Lab Animal
- 35, 7 (2006)
doi:10.1038/laban0706-47
Use of an adjustable restraint device for prolonged and intermittent intravenous infusion and blood sampling in ferretsDaniel E. McLain, PhD, CNS (FACN), DABFEWalker Downey & Associates, Inc., Product Safety and Development Consulting, 7565 Heather Knoll Lane, Sauk Prairie Estates, Verona, WI 53593. demclain@charter.net. For experiments requiring repeated venipuncture or prolonged intravenous infusion or sampling techniques, the ferret can serve successfully as a surrogate nonrodent animal model. Because experimental protocols must be suitable for the test system or model used, the author describes specific restraint equipment designed to facilitate the use of ferrets in the research laboratory.Interest in the ferret as a laboratory animal seems to have grown in response to the need to find smaller nonrodents for use in testing new biotechnology-derived products or limited-quantity biologics. To this end, conventional nonrodents, such as dogs, are increasingly excluded from testing because a sufficient quantity of test material may simply not be available. The smaller, laboratory-adapted ferret seems to have found an increasing number of applications in this present environment, and extensive reviews of its use in the research laboratory have appeared in the literature1,
2,
3. To standardize these investigations, data on the nutritional requirements of the species are also available4, as are extensive reviews of its physiology and diseases5,
6.
Restraint devices used for long-term and short-term animal studies should meet published standards for comfort and stress7,
8. At the same time, however, the system or device used must meet the practical needs of the investigator in terms of efficiency and control of the animal, two variables that can affect the reliability of the research data. For the ferret in particular, a restraint device must be durable enough to withstand repeated cleanings, and its design must afford protection for both the animal and the handler from any injuries that may occur with repeated use. The device must also ensure restraint and preclude escape.
Restraint device construction
Under optimally controlled environmental conditions, the body weight of a sexually inactive adult ferret will generally range between 0.5 and 1.0 kg for females and between 1 and 2 kg for males at 5–7 months of age. Their marked sexual dimorphism, as well as the various ages of animals used in different research investigations and the species' seasonal deposition of body fat (during the March–August breeding season, body weight can fluctuate by as much as 40%), all make a single-sized restraint device impractical. Consequently, four different sizes of restraint devices are needed to cover the anticipated body weight ranges.
Components of the ferret restrainer (Fig. 1) are subdividable into eight sections or parts (see Table 1 and Fig. 2). The inside diameters (i.d.) of the restraining tubes corresponding to the various body weight ranges are 6.4 cm (400–700 g), 7.9 cm (650–1,100 g), 8.8 cm (1,000–1,750 g), and 10.1 cm (1,500–2,300 g). Construction of the device should be on scale with the i.d. of the restraining tube.
Animal restraint tube acclimation
Most commercial breeders will have selectively bred ferrets for an even disposition, and newly arrived animals will generally respond rapidly to standard environmental acclimation procedures, particularly if an end goal of animal-human socialization is included. In fact, it is this apparent need for physical and social contact that makes the ferret so amenable to restraint technology generally, and the device described here specifically.
Because of their innate attraction to enclosed spaces (such as small holes and tunnels), ferrets will readily enter the restraint tube without any indication of excitement or stress. On the contrary, it may at some point be necessary to end the 'game' and encourage the exit of those animals who take advantage of the tube's confines to sleep. Generally, the inquisitive and playful nature of the ferret facilitates rapid acclimation to the restraint tube. More importantly, ferrets do not typically seem to associate the device with any specific adverse treatment, allowing many treatments or manipulations per day to the same animal. Although the author has limited experience with respect to maximum treatment duration or chronicity, thrice-weekly treatments with a test agent by prolonged intravenous infusion for as long as 90 d (15–30 min/d) has yielded no apparent aversion to the device or procedures used13.
Introduction to the restraint device may require that the animal have some initial object-exploration time. A spare restraint tube (not an entire device) placed in the cage with socializing animals will greatly reduce, if not totally eliminate, initial introduction time and resistance, if any. Also, inclusion of a playful tail-lifting manipulation (see Restraint Device Use) during the animal-human socialization program will greatly facilitate restrainer acclimation. In the case of a ferret that persistently hesitates or resists the device, place its head in the restrainer before release, and instinct will cause the animal to move forward without pause.
Restraint device use
Referenced part numbers correspond to Figure 2 in the explanation given next. Set the restrainer on a flat surface so that the front (6) and rear (7) support legs are stabilized. Slide the nose cone (4) toward the front of the tube (8) a distance at least the length of the animal's body (minus the tail), and lightly tighten the clamping screw (3). Placing the animal at the rear of the device, remove the rear gate (5) and allow the animal to enter while gently grasping and lifting the tail in an upward direction. Reattach the rear gate while positioning the tail through the top center notch, then release the tail. Lightly tighten the wing-nuts for the rear gate to secure the attachment. Loosen but do not detach the clamping screw, and move the nose cone slide (2) toward the rear of the device, positioning the cone over the nose of the ferret. Adjust the position of the nose cone to achieve the degree of animal immobility required for the procedure by moving it slightly forward or backward. Animal observation is important for successful use of the device. If the animal can pull its tail completely into the tube, then the nose cone has been placed too far forward. Similarly, if the animal can turn around in the tube, then a smaller tube diameter is necessary. Once the desired position is achieved, tighten the clamping screw to complete the procedure.
Ferrets prefer a colder room temperature than most laboratory animals and rapidly succumb to heat stress. Therefore, the restraint tube and nose cone should have holes for adequate ventilation and air circulation. When necessary, a small tabletop fan can further enhance air circulation. The clear Lexan/Plexiglas tube material permits an unobstructed view of the animal so as to allow stress monitoring. A properly restrained animal will show no body jerking or other apparent escape attempts and may soon fall asleep. Animals restrained for prolonged periods should be frequently monitered for any signs of stress.
After completion of treatment, slide the nose cone assembly forward and off the device, allowing the animal an unobstructed exit. Food morsels, if permitted by protocol, will coax out stubborn animals and provide an incentive for repetitive restraint tube use.
Restraint tube procedures
Limited or initial intravenous dosing or blood sampling success may be possible with the crude ferret restraining device originally proposed by Curl and Curl9 for serial blood sampling of ferrets. However, as the intravenous access capabilities of the laboratory improve, and more prolonged and frequent infusions or samplings are necessary in animals of different sizes, a more sophisticated restraint system is valuable. The device described here has the dual purpose of accommodating the increasing skills of the laboratory's technical staff while providing safe and comfortable animal restraint. Treatment administration can involve a subcutaneously tunneled jugular catheter (at the nape of the neck) that exits from the top of the tube, or use of the caudal vein. For intravenous infusions longer than 2 h duration, it would be well to consider a jacket and swivel tethering system to allow for more freedom of movement10.
Venous access
Ferrets have notoriously thick skin, making venous (jugular, cephalic, or femoral) access difficult11 and prompting some investigators to recommend sedation, local anesthesia, and a surgical skin incision for placement of a catheter for short-term drug or fluid administration5,
13. Others have concluded that the only surface veins suitable for visible entry in the ferret are on the ventral surface of the tongue, but this procedure requires anesthetizing the animals before manipulation12. Contrary to these earlier reports, the caudal veins and arteries of the ferret are accessible, with practice, using techniques such as those originally developed for small rodents14,
15,
16,
17,
18. Capable technicians become proficient rapidly in these techniques, and, with the use of the restraint device, perform this procedure without additional assistance.
Although the caudal vessels are neither palpable nor visible to the naked eye, access to them is possible by viewing the dorsal surface of the shaved tail at the 12 o'clock position, then turning the tail in a clockwise or counterclockwise direction so that either of the large veins residing at the 10 o'clock or 2 o'clock positions assume the 12 o'clock position. To facilitate venous access in this position, insert a 23-gauge  1-in. needle (or Quik-Cath catheter, for example) at a depth and angle similar to that which would be used for rodents. The caudal artery assumes a 6 o'clock position when the animal is viewed dorsally. A previous publication provides a diagram of a transverse section of the ferret tail, showing the location of caudal arteries and veins19. Implantation of indwelling intravenous catheters in the ferret's caudal vein is possible when it is necessary to administer solutions at a very slow or constant rate using an infusion pump, or for chronic studies requiring many consecutive days of treatment.
An earlier method for the chronic jugular catheterization of the ferret has appeared in the literature20, but another study provides a more detailed description of catheter implantation and maintenance in ferrets21. Although different investigators may have developed their own methods for catheter placement, the latter authors used an interesting method to ensure catheter patency throughout the course of acute or chronic studies without the use of heparin. More specifically, they filled the catheter lumen with saline solution and clamped the catheter with a rubber-shod hemostat at the end of each infusion. They then removed the syringe containing a blunt needle from the lumen of the tubing, inserted an appropriately sized stainless-steel blocking pin, and then removed the rubber-shod hemostat. Pushing the stainless-steel pin slightly farther into the catheter forces the saline fluid to fill the lumen tip at the venous end, discouraging clot formation.
Venous infusion rates
Typical intravenous infusion volumes and rates used for ferrets depend, as with other species, upon the glomerular filtration rate (GFR) and the physical characteristics of the specific test article22. Ferrets can probably receive between 2 and 4 ml/kg/h of intravenous fluids without a notable increase in urine volume or frequency. This compares with an approximate continuous infusion rate of 0.5 ml/kg/h for dogs and 8 ml/kg/h for rats. Using the aforementioned ratios and 'reference' body weights of 1.5 kg for ferrets, the approximate total daily (24 h) fluid volumes that can be delivered to this test species without notable increase in urine volume or frequency is 70–140 ml.
Animal restraint best practices
Although it may be tempting to "let sleeping ferrets lie," it is ill advised to allow ferrets to stay enclosed in the restraint device unnecessarily. A ferret is the ultimate contortionist, and a determined individual will ultimately master even custom-fitted tubes. Although the sliding nose cone in the restraint device is designed to discourage unwanted turning, a balance with perceived animal comfort is necessary. In the author's experience, however, the tendency is to undercompensate for tightness so as not to cause unnecessary discomfort.
Author's note
Specifics of this ferret restraint device construction, including scaled engineering drawings with dimensions of each part, are available directly from the author (demclain@charter.net) or (dem@walkerdowney.com) upon request.
Received 2 March 2006; Accepted 24 May 2006
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Competing interests statement:
The authors declare that they have no competing financial interests. |
