Metabolic Bone Disease (MBD)

Opossum Society of the United states: http://www.opossumsocietyus.org

Note to finders/general public:

This is an article intended for use by licensed wildlife rehabilitators and others working on behalf of the opossum. If you have found an opossum and are trying to seek dietary advice then please go ahead and read this article. It will help you to understand why it is critical you turn the opossum over to a trained rehabilitator and not try to raise it yourself. Too many opossums have died or suffered painful, crippling disorders as a result of well-meaning but misinformed people attempting to raise the opossum themselves.  See photo of opossum fed a diet consisting of only hot dogs and the resulting facial deformities. Unfortunately, this opossum responded to initial treatment and proper diet but later succumbed due to a complication of his advanced disease.

What is MBD?

Nutritional Metabolic Bone Disease (MBD) is a serious disease resulting in severe crippling or death. It is sometimes seen by wildlife rehabilitators receiving an opossum from a well-meaning but misinformed finder. Often, a finder will not know what to feed the opossum and will offer an incorrect, grossly imbalanced diet. Once the finder notices that something is wrong with the opossum it can be too late unless the rehabilitator takes immediate action. The rehabilitator should always obtain a detailed diet history and suspect MBD if the opossum was fed a calcium deficient diet, an all-meat diet, or a commercial diet supplemented with meat off the bone, or if the diet history is unknown.

What causes the disease?

MBD is a complex disease with many etiologies. However, it is primarily due to poor husbandry, feeding an improper diet. Since the rehabilitator will most likely encounter the dietary form of MBD, only the nutritional-induced MBD will be discussed. The most common dietary errors are not enough calcium in the diet, too much phosphorous, too much protein and too many fruits. The opossum must have a certain amount of calcium and phosphorous in the diet with a specific calcium to phosphorous ratio (Ca:P). The desired ratio for the opossum is 1.5:1. If the opossum does not receive an adequate diet then dietary-induced disorders such as nutritional MBD may result.

At present, there is no commercially available diet formulated for the opossum. Feeding the OSUS recommended diet will provide the opossum with a nutritious diet and help prevent MBD. Nutrition-induced MBD is not seen in opossums fed the OSUS 90:10 diet. This diet is composed of 90% high quality, completely balanced cat or dog chow and 10% supplementary foods such as fruits, vegetables, yogurt, sardines (with bones) packed in water, chicken wings, egg, low or nonfat yogurt, etc. Do not feed the opossum meat without bones. When an opossum eats a prey, it consumes the entire animal, bones and all.

Commercial cat and dog food alone are not adequate diets for opossums but will provide a balanced dietary staple and help prevent diet-related disorders. A commercially prepared diet can more accurately guarantee that the opossum will receive a certain level of nutrients and on a consistent basis. This can not be guaranteed in “home made” diets or “smorgasbord” diets. Opossums permitted to select and choose favored foods will not put together a balanced diet. In general, homemade diets are not recommended if a reputable commercially available product is available to serve as a dietary staple. If you do plan to use a homemade diet then please evaluate the diet for completeness. It is important not to over-supplement with any food, nutrient, vitamin, etc.

Dietary-induced MBD may be PREVENTABLE and REVERSIBLE if caught early and corrected with a proper diet and supportive care.  Do not let this happen to the opossums in your care. Take the time to educate yourself about proper opossum nutrition.

Note: There are many opossum diets offered on the web and through different organizations. Some are acceptable diets and some are not. The OSUS diet has been proven to be a nutritious, balanced diet by a number of wildlife rehabilitators successfully raising and releasing opossums for over 25 years.  If you have had success with a different diet and wish to use it then you may continue to do so. Do not pick and choose the “best” parts from different diets. You are asking for trouble and an imbalance may result. If you find a diet that works for you then follow it in its entirety.

What happens?

As stated above, MBD is a complex disease and far too complex to describe here in detail. Basically, a low calcium diet can result in a decrease in the calcium in the blood. The parathyroid gland secretes parathyroid hormone (PTH) in response to a decrease in plasma calcium concentrations. When there is a calcium deficiency, high concentrations of circulating PTH (hyperparathyroidism) result. If the calcium deficiency is due to low dietary calcium intake then the increase PTH release is called Nutritional Secondary Hyperparathyroidism (NSHP), also known as Nutritional Metabolic Bone Disease. PTH prevents plasma calcium from falling too low by increasing conservation of calcium by the kidney, enhancing the absorption of calcium from the intestine, and by stimulating calcium mobilization from the bone. This results in a release of calcium into the blood to maintain the blood calcium level but also weakens the bone as minerals are lost thus subjecting the bones to increase risk of fractures. A sustained hyperparathyroidism can result in reduced appetite, interstitial nephritis, and can be toxic to the cardiac and skeletal muscles as well as the central and peripheral nervous systems.

What are the signs?

When first picking up opossums, obtain a detailed history from the finder. Knowledge of the dietary history will help you to recognize and correct MBD earlier.  The rehabilitator can easily miss early signs.

In the early stages (days to weeks) watch for a loss of grip in the hands or feet, tremors, twitching, decreased activity, sleeping more, uncoordinated gait, and a decreased appetite. You may notice as the disease advances that the opossum may have difficulty walking and climbing. It may take short, mincing steps, often with a hunched back. It may crawl or drag its limbs instead of walking. You may notice a bowing of the legs and a  “frog-like” appearance.  The limbs may appear chubby. Other skeletal deformities may be obvious. The bones at this stage are demineralizing and are extremely fragile. Bones easily fracture. Jerky movements and tremors may become more pronounced. Pain will be present.

As the disease progresses further, the opossum may be unable to eat and drink.  Changes in the skull occur resulting in an inability to use the mouth. The tongue may appear longer and protrude from the side of its mouth. Other organ systems may become affected, as the opossum gets closer to death. The rate of development of the disease is dependent upon the severity of the calcium deficiency and the age and health status of the opossum.

How is MBD treated?

If MBD is caught early enough then it can be treated and may be reversible. The diet must be corrected and supportive care offered. Feed a high quality, balanced diet with sufficient calcium and a proper Ca:P ratio. Do not feed meat without the bone.  Calcium is best obtained from the diet. Feed calcium-rich foods such as low or non-fat yogurt, kale, cabbage, collard greens, bok choy, etc. Calcium supplementation may be recommended in certain circumstances, especially advanced cases.  Caution must be taken to avoid over supplementation. Too much calcium can also be bad.

Consult your veterinarian, experienced rehabilitator, or OSUS for advice and dietary recommendations. In moderate to advanced cases make sure the opossum is not able to fall and hurt itself. Restrict climbing.

Make sure the opossum receives some exposure to sunlight for Vitamin D. They are nocturnal animals and receive Vitamin D in their diets but they also receive some exposure to sunlight while sleeping under woodpiles, dense shrubs, etc. Vitamin D is important for calcium metabolism. Make sure the opossum does not receive direct sunlight because it may suffer from the heat and may become sun burned.

The opossum did not develop MBD overnight and it will not be corrected overnight. Recovery will take at least as long as the time it took for the disease to cause the damage. Recovery may or may not be complete depending on a number of factors including duration of the disease, diet, overall health, age, parasite load, and any other complicating factors. Many opossums caught and treated early enough can be releasable. Others treated at more advanced states may recover sufficient motion but not sufficient enough to be released. These animals can be placed in wildlife centers for educational purposes, if of the proper temperament. Severe cases may require euthanasia, in the best interest of the opossum.

 This is meant for opossums that are to be re-released into the environment, non-releasables diet must be adjusted after symptoms are reversed.