ABSTRACT
Prevalence is measured by surveying a population, some of whose members are diseased and the remainder healthy, at a particular point in time. The severity of disease is dependent upon a number of factors, including breed of cattle, strain virulence, and age at time of infection. Mature cattle may develop severe disease with high mortality, whereas younger cattle show minimal clinical signs. The prevalence of a disease in a population may be higher or lower than incidence, depending on the average duration of the disease. Diseases that are rapidly fatal, such as rabies, or of short duration, such as bovine mastitis, might have a higher incidence than prevalence. Expressing the frequency of disease as a rate or proportion, using appropriate denominators rather than in terms of absolute numbers, i.e., dangling numerators, permits comparisons of disease frequency in comparable populations.
