The greatest risks for introduction of many infectious diseases into herds are the addition of subclinically infected animals and exposure through vectors other than cattle, such as wildlife and external parasites.
Herds are often classified as closed or open, based on their potential for pathogen exposure (see biosecurity). Most herds are open, and few are completely closed. Closed herds restrict the introduction of animals and vehicles from other livestock premises, as well as contact with other herds and animals. Open herds have a higher risk of introducing pathogens through:
introduction of purchased replacement stock (especially from commingled sale groups)
purchase of bulls
commingling of animals of different backgrounds
In general, all purchased or introduced animals should be separated from the home herd for a reasonable observation period, typically for 4 weeks or more, before being introduced to the herd. These animals should either have previously undergone, or should undergo after purchase, herd health procedures similar to those of their new 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 animals only from other herds negative for these diseases is vitally important.
Pubertal bulls should be tested for trichomoniasis. For artificial insemination programs, semen should be used only if it was collected and processed by an approved or certified program to minimize the risk of transmission of venereal diseases through frozen semen.
Shared equipment and facilities should be cleaned between use by different herds. Similarly, individuals that have recently spent time in areas known to have potential disease or health risks that are not endemic to the herd should be required to wear protective equipment or simply kept off the premises.
Tools that come into direct contact with blood or become dirty should be disinfected before use and again after each animal. Similarly, needles should not be reused across herds, and it is recommended to change needles as often as possible between animals, but preferably after each animal.
Any carcasses on the premises should be managed in a way that minimizes the risk of infection of other cattle as well as the risk of disease or hazard exposure to personnel or the environment.
