The present invention relates to a method of determining the presence of an infectious agent in one or more test animals. The method includes providing a cage housing a sentinel animal, the cage including a plurality of inlets to allow flow of air into the cage through each of the inlets; providing a plurality of cages each housing a test animal, each cage including an outlet to allow air to flow from the cage; connecting the plurality of cages each housing a test animal to the cage housing a sentinel animal so as to allow air to flow via the outlet of each cage housing a test animal into the cage housing a sentinel animal via an inlet on the cage housing a sentinel animal; and screening the sentinel animal for infection by the infectious agent after a period of time sufficient for the sentinel animal to be infected from one of the test animals, wherein infection of the sentinel animal is indicative of the presence of the infectious agent in one or more of the test animals.