Patent Publication Number: US-2006011143-A1

Title: Laboratory animal housing with euthanizing function

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to the field of laboratory animal handling, providing an animal housing cage rack system with a euthanizing capability. The animals in all or part of a cage rack of the type that normally houses the animals can be asphyxiated by automatic staged displacement of cage air with CO 2  gas. In this way the animals are euthanized with minimal associated stress and without danger to human operators.  
      2. Prior Art  
      High density animal housing facilities are known, for example, from U.S. Pat. Nos. 5,044,316; 4,690,100; 4,402,280; and 4,343,261, wherein small animals such as mice, rats, rabbits or the like are housed in molded plastic cage boxes that are supported on hollow shelves with integral air supply and air exhaust ducts. The cage boxes are supported on flanges under the shelves. Openings in the underside of the shelves allow the supply and exhaust ventilation air lines to be coupled through the open tops of the cage boxes, or in some embodiments couple through a filter cover or a valved lid that rests between the cage box and the underside of the associated shelf.  
      A lid or sealing filter cover between the cage box and shelf can help to seal the ventilation paths, but there is typically some leakage of ventilation air. It is possible to maintain the air pressure in such a cage rack at a higher or lower pressure than ambient atmospheric pressure. The result is some flow of leakage air, either outwardly from the cages into the surrounding air, or inwardly from the surrounding air into the cages.  
      The primary flow of air through the caging system originates at a preferably filtered supply, passes from the hollow ducted shelves through the cages and is extracted and exhausted or recirculated. Blowers upstream and downstream of the cage boxes can be employed. The pressure of the supply is balanced with the suction of the exhaust so as to maintain the cage internal pressure close to ambient pressure.  
      High density caging systems as described are particularly apt for laboratory experimentation involving a large number of individual animals that are subjected to experimental procedures or exist as control animals for purposes of comparison. In some facilities, quite a large number of animals are housed to support plural ongoing experiments.  
      In day-to-day maintenance of laboratory animals, in addition to ventilating the cages by means of a high density housing arrangement such as a cage rack, it is necessary to clean the cages, to change the bedding material, to provide supplies of food and water, etc. For animals such as rats and mice, this can be accomplished by removing a cage box from the rack onto a work surface, using forceps at the base of the tail to gently grasp and lift each of the animals bodily from the cage box into an adjacent clean cage box having new bedding material and supplies, and when all the animals have been transferred, replacing the cage box in the rack. This is accomplished with some excitement of the animals, but does not seem unduly traumatic. The animals are soon back with their familiar cohabitants in the familiar cage rack. The procedure becomes routine.  
      After an experiment has run its course or otherwise for animals that cannot be maintained, it may be necessary to euthanize the animals. For experimental subjects, for example, euthanasia may be a prerequisite to physical analyses associated with assaying experimental results. Insofar as animals may have participated in an experiment even as controls, euthanasia and physical analysis may be required for comparison with other subjects. Animals that may have lived through a given experiment are often unsuitable as subjects in a later experiment due to potential influence from the former experiment. Continued maintenance of such animals cannot be justified. In these circumstances and other similar circumstances, it may be necessary to euthanize the animals.  
      A common euthanasia technique, particularly for laboratory mice, is CO 2  gas overdose. However, the technique may be stressful to the animals. It is desirable that the animals not be traumatized. The CO2 gas technique also can be labor intensive, particularly where a large number of animals are to be euthanized.  
      U.S. Pat. No. 4,941,431—Anderson discusses prior art euthanizing techniques including asphyxiation using CO 2  gas. According to the patent, it has been known for this purpose to place a quantity of dry ice (frozen CO 2 ) into an animal cage of the type comprising an air impervious cage box. CO 2  gas that sublimes from the dry ice is more dense than air. The gas accumulates in the bottom of the cage, displacing oxygen and eventually immersing and asphyxiating the animals. This procedure requires processing of the cage boxes one at a time. Presumably the cage boxes are covered so that air currents do not diffuse the CO 2  gas from the cage box.  
      Dry ice is very cold and its use as a supply of CO 2  gas is considered likely to traumatize the animals. Use of dry ice for euthanasia is generally considered an unacceptable practice. Anderson uses a CO 2  gas supply from a gas cylinder, associated with a gas valve and timer that the operator adjusts and monitors. In Anderson, the pressurized gas is discharged into the cage box through a fitting in a cage box cover. It would appear that releasing gas pressure in this way would also substantially reduce the temperature of the gas, but at least there is no block of concentrated very cold CO 2  gas dry ice.  
      The Anderson technique is more humane than some other common euthanizing techniques, such as guillotining the animals one by one. Nevertheless, there is stress for the animals and work for the technician. Anderson teaches enclosing the animals in a box and airtight lid for application of the timed asphyxiation process. Although the animals might be processed in their individual home cage boxes, there is stress in removing the cage boxes from their normal location, sealing the gas fitting lid to each individual cage box in turn and proceeding with the timed process. Processing one box at a time is time consuming and inefficient for the operator, who is inclined to combine unfamiliar animals into one cage box for processing. Combining unfamiliar animals in a cage is stressful for the animals. One might extend Anderson to combining a number of cage boxes in some sort of sealed vault for application of the process, which would also be stressful for the animals and inefficient for the operator.  
      In an American Association of Laboratory Animal Science (AALAS) Abstract dated Feb. 26, 2002, entitled “Implementation of a ventilated cage rack for efficient, humane euthanasia of mice,” a technique is disclosed wherein a copper manifold pipe is attached to the air supply plenum at the end of a ventilating cage rack, and has nozzles of progressively different sizes arranged to emit CO 2  gas at different points along a flow path. According to the description, a cage rack that is partly or completely loaded with animal cages first is disconnected from the ventilation air supply arrangements. The air supply plenum at the end of the rack is opened and the manifold is clamped into place inside the plenum, so as to direct the nozzles toward the animal cages. The manifold is then coupled to a CO 2  gas supply that delivers gas at 30 psi, for a sufficient time to asphyxiate the mice. Afterwards, the clamped-in manifold is detached and removed. The rack is reconnected to the ventilation air supply and operated to expel residual CO 2  gas.  
      The AALAS Abstract solution has certain advantages in that the animals can be asphyxiated in their “home” rack, presumably without stress. However, this advantage is achieved with substantial inconvenience for the operators, who are to disassemble the air supply and air plenum structures for each cage rack to be processed, modify the cage rack by installing a CO 2  gas emission apparatus, operate the CO 2  gas supply with sufficient pressure, flow and timing controlled manually, and finally to remove these arrangements afterwards. It would be advantageous to provide an automated apparatus whereby animals can be euthanized without unnecessary stress, in their usual cage boxes and cage racks together with the same animals as usual, without moving the animals either individually or by transporting animal cages to a euthanasia facility, and without disturbing equipment installation steps conducted while the animals are decoupled from their ventilation air supplies. It would be most inefficient to build all cage ventilation apparatus with a euthanasia capability, as well as expensive and prone to accidents, simply to avoid the need to stress the animals by relocating them if and when euthanasia became necessary. What is needed is a way to resolve these issues in a way that is optimally efficient and yet empathetic to the animals.  
     SUMMARY OF THE INVENTION  
      It is an object of the invention to provide at least a part of a high density animal cage rack, of the type having ventilation air aspects whereby air is normally passed through cage boxes, with the capability to euthanize occupants by displacement of ventilation air with a gas. A standard cage rack is coupled by its standard ventilation couplings to an automatically controlled gas supply apparatus operable to substitute euthanizing gas for ventilation air in a series of controlled operations.  
      It is an object in a euthanizing apparatus employing CO 2  gas for asphyxiation using standard ventilating cage racks, to effect a programmed cycle of steps in which controls require certain sensed operational conditions to commence or continue a euthanasia process, wherein the gas can be applied in a sequence of operations, optionally with selectable time and gas concentration conditions that are maintained automatically when commenced by a user selection and start control. The device automatically obtains an initial anesthetizing concentration, proceeds to time a lethal soak concentration wherein the flow of air or gas is stopped, and finally vents the cage rack by resuming ventilation air flow, all automatically.  
      It is a further object to ensure that the operation of the euthanizing apparatus is both effective with respect to the subjects, is reliable and repeatable, and is safe for the operators of the apparatus.  
      These and other objects are met in a high density animal housing system with a ventilation system having an air source conduit and an air exhaust conduit that normally supply air for respiration of the animals. The system is coupled to a controller that activates and deactivates associated blowers and valves so as to substitute CO 2  gas for ventilation air. In a programmed sequence the system first anesthetizes and then euthanizes the animals via that same flow conduits that otherwise supply respiration air. The sequence progresses through insulation of the cages with gas, substituting the gas for inlet air over a timed interval while an exhaust blower continues to run. In a soak interval, the exhaust flow is discontinued. In a final venting phase the gas is flushed to the exhaust. Operator inputs can select among sequences. Status sensing inputs prevent initiation or continuation of a cycle in the event of certain faults.  
      The animal cages preferably are air impermeable boxes suspended from hollow shelves that define internal conduits for supply and exhaust of air, coupled to the supply and exhaust conduits. When initiating a euthanizing cycle, the supply air is discontinued and a gas, preferably CO 2  is supplied while continuing to operate the exhaust system for a defined period of time. This insulates the system, i.e., increases the concentration of CO 2  gas in the cages, first anesthetizing the animals and eventually displacing oxygen to a lethal concentration of gas. After a user defined timed interval, the CO 2  gas supply preferably is shut off, and the exhaust system is stopped for a further timed “soak” interval. The cessation of ventilation currents permits the CO 2  gas, which is heavier than air, to settle without turbulence in the cages, displacing any oxygen and further increasing the CO 2  gas concentration while removing oxygen, in the areas occupied by the animals. After a soak interval timed so that all the affected animals have expired, the inlet is recoupled to ventilation air system, the CO 2  gas supply remains shut off and the exhaust is operated to permit the cages to be vented. The cages and animals can be removed safely.  
      The invention provides a device that substitutes CO 2  gas for the cage ventilation air in a cage rack, by coupling a low pressure CO 2  gas supply to the ventilation paths of a standard and unmodified cage rack. The user has the option to select from among a number of pre-programmed cycles or sequences of operation that are controllably executed by a programmed ventilation supply facility that is coupleable to the standard ventilation fittings of a standard cage rack. It is an easy matter, particularly with cage racks having caster wheel chassis, to remove and exchange a rack coupled to the euthanisation controlled ventilation supply facility, with a different standard rack that is wheeled into place. The ventilation/euthanizing hardware remains in place for use with any selected rack and the animals and animal cages need not be disturbed other than to wheel the rack to the facility and to couple the ventilation ports of the rack to those of the euthanizing hardware.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      There are shown in the drawings examples of certain embodiments of the invention. It should be understood that the invention is not limited to the examples shown in the drawings but is capable of other embodiments in accordance with the scope of the invention claimed. Like reference numerals denote like features throughout the specification and drawings. In the drawings,  
       FIG. 1  is a schematic illustration of a system according to the present invention, having a ventilated cage rack coupled to a controller operable to substitute CO 2  gas for ventilation air in all or part of a cage rack and thereby euthanize animals housed therein.  
       FIG. 2  is a perspective illustration of high density animal housing system to which the invention is advantageously applied, having internally ducted cage racks supporting animal cages and providing ventilation.  
       FIG. 3  is a partial cutaway view showing the ventilation and exhaust conduits and illustrating the phases of ventilation, insufflation and CO 2  gas soaking that are established by the controller for timed periods.  
       FIG. 4  is a perspective view showing an embodiment wherein a fixture containing the gas valves and controlled blowers used for the euthanasia procedure are detachably affixed atop a cage rack in place of ventilating connections.  
       FIG. 5  is a detail view of the fixture shown in  FIG. 4  and including the bracket for supporting the fixture when not deployed.  
       FIG. 6  is an elevation view showing certain valve and sensing arrangements according to a preferred embodiment. 
    
    
     DETAILED DESCRIPTION  
      Carbon dioxide (CO 2 ) overdose is a known euthanasia technique and has been employed for mice as discussed in the background information above. Known techniques, for example as in U.S. Pat. No. 4,941,431—Anderson, process animals in air impervious cage boxes under a sealing cover containing gas fittings by which oxygen-displacing gas is injected into the cage box. Such a process of treating animals in units of cage boxes require that animals be moved into a cage box, or even if not move, that that cage boxes be removed individually from their normal environment as part of the process.  
      It may be stressful for animals to be displaced from their normal conditions. If moved into the same cage box the animals may become separated from their familiar cohabitant animals and mingled with strange animals. Moving the animals and/or processing cage boxes individually can be labor intensive and time consuming. The associated stress can excite the animals and complicate the procedure.  
      A more compassionate technique proposed in the above-cited MLAS Abstract entitled “Implementation of a ventilated cage rack for efficient, humane euthanasia of mice,” proposes processing the animals in their familiar environs but requires that fixtures be installed in a rack when the animals are to be euthanized. The rack is not a standard rack and instead needs to be modified for euthanasia and then un-modified to be returned to service. According to an aspect of the present invention, however, a gas application apparatus is provided that can be coupled to a standard rack in lieu of direct connections to the air supply and exhaust conduits. The invention is applicable to the racks that normally are used to house the animals, especially of the type having internally ducted shelves to which cage boxes are coupled for supply and exhaust of ventilation air. The operational elements and the controls are associated with the gas application apparatus.  
      Additionally, the invention provides certain automatic sensing and control arrangements that prevent a euthanasia cycle from starting or continuing in the absence of certain minimum requirements, that time and sequence operations without user control other than to select and trigger the start of a cycle, and that provides follow-up venting procedures to protect humans in the area from potentially problematic concentration of CO 2  gas.  
       FIG. 1  illustrates the invention in block diagram form. A cage rack  22 , shown schematically, carries a number of individual animal cages  24 . The rack has internal ducting as explained hereinafter, to supply ventilation air for respiration and to pass spent air through to an exhaust. The ventilation air flow aspects are illustrated in  FIG. 1  by a supply blower  32  and exhaust blower  34 ; however it should be appreciated that other types of relatively positively pressurized air source facilities and relatively negatively pressurized exhaust facilities can be used to produce a current of ventilation air for respiration.  
      According to an inventive aspect, a source  42  of CO 2  gas, shown schematically as a pressurized gas cylinder is coupled by a valve  44  to the air supply and is activated by a controller  50  to supply CO 2  gas to the cage rack in lieu of ventilation air for respiration. The CO 2  gas thereby displaces oxygen necessary for respiration and euthanizes the animals. Preferably the CO 2  gas is supplied in at least two phases and then flushed out, by switching from ventilation of the individual cages with room air (or a similar source containing oxygen) to CO 2  gas. The sequencing of the phases is fully automated.  
      Controller  50  can be a programmable logic controller (PLC), available from various suppliers including Allen Bradley, TI, GE, etc. The programmable controller operates to advance through states in which the outputs are varied as a function of inputs from switches and sensors, and the passage of time as determined by an internal clock. The inputs and outputs can involve switch closures, digital or analog signals and the like.  
      The controller  50  advances through phases including (1) a CO 2  gas insufflation phase during which the concentration of CO 2  gas is increased from ambient levels, progressively displacing oxygen; (2) a CO 2  gas exposure or soak phase in which the animals are exposed to the gas in a lethal concentration; and (3) a purge phase wherein the CO 2  gas is removed from the ventilated rack.  
      A shown in  FIG. 2 , in one embodiment the ventilated rack  24  is one of a number of racks used for the high density housing of laboratory animals. Each rack is normally coupled to ventilation facilities  60 , for example including a supply conduit  62  and an exhaust conduit  64 . These may be coupled to the rack through local blowers or directly, and may have associated valves and dampers (not shown) for balancing the supply pressure and exhaust suction.  
      As shown cutaway in  FIG. 3 , each cage box  24  fits into a space in the rack  22 , for example with flanges along the top edges of an air impervious box engaging flanges  71  for holding the boxes in position under openings  73  in shelves  74 . The openings  73  lead into internal ducts for supply and exhaust that extend adjacent to one another along the length of the hollow shelves  74 . The supply and exhaust ducts in the shelves are in communication with similar ducts in hollow end walls  76 , and the end walls have openings or couplings  78  where coupled to powered supply and exhaust facilities. It is also possible to have the normal ventilation air supply from the ambient room air into the system, or to vent from the system into the ambient, but connections for remote supply and exhaust venting are preferred, as are powered local blowers and high performance particle filtering such as HEPA filters.  
       FIG. 3  illustrates an exemplary succession from the normal ventilation state  81  to the insufflation phase  83 , soak phase  85  and resumed ventilation state  87 .  
      The invention can be applied to an animal housing system as an operating subsystem. Preferably, however, the euthanasia facilities are provided so as to coupled to a rack  22  when needed. Accordingly, a cage rack  24  as in  FIG. 2  can be detached from its connections to conduits  62 ,  64 , and wheeled into position for engagement with a detachable subsystem as shown in  FIGS. 4 and 5 . The respective elements of the system as shown include the ventilation system having at least an air source conduit  62  and air exhaust conduit  64  sufficient for bringing supply air for respiration of animals and removing exhaust air. In an example as shown in  FIG. 1 , the cage rack or its detachable ventilation subsystem include one or more blowers  32 ,  34 , preferably one for supply and one for exhaust.  
      The cage rack  24  supports a plurality of animal cages  22 . The cages each comprise a box shaped enclosure with air impermeable material at least partly surrounding a housing area for the animals. The support rack and animal cages are configured such that when the cages  22  are placed in said support rack  24 , the cages are coupled between the air source and air exhaust conduits  62 ,  64 . In the embodiment shown, the cages are disposed in an ventilation air path that passes through hollow ducting in the cage rack, including the shelves, into the cages from supply opening in the shelves, and back from the cages into openings leading into the exhaust ducts in the shelves and in due course to the exhaust conduit  64 . This route of air passing through the ventilation system is the same route whereby the cage rack  22  normally supplies occupants of the animal cages with air for respiration.  
      A supply of gas is coupled to the rack  22  in association with the air source flow path from the air inlet conduit  62 . In the example of  FIG. 1 , the gas supply  42  is coupled through a controllable valve  44 , which in turn is electrically controlled from the programmable controller  50 . Controller  50  also controls the operation of the inlet and/or outlet blowers  32 ,  34  and any flow regulating valves that are operable to stop, wherein the controllable valve is operable to displace at least part of said air for respiration, for at least one of anesthetizing and asphyxiating said animals while disposed in said cages in said support rack.  
      In the preferred embodiment discussed, the composition used as the means for euthanizing the animals is CO 2  gas and the gas is introduced in stages. At first, the gas is introduced at a concentration effective to anesthetize the animals, i.e., to render the animals sleepy and then unconscious. The next stage is displace air from the cages to the extent that the concentration of CO 2  gas is lethal, and to hold the concentration for a sufficient time to asphyxiate the animals. The stages are timed by operation of the programmable controller  50 .  
      The CO 2  gas is confined because the cages comprise impermeable boxes suspended from the hollow shelves with internal ducts or conduits that lead to the supply and exhaust conduits and blowers, but in a manner that is controllable by controller  50 .  
      Although the exemplary supply of gas comprises pressurized CO 2  gas, other gases are not excluded, to be used instead of or in addition to the CO 2  gas. Other oxygen displacing gases such as nitrogen are possible. In order to further reduce stress in the animals nitrous oxide can be injected prior to the oxygen displacing gas. Nevertheless, CO 2  gas is advantageous because it is more dense than air. When the CO 2  gas is injected and air turbulence is stopped by decoupling the inlet and outlet flows, the CO 2  gas settles in the bottoms of the cage boxes to provide locally high concentrations of CO 2  gas and low concentrations of oxygen.  
      It is possible to commence and stop air flow in the embodiment shown in Gif.  1  simply by turning the blowers on or off. Additionally or instead of relying on stopped blowers to block gas flow, one or more electrically operated gate valves can be coupled to the controller at the inlet and outlet conduits  62 ,  64  for controllably coupling one of the respiration air and the supply of gas to the air source conduit. Likewise, check valves associated with the exhaust arrangements can ensure that in the venting phase the CO 2  gas is not released into the ambient air and instead is carried away. In the embodiment discussed below, additional safety elements for ensuring exhaust of the CO 2  gas include a thimble connection with the downstream exhaust conduits, and an optional supplementary exhaust inlet to remove air at a low elevation in the room, etc.  
      The controller is coupled at least to the controllable valve operable to inject the CO 2  gas, and also controls the blowers  32 ,  34  and/or associated valves to selectively activate and deactivate the air (or gas) flow into the ventilation pathways leading to the animal cages, as well as the exhaust flow from the cages to a downstream point of discharge. The controller is programmed and coupled to operate the blowers and/or valves in a series of successive operations, shown schematically in  FIG. 3 . For this purpose, the controller comprises a timer  110 , for stepping through the series of successive operations in a timed sequence. The sequence can be more or less variable and programmed to effect two or more different cycles, selected by operation of switch inputs from user inputs  110 , and/or as a function of sensed conditions of gas pressure, flow and the like via sense inputs  112 .  
      In the embodiment shown in  FIG. 3 , the blowers  32 ,  34  are activated and deactivated by the controller, in order to start and stop flow. In this respect, stopping flow (halting or decoupling the respective blower) is equivalent to operating a gate valve to close off the associated air/gas passageway. The controller effects a coordinated activation and deactivation of flows in a sequence. The sequence comprises replacing ventilation air as the source of flow with the CO 2  or other operative gas for a sufficient time to render lethal the atmosphere in the animal cages. More particularly, starting from the state  81  of normal ventilation, the preferred sequence comprises timed periods of insufflation  83  wherein the concentration of CO 2  gas is increased, soak  85  wherein a lethal concentration is maintained for an effective period of time, and venting  87 , wherein ventilation flow is resumed for a sufficient time to clear the CO 2  gas to the extent that it is safe to remove the cages  24  from the rack  22 , and ensure that the process was effective, finally exposing the ambient atmosphere to the contents of the cages.  
      Advantageously, the time periods chosen are planned for initially anesthetizing the animals, i.e., rendering the animals unconscious before the concentration of CO 2  is such that the atmosphere is not breathable. This eliminates or substantially reduces stress on the animals.  
       FIGS. 4 and 5  illustrate a practical embodiment of the invention. In this arrangement, the supply and exhaust blowers  32 ,  34  are normally kept suspended in a fixture that is removably coupled by flexible ducts  120 ,  122  to the cage rack  22 , preferably to a hollow end wall  76  coupled to internally ducted shelves as also shown in  FIG. 3 . The arrangement can lowered from a hook and bracket arrangement  125  mounted on a building wall such that a rack  22  can be wheeled into position at which the fixture is conveniently deployed, e.g., at least the connecting portions and optionally also the blowers  32 ,  34 , are lowered onto the cage rack. With power coupled to the blowers  32 ,  34  through the controller  50 , normal ventilation ensues.  
      In the normal ventilation state, the animals move about in the cage box as they desire. The area under the incoming air orifice in the shelf duct is a preferred gathering place, apparently due to the ventilation air currents. A gas cycle begins by a user initiating a “start” input to controller  50  (e.g., selecting an operation and activating a key-operated switch). There is a brief interruption of the air stream as the air supply blower  32  is shut off. Preferably, the supply duct  62  is decoupled by closing a gate valve  127 . Such a gate valve can be located upstream or downstream of the supply blower  32  and in the embodiment shown is between the supply blower and the ducting in the cage rack  22 .  
      After the brief interruption, controller opens the CO 2  gas supply valve to insert CO 2  gas at a point downstream of the gate valve and leading into the air supply ducting in rack  22 . The air stream into the individual cages resumes through the inlets  73 , but now the incoming stream is CO 2  gas rather then air for respiration. Meanwhile, the exhaust blower  34  continues to operate.  
      The CO 2  gas stream mixes with the air in the cage boxes  24 . Over a period of time of suffusing the cage boxes with CO 2  gas, more and more oxygen is displaced from the cage boxes. The concentration of CO 2  in the cages increases and the concentration of oxygen decreases. As the concentration of CO 2  gas increases, the gas acts as a general anesthetic, eventually putting the animals into a deep sleep. This phase is continued for a time sufficient to anesthetize the animals and to substantially replace the air in the cage boxes with CO 2  gas. Over a period of about two minutes, the cage box is progressively charged with CO 2  gas. There is some turbulence due to continuing flow. However, CO 2  gas is heavier than air, tending to accumulate in the bottoms of the cages and to float any remaining oxygen carrying air toward the tops of the cage boxes. The cage boxes are mounted underneath the ducted shelves. Thus the floating oxygen carrying air is extracted into the exhaust ducts as the CO 2  gas settles.  
      The foregoing insufflation phase is maintained for a sufficient time to replace the air in the cages with CO 2  gas. The period can be longer or shorter depending on the rate of gas supply and through current. Two minutes is an example. The controller  50  then steps to the next stage of operation, which is a soak or dwell phase during which the cage boxes are kept charged with lethal levels of CO 2  gas. In this soak phase, the controller  50  turns off the exhaust blower. The controller  50  can close the CO 2  gas supply valve  44  at the same time, leaving the cage boxes in a stagnant air condition with a high concentration of CO 2  gas. It is also possible to permit the CO 2  gas supply to remain open or to remain open for a time. The soak phase is timed to last, for example for 15 minutes, during which the dense CO 2  gas settles to the cage bottoms and the animals are completely euthanized by asphyxiation. Generally, the asphyxiation is complete in a shorter time, but it is advantageous to extend the soak phase for a more than sufficient time so as to euthanize the unconscious animals with a very high degree of effectiveness.  
      After the timed CO 2  gas soak interval, the controller  50  enters a purge or venting phase. The controller opens the air supply gate valve  127  and couples power to the exhaust blower  34 . At the same time, or after a delay, the air supply blower  32  is powered as well. Operating the exhaust blower  34  before the supply blower for a time produces a negative pressure that tends to draw room air into the exhaust path, as well as air from the supply conduit  62 , producing little or no leakage of CO 2  gas into the room. The CO 2  gas is discharged from the cages into the exhaust ducts. Preferably, the supply blower  32  is used for this venting phase, at least after a delay, so that the CO 2  gas is dependably flushed from the cage boxes into the exhaust. A timed interval of five minutes is generally sufficient to clear the CO 2  gas from the cage boxes. After the timed purge phase, the controller can operate a warning light or buzzer to indicate that the cycle has been completed. At that point, the cage boxes can safely be removed for processing of the euthanized animals.  
      In order to apply the present method and apparatus for euthanizing infant mice pups, relatively long soak periods are needed, optionally with periodic addition of CO 2  gas to keep up the necessary concentration. For natural reasons, infant mice are able to survive a relatively longer period of oxygen deprivation than adults. It is possible to provide user selection inputs via inputs  110  to select different cycles or to select specific time intervals for the respective operational phases. In any case, the controller is programmed to effect an insulation phase wherein the supply of gas is substituted said air for respiration during a first time period that generally accomplishes general anesthesia, and a timed for distribution of the gas in a soak phase that generally asphyxiates the animal subjects.  
      In the embodiment of  FIGS. 4 and 5 , the cage support rack is detachable from an existing set of air supply and air exhaust conduits  62 ,  64  and coupleable to a set of air supply and air exhaust conduits of a fixture in communication with the supply of gas and comprising the controllable gas valve. After completion of a euthanasia cycle as described, the cage rack is ready to resume use as an animal housing system as shown in  FIG. 2 .  
      A practical embodiment of the invention is shown in  FIGS. 4-6 . The controller  50  is generally contained in a wall mounted cabinet having indicators for gas pressure, indicator lights for status indications and switch controls for operation. The face of the controller cabinet defines a control panel and indicates on/off status, the selected operation and the present phase in the operation. The controller includes indications of when the process is operated and when the process has been completed, for example a green light to show that the cages can safely be removed, e.g., for disposal of animal carcasses.  
      In a preferred arrangement, a switch key is used for activation. Upon commencement of a cycle, the PLC  50  turns off the supply blower  32  which has been providing cages with respiration air, such as HEPA-filtered room air; activates closure of the gate damper valve  127 , thereby isolating the supply blower conduit so as to prevent backflow of gas along the supply conduit. A solenoid valve is opened, allowing CO 2  to enter the supply plenum  76 , flowing from there to the shelf ducts and into the cages  24 . The solenoid valve can be mounted in series with a high flow pressure regulator  132 , a manual shutoff valve  134 , and a quick connect fitting for coupling with a pressurized gas cylinder  42  (shown in  FIG. 1 ).  
      The exhaust blower  34  preferably remains on for almost the entire insulation phase and aids gas distribution into the cages  24 . At the end of this phase, the PLC turns off the exhaust blower  34 . Preferably at this point, especially if the various connections along the ventilation flow path are snugly sealed, the solenoid valve  44  controlling CO 2  gas inflow is closed. It is also possible to continue the CO 2  gas inflow pressure, although the exhaust blower is off, but it should be appreciated that leakage gas could be released into the room. To deal with such leakage, a floor inlet  142  can be provided in the general area to extract air near the floor level containing CO 2  gas. Normally with no flow being driven by the blowers  32 ,  34 , the CO 2  gas in the cages settles to the lowest elevation and remains at a lethal concentration.  
      The animals in the cages  24  are exposed to CO 2  gas during the insufflation phase, and to lethal concentrations of CO 2  gas in the subsequent soak phase, which is held for a sufficient time to ensure euthanasia, e.g., 15 minutes.  
      At the end of the timed soak phase, the gate damper  127  is opened. Both the exhaust and supply blowers  34 ,  32  are activated. Room air is drawn in through the filters associated with the inlet conduit  62 , passed through the respective ducts and cages to the exhaust conduit  64 , and purges the cages of CO 2  gas.  
      The system of the invention can be ducted to an outdoor discharge, or can be simply ducted to the building HVAC system exhaust line, provided the capacity is such that the volume of CO 2  gas is appropriately diluted and/or safely routed. In the vicinity of the euthanasia system, where a concentration of CO 2  gas could present problems, the exhaust is preferably passed, through a thimble connection to prevent local CO 2  gas release into the room air.  
      The controller  50  preferably can be set to different timed cycles depending on the operator&#39;s selection, e.g., programmed to selectively execute up to six different cycles. The duration of each cycle can be varied. The valves and the blowers can be selectively opened/closed or turned on/off during these cycles. The valves also can be arranged to open by incremental or proportional amounts, and the blowers can be operated at less than full power or at selected rates in a range of available variable speeds, in phases of operation maintained by outputs from controller  50 .  
      A two minute insufflation period, and a 15 minute soak cycle were found to be effective for euthanizing mice greater than seven days of age. Neonates were found to require excessive soak time and/or substantial use of CO 2  gas.  
      According to the invention, the mice or other animals need not be removed from their home cage to another cage. No additional animals typically are added to the cage prior to euthanasia. There is little unusual activity. As a result of these factors, animal stress is substantially eliminated and personnel labor is reduced compared to other techniques. As described above, the system incorporates a number of features to ensure compassionate animal treatment and personnel safety and convenience.  
      Among the safety and operational aspects, the CO 2  gas application and control apparatus has a standard exhaust blower box with a keyed plug and a modified supply blower box. Neither blower box is standard to any rack, making it unlikely that the CO 2  gas system can be inadvertently engaged to a cage rack by an inexperienced worker.  
      Modifications to the supply blower and conduit include the added supply gate valve  127  and the high flow pressure regulator valve  136  that has been specially configured and sized to dispense CO 2  gas at a flow rate and pressure that approximates the conditions at which the normal supply blower would provide respiration air if activated.  
      The safety features built in to prevent dangerous operating conditions include two pressure sensing switches (one at the supply and one at the exhaust) to monitor the integrity of the supply and exhaust connections during the respiration air ventilation state. Such pressure monitoring is known in caging systems and can generate an alarm condition if pressure and suction levels suggest a blockage or leak. However, according to the invention, an alarm state in these pressure and suction levels is sensed by controller  50 . The controller is programmed so as not to allow a CO 2  gas cycle to commence if a connection problem or anomaly is sensed.  
      Similarly, if a problem in the availability of CO 2  gas through the fixture is sensed, the controller can refrain from commencing a cycle. Thus if the available gas supply pressure is inadequate, if a connection line is decoupled or leaking, etc., the cycle is not started and a fault light or alarm can so indicate. As another fail safe, the position of the gate valve  127  can be sensed using a limit switch contact closure coupled to controller  50 . If the gate valve  127  is not sensed to be closed, the controller can refrain from opening control valve  132  for release of CO 2  gas, thereby preventing backflow of CO 2  gas on the supply side.  
      The CO 2  gas valve is spring biased to close. In the event of a power failure, the CO 2  gas valve closes immediately. When power resumes, the controller assumes a default animal-maintenance starting state. An alarm light can be activated to indicate this occurrence as another error condition. For humane reasons, it is desirable in the event of a commence euthanasia cycle, to complete the cycle and not to risk the possibility of injuring but not euthanizing the animals. Also for this reason, the available CO 2  gas volume can be sensed, e.g., by available pressure or by tank weight, and no cycle commenced if the pressure or volume appear to be marginal for completing a cycle until the CO 2  tank is changed.  
      The PLC monitors the safety features as inputs, together with user selections and switches. The PLC outputs closures for application of power to the valves and blowers or to motor starter relays or solid state switches, etc. The PLC sequence is selected in part from programming and in part from user selection switches, and an internal clock is used for timing the sequencing from one selected stage or phase to the next.  
      The PLC program can be arranged to be downloaded remotely over a phone modem (not shown). The program can be contained in nonvolatile memory such as a programmed ROM. The control of the euthanasia system can be one of a number of control aspects associated with controls in an animal housing facility, e.g., as subroutines in a larger control scheme run on a PC or the like.  
      In the practical embodiment of  FIG. 6 , the initial setup procedure includes adjusting the CO 2  gas pressure. First the manual gas valve on the tank is opened. The CO 2  gas pressure from the tank is set to a nominal level, e.g., 8 PSI, that is higher than the regulated operational pressure that will be used. The controller  50  has a selector switch for selecting the program or sequence to be operated, and for setup the selector is switched, e.g., to Program #1. The operator turns the “start” key switch, opening gas valve  44 . At this point, the high flow rate regulator valve  132  is manually adjusted, preferably quickly, to get an 0.20 to 0.30″ H 2 O column pressure (preferably 0.20-0.25″) on the magnehelic pressure meter of the controller  50 . The gas system is then set up. The operator presses “stop” and the euthanasia system is ready to be coupled to a cage rack.  
      A standard unmodified cage rack is wheeled into position under the euthanasia transition hookups, normally suspended using wall bracket arrangement  125 . The transition is dropped from the bracket and clamped in place on the rack with quick connect clamps that correspond to the standard ventilation hookups on the racks. The supply and exhaust blowers are powered and operate to ventilate the cage boxes in the rack. The operator should verify that supply air pressure is close to nominal (e.g., 0.2-0.25″ H 2 O) and exhaust pressure is close to nominal (e.g., 0.15-0.20″ H 2 O). If not, the respective connections should be checked and necessary damper adjustments made. The controller  50  will decline to initiate operation for the euthanasia cycle if sensed pressures are not near nominal.  
      It is possible to inject CO 2  at a lower pressure during a euthanasia cycle (for a slower gas feed) than in the ventilation air “maintain” state. For this purpose, a variable speed exhaust blower and/or damper arrangement can be substituted for a simple on/off exhaust blower arrangement. In that case, the controller operates the exhaust blower at a higher level in the maintain state and at a lower level at the beginning of the euthanasia cycle, so as to match the lower CO 2  injection pressure.  
      It is an aspect of the invention that the CO 2  gas is handled at pressure differentials that are comparable to the pressure differentials in the ventilation air “maintain” state. Thus the high flow rate—low pressure regulator valve  136  provides CO 2  gas in lieu of oxygen carrying air during the euthanasia cycle, but at very similar flow conditions. This is an improvement over injecting CO 2  gas simply by opening flow to orifices from a pressurized gas supply into the ventilation air supply or into the individual shelf ducts. Releasing gas at orifices near the cages can cause rapid substantial cooling (due to the gas pressure drop), noise and other adverse effects.  
      There are potential different gas application requirements for different animals. As mentioned above, the CO 2  gas euthanasia technique may not be suitable for infant mice, due to the long CO 2  gas soak time that might be necessary for complete effectiveness. On the other hand, certain genetically altered or weakened animals, being more sensitive, may advantageously be subjected to a slower gas feed to further minimize stress.  
      In the maintain state, the rack can remain in place and coupled to the inoperative euthanasia system while being supplied with ventilation air for respiration. This state can remain as long as desired.  
      In commencing a euthanasia cycle, connections should be checked and pressures monitored before pressing the start button, and status indicators should be observed when attempting to commence a cycle, to take due note of fault indications requiring attention. As the cycle commences, the controller  50  monitors operations and provides the necessary outputs to complete the cycle without continued attention.  
      The invention has been described in connection with certain examples and advantageous features. These examples are not intended to be limiting, and reference should be made to the appended claims rather then foregoing discussion of examples, to assess the scope or the invention in which exclusive rights are claimed.