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Microorganisms that are pathogenic to insects provide a wealth of biological material that can be exploited by humans to control insect pests. Innovative applications of a few such entomopathogens are found throughout the world, but widespread commercial production of microbial insecticides awaits further studies of the biology, ecology, and pathogenicity of the agents. Genetic engineering techniques may be used to increase the virulence of these microorganisms, as well as to make them more tolerant of physical and chemical conditions and perhaps to broaden their host ranges. The use of microbial insecticides could decrease our dependence on chemical pesticides.

The dynamics of establishment, pathogenesis, and continued maintenance of Enteric Redmouth (ERM) disease in rainbow trout (Salmo gairdneri) were examined. Groups were infected by parenteral and oral routes and subsampled at weekly intervals. Both modes of infection were standardized to obtain ID100 levels of infection with an average of 30 bacteria constituting an LD50 dosage when administered parenterally. The clinical course of the disease was monitored up to 102 days postinfection. A clinically asymptomatic carrier state infection was established in the lower intestine of 25% of survivors 45 days postinfection at 14.5 C. Thereafter, regular intestinal shedding of the pathogen, causing recurrent infection and mortality within the population, occurred on acyclic basis of 30–40-day intervals. Clinical recovery, establishment of the asymptomatic condition, and periodicity of intestinal shedding and the clinical course of the disease to convalescence were described.

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