The greatest risk for initial introduction of many infectious diseases into a herd is the addition of subclinically infected animals, although some risk is also attributable to wildlife carriers. Potential sources of infection may be seen with herd additions, or through intentional or inadvertent movements and contacts. Herds may be classified as “closed” or “open,” based on their potential for pathogen exposure. (Also see Biosecurity.) Closed herds restrict the introduction of animals and vehicles from livestock sources as well as contact with other herds and animals. Open herds have a higher risk of introducing pathogens through such practices as introduction of purchased replacements (especially from commingled sale groups), purchase of bulls, direct introduction of high-risk stocker calves (especially into high population densities), and mingling of animals of different backgrounds, through either cooperative breeding programs or poor herd biosecurity.
In general, all purchased or introduced animals should be separated from the home herd for a reasonable observation period (eg, 4 wk); these animals should undergo the same health procedures as the home herd. It is prudent to obtain animals from herds in which the herd health history is known and to have a record of vaccinations and treatments. Before purchasing animals, buyers should be sure that herds have tested negative for paratuberculosis and are free of persistently infected BVD, tuberculosis, and brucellosis. If the herd is free of diseases such as bovine leukosis and anaplasmosis, purchasing only animals from other herds negative for these diseases is vitally important. Pubertal bulls should be tested for trichomoniasis and vibriosis when indicated. For artificial insemination (AI) programs, semen should be used only if it was processed by an approved AI center with a comprehensive health program for minimizing the risk of transmission of venereal (and other) diseases through frozen semen.