3 Barrier Programs
The term barrier (used as either an adjective or noun), like some of the terms for pathogen status, identifies a general concept rather than a definitive qualitative standard. In essence, the concept requires beginning with formerly germfree or defined flora animals and maintaining them as a breeding or experimental population under conditions that exclude pathogens. A barrier is not merely a facility of a certain design. A barrier is a systematic, comprehensive program for the prevention of pathogen contamination. Facility design is one aspect of such a program (Jonas, 1965; Brick et al., 1969; Serrano, 1971). An effective or true barrier cannot exist in the absence of a systematic, comprehensive program.
Barrier programs consist of several essential elements: stocking with animals known to be free of pathogens, appropriate design of housing environments, rigorous management of the physical plant and caging environments, regular monitoring of pathogen status, and corrective action (e.g., elimination of pathogen-contaminated subpopulations) to ensure program effectiveness.
The barrier concept can be applied at different levels: an entire facility, part of a facility, room(s), groups of cages, or a single cage. As indicated above in Option 4, these alternative approaches can be used singly or in combination in accordance with the specific needs and research objectives of individual investigators or entire institutions.
An entire facility or part of a facility can be designed and operated as a barrier. However, this is only practical when a facility or part of a facility is fully committed to a specific objective (e.g., breeding pathogen-free animals) or a community of investigators has agreed to conduct their research under the constraints of a single barrier program.
Barrier facilities are classified on the basis of operational criteria and the degree of security provided (ILAR, 1976a): type I (maximum-security barrier), type II (high-security barrier), type III (moderate-security barrier), and type IV (minimal-security barrier). The criteria for classification include the following (ILAR, 1976a):
- quality, quantity, and source of animals;
- frequency and method of introducing animals and animal-derived biologic materials into the barrier;
- processing of materials into the barrier;
- entry of animal technicians into the barrier;
- entry of investigators and laboratory technicians into the barrier;
- method of housing and handling animals;
- the environmental systems, particularly heating and air-conditioning; and
- monitoring practices.
The effective operation of a barrier facility generally requires that written standard operating procedures be followed to maintain each of the criteria (from the list above) established for that facility. In addition, a written statement of key objectives and principles of operation should be prepared and posted in conspicuous locations accessible to all personnel. It is essential that all personnel know and support the objectives and principles of operation of a barrier facility.
Most animal research facilities are multipurpose, i.e., they must house animals of variable pathogen status, from many sources, and for many different research purposes. The principles of barrier maintenance can be successfully applied to individual rooms or groups of rooms within a multipurpose facility; however, even if appropriate operating procedures are followed, the risk of pathogen contamination is greater than it would be in a properly managed barrier facility. Emphasis is given to protecting the room population(s) from known or potential pathogens in all other rodent populations in the facility. As with barrier facilities, written standard
operating procedures, based on the criteria listed above, should be prepared and followed for each barrier room.
Isolators as Barriers
In addition to their usefulness in the maintenance of germfree or defined flora animals, isolators can be used as absolute barriers for populations of pathogen-free rodents consisting of one to many cages. Isolators made of clear plastic are the most versatile and economical (Trexler and Reynolds, 1957) and can be fabricated in varying dimensions up to room size (Lattuada et al., 1981). They may be used as barriers for breeding or experimental populations. By comparison with most other housing systems, they tend to be expensive and labor intensive. For these reasons, many laboratories use isolators primarily for maintaining small numbers of "seed stock" of their most valuable strains in reserve for use in the eventuality that their regular breeding stocks become contaminated. The isolators are serviced by standard gnotobiotic methods, and the animals are monitored regularly (Newton, 1965; ILAR, 1970). Again, written standard operating procedures are essential.
Airflow Systems as Barriers
A number of different systems have been developed that use airflow to prevent or control airborne infection of laboratory animals. Incoming air is passed through high-efficiency particulate air (HEPA) filters that remove 99.9% of 0.3 um particles. Laminar airflow (LAF) rooms or cabinets move the filtered air vertically or horizontally over the animal cages at an average velocity of 30.5 m/min (100 ft/min). Mass airflow (MAF) involves the flow of the filtered air through orifices in the ceilings of entire rooms and vertical flow of the air at average velocities of only 6.1-10.7 m/min (20-35 ft/min) (McGarrity and Coriell, 1976).
Limited data are available on the effectiveness of these systems. LAF has been demonstrated to be effective in the prevention of cage-to-cage transmission of the intestinal bacterial flora of rodents (van der Waaij and Andreas, 1971) but has been tested only superficially for the prevention of rodent pathogen transmission (Beall et al., 1971; Coriell and McGarrity, 1973). MAF has been shown to be effective in reducing or preventing the transmission of the bacterial flora (McGarrity et al., 1969; McGarrity and Coriell, 1976), reovirus-3 (McGarrity and Coriell, 1973), and polyoma virus (McGarrity et al., 1976). In one study, MAF was found to be ineffective in preventing cage-to-cage and rack-to-rack transmission of indigenous virus infections in rats, particularly sialodacryoadenitis virus/rat coronavirus, Sendai virus, and Kilham rat virus (Thigpen and Ross, 1983).
Based on the currently available evidence, LAF and MAF may serve as useful barrier systems for selected research objectives. Animals of known pathogen status should be used, and written standard operating procedures should be followed. Disadvantages of LAF and MAF are the added costs of the purchase, maintenance, and operation of the equipment.
Filter cage systems provide barriers at the level of the individual cage. Factors that influence cage-to-cage transmission of airborne infection include type of cage used, use of bedding, distance between cages, number of animals per cage, relative humidity, and the generation of airborne dust particles during cage changing and cleaning (McGarrity et al., 1969; van der Veen et al., 1972). Filter cage systems, if properly used, act as barriers by preventing the transmission of contaminated particulates and aerosols between cages.
In order to be effective, filter cage systems must be used according to the following principles. Each cage within a room is fitted with a filter, usually made of fiberglass. The filter is removed from only one cage in the room at any given time, and filter removal is permitted only in a transfer hood. The animals and inner surfaces of each cage are handled only with disinfected forceps or sterile gloves. Changing of cages is accomplished by transferring animals to autoclaved cages supplied with autoclaved bedding, food, and water. After the filter on each cage is replaced, the inner surfaces of the transfer hood are disinfected before the next cage is serviced (Kraft, 1958; Kraft et al., 1964).
Kraft (1958) originally introduced the filter cage system for the control of mouse rotavirus and mouse hepatitis virus infections. Subsequently, this methodology has been tested extensively and found to be effective in preventing transmission of mouse diarrheal diseases (Jennings and Rumpf, 1965; Schneider and Collins, 1966), pinworm infection (Wescott et al., 1976), normal intestinal bacterial flora (Sedlacek and Mason, 1977; Sedlacek et al., 1981), and various pathogens (Simmons et al., 1967). In addition to being highly effective, filter cage systems are relatively inexpensive, simple to use, and readily adapted to a wide range of research needs and objectives. As with other barrier systems, written standard operating procedures for their use should be prepared and followed.