Medical Corner:
Weakness in Older Rats: A Paralysis Primer

Kathy Barrett
From the January/February 2000 Rat & Mouse Gazette

Congratulations, your rat has just qualified for the two year club (or for that matter, the 18 month milestone). While this may be a laudable accomplishment on the rat's part, a responsible rat owner has to be on the lookout for medical problems. Although elderly rats are more commonly afflicted with respiratory illness and tumors (usually mammary), the incidence of paralysis is also quite high. Causes for paralysis are numerous, and, like respiratory infection and tumors, early detection can minimize suffering for you and your rat.


Okay, so you suspect your elderly rat may have weakness. What are the possible causes, treatments, and prognoses? The list of possibilities is extensive; however, it can usually be narrowed down to three major players: stroke, pituitary adenoma, and spinal compression due to vertebral degeneration. Two factors help to distinguish among these disorders. Generally, only stroke will present with an acute onset, so it's important to observe how suddenly the weakness has occurred. Second, note the pattern of the weakness. Are the forelimbs primarily affected? Is one side of the body weak?


Strokes are fairly common in elderly rats, and may be the most treatable cause of paralysis. As mentioned, paralysis due to stroke is sudden. Your perfectly healthy rat may be frolicking with cagemates one day and be stricken the next. Due to the anatomy of the nervous system, stroke commonly affects one side of the body, but not always.

Stroke is caused by the interruption of blood flow to the brain or spinal cord. This may happen in one of two ways. First, a blood clot can plug vessels, starving nervous tissue, or a vessel can rupture causing bleeding. It is for this second reason that preventive treatment with aspirin (or other anticoagulants) is controversial and generally not recommended. The extent of damage will depend upon the size and location of the area affected. Strokes to the motor areas of the brain will cause the one-sided weakness noted above. More ominous are strokes that affect the brainstem, which contains areas vital for life, such as heart and respiratory centers. Brainstem strokes may cause a head tilt, as balance is mediated by this brain region.


Since our rats cannot talk to us and emotional changes are difficult to detect, the first signs of central nervous system damage will be motor, that is, difficulty moving or coordinating movement. Identifying moderate or severe paralysis is straightforward. The rat has trouble moving, cannot groom himself, and certainly cannot keep up with healthy cagemates. A number of conditions causing weakness in rats exhibit a gradual onset, therefore it is possible and advantageous to recognize (and treat) mild weakness as soon as possible.

Weakness in its earliest stages may manifest as clumsiness. Your rat will lose his grip on the wall of his wire cage, or may pratfall off objects in his home, such as cardboard boxes or ladders. A reluctance to climb by a formerly active rat may also be an indication of weakness. A rat with mild forelimb weakness may have trouble holding food (he is unable to curl his fingers around the food), and may contort himself during feeding. For instance, he may hunch over so that food is wedged securely between his body or the floor of the cage and his weak limb. During excursions outside his cage, watch your rat walk. Weak hindlimbs may cause the rat's back feet to slip, giving him the appearance of walking on an icy surface. In contrast, a rat favoring an injured limb will tend to lift it, sparing pressure from body weight. A weak rat, of course, is unable to lift the limb.


This is usually a benign tumor of the pituitary gland, which is situated at the base of the brain. Pituitary tumors primarily affect older female rats. The location makes the tumor inoperable, and the compression of nearby brain tissue likely. In contrast to strokes, the onset of weakness in an expanding pituitary mass is gradual. Commonly, the forelimbs will become weak first, and the rat will have trouble holding food. Weakness can then spread to the hindlimbs. Pituitary tumors carry a poor prognosis, but early treatment with prednisone can temporarily relieve symptoms. In addition, there has been some evidence that antibiotics can shrink tumors, and these may be added to the treatment regimen. Perhaps the best treatment for pituitary tumors is preventive. These are hormone-driven tumors, as are mammary tumors, therefore spaying greatly reduces their incidence. Spaying rats, of course, is a controversial and risky procedure, and not all rat owners condone it.


As a rat (or human, for that matter) ages, connective tissue such as cartilage degenerates. Vertebrae, the bones of the spinal column, are separated by pads of cartilage. When these degenerate, the vertebrae collapse on one another and the vertebral canal, where the spinal cord passes, can become narrow. (Or a disc can herniate from an unstable vertebral column and have the same effect.) The spinal cord may be compressed and cause weakness. Weakness from vertebral degeneration commonly affects the lower spine, and hindlimbs on both sides may be involved. Forelimb weakness may be caused by compression of upper regions of the spinal cord, which is less common. As with pituitary tumors, weakness is gradual in onset. Prednisone helps to minimize compression and any swelling due to arthritis, which is actually quite rare in rats.


In medicine, "zebras" are rare causes of common symptoms. Regarding weakness in rats, zebras may include infection of the central nervous system (mycoplasma has been known to infect the brain), including abscesses, which may behave like growing tumors. Spinal cord tumors have also been observed in rats. A rare case of metabolic disorder causing weakness has also been reported. In this rat, specific foods (such as broccoli) could not be properly metabolized, resulting in severe paralysis. Ingestion of toxins may also cause central nervous system damage, as will overdosing with certain drugs (such as Ivermectin).


The major causes of paralysis in rats can be treated with the anti-inflammatory agent, prednisone. Note that prednisone has its drawbacks, namely side effects, a major one being possible diabetes. Severe weakness will require nursing care, including bathing and feeding. Separation from cagemates may be necessary, depending on how the other rats are responding to the sick one. It is, of course, up to the owner to decide when and if to euthanize a gravely ill rat, paralyzed or not.