Hind Limb Weakness in Ferrets

         Hind limb weakness is a clinical sign, meaning that it is an indication of a problem, but not an actual disease entity itself. There can be varying degrees of weakness from a mild, transient unsteady gait, to complete loss of function of the hind limbs. I would like to approach this subject by going through some of the potential diseases and the types of weakness that result. Rather than trying to list all the disorders that can cause hind limb weakness, I will highlight those that are seen most frequently. You will note that the great majority of diseases that cause hind limb weakness in the ferret are not neurological, but are related to other metabolic disturbances in the body.

       It is important to note that ferrets affected by any condition that causes generalized weakness may exhibit the weakness more prominently in the hindquarters. This can lead to a misdiagnosis of a hind limb disease. This phenomenon may be due to the ferret's anatomy, the long body shape and short legs, and the conservation of energy or blood flow into the front legs, and vital organs such as heart, liver, lungs and brain.

                         DETERMINING THE CAUSE OF HIND LIMB WEAKNESS

The first two steps in diagnosing the source of hind limb weakness in the ferret are to obtain a detailed history and perform a thorough physical examination. An important part of the physical examination is the part that detects deficits in neurological (nerve) function. The neurological tests are simple and can help to determine the difference between a nerve related and a non-nerve related problem. Occasionally it is impossible to tell the difference between these two types of problems if the ferret is extremely weak or debilitated and unable to respond to any tests.

A neurological examination for hind limb weakness may consist of any combination of the following tests:

Gait and Stance - observing how the ferret moves around the floor. Abnormalities can include limping, one-sided weakness or dragging the limbs.
Righting Reaction - placing the patient on his/her side or back and seeing if he/ can get back into the normal standing position fairly quickly.
Toe Pinch - firmly pinching the skin web between the toes to detect pain sensation. A normal ferret will pull the foot away and will often turn around and look at what is causing the discomfort. Abnormalities noted can include absence of any reaction (no pain felt), hyperesthesia or exaggerated sensitivity of the limb, or the opposite leg reacts instead of the leg where the toe is being pinched.
Proprioception Positioning - turning the ferret's foot under as he/she is walking on the knuckles. A normal ferret will quickly turn its foot back to the normal position
Placing Reflex – a test that determines if the ferret knows where to put its foot when it touches a surface. Normal ferrets can difficult to evaluate with this test and it may be inconclusive. Usually the ferret is lifted off the table and held upright and then lowered to the table and allowed to touch it. A normal ferret will then “tighten” up the muscles in the legs and attempt to stand. Some ferrets “dance” and jump on the table. It is abnormal for the ferret's leg to collapse and be unable to bear weight.
Patellar Reflex - lightly tapping the ligament attaching the knee cap to the tibia to see if there is a spontaneous “jerk” of the lower leg. This test can be difficult to perform in an active ferret.
Anal Sphincter Tone - lightly touching the anal tissue with a thin blunt object to see if it responds by “flinching”. In the normal ferret there is not only a tightening of the sphincter muscle when the anus is touched, but often the ferret will look around to see what is touching him/her. Abnormalities include no response, or a sphincter muscle that is constantly relaxed leaving the anus open. These animals will have no control over bowel movements and don't know when they are defecating. The urinary bladder and urethral muscles can also lose their tone due to disease leading to urine leakage and a large flaccid bladder.

                DISEASES THAT CAN RESULT IN HIND LIMB WEAKNESS

NEUROLOGICAL DISEASES

Spinal Disease - The bony vertebrae of the spine enclose the spinal cord, which is the large nerve bundle that allows the brain to communicate with the rest of the body. Any disease that disrupts the flow of electrical activity along the spinal cord can result in varying degrees of weakness. Diseases that can affect the spinal cord include prolapsed intervetebral disc material (“slipped disc”) which pinches the spinal cord, cancer of the vertebrae, congenital weakness of the spine such as hemi-vertebrae or malformed vertebrae and trauma to the spine. The degree of weakness can range from mild to complete paralysis and can be present on one or both sides. Weakness associated with nerve damage is constant and rarely fluctuates. This is in contrast to diseases such as insulinoma or cardiovascular where weakness is intermittent. There can be pain associated with spinal injuries and the ferret may be very sensitive to touch in the area of the spinal lesion. Spinal disease is diagnosed with a combination of a neurological examination, x-rays, ultrasound and myelograms (dye studies) of the spinal cord.

Brain Disease- The brain is a complex organ that coordinates the body's neurological functions. The signs of brain injury or disease can vary greatly depending on the area of the brain affected. Diseases of the brain include congenital defects, trauma, cancer, cardiovascular accidents (“strokes”), infections and parasites. Disease of the brain often produces additional clinical signs besides weakness which can include one or more of the following; loss of balance, head tilt, blindness, radical personality change, tremors or seizures. The weakness is usually constant as in spinal disease. A neurological examination is a vital part of the diagnosis however, definitive diagnosis of the specific brain disorder can be difficult without the use of sophisticated diagnostic equipment such as MRI or CT scans. Your veterinarian will advise you if these tests are available in your area.

Broken Hind Limbs or Pelvis - This is a disease of the bone usually caused by trauma, but it can also affect the nerves near the fracture site. Bone cancer or nutritional deficiencies can also cause weakness of the bone ultimately resulting in pathological fractures. Severe bruising or tearing of the muscles or ligaments of the hind limbs, usually caused by trauma, can also cause weakness. There is usually moderate to severe pain in the area of the injury, which is also accompanied by tissue swelling. In addition, unless the pelvic bone or both legs are simultaneously affected, the weakness will be one-sided. X-rays are used to diagnose pelvic or hind leg bone injuries.

NON-NEUROLOGICAL DISEASES

Insulinoma - This cancer of the beta cells of the pancreas causes intermittent low blood glucose due to the release of abnormally large amounts of insulin. Low blood glucose deprives the brain and muscles of the “fuel” they need to function. The result is muscle weakness and sluggish activity. The body, however, is constantly trying to counteract this condition and will produce more glucose from the liver. The ferret will return to normal without medication early in the disease. As the disease becomes more severe, it becomes more difficult for the ferret to come out of the hypoglycemic attacks and eventually he/she goes into seizures. The type of weakness produced by insulinoma can be very mild to severe. The key is that the weakness produced in insulinoma is not constant and can vary in intensity many times during the day. It is usually most noticeable right after waking from sleep or after fasting for several hours. Normal strength often improves after a meal or glucose administration. In ferrets that are sluggish upon waking, the weakness may disappear after they have moved around for a few minutes. As the disease advances generalized weakness may become constant followed by seizures. Insulinoma is diagnosed by checking the fasting blood glucose. It may be necessary to check this more then once to demonstrate lower than normal values.

Cardiovascular Disease - This includes any disease of the heart or blood vessels. Cardiomyopathy, or heart muscle disease, is very common in ferrets and results in poor blood circulation and poor oxygenation of tissues. Other heart disease includes valvular disorders, electrical disorders and congenital defects. Since the muscles and brain need a constant supply of oxygen, a sluggish circulation can result in varying degrees of weakness. The weakness seen with heart disease can vary during the day and frequently occurs after exercise. Strength often returns after a period of rest. In addition, the pulse can be weak and irregular, the body temperature may be cooler than normal and the ferret's gums may be bluish in color during periods of weakness. As heart disease progresses whole body weakness becomes constant.
Another cardiovascular disease occasionally seen in ferrets is the formation of blood clots in the blood vessels of the hind limbs. These can form spontaneously or can be the result of trauma to the vena cava, which is the large vein returning blood from the body to the heart. The vena cava can be disturbed either through trauma such as the ferret being stepped on, through surgery in this area or a tumor causing pressure on the vein. Blood clots can form in the major blood vessels of the hind limbs and thus cut off the blood supply resulting in total loss of function of the hind legs. The keys here are that the weakness is confined to the hind limbs and the front half of the ferret is bright and alert, there is an absence of a pulse in the femoral vein (found on the inside of the thigh), and the hind legs are considerably colder then the rest of the ferret. In addition, when the blood clots first form the ferret will exhibit pain in the hind legs. The pain subsides as the tissues die from lack of oxygen. This is a dire situation and should be attended to immediately. Cardiovascular disease is diagnosed with one or more of the following: x-rays, ultrasound, ECG and vascular dye studies.

Anemia - Anemia is not a disease in itself but is a condition where there is a lack of sufficient red blood cells (RBCs). RBCs carry oxygen to tissues so if there are not enough RBCs to deliver the oxygen the body's tissues become starved and will not function properly. As described under the section Cardiovascular Disease, the muscles and brain have a high oxygen requirement in order to function. Anemia can be caused by a variety of diseases including; kidney disease, liver disease, heart disease, trauma (loss of blood through an injury), cancer of the blood cells, toxins that destroy RBCs, infections, nutrition, parasites and bleeding gastric ulcers. Hyperestrogenism, which is a condition where a female ferret is in heat too long and the excess estrogen in her system suppresses blood cell production in her bone marrow, can result in a fatal anemia if not treated. The weakness associated with anemia is usually more constant then insulinoma or cardiovascular disease, but may show a pattern of becoming more obvious with exercise. If the anemia is severe enough to cause noticeable weakness then you will probably be able to see other signs such as a pale pink or white color of the tissues in the mouth and the tissues around the eye. The presence of anemia is diagnosed through a complete blood cell count. In particular, the RBCs are examined closely. The diagnosis of the cause of the anemia can include any of the following: blood biochemistries, x-rays, ultrasound, urinalysis, fecal examination and dye studies.

Adrenal Tumors - Adrenal disease does not usually cause significant weakness in the ferret. Since insulinoma often occurs simultaneously with and may be masked by adrenal disease, weakness caused by insulinoma may be erroneously blamed on adrenal disease. However, in some rare cases, weakness can be attributed directly to adrenal tumors. Both adrenal glands lie very close to the vena cava, which is the main vein returning blood from the body to the heart. If the adrenal tumor is large enough, it may cause pressure on this vein and disrupt blood flow to the hind legs. This will cause a lack of oxygen in the muscles of the hind legs with resultant weakness. As in the case of other cardiovascular disease already described, the pulse in the hind limbs will be weak or irregular and the limbs may be colder than the rest of the body. Rarely, anemia can be caused by adrenal disease. Adrenal disease is diagnosed via a variety of methods including clinical signs, blood tests called hormone assays, ultrasound and exploratory surgery.

Aleutian Disease (AD) – AD is caused by a parvovirus that causes an autoimmune-type disease in the ferret. Because so many different organ systems can be involved, generalized or hind limb weakness can be part of the clinical picture. AD is diagnosed by blood tests that detect antibodies to the virus. In future there may be more accurate tests available to detect the virus itself. For more information on this disease, see the handout Aleutian Disease in Ferrets.

Toxic Conditions - There are several toxic conditions that can cause weakness by either killing cells or disrupting normal cell function in various organs of the body. Toxins can be produced by the body itself in, for example, kidney or liver disease. These organs normally remove or neutralize natural metabolic waste products in the body but when they are diseased the waste products build up and poison the body. Toxic infectious conditions include septicemia (overwhelming bacterial infections) and viremia (overwhelming viral infections). Toxins can also be acquired by being eaten, inhaled or absorbed through the skin. These include, heavy metal, insecticides, rodenticides, overdose of veterinary medications and access to human medications or household cleaning agents. Hind limb weakness will not be the only clinical sign noted with toxicosis. Toxic ferrets are usually lethargic, depressed, have a loss of appetite and may exhibit vomiting or diarrhea. The diagnostic tests used depend on the suspected cause of toxicity. They can include any combination of CBC, serum biochemistries, x-rays, ultrasound, urinalysis and serology for specific toxins.

           In conclusion, hind limb weakness is a common sign associated with many diseases and it will be necessary to work with your veterinarian to determine the cause and hopefully the treatment for the underlying condition. You can be of greatest assistant by being a careful observer and recording your observations on the onset, frequency and duration of the weakness and if the weakness improves with anything you gave your ferret. Your veterinarian should investigate any sign of weakness as soon as possible, however a sudden or severe weakness should be considered an emergency.