Chronic Murine Pneumonia (Murine Mycoplasmosis)

Mycoplasma pulmonis is a very elusive bacteria that 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 sniffling, sneezing, labored breathing, squinting, red-brown tearing and a rough hair coat. If the inner ear becomes infected, a head tilt and neurologic signs 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 to the respiratory tract by other bacterial or viral infections or exposure to inhalant irritants can increase the severity of mycoplasmosis. The disease runs a chronic course, which may result in death if not treated early.


Antibiotic therapy should be initiated at the first suspicion of infection. Severely affected individuals may need injectable medications and extensive supportive care. In addition, secondary infections with other organisms are common, sometimes requiring 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 are spread by direct contact with affected individuals or from an affected mother to her unborn young while still in the womb. Transmission usually occurs through respiratory aerosol and 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, caused by a bacteria (Bacillus 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 can be observed at post mortem.


Sendai virus

The 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 affected colony.


Signs of infection are usually expressed in nursing mice, while affected 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 sub-clinical 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 signs. A vaccine is available, but it is only practical for use with large colonies of affected mice. Prevention involves selecting pet mice from a Sendai virus-free source and keeping them isolated from mice of unknown backgrounds and other susceptible rodents, which may carry the disease.



Rats are the natural host for this highly contagious viral disease. 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 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 to 4 week old rats with no maternal antibody protection. Initial symptoms include squinting, blinking and rubbing of the eyes. Sneezing and swelling in the neck area develop later. Finally, swellings below or around the eyes, bulging of the eyes, production of red-brown tears and self-trauma to the eyes are noted. 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 the development of tumors. It is reported that rats over two years of age have an 87% chance of developing tumors. The most common type of cancer in the rat is mammary fibroadenoma (breast cancer). Numerous other forms of cancer occur, but to a lesser degree. Mice develop tumors in a wide variety of tissues, both internal and external. Leukemia, cancer of white blood cells, is also common in the mouse.


The most common type of cancer in these pets is the mammary tumor of rats. This form of breast cancer can occur in both female and male rats. Since rats have widely distributed mammary tissue beneath the skin, it is not unusual to find these tumor lumps behind the front legs, along the sides, in the flanks, as well as along the underside of the body. These tumors can be removed surgically, but often recur. If not treated and surgically removed, 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.


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 the harderian gland. These tears are often mistaken for blood. They usually appear during stressful situations and disease. The eyelids, nares and forepaws may be smeared with pigment. When present, the underlying cause of stress should be sought and relieved.


Ring Tail

Low humidity and high temperatures may result in ring tail of young rats. Ring tail presents as constrictive bands along the tail. Other factors that have been implicated in this condition include the vascular structure of the tail, the presence of endotoxins and high dietary lipids. Treatment involves correcting the environmental conditions.