THE MERCK VETERINARY MANUAL
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Preventing Introduction of Infectious Diseases

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Introduction of disease-causing pathogens onto farms where the animals are naive can be catastrophic to the health of all exposed animals. Therefore, careful attention must be paid to preventing the entry of such pathogens.

Animals should be acquired from sources for which the status of disease-causing pathogens is known. A complete history of the herd of origin should be provided to the operator(s) of the recipient herd. This typically requires that veterinarians providing health services to the respective farms communicate about information relevant to health status, disease outbreaks, results of serologic testing, postmortem findings, animal performance, and when available, slaughter checks. Information related to disease control measures in the herd of origin is critical for formulating a complete picture of the herd's health status and properly interpreting the results of some diagnostic tests. Results of diagnostic testing should be provided to the veterinarian of the recipient herd before animal shipment to facilitate preparations for receipt of the purchased animals. The timing of diagnostic testing may vary; the results of quarterly testing may suffice for some herds, while testing immediately before shipment may be required for other herds.

When animals arrive at the farm, they should be isolated from the existing herd. This is necessary because the new arrivals may be incubating and/or shedding infectious agents immediately after transport and could pose an increased risk to the health of animals in the existing herd. The length of the isolation period will vary based on the diseases under consideration. Isolation periods >60 days may be necessary to prevent introducing animals that are actively shedding infectious agents.

The isolation facility should be a sufficient distance away from the recipient herd to minimize the transfer of pathogens; a distance of 1.86 miles (3 km) is considered ideal. The distance reflects the fact that some flying insects can travel up to 1.5 miles (2.4 km) between facilities and potentially transmit infectious agents from one facility to another. The minimum recommended distance to minimize disease transmission between facilities is ~330 yards (300 m).

During the isolation period, animals should be monitored for disease and observed daily for clinical signs of illness. Testing may include serology, PCR, culture and sensitivity, and virus isolation. Medications such as antibiotics, chemotherapeutics, and anthelmintics should be administered during the isolation period for control of bacterial pathogens of concern, ectoparasites, and enteric parasites. Any necessary vaccines should be administered during the isolation period to permit new animals to build immunity to diseases present in the recipient herd. Controlled exposure of animals in isolation to diseases in the recipient herd using serum, feces, or live animals should be performed under the guidance of the herd veterinarian.

When possible, a dedicated caretaker should provide the daily care to animals housed in the isolation facility. This reduces the likelihood of tracking pathogens from the isolation facility to the recipient herd. Equipment, clothing, and footwear that can be cleaned and disinfected or washed should be used for and not removed from the isolation facility. If it becomes necessary to remove items from the isolation facility, they should not be transported to the recipient herd.

Last full review/revision July 2011 by Darryl Ragland, DVM, PhD

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