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Viral and bacterial infections in mice and rats

Mice and rats suffer from a number of viral and bacterial infections. Here are some of the more commonly seen infections that you should keep an eye out for.

Your questions answered

What is chroic murine pneumonia (CMP)?

This is the most significant and serious bacterial infection in mice and rats. It is caused by the rather unusual bacterium, Mycoplasma pulmonis. This organism is relatively difficult to isolate because it cannot be grown in a laboratory using ordinary culture methods. This makes diagnosis of CMP more difficult, except for the fact that the disease is very common and well recognised. For this reason, CMP is usually diagnosed by signs of illness, without attempts to isolate the causative bacterium.

Caution must, therefore, be exercised when allowing contact between murine rodents and these potential "carriers." Mice and rats, too, may carry the Mycoplasma pulmonis organism without showing obvious signs of illness. This is especially true of newly acquired mice and rats. This fact underscores the importance of restricting contact between mice and rats of unknown health status and those whose health status has been proven by remaining disease-free for relatively long periods.

Furthermore, all newly acquired rats and mice should be quarantined (strictly confined from other pet rodents) for at least 4-6 weeks before contact with them is permitted. Any mouse or rat exhibiting respiratory signs (no matter how mild) should never be housed with or near a healthy pet mouse or rat. The severity of CMP can be increased substantially by any agent that harms the respiratory linings.

Other bacterial and/or viral infections and exposure to the irritating chemical effects of ammonia from urine within poorly maintained enclosures can complicate CMP, making the disease far more deadly.

How will I know if my pet has CMP?

Signs of CMP include sniffling, sneezing, squinting, red-brown tears, rough hair coat and laboured and audible respiration. If the inner ear becomes involved, a severe, often incapacitating head tilt usually develops. This disease can also seriously affect the reproductive capacity of female rodents, resulting in infertility and reduced litter sizes.

The bacterium responsible for CMP, Mycoplasma pulmonis, is highly contagious and can be transmitted between mother and offspring in the womb during embryonic life and by direct contact after birth. Transmission among infected and uninfected older rodents results from exchange of respiratory aerosols and sexual activity.

Can my pet be treated?

Because this disease tends to have a very chronic (long-lasting) course, afflicted individuals should receive antibiotic treatment as soon as the first signs are recognised. Antibiotics can be added to their drinking water for long periods. Individuals exhibiting serious, life-threatening signs must be treated aggressively with injectable antibiotics if there is any hope of helping them. It is also possible that other harmful bacteria complicate CMP which often necessitates use of multiple antibiotics.

Elimination of the Mycoplasma pulmonis organism from infected individuals is regarded by most experts as a practical impossibility. However, early treatment reduces the severity of the disease in affected rodents. The outcome of treatment is always unpredictable because there are so many factors that can have an influence on it, including individual susceptibility and resistance to the causative agent, age, physical condition, nutritional status, and presence of complicating factors, eg other bacterial and/or viral infections, high levels of ammonia within the enclosure, etc.

What other bacterial disease affect mice and rats?

A wide variety of other bacteria can cause illness in pet mice and rats. Your vet is best equipped to diagnose and prescribe medications for these diseases. Wounds (from fighting and other forms of trauma) are commonly infected with bacteria that already exist within the living quarters.

Abscesses commonly result from wounds when they have gone Unnoticed and untreated. Successful treatment of certain wounds, especially long and deep cuts, and abscesses requires veterinary intervention. Abscesses usually must be surgically opened because the relatively solid nature of rodent pus precludes lancing and draining them.

What about viruses?

In many mouse colonies, Sendai virus infection (also called parainfluenza 1) is the most significant. Sendai virus is closely related to parainfluenza viruses of human origin and is named after Sendai, Japan, where it was first isolated. Nursing mice and rats, and those being weaned are the most commonly and seriously infected. Adults may become infected but rarely show signs.

Like human flu, the clinical illness is caused by the animal's immune system trying to clear the body of the infection. Signs are variable, from none, to mild snuffles, laboured breathing, severe pneumonia, rough haircoat, lack of appetite, weight loss and even death. Bacterial infections complicate the infection and usually increase the death rate.

It has also been seen to affect litter size and cause growth retardation in kittens. The decreased litter sizes are caused due to the doe being so ill that not enough oxygen is getting to the litter in the uterus.

Unfortunately there is no specific treatment for this infection.

What is sialodacryoadenitis?

Sialodacryoadenitis is a highly contagious viral disease of rats and recently weaned mice. Initial signs include squinting, blinking and rubbing of the eyes. Later, sneezing and swelling in the neck region are noted. As the disease progresses, swellings below or around one or both eyes, bulging of the eyes, red-brown tears and self-trauma to the eyes are seen. Respiratory signs may also occur.

There is no specific treatment for this viral disease. This virus is very unlikely to infect pet rats and mice unless they were acquired from a colony with this infection already established within its members.

What is mousepox?

Mousepox is another highly contagious viral disease of mice that was only recently recognised in the United States. The mouse is the only natural host of the virus, and the acute (sudden onset) form of the disease affects the entire body.

Clinical signs include lethargy, hunched posture, rough haircoat, diarrhoea, inflammation of the eye membranes, swelling of the face and legs, and death. Another form of the disease results in a body-wide skin rash. Soon, the skin becomes swollen and ulcerated. Because of the resulting pain and discomfort, afflicted mice begin to chew on themselves. This behaviour often becomes obsessive, resulting in amputation of appendages. There is no specific treatment for this viral disease, however it is very unlikely to infect pet mice unless they were acquired from a colony with this infection already established within it.

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