Clinical Techniques in Ferrets
Heidi L. Hoefer, DVM, Dip ABVP
Pet ferrets are easy to handle and restrain for examination. Scruffing provides additional restraint. To immobilize a ferret for treatment or venipuncture, 2 assistants are needed: one to restrain the head and forelegs and the other to hold the rear legs. If the ferret continues to struggle in this two-person hold, try syringe feeding Nutri-Cal® or Ferret-tone® during the restraint. Chemical restraint is usually not needed for venipuncture and treatment techniques but will be needed for catheter placement and radiographs in most cases. Isoflurane works well for short procedures but for surgery and lengthy anesthetic procedures, ferrets must be fasted a minimum of 8-10 hours. For short procedures, a facemask can be used for induction and maintenance of anesthesia. For long procedures and surgery, endotracheal intubation is recommended. Intubation is straightforward; a 2.5-3.5 cuffed tube works for most ferrets. Isoflurane is not recommended during venipuncture because it can temporarily depress the red blood cell values in ferrets and the hematocrit can drop up to 40%.
If a ferret needs chemical restraint and isoflurane is not available, there are several protocols that can be followed. A small intravenous bolus of ketamine (5 mg/kg) plus equal or double volumes of diazepam (0.5 mg/kg) mixed in the same syringe might be useful for short, non-painful procedures like radiographs or ultrasound. Midazolam (Versed®) is an anxiolytic drug similar to diazepam but has better intramuscular absorption. Midazolam (0.5-1.0 mg/kg IM) works well to "take the edge off" a very fractious ferret but won't be immobilizing. Pretreatment with midazolam can then be topped off with a small IV bolus of ketamine and/or diazepam for short procedures. Ferrets are very sensitive to injectable analgesics like butorphanol or buprenorphine; these drugs prolong anesthetic recovery and can interfere with thermoregulation. Use at the low end of the dose and use cautiously with other sedatives.
Ferrets are vaccinated for canine distemper (CDV) and rabies virus. The only USDA approved vaccine for use in ferrets for protection against canine distemper virus is Fervac. This is a modified live vaccine produced by United Vaccines (Madison, WI). An alternative vaccine is Galaxy-D by Schering-Plough that is made in a primate cell line. The vaccine appears safe and anecdotally effective. Killed vaccines are not recommended due to their inability to stimulate long term immunity. Canine cell propagated vaccines are not recommended as they may induce distemper virus in the ferret. Ferrets follow the schedule used for dogs vaccinated with CDV: at 6 to 8 weeks of age and then every 3 to 4 weeks until 16 weeks of age and then yearly. Previously unimmunized adults require 2 vaccines 3 weeks apart before the annual booster.
Rabies vaccines are recommended for ferrets in certain locations. Local health department regulations in some states may not recognize the rabies vaccine. There is one approved product available, Imrab 3 (Merial), available as a subcutaneous injection. This is given at 3 months of age and then annually. Ferrets are not susceptible to feline distemper or feline leukemia virus. There is also no evidence to vaccinate ferrets with the Bordetella vaccine.
There is always a risk of an adverse response or reaction to a vaccine. While the rabies shot can result in an injection reaction, this is seen more commonly with the distemper vaccines. This anaphylactic reaction is characterized by profuse vomiting, diarrhea, and lethargy. The reaction usually occurs within 12 hours of the shot and most cases are within the first 60 minutes. Steroids, antihistamines, and sometimes epinephrine must be given by injection to the affected ferret. Most reactions are controllable with these drugs and most ferrets quickly respond to treatment. Ferrets that have vaccine reactions are still protected against the virus. The decision to re-vaccinate a ferret with a history of past reactions should be discussed your veterinarian. One must weigh the potential risks of vaccine reactions against the risk of contracting the distemper virus.
The blood volume of a healthy ferret is approximately 40 ml in females weighing 750g and 60 ml in the larger males (1000g). Up to 10% of blood volume can be taken safely at one time. Unless the ferret is serving as a blood donor where larger amounts of blood need to be collected, most assays can be performed on 1-2 ml of whole blood.
Blood samples need to be properly handled for accuracy in testing. Small blood tubes holding up to 1 ml are available through most labs or can be purchased (Microtainer®, Becton-Dickenson or Microvette®, Sarstedt). EDTA (purple tops) are adequate for complete blood count testing and heparin samples (green tops) are recommended for biochemical analysis. Samples for plasma testing should be centrifuged shortly after collection if possible.
There are two sites routinely used for collecting large amounts of blood in the ferret: the jugular vein and the cranial vena cava. The jugular vein may be difficult to palpate in some ferrets, especially obese ones. Restraint and positioning is like that used for a cat: forelegs pulled over the end of a table with head up and rear legs secured. A 3 cc syringe and 22 g hypodermic needle can be used. Blood can sometimes flow slowly from this site; try repositioning the head or heparinizing the syringe beforehand to prevent premature clotting of the sample.
The cranial vena cava is the preferred site for large sample collection, but it usually takes some practice. Using a 3 cc syringe and 25 g needle, the vein is entered just lateral and alongside the sternal manubrium and at an angle directed toward the opposite hip. It is a" blind" technique and negative pressure must be maintained in the syringe during the procedure.
There are several peripheral veins that can be used for small samples like a blood glucose or hematocrit: the cephalic veins and the lateral saphenous veins. An insulin syringe works best in these sites.
Published reference values for ferrets are available in the literature. Some labs will provide a range of normal values. Generally speaking, the hematocrit in ferrets is slightly higher than the dog and cat (range 44-63%) and the white cell count can be lower (range 3 -8 x 103/ul) and is rarely elevated above 15 x 103/ul. The lymphocyte count in ferrets can be higher than in the cat (up to 54%).
Clinical chemistries in the ferret have several differences when compared to the dog and cat. Most notably is the alanine aminotransferase (ALT or SGPT) and the blood urea nitrogen (BUN) and creatinine (Cr). The normal ALT is higher (up tp 280 IU/L) and should be considered liver specific. The creatinine in ferrets has a narrow normal range (0.2 -0.8 mg/dl) and any elevation, however slight, may be significant, especially in combination with elevations in BUN. It is not unusual to have a ferret in renal failure where the BUN is > 200 mg/dl and the creatinine may only be 1.1 mg/dl. Of course, other parameters must be considered (urinalysis e.g.) before diagnosing renal failure in the ferret.
Urine collection can be through a "table-top" sample or cystocentesis. Urinary tract catheterization is difficult in ferrets and is usually only performed to relieve lower urinary tract obstruction. Most ferrets will urinate during restraint and venipuncture, making a table top collection possible. Cystocentesis can be performed using a 25 g needle, but some ferrets can have a transient, mild hematuria following the procedure if not properly restrained.
Normal urine pH of the ferret reflects the diet: those on a meat-based, high quality ferret or cat food should have a pH 5.5-6.5. The pH will be higher on a grain-based diet which is not recommended due to the propensity of ferrets to develop sand, grit, and discrete calculi (struvite most common) in the lower urinary tract. Other urine reference ranges are similar to the dog and cat. There is very little in the literature in regards to normal urine specific gravity in the ferret.
Urinary Tract Catheterization/Obstruction
Urinary tract catheterization is most often indicated for neutered male ferrets with urethral obstruction (UO). Obstructions are most commonly caused by prostatic enlargement secondary to adrenal tumor hormone production, or from urolithiasis. A ferret with a UO needs immediate intervention to relieve the obstruction. The ferret must be anesthetized for passage of a urinary catheter. Isoflurane inhalation is the preferred anesthetic agent in the sick ferret. With the ferret in dorsal recumbency, the prepuce is gently grasped and the penis extruded. There is a crooked os penis in the ferret that obscures visualization of the urethral opening, especially when the penis is held taught. A 3.5 French red rubber catheter with a stylet can often be passed. In cases of urethral blockage from gritty material, a tomcat catheter or 24 gauge intravenous catheter (without the needle) can be introduced first to flush through the distal aspect of the urethra. Alternatively, a through-the-needle jugular catheter can be used (again, without the needle!). The urinary collection system can be attached with tape and sutured to the skin. Be forewarned: it is very difficult to keep these systems in place once the ferret begins to feel better and elizabethan collars and other protection devices are necessary at all times!
Intravenous Catheterization And Fluid Therapy
Indications for intravenous (IV) catheters include vomiting, dehydration, hypoglycemia, transfusions, and prolonged anesthetic procedures. Most ferrets require full anesthesia for catheter placement (unless extremely moribund). Place intravenous catheters in the cephalic, jugular, or saphenous vein. Use small gauge catheters (24 or 26 gauge) for the peripheral vessels and immobilize the ferrets with anesthesia. Pre-puncture the skin with a 22 gauge needle to allow easier catheter placement. Jugular catheters are possible but are more difficult to place and are very ackward for the ferret once up and ambulating. For jugular venipuncture, 22 gauge catheters can be used. A cut down may be needed to catheterize the jugular. Intraosseous (IO) catheterization is uncommon in pet ferrets. If it is impossible to get a vein, this can be attempted. The femur (via the trochanteric fossa) and tibia (tibial crest) are common places in which to insert an intraosseous catheter. A 22g spinal needle can be used for this purpose. IO catheters are very painful and should only be used for short term.
Fluid therapy guidelines are similar to the dog and cat. Maintenance fluids are usually given @ 75 ml/kg/day with additional volumes given to correct dehydration. Lactacted Ringer's can be used in most cases and debilitated or anorectic ferrets are given a 2.5% dextrose solution. Use of a buretrol fluid administration set or a continuous rate infusion pump is recommended to prevent fluid overload in the small patient. Most sick ferrets will not chew on the catheters or IV lines but can become entangled in their bedding and most be monitored.
Subcutaneous fluids can be used in ferrets with only mild dehydration or for maintenance fluids. Daily dose is divided for B-TID doses. Most ferrets rigorously reject the administration of subcutaneous fluids and for prolonged fluid therapy, an IV should be considered.
Ferrets with acute blood loss from a gastrointestinal bleed, intraoperative hemorrhage, or bone marrow suppression may need a transfusion. The decision to administer blood to a ferret is based on clinical signs, PCV, and disease state. Consider the transfusion when acute blood loss results in a PCV below 15-18%.
Ferrets lack detectable blood groups and even multiple transfusions can be given with little risk of transfusion incompatibility reactions. Because of this, crossmatching is not usually performed. Blood from the donor ferret is usually drawn and transfused into the recipient ferret immediately, although short term storage (< 12 hours) may be possible. Large healthy males are selected as donors whenever possible because 6-10 ml. can be safely taken from these ferrets. Heparin or acid-citrate-dextrose (ACD @ 1 ml to 6 ml blood) can be used as anticoagulants for the donors' blood. The donor is usually sedated and a 25 gauge needle or butterfly catheter is used to draw the blood from the cranial vena cava or the jugular vein.
The recipient of the blood donation should have a hematocrit performed immediately prior and 1-2 hours after the transfusion. Pre-treatment with a short-acting steroid may not be necessary but is often recommended.
Normal adult ferrets often present with varying degrees of splenic enlargement that can be quite dramatic in some cases. The most common cause for enlargement is extramedullary hematopoesis (EMH), although neoplasia or hematomas can occur sporadically. Lymphosarcoma is the most common tumor type seen in the spleen and rarely, hemangiosarcoma. Mast cell tumors are not reported in the ferret spleen.
Ferrets with irregular or nodular spleens or those with a clinical picture compatible with lymphosarcoma (ie, fever, lymphocytosis) can have splenic cytology performed via a fine needle aspirate. The aspirate is usually done as an outpatient procedure and usually does not require sedation or ultrasonography for guidance. With the ferret properly restrained, the spleen is isolated and brought to the lateral or ventral body wall. The aspirate is per cutaneous with a 25 gauge hypodermic needle and 6 cc syringe. The needle hub fills instantly with blood, and slides are smeared immediately. The procedure has a fairly good "hit-miss" ratio for lymphosarcoma which tends to have a diffuse infiltrative pattern in the spleen. Morbidity associated with the procedure is very low if the ferret is rigidly restrained.
There are very few drugs that have been pharmocokinetically studied in the ferret. Most drug doses are scaled down from the cat and dog (i.e., used at the same dose rate but for the 2-3 lb. ferret). The ferret is a carnivore with a simple gastrointestinal tract, so it tolerates most antibiotics without the problems of flora imbalance seen in other small mammals. Medications are administered as an oral suspension (ferrets can't be pilled) or given parenterally as indicated by clinical signs and disease state. Ferrets can paw violently at the mouth and palate following administration of some medications.
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2. Kawasaki TA: Normal Parameters and Laboratory Interpretation of Disease States in the Domestic Ferret. Semin Avian Exotic Pet Med 3(1):40-47, 1994.
3. Marini RP, Esteves MI, Fox JG: A technique for catheterization of the urinary bladder in the ferret. Lab Anim 28(4):155-157, 1994.
4. Quesenberry KE: Basic approach to veterinary care. In E. V. Hillyer and K. E. Quesenberry (eds): Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery. Philadelphia, WB Saunders, 1997, 21.
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