Epidemiology of Ostertagia ostertagi in warm temperate regions of the United States.

For many years our general knowledge on the epidemiology of Ostertagia ostertagi in the U.S.A. was based on research conducted in Scotland and England. During the last 10 years, epidemiologic investigations on O. ostertagi and other gastrointestinal nematodes of cattle have been conducted in various sections of the U.S.A. and Canada. Definite seasonal patterns of O. ostertagi larval inhibition have been demonstrated, but occurrences of the Type I and Type II clinical entities have not been adequately described and associated with the epidemiology of the parasite. All information to the present time suggests that there are seasonal differences in ostertagiasis based on a north and south plane, with larval inhibition occurring during autumn in the north and during spring in the south and parts of the west. Under environmental and management conditions of the south and those other regions where O. ostertagi is significantly and summer is the dominant season, some minimal numbers of inhibition-prone O. ostertagi may be acquired during autumn and winter. However, the early pre-Type II phase (acquisition of inhibition-prone larvae) begins in late winter and reaches peak levels during spring. From January to April, young cattle between weaning and 16 months of age may be affected by Type I disease. Pasture transmission basically ceases from late spring into autumn with consistent high temperature and alternating wet and dry periods. From late summer through autumn (occasionally earlier), Type II disease is possible primarily in yearling cattle and also in older cattle. September and October are months of greatest prevalence. The probability of Type II disease effects occurring, i.e. effects ranging from mild clinical signs and minimal production loss to acute disease and deaths, is dictated by numbers of worms present and characteristics of maturation of inhibited larvae, which in turn may be influenced by current weather factors and management conditions, as well as those of the previous winter and spring.