Ventilated animal caging system with cage racks and filter covers including valves operable by rack

Abstract: A ventilated caging system of the type having ducted shelves with lower planar surfaces for box-like animal enclosures, the shelves (20) having duct openings (28) for powered supply and exhaust of air to the animal cages (40), has valved filter covers (50) operable to isolate the animal enclosure from ingress and egress of airborne particles. The filter cover (50) has a frame (56) sealingly engaging a perimeter of the cage (40), the frame (56) carrying a filter membrane (60) with at least one opening. A support web (70) integral with the frame (56) extends inwardly of the perimeter within the frame (56) to support a valve. A movable valve body (74) in the form of a leaf spring is mounted on the support web (70), the valve body (74) including a blocking portion resiliently urged to block said opening in the filter membrane (60), and a contact portion (88) protruding through the opening by an opening displacement of the valve body (74), the blocking portion being displaced by the contact portion (88) when the cage (40) is fitted in the ventilating rack, wehreupon the valve is opened, and the blocking portion being resiliently released to cover the opening when the cage (40) is removed from contact with the ventilating rack, whereupon the valve is closed.

Description: The invention relates to a ventilating cover for an animal cage box according to the generic portion of claim 1. A ventilating cover of this kind is known from US-A-4 343 261 - Thomas. Thomas teaches a high density animal caging system, wherein the animals are housed in cage boxes having the form of box-like enclosures which are airtight on their sides and bottoms and are adapted to fit to a ventilating rack in the form of ducted shelves such that open tops of the cage boxes fit against the ducted shelves in the area of air supply and air exhaust openings. When the cages boxes are fitted into place under the shelves, for example being slid under the shelves along complementary supporting flanges on the cage boxes and shelves, respectively, air flow is established through the cage box. The internal ducts of the shelves are connected to a powered airflow means typically including a HEPA filter or the like for removing particulate matter. Clean air is carried along the shelf ducts to each cage box and used air, which may also contain particulate matter, is carried away via the exhaust ducts. This arrangement accomplishes individual ventilation of the cage boxes, allowing for high density animal housing as appropriate for laboratories and the like. Animal cage boxes require regular maintenance. Used bedding material must be removed, and food and water must be added. If the animals are housed in open topped cage boxes, there is a period of exposure to particulate material from other animals when a cage box is removed from the ducted shelf rack for such maintenance. Particulate matter in the form of airborne dust and dander can carry various diseases, a very serious example being infantile diarrhea in mice, which can cause extensive mortality in breeding colonies. Typically, clean cage boxes and bedding are made available and upon removal of a cage box from the ducted rack the animals are transferred to a clean cage box with required food and water, whereupon the cage box is replaced in the rack. The original bedding is discarded and the original cage box is cleaned. The transfer operation can be conducted in a protected environment, for example in a hooded maintenance enclosure with a laminar air flow pattern to reduce the escape of dander, bedding particles and other potential disease vectors. However, the cage box and its occupants still are exposed at least when being slid into or out of engagement with the ducted rack. According to the aforesaid patent to Thomas, it is possible to include a cover on the cages boxes to prevent this period of exposure. A cage cover having valves which align with the air inlets and outlets is placed over each cage box, the cover fitting between the cage and the rack. The valves are spring biased to close and are opened against the spring bias by contact with the underside of the rack. When the cage box is inserted, the valves open; and when the cage box is removed, the valves close. The particular valves employed in Thomas have cylindrical valve bodies movable in tubular valve housings in a direction perpendicular to the plane of the cage cover for opening and closing the air flow path. The remainder of the cover is airtight plastic, being sealed to the perimeter of the cage box by an appropriate resilient seal means which engages with the cage box along its upper edges. Helical springs urge the valve bodies upwardly into the closed position. In the open position, a piece of filter material within the valve prevents any particulate material from being drawn out of the cage box and into the exhaust ducts of the rack. The valves in the Thomas disclosure are effective, but expensive to provide and to maintain. Moreover, when the valves are closed, there is no possibility of air flow to the animals, presenting a danger of suffocation if the cage box is left too long out of the ducted rack and/or if a valve is not operated properly, for example due to clogging or due to an improper fit between the cage box and the rack. In order to ensure a proper fit, the cage flanges and the complementary shelf flanges must be relatively precisely positioned and dimensioned. The animal maintenance technician must ensure that each cover is properly in place such that the covered cage box will fit into the shelf. Each cage box must be inserted fully to align the valves and their respective ducts. Care must be taken that the valves do not become clogged, because air flow can occur only through the valves. It is the object of the invention, to improve the ventilating cover of Thomas in such a way, that the danger of suffocation if the cage box is left to long out of the ventilating rack and/or if the valve is not operated properly is avoided. This object is solved by a ventilating cover with the features of patent claim 1. Animal housing systems are known wherein the cage boxes are covered by filter material rather than by an airtight cover. Examples are disclosed in US-A-3,343,520 - Schwarz, Jr.; US-A-3,528,277 - Lee et al; US-A-4,480,587 - Sedlacek; and GB-A-2,065,440 - Bernardini. The covered cage boxes are no adapted for use in racks with internal air ducts. In filter covered cage boxes the body heat of the occupants produces thermal currents that cause an exchange of air with the outside. Over time, filter covers can become clogged with particles, reducing the air exchange; however, the surface area available for air exchange is much larger than the surface area available through a valve as in Thomas. US-A-4,593,650 - Lattuada discloses a filter covered cage box with a powered air inlet which is plugged into the cage box, thereby forcing air outwardly through the cage box and ensuring ventilation. However, the plugged connection requires individual attention in aligning the cage box and the air supply plug, and in ensuring an operative connection. Filter covers which are self supporting are typically made of bonded fibers and adhesive. The adhesive tends to occupy spaces between the fibers and to block air flow. The foregoing patents to Sedlacek, Lattuada and Bernardini support or sandwich a sheet of filter material between frame elements, for example perforated plates. The perforated plates also tend to limit air flow. Moreover, such composite structures are unduly thick. The filter covers also must be assembled and used filter sheets must be replaced, making it difficult to simply process a used filter cover via a cleaning process such as autoclaving. The present invention employs a cage cover wherein a filter material of spunbonded polyester, which is quite durable, is bonded directly to the upper surface of a limited frame. The frame has peripheral edges that snap over the top edge of the cage box to ensure a good seal. The vertical thickness of the frame is minimal, allowing the cover to be placed over a cage box fittable in a rack. The area of the cover over the open top of the cage box is substantially defined by filter material, which extends over the full area of the top of the cage box and frame, save an opening for a valve to align with the air supply opening in the rack. Thin webs extend across the cage top from the periphery of the frame of the cover to support the filter material without substantially blocking the air exchange area. A valve structure in the form of a spring leaf is carried in the frame by means of support webs that extend inwardly from the frame edge, the spring leaf blocking and unblocking the opening in the filter material for air supply when the cage box is removed and inserted into the cage rack, respectively. Air is exhausted to the rack exhaust opening directly through an area of the filter material without webs. An upward protrusion which may be attached to the spring leaf or simply placed on the spring leaf, contacts the underside of the rack shelf for uncovering the supply opening. The filter cover as a whole is strong and effective, can be simply autoclaved for cleaning, and protects the animals from contagion while freeing the animal maintenance technician from demanding requirements of cage-cover assembly, cage positioning and the like. The supply valve allows unrestricted air flow into the cage box eliminating the diffusion of air across the filter media as well as the associated pressure drop that would occur through the filter media if the filter material were disposed across the area of the supply duct opening. This allows the micro-environment within the cage box in the rack to remain positively pressurized relative to the room in which the rack is located, even when the exhaust orifice static pressure on the filter media is raised to a level greater than the supply orifice pressure, as long as the differential pressure is less than the pressure drop across the filter media. There are shown in the drawings the embodiments of the invention as presently preferred. It should be understood that the depicted embodiments are exemplary and that the invention is subject to certain variations within the scope of the present disclosure and claims. In the drawings: Fig. 1 is a partial perspective view of an animal caging system; Fig. 2 is an exploded perspective view of a valved filter cover according to the invention; Fig. 3 is a plan view of the valved filter cover according to Fig. 2, from the underside; Fig. 4 is a partial perspective view of the filter cover according to the invention, showing details of the valve structure, from the underside; Fig. 5 is a partial perspective view of the embodiment of the Fig. 4, from the top; Fig. 6 is a section view of an alternative embodiment of the valved cover, taken along line 6-6 in Fig. 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 illustrates an animal caging system according to the invention, in a partial perspective view. A plurality of cage boxes 40 are supported against a ventilating rack 20 such that air supply and air exhaust ducts in the rack lead to and from an area within the open periphery at the top of the each of the cages 40. The cage boxes 40 are airtight boxes having a body 44 including four integral sidewalls and a bottom, for example of molded polycarbonate or the like, and an open top. The top is closed by a cover 50 that snaps onto the cage box 40 at its upper open periphery. The cover has large areas of filter material supported by the cover and thereby allows air exchange but prevents airborne particles from falling into the internal area occupied by the animals, for example laboratory mice, rats, etc. The ventilating rack 20 may be one of a number of vertically stacked shelves which attach to a side plenum with ducts corresponding in position to ends of duct means 22, 24, for providing a powered supply of fresh air to the duct means 22, 24 of the ventilating rack 20. It can be a sheet metal structure having walls folded and attached together via partitions 26 to keep the supply and exhaust sides isolated from one another. In the embodiment shown, a central duct means 24 supplies air to cage boxes 40 on both sides of the rack (only one cage box on one side is shown), and a laterally spaced duct means 22 exhausts air from the cage box on the side shown, another similar duct means being provided on the other side of central duct means 24. Each of the ducts means 22, 24 communicates with the cage boxes 40 by means of openings 28 in the duct wall. The cage box 40 is arranged to be removably mounted to the rack 20 such that at least one opening to supply duct means 24 and at least one opening to exhaust duct means 22 communicates with each cage box 40. In the example of Fig. 1, two supply openings and two exhaust openings communicate through the filter cover with the cage box 40. The cage boxes can be slid under the shelves of the rack to align the top of the airtight body 44 of each cage box to the openings 28 in the duct means. The periphery of the airtight body portion occupies a plane, as does the underside of the rack 20. The cage boxes 40 can be urged against the duct means by flange members 30, rigidly mounted to the rack 20, and having flanges 34 which engage under the laterally protruding flanges 42 of the cage boxes 40. The cage flanges are integral with the bodies 44, and are of substantially the same width as the spacing between the shelf flanges 34 and the underside of the rack. Accordingly, each cage box is urged against the rack 20 when slid onto the flanges 34. The flanges 34 can be the ends of the legs of U-shaped structures 30, which attach to the shelf by fasteners (not shown) passed through the central portions of members 30. The cage boxes 40 are each covered on their otherwise open tops by filter covers 50. Therefore, when removed from the positions under the rack 20, the cage boxes remain protected against airborne particles which may carry pathogens. As so protected, the cage boxes can be transported to a service station and uncovered only therein for servicing the occupants. Preferably, the cage covers have a substantial area of filter material 60, allowing passage of air but blocking passage of particulate material. The preferred filter material is spunbonded polyester, for example as marketed by the Dupont Company, Wilmington, Delaware, under the trademark Reemay. This material is quite durable, and although flexible has structural strength when fastened in tension in a frame. According to the invention, the filter material is bonded to a frame 56 such that the filter material is disposed on the uppermost face of the frame 56 and thus comes directly into contact with the underside of the rack 20. The frame 56 is a plastic resin material and the spunbonded polyester filter material 60 can be heat bonded to the frame 56, whereby the filter material is placed in tension in the event any forces are applied thereto. Inasmuch as the filter material is at least slightly compressible, the filter material functions in part as a resilient seal between the frame 56 and the underside of the rack 20. The frame 56 has a peripheral portion with depending edges 62, enclosing around the outside of the planar open top of the body 40. One or more webs 58, which preferably are integral with the peripheral parts of the frame, bridge across the central area over the open top of the cage box 40. At each area where the filter material is disposed over a portion of the frame 56 or the webs 58, the filter material is heat bonded thereto. As a result, the filter material rends to reinforce the frame, providing a relatively strong self-sustaining structure which can be processed apart from the cage box 40, for example for cleaning and/or autoclaving. The cage and filter cover shown in Fig. 1 are double width versions, as used for breeding cages. The cage box 40 can also be a single width variety as shown in Figs. 2 and 3, being substantially identical to the double width version except adapted for mounting on more closely-spaced shelf flanges 34. In each case the filter cover is thin but substantially stiff arrangement due to the bonded filter and frame, that is placed over the open top of the cage. When the cage is inserted in the rack, the cover remains in place. Fig. 2 illustrates the filter cover in exploded view and Fig. 3 shows the filter cover from the underside. The filter cover is valved in a manner permitting the cage to be sealed automatically when removed from its position under the rack or shelf 20. The frame 56 is dimensioned to sealingly engage the perimeter of the cage box 40, having a minimal thickness in an area overlying the perimeter of the cage, and enclosing over the cage edge. A support web 70 is attached to the frame, the support web 70 being adapted to carry a movable valve body 74 operable to uncover an opening through the filter material. The valve body 74 and/or a contact member 88 operative to move the valve body 74 are placed at a position where the valve body will be opened due to contact between the inserted cage and the rack 20, and allowed to close due to lack of such contact when the cage is removed from the shelf. The support web extends inwardly of the perimeter within the frame, for example bridging between the peripheral frame 56 and one or more of the webs 58 which extend inwardly from the frame 56. The filter membrane 60 is sealed to the frame 56 and preferably also to webs 56, 58. Across the full area of the filter membrane (i.e., at all the areas 78 over the cover between the webs 56, 58, as well as areas bonded to the frame and webs), the filter membrane, frame and webs block passage of airborne particles. The filter membrane has at least one opening 78, adjacent the support web 70, where the valve body movably opens and closes an air passage. This passage communicates with the supply side internal duct means 24 of the rack 20. The movable valve body 74 is mounted on the support web 70, the valve body including a blocking portion resiliently urged to block said opening 78 in the filter membrane 60. A contact portion 88 protruding through the opening by an opening displacement of the valve body, the blocking portion being displaced by the contact portion when the cage box is fitted in the ventilating rack 20, whereupon the valve is opened, and the blocking portion being resiliently released to cover the opening when the cage box is removed from contact with the ventilating rack, whereupon the valve is closed. It is possible to arrange valve structures operable by contact with the rack 20 on both the air inlet and air exhaust sides of the cage box 40. According to the preferred embodiment, only the supply side is valved. The exhaust opening, which is more likely to release airborne dust, dander and the like into the duct system, is preferably permanently protected by areas of filter material. Inasmuch as the air supply side is powered and not subject to blocking or clogging, omitting the valve on the exhaust side enables some escape of air even if the exhaust side area (i.e., the area of the filter cover under the respective exhaust passage into the duct means 22) becomes clogged. When the supply valve opens by contact with the rack 20, it allows unrestricted air flow into the cage box. This eliminates the diffusion of air across the filter media as well as the associated pressure drop that would occur through the filter media if the filter material were disposed across the area of the supply duct opening. On the exhaust side, the filter material does produce a pressure drop. As a result of the unrestricted supply and restricted exhaust relative to the cage, the micro-environment within the cage box 40 in the rack 20 remains positively pressurized relative to the room in which the rack 20 is located. Airborne dust and dander cannot be drawn to the cage to accumulate on the filter cover. The positive pressure differential relative to the room exists even when the exhaust duct is operated at a higher static pressure (vacuum) on the filter media than the unrestricted supply orifice pressure, as long as the differential pressure between the supply and the exhaust is less than the pressure drop across the filter media. Therefore, the exhaust side can be operated at sufficient vacuum to draw odors and the like from the room, while the positive pressure in the cage box prevents it from picking up dust. Preferably, the respective duct means are provided with manometers and the like for sensing and adjustment of the static pressures in the supply and exhaust ducts means to maintain this condition. In the preferred embodiment shown, the frame includes a plurality of webs 58 disposed across the open perimeter edge of the cage box 40. The support web 70 carrying the valve is attached to at least one of the frame and the plurality of webs. As shown, the webs 58 can define a supporting spider passing between opposite areas of the frame. The support web 70 runs from the peripheral frame 56 to one of the central webs of the spider. As shown in Fig. 2, the filter membrane 60 is fully as wide as the outer edges of the frame 56. The filter material is heat bonded or otherwise sealingly attached to the frame elements over the full area of the top of the cage box 40, at least along substantially the entire perimeter of the cage box 40 on the side directed outwardly of the cage box 40 toward the ventilating rack 20. Preferably, the filter membrane 60 is bonded to the the frame 56, the support web 70 and the plurality of webs 58, at each point where these parts overlap. The valve body 74 preferably includes a spring strip, for example a length of stainless steel leaf spring, attached to the support web 70 at a space from the opening 78 passing through the filter membrane 60. The spring strip is normally disposed coplanar with the frame 56. The contact portion 88 is attached to the spring strip or engaged against the spring strip at a space from an attachment of the spring strip to the support web 70. The contact portion 88 protrudes through the opening 78 in the filter membrane 60. When the cage box 40 is inserted under the rack 20, the lower portion thereof contacts the contact portion 88, and forces the valve open as the cage goes under the rack 20. Fig. 4 illustrates the valve mounting from the underside of the cover, in detail. The support web 70 forms a box-like depression 86 for the spring strip and thus laterally encloses the spring strip when the valve is closed. The depression 86 preferably is such that the support web 70 has an extension inwardly of the cage box that is less than the opening displacement of the valve body. The opening displacement of the valve body is due in part to the vertical extension of the contact member 88 that protrudes upwardly from the filter cover and contacts the rack 20. As shown in Fig. 5, which illustrates the valve area of the cage cover from the top, the contact member can be a sloping or tapered body attached to the end of the spring leaf such that insertion of the cage box causes the rack edge to ride over the taper and force the contact member 88 downwardly. Figs. 2-6 show the valve body as a metal spring strip, attached to the support web by means of screws. It will be appreciated that other fasteners and fastening means as well as other materials are possible. For example, a metal spring strip can be riveted to the frame or merely received in a receptacle in the frame. The strip can also be a plastic resin and can be heat welded or adhesively bonded to the frame. The strip as shown is attached to the frame simply via screws. A cover plate (not shown) in the area of the frame attachment screws 92 can be provided as well. Likewise the contact member 88 can be attached by a different type of fastener than the illustrated screw 94, or as shown in Fig. 6, the contact member can open the valve without being rigidly attached to the spring strip at all. In Fig. 6, the area of the filter cover at least directly over the valve body is not covered with filter material (i.e., the valve could be used with a wholly airtight cover as shown). Preferably, however, the areas of the cover remote from the valve are provided with open areas between frame and web elements, which areas are covered by a filter membrane such a spunbonded polyester, bonded to all the upper surfaces of an integral frame and web of plastic resin. A hole 106 in the cover over the valve strip 100 is either opened or closed by action of a ball shaped contact member 102, which is not attached directly to the valve strip 100. In this embodiment, the contact ball 102 is captured between the spring strip 100 and the filter cover by means of a tapering or conical hole 104 which allows the ball 102 to move downwardly but limits the extent of upward movement of the ball 102. When the valve is at rest (i.e., when the cage has been removed from the rack), spring strip 100 forces the ball upwardly in the tapered hole 104, whereupon the valve is closed as shown on the left in Fig. 6, the spring strip 100 blocking the opening 106 in the cover. As the cage is slid under the rack 20, as shown on the right in Fig. 6, the lower wall of the shelf pushes the ball 102 downwardly against the spring strip 100. The ball is preferably disposed closer to the point of attachment of the spring strip 100 than is the opening 106. Therefore, a relatively smaller displacement of the ball 102 results in a relatively larger displacement of the spring strip in the area of the opening 106. As shown in Fig. 6, the displacement can be arranged such that the ball motion causes the distal end of the spring strip to be pushed inwardly of the cage to a point at which the end of the spring strip is disposed inwardly of the sides of the receptacle box 86 formed in the cover or the support web therein. In Fig. 6 an alternative embodiment of the filter material is shown on the right side. According to this embodiment, a limited area of filter material 108 is placed in a counterbore in the upper surface of the cage cover, overlapping the opening 106. As shown on the left side of Fig. 6, the opening can be unobstructed. It is presently preferred that an opening 106 leading to the air supply duct means of rack 20 be unobstructed. If a valved opening is provided to the exhaust duct means, then a supplemental filter pad can be included to prevent airborne dust from entering the exhaust duct and possibly being passed to a cage downstream along the airflow path in the exhaust duct means. According to the embodiment of Fig. 6, the valved cover 50 comprises a frame engageable with the perimeter of the animal cage box, a valve body 100 movably disposed over an opening 106, the valve body including a strip attached to the frame at a space from the opening, the strip at rest residing across the opening 106 to block airflow through the opening, and, a contact member 102 dimensioned to protrude from the valve cover, the contact member being operative when depressed to press the strip back from the opening. The contact member 102 is captive between the strip and a tapering opening 104 in a member attached to the frame, in the form of a ball captured between the strip and a tapering opening in said member attached to the frame. The ball is disposed between the opening and a point of attachment between the strip and the cover.

Claims: A ventilating cover for an animal cage box (40) for insertion into a ventilating rack (20), the cage box (40) having a body (44) with substantially airtight walls around four side walls, and a bottom, the side walls having an upper edge defining an open top of the cage box (40), the cage box (40) being removably fitted into the ventilating rack (20) to position the top of the cage box (40) adjacent an opening (28) to duct means (22, 24) in the ventilating rack (20), the duct means (22, 24) being coupled to a powered airflow means for providing airflow through the cage box (40), wherein the cover comprises (a) a frame (56) having a peripheral portion operable to seal with the side walls around the top of the cage box (40), (b) at least one web (58) extending laterally inwardly from the peripheral portion the cover being dimensioned such that the cage box (40) can be fitted into the ventilating rack (20) with the frame (56) in place on the top of the cage box (40), between the side walls and the duct means (20) to thereby seal the cage box (40) to the ventilating rack (20), (c) an opening (78) through the cover aligned to said opening (28) to the duct means (22, 24); and, (d) a valve including a movable valve body (74) mounted on the frame (56), the valve body (74) including a blocking portion resiliently urged to block said opening (78) through the cover, and a contact portion (88) coupled to the blocking portion, the contact portion (88) protruding through the opening (78) for contact with the ventilating rack (20), the blocking portion being displaced by the contact portion (8) when the cage box (40) is fitted into the ventilating rack (20) whereupon the valve is opened, and the blocking portion being resiliently released to cover the opening (78) when the cage box (40) is removed from contact with the ventilating rack (20), whereupon the valve is closed, characterized in that open areas are provided between the peripheral portion and the at least one web (58) which open areas are remote from the valve body (74), in that the cover further comprises a filter membrane (60) for blocking passage of airborne particles while allowing passage of air, which is sealed to the frame (56) and covers the open areas, whereby ventilation of the cage box (40) occurs substantially only through the opening (78) when the cage box (40) is placed in the ventilating rack (20), and ventilation occurs substantially only through the filter membrane (60) when the cage box (40) is removed from the ventilating rack (20), and in that the filter membrane (60) is bonded to the frame (56) substantially entirely around the peripheral portion of the frame (56) on a side of the frame (56) directed outwardly of the cage box (40) and toward the ventilating rack (20), whereby the filter membrane (60) assists in sealing between the cage box (40) and the ventilating rack (20). The ventilating cover according to claim 1, characterized in that the frame (56) includes a plurality of webs (58, 70) extending laterally inwardly along the top of the cage box (40), said webs supporting the filter membrane (60). The ventilating cover according to claim 1, characterized in that the filter membrane (60) is bonded to the frame (56) adjacent the side walls of the cage box (40) and at the plurality of webs (56, 70). The ventilating cover according to claim 2, wherein the valve includes a resilient strip attached to one of the webs (70) at a space from the opening (78) through the cover, the resilient strip being normally coplanar with the frame (56), the contact portion (88) being attached to the resilient strip at a space from its attachment (92) to said one of the webs (70), the contact portion (88) protruding through the opening (78) in the cover. The ventilating cover according to claim 4, characterized in that the resilient strip is laterally enclosed by said one of the webs (70) to which the resilient strip is attached. The ventilating cover according to claim 5, characterized in that the filter membrane (60) is bonded to the frame (56) and to said one of the webs (70) along all surfaces thereof directed outwardly of the cage box (40). The ventilating cover according to claim 6, characterized in that the frame (56) is plastic resin and the filter membrane (60) is spunbonded polyester resin and is heat bonded to the frame (56). The ventilating cover according to claim 7, characterized in that the frame (56) and the webs (58, 70) are integrally molded plastic resin, at least some of the webs (58) crossing between opposite side walls of the cage box (40), and wherein said one of the webs (70) to which the resilient strip is attached extends from one of the webs (58) that is central to the frame (56). The ventilating cover according to one of the claims 1 to 8, characterized in that the filter membrane (60) extends over the full area of the top of the cage box (40) and frame (56), save the opening (78) for the valve.

Inventors: THOMAS WILLIAM R, THOMAS, WILLIAM R.
Applicants: THOREN CAGING SYSTEMS INC, THOREN CAGING SYSTEMS, INC.
Application year: 1991
Main classification: A01K1
IPC codes: B01D46, A01K1