Diseases and disorders commonly seen in pet mice include infectious diseases, injuries due to trauma, and problems related to nutrition and aging. Although the treatment of disorders in rodents is becoming more sophisticated, prevention of disease (by providing an appropriate diet and practicing good management and hygiene) is usually more successful than treatment.
A number of disorders can affect the digestive tract, including infections by bacteria, viruses, or parasites. Signs of illness include loss of appetite, weight loss, and rough or ruffled fur. Diarrhea may be seen.
Often it is not possible to determine the exact cause of the disorder. If a bacterial infection is suspected, your veterinarian may recommend an antibiotic to eliminate the infection, along with supportive treatment. If parasites (such as pinworms) are the cause of illness, they can usually be eliminated with appropriate treatment. Good sanitation helps prevent many diseases.
Infections Caused by Bacteria
Bacterial infections of the digestive system are not common in mice kept as pets, although they occur frequently in breeding or research colonies. Young, stressed animals are most likely to be infected. Signs of illness may include loss of appetite, dehydration, watery diarrhea, ruffled fur, lethargy, and hunched posture. In most cases, the bacteria are transmitted by exposure to the feces of infected animals. Bacterial infection is treated with appropriate antibiotics and fluids or antidiarrheal drugs if needed. Follow your veterinarian’s advice regarding the isolation of infected animals and disinfection of cages and equipment.
Infections Caused by Viruses
Like bacterial infections, viral infections of the digestive system are seen more commonly in colonies of mice than in mice kept as pets. Distinguishing whether an infection is viral, bacterial, or caused by other organisms can be difficult. Young mice are more likely to be infected than adult mice. Signs of illness may include watery diarrhea and weight loss. Only supportive treatments are available.
Infections Caused by Protozoa
Various types of protozoa (single-celled microorganisms) are normally present in the digestive tract of mice and do not usually cause disease. However, in younger or stressed mice, these protozoa (most commonly Spironucleus muris and Giardia muris) can cause intestinal infections. Infection is transmitted by contaminated feces. Infected mice have diarrhea, weight loss, lethargy, and a rough hair coat. Young mice may have stunted growth. The protozoa can be controlled with appropriate drugs but cannot always be eliminated.
Pinworms are common intestinal parasites in mice. Mice normally carry pinworms with no sign of disease, but a heavy infection can cause diarrhea due to intestinal inflammation. The disease is transmitted by eating materials contaminated with feces, and infected mice usually show few if any specific signs. The disease is diagnosed by identifying worms or their eggs in infected feces or on the area around the anus of the mouse. Medications available from a veterinarian can eliminate pinworm infection. Because pinworm eggs are light and may float in the air, it is important to sanitize and disinfect the cage as a part of the treatment program. Follow the advice of your veterinarian regarding medication and cage cleaning.
The infection of mice with tapeworms is relatively uncommon and there are usually no clinical signs of infection. However, diarrhea and weight loss may occur with a heavy infestation. The dwarf tapeworm can potentially infect humans if it is ingested. Care should be used when handling infected mice. Tapeworms are transmitted indirectly by cockroaches, beetles, or fleas. The infection is diagnosed by identifying tapeworm eggs in infected feces. Medications are available for treatment. The cage should be sanitized and disinfected. Follow the advice of your veterinarian regarding medication and cage cleaning.
Lung and Airway Disorders
Disorders of the respiratory tract are common in pet mice. General signs of illness may include sniffling, difficulty breathing, discharge from the nose, or sneezing. Mice with respiratory disease also tend to make a noise called chattering. Infections may be caused by bacteria, viruses, or sometimes by several different microorganisms (known as a mixed infection).
This bacterial infection causes sudden and severe respiratory disease—as well as long-lasting respiratory and other problems—in mice. The infection is transmitted by direct contact, airborne bacteria, and sexual contact. It can also be passed from a mother to her offspring during birth. The signs of upper respiratory disease (in the nasal passages and middle ears) include sneezing, sniffling, rough hair coat, lethargy, labored breathing, and weight loss. As the disease progresses, it will affect the lungs. The infection can become more severe in the presence of other bacterial and viral infections, often leading to pneumonia. Infection of the uterus and ovaries may occur in female mice infected with these bacteria, and genital infection may reduce fertility.
There is no cure or vaccine for mycoplasmal infection. However, infection and its signs can be suppressed with antibiotics. Keeping your mouse’s home clean and ensuring early treatment of the infection are the only ways that you can fight this disease.
Other Respiratory Infections
Several other bacteria and viruses can cause respiratory infections in mice and all may have very similar signs of illness. Of these, Sendai virus is the most likely to cause disease.
Signs common in these infections include sneezing, sniffling, labored breathing, rough hair coat, inactivity, weight loss, lack of appetite, and discharge from the eyes or nose. If you notice any of these signs, you should take your mouse to the veterinarian.
The diseases may be transmitted between mice by direct contact with infected animals, contaminated feces, or sneezing or coughing, depending on the organism. While most of these infections cannot be cured, your veterinarian may prescribe antibiotics to help reduce the severity of illness. Supportive care and keeping the mouse’s environment clean will also be helpful. Individuals showing signs of respiratory infection should be kept separate from other mice to reduce the spread of disease.
Some of the most common health problems in pet mice are disorders affecting the skin or fur. Skin inflammation and fur loss may be caused be infection, infestation with mites or other parasites, or barbering from incompatible cage mates.
This infection is caused by bacteria that are commonly found on the skin of most animals, including mice. Infection occurs when the skin is damaged by scratches or bite wounds. Mice with weakened immune systems are more susceptible to the infection. Inflamed skin and sores may be observed on the head and neck, and the resulting abscesses may enlarge and spread under the skin to form lumps (tumors) around the face and head. The infection is treated with antibiotics or antibiotic/steroid ointments applied as directed. To prevent further damage caused by scratching, the toenails of the back feet should be clipped.
Scaly Skin Disease (Corynebacterium bovis Infection)
This bacterial infection causes no signs in most mice, but can cause scaly skin in hairless (nude) mice. Bacteria are present in the skin or in the mouth. This disease is transmitted by direct contact with infected mice or with contaminated objects such as bedding. Infected mice have yellow-white flakes on the skin and lose weight. Most mice recover. The infection is diagnosed with blood tests. Treatment with appropriate antibiotics decreases the signs.
Ringworm is caused not by worms but by fungi called dermatophytes that parasitize the skin. The infection is contagious and can infect humans and other animals. Fungal skin infections occur infrequently in mice. Usually, infected mice do not have any visible signs. However, some affected mice have areas of hair loss and reddened, irritated, or flaky skin. The infection is transmitted by direct contact or by contaminated bedding, litter, or cage supplies. Treatment should be directed by your veterinarian and includes topical or oral antifungal agents. Even though a fungal infection may clear on its own in several weeks, treatment can reduce the chance the affected animal will spread the infection to people or other animals.
To control the spread of infection, other household pets should be checked for ringworm and treated if needed. It is necessary to wash your hands and all materials that have come in contact with an infected animal. The use of disposable gloves is also helpful. If you notice signs of the fungal infection on yourself, you should check with your own physician regarding appropriate treatment.
Skin and Fur Mites
Several types of mites may infest the skin and fur of mice. Mites are external parasites that often produce no visible signs in mice. Heavily infested mice may have inflammation of the skin, and mites can be seen as white specks of dust on their hair follicles. In addition, they experience intense itching that may cause scabbing on the shoulders, neck, and face. Mouse fur mites do not infest humans or other animals. Infestation is diagnosed by identifying the mites or their eggs from the hair and skin of the affected animal. Mite infestations are treated with medication given by mouth or applied to the skin of the mouse. Your veterinarian will advise you on the best treatment.
Under normal conditions mites are present in small numbers and do not bother their host; however, their numbers increase when the mouse is stressed, has decreased immunity due to other illnesses, or is unable to keep the numbers reduced by normal grooming. Therefore it is important that you provide proper regular care for your pet, including monitoring for illness. After treatment, the cage and all cage materials should be thoroughly cleaned and disinfected, because unhatched eggs may lead to reinfection.
Infestations of blood-sucking lice are common in wild mice but are rarely seen in pet mice. People are not affected by mouse lice because the lice do not cross from one species to another. Heavily infested mice have intense itching, restlessness, weakness, and anemia (lack of red blood cells). Infestation is diagnosed by identification of adult lice or eggs on the fur. Lice are treated similarly to mites (see above).
Rodent fleas are uncommon in pet mice but are sometimes seen if pets come into contact with wild rodents. Fleas are diagnosed by identifying them on the infested mice. Ask your veterinarian about treatment. To prevent reinfestation, disinfect and clean the cage thoroughly. When holding or playing with mice other than your own, it is recommended that you wash your hands and change clothes before handling your own mice.
Low humidity, high temperatures, and drafts predispose young mice to develop a ring-like constriction of the tail called ringtail. This condition can also involve the feet or toes. Ringtail can lead to swelling and eventually to loss of the part of the tail below the constriction. Ringtail is diagnosed through medical history and signs. The condition is not common in mice kept as pets. Surgical removal of all or part of the tail is often necessary, and tail stumps usually heal without complication. Ringtail can be prevented by providing an environmental humidity of 30% to 70%, reducing drafts, and maintaining the cage temperature between 64°F and 79°F (18°C to 26°C).
Barbering (Fur Chewing)
This abnormal grooming behavior is occasionally seen in groups of male or female mice. Dominant members of the group chew the hair and whiskers of less dominant mice. Because the mouse chews the hair so close to the skin, it gives the appearance of being clean-shaven, hence the term barbering. Stress, boredom, and even heredity can lead to this behavior, and mice sometimes barber themselves. The most common locations on the body for barbering are the stomach and front legs if caused by self-grooming, or on the muzzle, head, or shoulders of a cage mate. The skin is generally not affected, and its appearance will be normal, without signs of inflammation, irritation, or cuts. Unless irritation develops, this condition does not require treatment. If barbering occurs because of the presence of a dominant mouse, the dominant animal should be removed for the well-being of the other cage mates.
Barbering (fur chewing) may be due to boredom, stress, or a dominant mouse.
Male mice often fight each other and cause injuries to the face, back, and genital areas. The skin will have patches of hair loss and scabs. The injury can become infected with bacteria and turn into an abscess. Tail biting can lead to gangrene. Affected mice lose weight and sometimes die. Fight wounds are treated by cleaning them with an antiseptic solution, draining the abscesses, and applying appropriate antibiotics as recommended or prescribed by your veterinarian. Mice that fight frequently should be separated.
Disorders Affecting Multiple Body Systems
Some disorders of mice can affect more than one body system. These conditions are also called generalized or systemic disorders.
This disease, caused by Salmonella bacteria, is uncommon in pet mice. However, pregnant females and infants are at higher risk of infection. The infection is transmitted by eating food contaminated by feces and is often associated with food, water, or bedding contaminated by wild rodents. Signs of infection include diarrhea, dehydration, loss of appetite, weight loss, rough hair coat, depression, and sudden death. There is no treatment. Infected mice should be isolated and the cage should be sanitized and disinfected to eliminate any potential source of contamination.
Salmonellosis can infect humans. Washing your hands before and after every contact with your mouse and its cage will help prevent the spread of this disease. Using disposable gloves when handling infected animals or cage contents provides additional protection.
Streptobacillus moniliformis Infection
This infection is caused by bacteria normally found in the upper respiratory tract of rodents. Rats are the natural host for the bacteria, but infection can occur in mice. Infected mice may develop acute septicemia (bacterial infection of the blood), which is often fatal. Mice that survive may have chronic arthritis and deformities in their legs and feet. Follow your veterinarian’s advice regarding medication, supportive care for your pet, and cage cleaning. Streptobacillus moniliformis can also infect humans.
Pseudomonas aeruginosa Infection
This infection of the nasal cavities and intestines is caused by drinking contaminated water. It does not cause signs in healthy mice. However, those with weakened immune systems may develop discharge from the nose as well as a rough hair coat, hunched posture, loss of appetite, or sudden death with no indication of infection. There is no treatment, and every effort should be made to prevent infection by supplying clean drinking water and housing. Change your pet’s water at least once a day. Be sure that all water containers have been thoroughly cleaned before refilling them.
Mousepox (ectromelia virus) infection is a highly contagious viral disease that only affects mice. Pet mice are rarely infected unless they came from a colony with infected members. Therefore, it is important to get your pet from a reliable source. In the early stages of infection, mice lack energy and may have a hunched posture, rough hair coat, diarrhea, inflammation of the eyes, and swelling of the face. Death often follows. Mice with chronic (longterm) infection develop a body-wide skin rash. The skin becomes swollen and wounds appear on its surface. Because of the resulting pain, afflicted mice begin to chew on themselves, even to the point of amputating toes. The infection is transmitted through contact with contaminated feces or urine or by direct contact with skin wounds. Infection is diagnosed by physical examination and blood tests. There is no specific treatment for this infection. Consult your veterinarian regarding supportive care for an infected mouse and control of the disease.
Several parvoviruses can infect mice, including mouse parvovirus and minute virus of mice. However, these infections are not likely to cause signs in healthy mice. All parvoviruses are highly contagious among mice. Transmission occurs through direct contact with infected urine or feces or by contamination of objects (such as bedding) in the environment. Disinfection of the cage is required to eliminate the virus. There is as yet no treatment.
Lymphocytic Choriomeningitis Virus Infection
This disease occurs primarily in wild mice. However, mice can carry the virus and transmit it without becoming ill. Mice can become infected at a pet store by contact with other infected rodents (rats and hamsters) or from contact with urine or feces from infected wild rodents. The virus is transmitted by coughing, sneezing, or direct contact with the urine or saliva of infected animals. Infection is diagnosed with blood tests. There is no effective treatment. Affected animals should be euthanized and the cage should be appropriately sanitized and disinfected.
Mice with this virus can infect humans, in whom it can cause serious illness. It may cause influenza-like signs or viral meningitis, an inflammation of the brain and spinal cord. Infection in pregnant women may result in miscarriage or birth defects.
Atrial thrombosis is a heart disease that can occur in mice. Affected mice have an abnormally fast heart rate, bluish gums, and difficulty breathing. There is no effective treatment.
Osteoarthrosis is a type of arthritis that affects the joints in aged mice and can lead to difficulty of movement. Ask your veterinarian about appropriate treatments.
Mammary tumors are common in mice.
Degeneration of the retina is often genetic (inherited). It may also be caused by exposure to high light levels. Retinal degeneration often causes blindness. Most mice compensate by using other senses such as smell and touch.
Cataracts may occur in older mice or after a decrease in tear production.
Cancers and Tumors
Tumor development in mice depends on various factors, including the breed of mouse, environment, age, and infection by viruses that cause cancer. Treatment usually requires surgery to remove the tumor because tumors may grow and spread to other locations in the body. Early removal allows for the best outcome with the least chance of complication and recurrence of the tumor.
Mammary tumors are common in mice and can develop anywhere under the skin because mice have widely distributed mammary (breast) tissues. These tumors can be caused by infection with viruses that can be transmitted to baby mice through their mother’s milk and placenta. Your veterinarian can remove the tumors surgically, but the cancer often spreads to the lungs, and overall outcomes tend to be poor.