Animals Used to Monitor Other Animals

You have a colony of laboratory animals that must be kept isolated from all other animals.  Every few weeks, you sacrifice one or two animals in order to verify that they have not become contaminated by outside contact.  How are these “monitor” animals different from the other animals actually used for research?

K5R says:  Guidelines have been established by the Association for Assessment of Laboratory Animal Care (AALAC) for keeping and maintaining sentinel animals within a facility.  In theory and practice, sentinel animals should be housed, manipulated and treated in all respects like the other study animals within the same environment.