one, and knowledge of techniques in both areas is required. The term "ectoparasite" groups a broad array of invertebrate animals externally parasitic on larger animals, many of them blood feeders in at least one stage of their life cycles. The ecological relationships between them and their hosts may be exceedingly complex, involving ectoparasites as vectors of parasitic micro-organisms, and in some cases as reservoirs of infection as well. In their role as vectors and also as bloodsucking parasites, they have a great impact on the ecology of animal and human populations. The importance of many ectoparasite species, especially fleas, ticks, mites, and lice, to human welfare cannot be overemphasized. The roles of fleas in the transmission of plague and murine typhus are well known, as are those of ticks in a variety of viral, rickettsial, spirochaetal, and bacterial diseases, trombiculid mites in scrub typhus and lice in epidemic typhus. In addition, man may be exposed to painful bites resulting in direct pathological effects, both from wild animal ectoparasites or from others more directly associated with man and domestic animals. The need for adequate ectoparasite control methods is manifest. Ectoparasite control ranks with control of vertebrates and with immunology and clinical treatment as a potent tool in protecting man from zoonoses. In many cases, the ectoparasite is the most susceptible link in the chain of man transmission of diseases from sources in nature to man. In others, control of ectoparasites is capable of immediately alleviating potentially dangerous situations until more lasting control measures can be carried out. It should be borne in mind that human discomfort from ectoparasites and vector-borne disease stems from a complex ecological situation and can be solved ultimately only by environmental management practices in which ecological factors are separated, analyzed, related, and adjusted in favor of man. The decision of how to control or whether to control ectoparasites should be based on a knowledge of these factors.