From the Archives, in Recognition of the 75th Anniversary of AALAS: The Development of Practical and Effective Filtered Cage Caps to Exclude Viral Diseases from Mouse Breeding Colonies (1964–1967)

Commentary

Previous articles republished as part of the From the Archives series documented historical considerations of laboratory rodent housing options such as proper diet and bedding as well as practical cage construction. Plastic shoebox-style cages in various sizes were being used in the 1950s, but these were provided in an open-top configuration. The specific-pathogen-free rat colony described in the last installment was managed using exclusion methods focused on maintaining a barrier at the room level. However, it was soon recognized that, in both academic and commercial colonies, there were situations that would benefit greatly from a system that significantly reduced or eliminated cage-to-cage transmission within a single room. This would increase the efficiency of both space utilization and labor and also allow scientists to conduct more robust infectious disease experiments without inadvertent cross-contamination between groups.

Dr. Lisbeth Kraft, DVM (1920 to 2002) was an AALAS Griffin Award winner (1972) and has been acknowledged as the inventor of the barrier at the cage level. ¹ Dr. Kraft used her first filter cages as part of research studies investigating the diarrheal diseases of infant mice. Much of her work involved study of a disease referred to as the “epizootic diarrhea of infant mice” (or EDIM), which she characterized for many years, eventually identifying a virus as the etiological agent in an article published in the journal Science. ² Electron microscopy of the causative virus revealed a reovirus-like particle that was later classified as mouse rotavirus. While she was studying EDIM, another viral diarrheal disease of neonatal mice became recognized, but this syndrome was associated with much more mortality so it was referred to as the “lethal intestinal virus of infant mice” (or LIVIM), which she described in a paper also published in Science. ³

For her own viral research projects, the filtered cages used were cylinders that had been custom-made from galvanized wire mesh with the curved sides fully covered with thick layers of fiberglass. These cages were only opened within a transfer hood consisting of a filtered glove box maintained under negative pressure. ⁴ But to adapt the principles of filtered caging more broadly in commercial or academic colonies, it was recognized that what would be more useful would be a filtered cap that could be placed on top of existing shoebox caging.

The first of the following papers describes such a system designed by Dr. Kraft and her collaborators that used standard caging with a fiberglass-covered wire mesh cap added. ⁵ In order to allow clear visualization of the food and water above the cage top, these filter caps were designed with one clear side toward the front that was created using a transparent polyester film approximately 0.2 mm thick. The transfer hood remained an important part of cage-level exclusion practices, but rather than the complicated glove box she originally described, the system depicted here used a filtered positive-pressure hood with a basic design that is quite familiar to us 60 years later.

The second paper reproduced in this issue was published in Laboratory Animal Care a few years later, and it describes a number of additional approaches that had been taken to develop effective filter tops that were as durable and economical as possible. ⁶ The author is Mr. Samuel Poiley, a 1965 AALAS Collins Award winner who was head of the Mammalian Genetics and Animal Production Section for one of the NIH institutes at the time. He had worked his way up to that position after 30+ years working in animal care management positions at the NIH, so he was able to provide detailed background information on EDIM in mouse colonies and to describe various prevention and remediation strategies that had been attempted. One of the important developments in cage-level exclusion he described was the use of thin and pliable nonwoven synthetic fabric filter materials as a substitute for the more bulky and friable fiberglass pads used previously in filter tops. This sheet-like material could be cut and sewn into a rectangular shape that used a minimal wire frame for internal support, rather than the full wire mesh supports previously used. Looking at the figures from the Poiley paper, it is not difficult to envision the final change from a wire frame to a polymeric support shell, which led to the modern plastic microisolation-style cage design.

The final reprinted article brings the story of LIVIM to a conclusion. Although the full scientific report on the etiological agent would eventually be published in Infection and Immunity in 1979, ⁷ the authors from the Centers for Disease Control knew that this was an important finding to those in the laboratory animal and comparative medicine field, so they quickly provided a preview to the readership of Laboratory Animal Science by publishing a Brief Report that identified a strain of mouse hepatitis virus (MHV) as the causative agent of the transmissible and lethal enteritis. ⁸ This was the beginning of our understanding of the pathobiology of enterotropic strains of MHV, which would go on to cause significant issues for research mouse colonies in years to come. As time went on and the use of filter tops began to be more common, the enterotropic strains of MHV proved more difficult to exclude than the more classically studied polytropic (also known as respiratory-tropic) strains. ⁹

In the ‘Letters’ section of that same issue of Laboratory Animal Science, Dr. Kraft commented on this news regarding MHV as the etiological agent, and she graciously concurred that the designation ‘LIVIM’ should be retired. Her note is appended below.