Mouse and Rat Medical Conditions


Chronic Murine Pneumonia (Murine  Mycoplasmosis)

Mycoplasma pulmonis is a very elusive bacteria which causes one of the most common and serious infections of rats and mice.  The organism is difficult to isolate by standard laboratory culture procedures.  As a result, a presumptive diagnosis is typically made based on the patient’s signs and symptoms.

Signs of mycoplasmosis include sniffing, sneezing, labored breathing, squinting, red-brown tearing, and a rough hair coat.  If the inner ear becomes infected, a head tilt and neurologic signs may develop.  In addition to respiratory signs, a genital infection may occur.  Manifestations of the genital form include infertility, embryonic resorption, and small litter size.  Compromise of the respiratory tract by other bacterial or viral agents or exposure to inhalant irritants can increase the severity of mycoplasmosis.  The disease usually runs a chronic course, which often results in death if not treated early on.

Antibiotic therapy should be initiated at the first suspicion of infection.  Due to the chronicity, long-term treatment by antibiotics in the drinking water may be necessary to suppress the infection.  Severely affected individuals may need injectable medications and extensive supportive care.  In addition, secondary infections with other organisms are common, sometimes necessitating the use of multiple medications.  The goal of therapy is to reduce the severity of symptoms, but complete elimination of the infective bacteria is practically impossible.

The disease is highly contagious.  The bacteria is spread by direct contact with affected individuals or from an affected mother to her unborn young in the womb.  Transmission usually occurs through respiratory aerosol and/or sexual activity. Rabbits, guinea pigs, and other rodents can serve as carriers of the disease without exhibiting clinical signs.  Other mice and rats can also serve as carriers.  It is extremely important to restrict contact between mice and rats of unknown health status until a quarantine period has elapsed.  A quarantine period of four to six weeks is recommended.  Any animal exhibiting even the slightest signs of respiratory illness should remain isolated.

Tyzzer’s Disease

A common infectious disease of rodents is Tyzzer’s disease  It is caused by a bacteria (Bacilus piliformis) that infects living cells.  The disease causes a high death rate in young, stressed rodents; particularly mice and gerbils.  Clinical signs are nonspecific, but primarily appear as ruffled fur, lethargy, hunched posture, and poor appetite.  Diarrhea may also be present.  The disease causes changes in the heart, liver, lymph nodes, and digestive tract, which are often noted at necropsy.

Prevention is the key to this disease.  Strict sanitation, and minimal stress greatly reduces the occurrence of this disease in colony situations.  This disease can be harbored by outwardly normal looking rodents, and then spread to cause disease in others.  Tyzzer’s disease usually affects rodents that are stressed by weaning, shipping, and adjusting to new environments.  Thorough sanitation prior to introduction of new animals is important in preventing outbreaks.

Sendai Virus

Sendai virus causes one of the most significant and severe respiratory infections of laboratory rodents.  Suckling and weaning mice are most commonly affected, posing a serious problem to mouse colonies.  Other affected species include rats, hamsters, guinea pigs, and swine.  It is unlikely for a pet mouse to become infected unless it was acquired from an colony that contained the disease.

Signs of infection are usually apparent in nursing mice, while infected adult mice rarely show symptoms.  Signs include labored breathing, chattering, rough hair coat, weight loss, and death.  Secondary bacterial infections often worsen the disease, resulting in a higher death rate.  Sendai virus infections are usually subclinical (without signs) in other susceptible rodents, but these species may be a source for infection in young mice.

There is no specific treatment for this disease.  Supportive care and treatment of secondary bacterial infections may lessen the severity of symptoms.  A vaccine is available, but only practical for use in large colonies of affected mice.  Prevention involves selecting mice from Sendai virus-free source and keeping them isolated from mice and other potential carriers of unknown backgrounds.


The natural host for this highly contagious viral disease are rare.  The disease is usually self-limiting in young rats.  Recently weaned mice may also be affected.  The disease is spread from affected individuals through respiratory aerosol or direct contact with the respiratory secretions.  Infected rodents carry and secrete the virus for about seven days.

Signs are variable depending on the age and immune status of the affected rat or mouse.  The most serious signs are seen in 2-4 week-old rats that did not receive maternal antibody protection.  Initial symptoms include squinting, blinking, and rubbing of the eyes.  Sneezing and swelling in the neck area develop subsequently.  Finally, swellings below or around the eyes, production of red-brown tears, and self trauma to the eyes may occur.  Respiratory signs may be present, especially if complicated by Sendai virus or murine mycoplasmosis.  The affected rat usually remains active and eating during the course of this disease.


Rats and mice are very susceptible to certain types of tumors.  It is reported that rats over two years of age have an 87% chance of developing a tumor.  The most common type of tumor in the rat is mammary fibroadenoma (breast tumors).  Numerous other forms of tumors occur, but to a lesser degree.  Mice develop tumors in a wide variety of tissues, both internal and external.  Leukemia, cancer of the white blood cells, is common in the mouse.  More often tumors in a mouse are more likely cancerous versus the benign tumors that rats typically get.

In rats, mammary tumors can occur in both the female and the male.  Since rats have widely distributed mammary tissue, it is not unusual to find these tumors behind the front legs, along the sides and flanks, and along the underside of the body.  These tumors can be removed surgically but often additional tumors occur.  When not treated, these masses continue to enlarge, ulcerate, and become infected.  Early surgical removal allows for the best outcome with the least chance of complications or recurrence.  Early spaying of the females (4 months) may reduce the incidence of these tumors, and spaying adults may also help.

Red-Brown Tears of Rats

Rats secrete red tears from a gland behind their eyes.  This is a normal secretion of porphyrin pigments produced by this “harderian” gland.  These tears are often mistaken for blood.  Most commonly they appear during stressful situations and disease states.  The eyelids, nares (nostrils), and forepaws may be smeared with the pigment.  When these tears are present the underlying cause of stress should be evaluated.

Ring Tail

Low humidity and high temperatures may contribute to ”ring tail” of young rats.  Ring tail is a condition where constricting bands along the tail length cause compression, tissue death, and sloughing.  Other associated factors include vascular problems within the tail, endotoxins (toxins from the digestive system), and high dietary lipids.  Treatment involves correcting the environmental factors, but often necessitates tail amputation.

Overgrown Incisors

The normal ratio of incisor tooth size is 3:1 (lower to upper).  These teeth can become overgrown if there is a mall alignment of the teeth rows.  Treatment includes regular trimming (often requiring sedation), and sometimes extraction.

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