Merck Manual

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Pet Owner Version

Disorders and Diseases of Rats


Katherine E. Quesenberry

, DVM, MPH, DABVP (Avian);

Kenneth R. Boschert

, DVM, DACLAM, Washington University

Reviewed/Revised Apr 2020 | Modified Oct 2022
Topic Resources

Infections, parasites, and cancer are some of the most common types of diseases and disorders in rats. Providing a balanced diet, proper housing, and routine veterinary care will help maintain your rat in the best condition possible.

Digestive Disorders of Rats

The most common digestive disorders of rats are caused by intestinal parasites or bacterial infection. These conditions are described in this section.

Intestinal Parasites

Pinworms are common intestinal parasites in rats. They only require one host (in this case, rats) and are transmitted through infected feces. Most infected rats have no signs, but a heavy infection of pinworms can cause diarrhea due to intestinal inflammation. The disease is diagnosed by identifying worms or their eggs in infected feces or on the area around the anus of the rat. Pinworm infections can be treated and controlled by using appropriate drugs prescribed by a veterinarian. Because pinworm eggs are light and may float in the air, it is important to sanitize and disinfect the cage on a regular basis.

The infection of rats with tapeworms is relatively uncommon, and there is usually no sign of infection. However, diarrhea and weight loss may occur with heavy infestation. The dwarf tapeworm can potentially infect humans if it is ingested. Tapeworms are transmitted indirectly through cockroaches, beetles, or fleas. The infection is diagnosed by identifying tapeworm eggs in infected feces. The worm infection is treated by using appropriate antiparasitic drugs. The cage should be sanitized and disinfected.

Rats are the intermediate host for the cat tapeworm, Taenia taeniaeformis. Cat tapeworms can infect rats. Infection occurs when rats eat feed or come in contact with bedding contaminated with cat feces. Tapeworm cysts embed in the rat’s liver, which becomes enlarged. It is important for those who own both rodents and cats to eliminate potential sources of infection.


Various types of protozoa (micro-organisms) are normally present in the digestive tract of rats and do not usually cause disease. However, in younger or stressed rats, these protozoa can cause intestinal infections. The infection is transmitted by contaminated feces, and infected rats have diarrhea, lethargy, rough hair coat, weight loss, and, in severe cases, heavy bleeding that can lead to death. The protozoa can be controlled with appropriate drugs but cannot always be eliminated.

Brain, Nerve, and Spinal Cord Disorders of Rats

Spinal cord degeneration can occur in rats older than 2 years. Affected rats’ hind limbs are paralyzed.

Lung and Airway Disorders of Rats

Chronic respiratory disease (murine respiratory mycoplasmosis) is caused by a bacterial infection. It causes both short- and longterm respiratory signs and other problems in rats. The infection is transmitted by direct contact, airborne bacteria, and sexual contact. It can also be passed on from a mother to her offspring during birth. The signs of infection vary but may include sneezing, sniffling, rough hair coat, lethargy, labored breathing, weight loss, head tilt, and reddish-brown staining around the eyes and nose. As the disease progresses, it affects the lungs.

The infection can become more severe in the presence of other bacterial and viral infections. Infection of the uterus and ovaries may occur in female rats with chronic respiratory disease.

There is no cure for this condition. The signs of infection may be improved with antibiotics. Rats with chronic respiratory disease rarely live longer than 2 years. Keeping your rat’s home clean—in particular by reducing ammonia levels in the cage—and early treatment of the infection are the best ways to fight this disease.

Several other bacteria and viruses can cause respiratory infections and pneumonia in rats. These all lead to similar signs, such as 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 rat to the veterinarian.

The diseases may be transmitted between rats by several routes, depending on the organism, including direct contact with infected animals, contaminated feces, or sneezing or coughing. Although most of these infections cannot be cured, your veterinarian may prescribe antibiotics to help reduce the severity of illness. Supportive care and keeping the rat’s environment clean will also be helpful. Individuals showing signs of respiratory infection should be kept separate from other rats to reduce the spread of disease.

Rats are the natural hosts of the bacterium Streptobacillus moniliformis, which causes rat bite fever in people. Rats carry these bacteria in their upper respiratory tracts (nose and throat). The bacteria do not cause disease in rats but can cause serious illness in humans.

Sialodacryoadenitis and Rat Coronavirus Infection

These related viruses infect the nasal cavities, lungs, Harderian glands (near the eyes), and salivary glands of rats. They are highly contagious and are transmitted by direct contact, airborne virus particles (such as from an infected rat’s sneeze), or by exposure to contaminated bedding, feces, or other objects in the cage. To prevent the transmission of these diseases, it is important to wash your hands after handling animals in a pet shop or at a friend’s house before handling your rat. Infection usually lasts 2 to 3 weeks.

Affected rats will sneeze and may seek to avoid direct or bright light. Enlarged salivary glands and lymph nodes can sometimes be felt. A rat may look as if it has mumps. Reddish-brown pigments and discharge may be seen around the eyes, and eye involvement such as inflammation of the cornea or conjunctiva may occur; however, not all infected animals show these signs. There is no treatment, but infected animals usually recover and will develop resistance to future infections by these viruses.

Skin Disorders of Rats

Skin disorders are common in rats. They can be caused by parasites, bacteria, or injury.


This abnormal grooming behavior is occasionally seen in groups of male or female rats. Dominant members of the group chew the hair and whiskers of less dominant rats. Because the rat 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 rats 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 rat, the dominant rat should be removed for the well-being of the other cage mates.

Fight Wounds

Male rats often fight and cause injuries to the face, back, and genital areas. The skin will have patches of hair loss and scabs. Such injuries can become infected with bacteria, leading to the formation of abscesses. Tail biting can lead to gangrene. Affected rats lose weight and sometimes die. Fight wounds can be treated by cleaning them with an antiseptic solution, draining the abscesses, and applying appropriate antibiotic ointments as recommended or prescribed by your veterinarian. Rats that fight frequently should be separated.


Rodent fleas are uncommon in pet rats but are sometimes seen if pets come into contact with wild rodents. Fleas are diagnosed by identifying them on the infested rats. Ask your veterinarian about treatment. To prevent reinfestation, disinfect and clean the cage thoroughly. When holding or playing with rats other than your own, it is recommended that you wash and change clothes prior to handling your own rats.


Infestations of blood-sucking lice are common in wild rats but are rarely seen in pet rats. Human beings are not affected by rat lice because the lice do not cross from one species to another. Heavily infested rats have intense itching, restlessness, weakness, and anemia (lack of red blood cells). Infestation is diagnosed by identification of adult lice or eggs on the rat fur. Lice are treated similarly to mites (see below).


Several types of mites may infest the skin and fur of rats. Mites are external parasites that often produce no visible signs in rats. Heavily infested rats may have inflammation of the skin, and mites can be seen as white specks of dust on their hair follicles. In addition, mites can cause intense itching that may cause scabbing on the shoulders, neck, and face. Rat 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 rat. Mites are treated with medication given by mouth or applied to the skin. 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 rat 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 care for your pet, including monitoring for illness. After treatment, the rat’s cage and all cage materials should be thoroughly cleaned and disinfected because unhatched eggs may lead to reinfection.

Ringtail Syndrome

Low humidity, high temperatures, and drafts predispose young rats to develop a ring-like constriction of the tail called ringtail. This condition can also involve the feet or toes. Ringtail is most often seen in laboratory rats and is fairly rare in rats kept as pets. Ringtail can lead to swelling and eventually to loss of the part of the tail below the constriction. 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 cage temperature between 64°F and 79°F (18°C and 26°C).


Ringworm is caused by fungi called dermatophytes that parasitize the skin. The infection is spread by direct contact or by contaminated bedding, litter, or cage supplies, and it can infect humans and other animals. Ringworm occurs infrequently in rats. Infected rats may not have any visible signs. However, some affected rats have areas of hair loss and reddened, irritated, or flaky skin. Treatment should be directed by your veterinarian and includes eliminating the fungus by using 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 disease to people or other animals.

Staphylococcal Infection

This infection is caused by Staphylococcus bacteria that are commonly found on the skin of most animals, including rats. Infection occurs when the skin is damaged by scratches or bite wounds. Rats with weakened immune systems are more likely to become infected. 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 hind toenails should be clipped.

Kidney and Urinary Disorders of Rats

Chronic progressive nephrosis (glomerulonephrosis) is a common disease of older rats. It involves inflammation of the blood vessels in the kidney. The disease and its severity are influenced by the rat’s sex and hereditary background and by dietary factors such as protein content and total calorie consumption. The disease occurs earlier and is more severe in male rats than in female rats. Affected rats are lethargic and lose weight, and they may have increased thirst and urination. Unfortunately, there is no treatment, and the condition is always fatal. Supportive treatment may be able to decrease the signs.

Diseases that can be Spread from Rats to People

  • Borreliosis

  • Leptospirosis

  • Melioidosis

  • Plague

  • Rat bite fever

  • Tularemia

  • Murine typhus

  • Hymenolepiasis (rat tapeworm)

  • Salmonellosis

  • Ringworm

Leptospirosis is a bacterial infection of the urinary tract that is most often reported in wild rats and mice. It can potentially be transmitted to pet rats and humans. Diagnosis is based on blood tests or isolating the bacteria from the urine. Treatment is not recommended because of the risk of human infection.

Certainroundworms (nematodes) occasionally infect the rat’s bladder, but these parasites are rarely reported. The treatment is similar to that for cases of intestinal pinworms ( see Intestinal Parasites Intestinal Parasites Infections, parasites, and cancer are some of the most common types of diseases and disorders in rats. Providing a balanced diet, proper housing, and routine veterinary care will help maintain... read more , above).

Uroliths (stones) occur in the kidneys and bladders of older rats. Affected rats either show no signs or can have blood in the urine and other signs of inflammation or infection of the bladder. Surgery may be necessary to remove the stones. If stones block the passage of urine or damage the kidneys, death may occur.

Disorders Affecting Multiple Body Systems of Rats

Some disorders of rats can affect more than one body system. These conditions are also called generalized or systemic disorders. The disorders that affect multiple body systems of rats are listed below.

Lymphocytic Choriomeningitis Virus Infection

Infection with this virus occurs occasionally in rats. Rats can become infected at pet stores by contact with other infected rodents (mice, guinea pigs, or hamsters) or from contact with the urine or feces of wild rodents, such as house mice. Infection is transmitted by coughing, sneezing, or direct contact with the urine or saliva of infected animals. Most infected rats do not show signs. However, some rats carry the virus and shed it in high quantities through urine. There is no effective treatment. Affected animals should be euthanized and the cage should be appropriately sanitized and disinfected.

It is possible that rats with this virus could pass the infection to 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. However, human infections from pet rodents are rare.

Parvovirus Infection

This viral infection is uncommon in rats kept as pets. During the active stage of the infection, infected rats have small litters, stillborn pups, infertility, and runting. Infected rats may develop fatal hemorrhage (bleeding) and cell death in the brain and gonads. All parvoviruses are highly contagious. 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 currently no treatment.

Polyarteritis Nodosa

Polyarteritis nodosa involves inflammation of the walls of the arteries and can affect many organs. The cause of disease is unknown. Heart attacks and aneurysms may occur in rats with polyarteritis nodosa.


This disease, caused by Salmonella bacteria, is uncommon in pet rats. However, pregnant females and infant rats 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. Affected rats may have a distended abdomen (belly), diarrhea, dehydration, weight loss, rough hair coat, depression, and sudden death. Miscarriage may occur in pregnant rats. Infected rats can also transmit the disease to people. There is no treatment. Affected rats should be isolated and the cage sanitized and disinfected to eliminate any potential source of contamination.

Cancers and Tumors of Rats

Rats are very susceptible to the development of tumors. Your veterinarian will likely recommend surgical removal of 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 complications and recurrence.

Mammary tumor, rat

Mammary tumor, rat

Keratoacanthomas are benign tumors of the skin. They develop on the chest, back, or tail.

Mammary fibroadenomas are the most common tumors in rats. Because rats have widely distributed mammary (breast) tissue, tumors may be found under the skin anywhere on the belly side of the body, from chin to tail. Both female and male rats can develop tumors. Typically these tumors are soft, round, or somewhat flat growths that can be moved by firm pressure. Your veterinarian will perform surgery to remove the tumor, but recurrence in other parts of the body is common. Most of these tumors do not become malignant (cancerous).

Tumors of the pituitary gland, a gland linked to the brain that controls hormonal secretion, are common in rats, especially females. The development of these tumors increases with the consumption of high-calorie diets. Affected rats have head tilt and depression and may die suddenly.

Most testicular tumors in rats are benign. The recommended treatment, when necessary, is the surgical removal of the testicle.

Tumors of the Zymbal gland are infrequent in rats. They develop at the base of the ear in older rats. These tumors may be malignant.

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