Twenty-Five Years of Progress in Understanding Major Infectious Diseases of Dairy Cattle

Disease, whether it occurs in cats, cattle, caribou, or other species, is the manifestation of insult to the structural or functional integrity of the living being. Infection may occur with or without evidence of disease. Disease may represent only the “tip of the iceberg” of infection. Prevalence of disease is the proportion of a defined population that meets specific criteria at a point in time, and incidence refers to the proportion of a defined population in which the onset occurs during a specifiable interval. Risk is an instantaneous rate of incidence. Prevalence, incidence, and risk in infectious diseases are quantitative inferences derived from assessment of implications — caveats derived from variables that often are elusive, such as antibiotic resistance to plasmid mediated determinants transferable between species of enteric bacteria, or such as increases in population density among insect vectors influenced by climatic conditions favorable to the insects. Carefully orchestrated manipulation of the etiologic agent or the host response is made possible by ever-expanding understanding of the molecular biology of pathogenicity on the one hand and cellular and humoral immunity on the other. Such understanding often leads to new caveats or destroys old ones based on incomplete information. This paper deals with a selected limited number of infectious diseases of dairy cattle. Knowledge developed around the cited parameters in 13 infectious diseases during the past quarter century is brought into focus. Ten geographically dispersed specialists in infectious diseases of cattle were asked to identify priorities among the array of transmissible diseases important to North America. Except for the mastitis syndrome, which will be given special attention elsewhere in these anniversary papers, the diseases discussed are those wherein major advances in knowledge or major needs for advanced knowledge were of special importance.