The ferret (Mustela putorius furo) is a domesticated animal of which the closest counterparts in the wild are the European polecat (Mustela putorius) and possibly the steppe polecat (Mustela eversmanni). The first reports on ferrets being kept by people are from approximately 2500 years ago. Ferrets were originally kept for hunting rabbits in the field and controlling rodents in areas where shooting is too risky. Nowadays ferrets are predominantly kept as pets, although hunting is still very popular.

The ferret is a carnivore with an average body weight of 1200 grams in hobs (male) and 600 grams in jills (female). Over recent years, the size of pet ferrets has increased. The life span is a little shorter than that of the dog and cat but much longer than most other pet mammals. In the Netherlands, the average life expectancy of a ferret is 6-8 years. There are reports, however, that ferrets may reach the age of 15 years. The odour of ferrets is produced by their sebaceous glands. Removal of their anal glands does not have any influence on their odour.

When ferrets our housed outdoors they should be given enough protection against the elements. Direct sunlight should be avoided as ferrets are prone to heat stroke. The size of the cage will never provide the amount of space the ferret actually needs. It is therefore important that owners regularly give their ferrets the opportunity to play, under supervision, outside their cages in an escape-proof environment. The height of the cage should enable the ferret to investigate the surroundings while standing on its hindlegs. The cage encompasses a sleeping area (nest box), a litter box, a feeding area and some extra room to play. The litter box should be placed at the far end of the cage, away from the food, water and sleeping area. Water can be provided in either a bowl or a bottle. Most owners prefer bottles because ferrets will play with a water bowl and make a mess of the cage. Food can be offered in a bowl which is either attached to the cage or heavy enough so that the ferret cannot tip the bowl over or drag it around its cage. Ferrets like to hide in their sleeping area. A box may give privacy, while cloths, towels or old T-shirts make excellent bedding. Hay, straw and wood shavings have been associated with chronic upper respiratory irritation.

Ferrets are strict carnivores. In the past, high-quality cat and ferret foods were advised. Recently, however, some veterinarians have started advising feeding entire prey animals. The idea behind this advice is that the high concentration of carbohydrates which is present in most commercial diets plays an important role in the high incidence of insulinomas found in ferrets. Prey animals contain much less carbohydrate and may therefore prevent this common disease. At this moment, no scientific data can support this assumption, but it is certainly not harmful to feed entire prey animals to ferrets. An alternative may be a pelleted diet which is low in carbohydrates. Ferrets should always have access to fresh water, especially when pellets are fed. Many owners believe that ferrets should have continuous access to food. The general impression of owners is that withholding food for more than 4 hours is dangerous to their pets, because their gastrointestinal tract would be empty by then. Abdominal ultrasound investigations which have been performed in ferrets after withholding food for 4 hours have shown that in many of them food was still present in their stomachs. It is therefore safe to say that food can be withheld for more than 4 hours. Extra caution is only necessary in ferrets with an insulinoma.

Handling and Clinical Techniques

Restraint

Most ferrets kept as pets are used to being handled. The risk of being bitten by a ferret is therefore no greater than by a cat or dog. To pick up a ferret it is best to hold it around the thorax with one hand and support the hindlegs with the other. Ferrets will struggle if the hindlegs are held too firmly. If the ferret struggles too much a firmer grip around the neck may be necessary. Scruffing of the loose skin on the back of the neck is sometimes necessary. A pleasant way for both ferret and owner to distract a ferret is by feeding it a liquid diet like Royal Canin Feline Convalescence Support instant diet (Royal Canin, Crown Pet Foods (Ltd), Somerset, United Kingdom) or Carnivore Care (Oxbow Enterprises, Inc., Murdock, New England, USA). If the liquid diet is applied to the belly of the ferret most owners can clip the nails of their ferret while it is licking its belly.

Physical Examination

The physical examination of the ferret is very similar to that of the dog and cat. As with any consultation a thorough anamnesis is mandatory. Alertness and attitude should then be assessed. In the carrier the ferret may appear quite sleepy, but once out of the carrier the ferret should be highly active. Also note that the normal locomotion of a ferret is with an arched back. Handling of the ferret may alter its body temperature and its heart and respiratory rate. Therefore the author prefers to start with the respiratory rate when the ferret is not handled. The normal respiration rate of a ferret is 33-36 breaths per minute. This is followed by the peripheral pulse frequency. Just as in dogs and cats, the peripheral pulse can be taken at the femoral arteries. This can most easily be performed when having the ferret rest on the lower arm and palpating the pulse with the other hand. The normal pulse rate in a ferret is 200-250 beats per minute. The pulse should be strong and regular, but a respiratory arrhythmia may be noticed. The rectal temperature ranges from 37.8 to 40°C and is preferably measured with a digital thermometer.

Checking the hydration state of any small mammal is one of the most important features of a physical examination. This can be performed by assessing the skin turgor of the upper eyelids, tenting of the skin in the neck and checking of the moistness of the oral mucosa. When the oral mucosa is checked attention should also be paid to the teeth. These should be free from tartar. The capillary refill time can be evaluated by pressing on a footpad and evaluating the return of colour within 1 second. Since ferrets often have ear mites, special attention should be paid to the external ear canals. The lymph nodes in the following areas should be checked: neck, axillary, inguinal and popliteal. In fat animals they may appear enlarged. If they are also firm, a fine needle aspiration biopsy should be taken to check for lymphoma. Finally both auscultation of the thorax and abdominal palpation are included in the standard physical examination.

Venepuncture

Bleeding techniques such as toenail clipping, cardiac and periorbital puncture should be considered obsolete. Much better locations are the lateral saphenous or the cephalic vein for amounts up to 1 ml of blood. The jugular vein or the cranial vena cava can be used for larger amounts.

The jugular vein is approached in a similar fashion to that used in cats and can be performed in docile ferrets without anaesthesia. Rolling the ferret in a towel makes handling a lot easier. The cranial vena cava approach is best used under isoflurane anaesthesia, but can also be performed in debilitated ferrets. The ferret is placed in dorsal recumbency. The needle is inserted at the junction between the manubrium and the first rib. The needle is directed towards the contralateral hindleg in a 30 degree angle. Due to the location of the heart, there is no chance of inadvertent cardiac puncture with the above described technique.

Medication

Liquid medications are the easiest to administer orally to ferrets. They are well accepted by ferrets and can be accurately dosed. It is therefore recommended that methylcellulose is held in stock so that suspensions can be prepared. Due to the low body weight of ferrets, no commercially available tablet or capsule will contain the correct amount of drug, making them unsuitable for use in ferrets. In addition, capsules are rather big for them to swallow and usually fall apart before that. Intramuscular injections can be given in ferrets, but the total muscle mass is a limiting factor. If injections have to be given, the subcutaneous route is preferred. Although the intravenous and intraosseous routes are the most direct ones for fluid therapy, the subcutaneous and intraperitoneal routes also work very well in most ferrets.

Preventive Medicine

All ferrets should be vaccinated against canine distemper (CDV). Pups should be vaccinated at 9 and 14 weeks of age. In addition, yearly boosters should be given. Standard modified live vaccines developed for use in dogs can be used as long as they are derived from non-ferret cell lines. In the USA myofasciitis has been associated with vaccinations. It is not yet known which vaccine is responsible for these reactions.

Ear mite infections caused by Otodectes cynotis are commonly diagnosed in ferrets. Ferrets should therefore be regularly checked. During a routine otoscopic examination ear mites may be overlooked. A scraping from the ear canals is therefore considered to be the best diagnostic technique. The author prefers the topical application of selamectin at the base of the skull as preventive treatment.

The reproductive season of ferrets is under the influence of light. When a ferret receives more than 12 hours of light, the suppression on the release of gonadotropin-releasing hormone (GnRH) is lost and pituitary and gonadal hormones are secreted. Oestrus in jills will persist until vaginal stimulation and neck gripping have taken place. If the jill is not mated, she will remain in oestrus and have elevated oestradiol levels until the end of the oestrous season. This may lead to a bone marrow suppression resulting in a pancytopenia and eventually death. The best preventive measure is a form of (nonsurgical) neutering the ferret within the first year of life before the oestrous season has begun.

Viral Diseases

Canine Distemper

On a worldwide scale canine distemper, caused by a paramyxovirus, is considered the most serious viral infection of ferrets in which the outcome is almost always fatal. Classical symptoms, in unvaccinated ferrets, often start with dermatitis on the lips, chin and around the inguinal region 7-9 days after infection. Mucopurulent ocular and nasal discharge, pyrexia (over 40°C), sneezing, coughing and anorexia are other common signs. Just as in dogs, ferrets can develop hyperkeratosis of the footpads (hardpad). Diagnosis is made with fluorescent antibody tests on conjunctival smears and/or brain tissue. Since vaccination is performed with a modified live virus, vaccinated ferrets will be positive for this test. Therefore only non-vaccinated ferrets can be tested.

Influenza

Ferrets are highly susceptible to several strains of human influenza virus and therefore influenza should be considered a zoonosis. The infection route runs from humans to ferrets, but also vice versa. When you, or one of your personnel, have minor symptoms of the flu it is best not to treat ferrets; otherwise wear a facial mask to prevent spreading the virus to your clients' pets.

Many of the symptoms of an influenza infection are similar to those of a canine distemper infection, but less severe. The nasal discharge is mucoserous instead of mucopurulent, the ferrets sneeze and cough more and the fever is usually over before the ferrets are presented to the veterinarian. Just as in people, the infection is self-limiting and usually not fatal.

Aleutian Disease

Aleutian disease is caused by a parvovirus, unrelated to the one causing bloody diarrhoea in dogs. Of the Mustelidae the mink is most susceptible, but ferret specific strains have been found. Aleutian disease is an immune complex-mediated disease, resulting in multiple organ failure. Clinical signs in mink include wasting, hepato- and splenomegaly, melaena, recurrent fevers and, eventually, hindleg paralysis and other neurological signs. Most mink eventually die within 5 months. Fortunately ferrets seldom develop such severe symptoms. Although serum countercurrent immunoelectrophoresis is necessary to confirm the diagnosis, a hypergammaglobulinaemia together with chronic wasting signs is suggestive of Aleutian disease. As with almost all viral infections there is no specific therapy, but antibiotics and steroids have been reported to give some relief.

Endocrine Diseases

Insulinoma

Insulinomas are a frequent finding in ferrets, which will be further addressed during the presentation on the approach to the ferret with hindlimb weakness.

Hyperadrenocorticism

Hyperadrenocorticism is a common disease in ferrets, which will be addressed during the presentation on the approach to the alopecic ferret.

Miscellaneous Clinical Conditions

Lymphoma

Lymphomas are the third most common tumours found in ferrets. Clinical presentation of ferrets with lymphomas is often non-specific and may include loss of appetite, weight loss and peripheral lymph node enlargement. More severe symptoms may include dyspnoea and coughing caused by pleural effusion and mediastinal masses. Finding atypical lymphocytes in any organ or lymph node is diagnostic for (malignant) lymphoma. Finding large numbers of lymphocytes (normal or abnormal) in organs in which they are not commonly found is open for debate.

Ultrasonography is the most useful technique for detecting masses in both the thoracic as well as the abdominal cavity. In addition, ultrasonography is very useful as a guiding tool for performing fine needle aspiration biopsies. These biopsies are essential for confirming the diagnosis, but false-negative diagnoses (reactive lymph node) do occur. A full-thickness biopsy or surgical removal of an enlarged external lymph node is therefore recommended. Immunohistochemical staining may help to distinguish a reactive lymph node from a mixed cell lymphoma.

Just as in the other companion animals, it is not possible to provide a clear-cut treatment protocol for lymphoma in ferrets. Surgery (splenectomy, lymph nodectomy) is the preferred treatment if only one organ is involved. When multiple organs are involved surgery in combination with chemotherapy may be considered. It is important to realise that administration of glucocorticoids to ferrets may result in resistance of lymphoid cells. Ferrets receiving glucocorticoids for treating insulinoma or immune-mediated diseases may therefore be refractory to chemotherapy. In case the owner declines chemotherapy, glucocorticosteroids may be used as an alternative. Initially, ferrets often respond well to these steroids but the duration of effect is limited.

Splenomegaly

Splenomegaly is a very common finding in ferrets, but seldom of clinical importance. It can be found in healthy ferrets as well as in animals with seemingly unrelated diseases. During abdominal palpation the spleen can easily be detected and in severe cases extends all the way to the pelvic inlet. The most common histological diagnosis of these enlarged spleens is extramedullary haematopoiesis. Tumours of the spleen, however, do occur. Therefore, when a ferret is presented with nonspecific symptoms and a large spleen is palpable at physical examination, abdominal ultrasonography is recommended. In case of lymphoma the spleen usually has an irregular aspect and hypoechoic areas during ultrasonographic examination. An ultrasound-guided biopsy is mandatory to confirm the diagnosis prior to surgery.

The only indication for splenectomy is when the spleen contains a tumour (e.g., lymphoma) or if the spleen is so large that it causes discomfort. Otherwise the spleen should be left in place. It has to be noted that an enlarged spleen may rupture should the ferret fall from a height.

Cardiomyopathy

Congestive cardiomyopathy is a common cardiac disease seen in ferrets. Clinical signs are often nonspecific and include lethargy and dyspnoea. Physical examination may reveal tachycardia, a systolic murmur and muffled heart sounds. Both pleural effusion as well as ascites can be seen with cardiomyopathy. The best diagnostic technique is ultrasonography, demonstrating similar results in ferrets with congestive cardiomyopathy as seen in other species. When ascites or pleural effusion is present treatment with furosemide (2-4 mg/kg, q12h) is indicated. To improve cardiac function both digoxin (0.01 mg/kg q24h) and enalapril (0.5 mg/kg q24h) can be used.

Anaesthesia and Surgery

Anaesthesia

A fasting period of only 4 hours prior to sedation or anaesthesia is considered to be sufficient. For non-painful straightforward procedures, ferrets can be simply brought under light anaesthesia with isoflurane. Mask induction with 100% oxygen and 4% isoflurane, and maintenance on 2% isoflurane, is both quick and often far less stressful than performing a procedure without anaesthesia. The great advantage is that the ferret quickly awakes after the anaesthesia and the owner can go home with a fully awake ferret. Another option is the use of medetomidine. A dose of 80-100 µg/kg is usually sufficient for a sedative effect for approximately 30 minutes. Atipamezole can be used to antagonise the effects of medetomidine.

For more extensive procedures ferrets can be premedicated with 100 µg/kg medetomidine followed by placement of an intravenous catheter in the cephalic vein through which the ferret is induced with 1-3 mg/kg propofol. Anaesthesia is maintained with isoflurane. Ferrets should always be intubated (size 2-2.5) during longer surgical procedures.

Just as in any other small animal, hypothermia can occur easily and should be prevented. Ferrets should therefore be kept warm during and after anaesthesia.

The analgesics meloxicam (0.2 mg/kg, orally or s.c.) and carprofen (2 mg/kg, orally) have proven to be very effective in ferrets.

Neutering

Since neutering in both male and female ferrets (commonly done in the Netherlands) increases the risk of inducing hyperadrenocorticism it is advised to manage reproduction in a different manner. If this is not possible, similar techniques can be used as in cats. A complicating factor in female ferrets may be the large amount of fat in the mesovarium. This might be one of the reasons why remnant ovaries are often seen in the Netherlands. It is advisable to make an incision of at least 3-4 cm so that good visual orientation is possible during surgery.

References

The BSAVA is currently in the process of producing a Manual of Rodents and Ferrets, which will contain all practical information involving ferret medicine.