Contagious and Infectious Feline Diseases (Part 1)

The Veterinarians’ Guide to Your Cat’s Symptoms: Contagious and Infectious Feline Diseases

Although cats are susceptible to a number of infectious and contagious diseases just as all other mammals are, for many years research into feline diseases and the development of preventive medicines for cats took a backseat to the study of canine diseases. Now, however, with the burgeoning growth of cat ownership, knowledge of cat diseases and how to prevent and treat them has grown by leaps and bounds.

Infectious diseases are caused when a cat’s body is invaded by a disease-producing entity or entities such as bacteria, fungi, protozoans, parasites, or viruses. Contagious diseases are infectious diseases that can be passed from one cat to another. Thus, contagious diseases are always infectious, but not all infectious diseases are contagious (that is, able to be “caught” by a cat from another cat). In general, bacterial diseases are less contagious than viral diseases, which are usually very contagious.

Contagious diseases can be transmitted from one cat to another by various means. These include direct contact with an infected cat or with her urine, feces, saliva, or blood; or the inhaling of airborne viruses or bacteria, which does not require direct contact between cats. Some infectious diseases are transmitted by an intermediate host such as a parasite. Viruses and bacteria can also be carried on people’s hands and clothing, shoes in particular.

Fortunately, routine immunizations are almost entirely able to prevent the majority of feline infectious viral diseases. Most parasites can also be avoided by the use of preventive medications. Diseases that cannot be prevented can usually be successfully treated, providing the treatment is begun as soon as symptoms appear.

Infectious Viral Diseases Against Which Cats Can Be Immunized Feline Infectious Peritonitis (FIP)

FIP is caused by a feline coronavirus and is a very strange disease. Like FeLV and FIV, the FIP virus is not highly contagious among cats but very young neonatal kittens may be highly susceptible to it. It is generally believed that most cats with FIP are infected early in life and the virus may be dormant in these cats for a very long time (months to many years). What eventually activates the virus is not understood at the present time. Because of the susceptibility of very young kittens, this virus is a significant problem in catteries with infected queens.

Coronavirus does not appear to directly attack a cat’s organs. Rather, a combination of the vitrus and antibodies to the virus produced by the cat himself form complexes that circulate through the cat’s bloodstream and eventually lodge in the blood vessels and tissues. These immune complexes then stimulate an inflammatory reaction that damages or destroys the tissues in which the immune complexes came to rest. In a very real sense, the cat’s immune system is responsible for most of the disease. There are also milder strains of coronavirus (enteric coronavirus) that cause only minor intestinal disease. It is unclear whether these are less virulent forms of the same coronavirus or a different coronavirus.

Like FeLV, FIP can produce a variety of syndromes in cats. The type of disease varies with the age of the cat, virulence of the virus, the organs in which immune complexes lodge, and possibly, susceptibility of the cat. A common syndrome seen in kittens and young adults is called “wet,” or effusive FIP. In this syndrome, the lining of the chest and/or abdominal cavities becomes inflamed. A high-protein, viscous watery fluid builds up in the chest or abdomen, causing difficult breathing or abdominal swelling. Accompanying signs include fever, vomiting, diarrhea, anorexia, and rapid loss of condition. This syndrome is usually quickly fatal.

In middle-aged or older cats, FIP is most often seen in the “dry,” or granulomatous form. In this form, chronic, granulomatous inflammation destroys the affected organ. These cats can virtually have any symptoms, including jaundice, kidney swelling and failure, brain swelling and seizures, vomiting, diarrhea, anorexia, weight loss, and fever. In other cats, FIP may produce a “fading kitten” syndrome, lameness from joint inflammation, or chronic respiratory problems.

Testing for FIP is problematic because we only have a blood test for coronavirus antibodies. An antibody test cannot accurately separate an exposed or immune animal from a currently infected animal. Theories about high antibody titers being more indicative of infection than lower antibody titers do not apply very well in individual cases and the test is often misinterpreted. The presence of enteric coronavirus, which cross- reacts with FIP virus, adds further confusion to the situation. It is the opinion of the authors that cats should not be euthanized only because of a high coronavirus antibody titer. The titer can be helpful when interpreted along with other blood tests, fluid analysis, cytology, or classic signs and symptoms.

FIP usually carries a poor prognosis because there are no specific therapies for this disease. Anti-inflammatory drugs can provide some relief in cats with “dry” FIP, and other symptomatic treatments may help for a while. The “wet” form of FIP is very resistant to therapeutic intervention.

A new FIP vaccine has recently been developed, but this vaccine is somewhat controversial. The vaccine is administered intranasally to stimulate immunity at the portal of entry of the virus. Because the FIP disease is produced in part by the body’s systemic immune response, this is probably a better vaccination attempt than giving an injection to stimulate systemic immunity. The vaccine should not be given until sixteen weeks of age. This is part of the controversy with this vaccine.

Since many, or most, cats are infected as newborns, this may be too late to protect them. The complicated technology of this vaccine does not, however, allow earlier administration.

Feline Immunodeficiency Virus (FIV—formerly called feline T- lymphotropic lentivirus, FTLV)

FIV is a cat retrovirus that is contagious to cats, but not to dogs and humans. Like FeLV, this virus is not highly contagious among cats but can be transmitted among those with intimate social contacts. The primary mode of transmission is through bite wounds and is, therefore, most commonly seen in male cats and cats who fight with other cats.
There may be other modes of transmission, however.

Like other retroviruses, FIV has a lengthy incubation period from infection to onset of symptoms. FIV is generally less aggressive than FeLV, but still will kill most cats eventually. Cancers and tumors are not caused by FIV, but serious immune-deficiency and bone marrow suppression occurs in infected cats.

There is no vaccine and no specific treatment available for FIV. Just as with FeLV, a very sensitive test is available and this can be used to limit the spread of the disease. New cats should not be introduced into households with uninfected cats until the new cat is found to be free of FIV. Outdoor cats and indoor-outdoor cats should be tested periodically and, if positive, should be isolated from uninfected cats or removed from the household.

Feline Leukemia Virus (FeLV)

The feline leukemia virus is one of the most devastating of all feline infectious diseases. The virus is classified as a retrovirus, the same type of virus as the feline immunodeficiency virus (FIV) and the human immunodeficiency virus (HIV). It is important to emphasize that these viruses are species-specific—FeLV and FIV cannot infect humans or dogs and HIV cannot infect cats or dogs. FeLV is a contagious virus between cats but is not as highly contagious as the upper respiratory viruses (see below). Casual contact between cats will not usually transmit the virus, but living together in the same house increases the risk of disease transmission over time. For this reason, uninfected cats should not be housed with infected cats.

As with other retroviruses, there may be a considerable time period (many years) from infection to development of disease symptoms. The FeLV virus can produce a variety of disease syndromes in cats. This is unpredictable from cat to cat. Cats infected with FeLV may develop cancers called lymphoma, lymphosarcoma, or leukemia, depending on the organs affected. Leukemia means cancerous lymphocytes circulating in the bloodstream. Lymphoma/lymphosarcoma are solid tumors affecting a cat’s lymph nodes, spleen, intestines, anterior chest cavity, kidneys, liver, brain, and/or virtually any other body organ.

Chemotherapy and/or radiation may help some cats with FeLV-induced cancer, but the eventual prognosis is not good. The FeLV virus can also produce a syndrome of immune deficiency, in which a cat becomes unable to protect himself from other infectious agents, bacteria, protozoans, fungi, or viruses. In these infected cats, death usually occurs from secondary infections. Antibiotics and other medications may help for a while, but none of the anti-infective drugs work very well without the body’s normal immune system.

The third syndrome seen in FeLV-infected cats is severe bone marrow suppression. In these cats, the bone marrow, which is responsible for manufacturing red blood cells (to carry oxygen), neutrophils (white blood cells that help fight infection), and platelets (small cells that help blood to clot), is suppressed and natural cell production is interrupted. A syndrome called pancytopenia (decrease in all blood cells) is seen, leaving the patient anemic, unable to fight infection, and unable to clot blood normally. There is no effective treatment for this syndrome and death eventually intervenes.

Because of the serious nature of this disease and poor available treatment, an owner must focus on prevention. There are very sensitive tests available that can detect the FeLV virus in cats long before they show disease symptoms. A new cat should not be introduced into a household with uninfected cats until the newcomer has been tested for FeLV. Outdoor cats and indoor-outdoor cats should be periodically tested according to a veterinarian’s advice. FeLV positive cats should be isolated from uninfected cats or removed from the household. This virus is not very persistent in the environment and common household disinfectants, including dilute bleach solution, will kill the virus in the environment.

There are several FeLV vaccines available but there is still a lot of controversy about their use. There are side effects from the use of these vaccines in many cats. The benefit of FeLV protection versus the risk of these side effects is situational, depending mostly on a particular cat’s lifestyle. This issue should be discussed with a veterinarian before a decision about vaccination is made. Most veterinarians will not recommend the vaccination for cats who are exclusively indoor cats. For these cats, the best protection is care when introducing new cats into the household. The issue is more complex for outdoor and indoor-outdoor cats. One rare side effect of the FeLV vaccine that has recently come to light is creation of a cancer called fibrosarcoma at the injection site. This is a very aggressive tumor that is probably not caused by the killed FeLV virus in the vaccine. Rather, it is probably caused by a chemical, called an adjuvant, which is added to the vaccine to stimulate the proper immune response to the FeLV virus. This side effect is rare, occurring in one in 5,000–10,000 cats, but can be fatal. Other vaccine side effects include immune system dysfunction, bone marrow suppression, and systemic illness.


This ancient, fatal viral disease occurs throughout the world. While it can affect any warm-blooded mammal, most rabies in the United States is carried by bats, skunks, foxes, and raccoons. Chance encounters with these animals are the most common source of rabies in dogs and cats.

While dogs were once the subject of most rabies legislation, rabid cats, while rare, outnumber rabid dogs and many municipalities now require rabies vaccination for cats. There is no cure for rabies and the disease is always fatal. Concern about rabies is focused mostly on the public health aspect of the disease.

The virus is transmitted primarily by animal bites with the virus present in the biting animal’s saliva. The incubation period of the rabies virus varies tremendously and can range anywhere from one week up to a year. However, the presence of the virus in the infected animal’s saliva occurs late in the infection, within ten days of death from the disease.

This is why biting animals must be confined or observed for ten days after biting a person. If the animal is still healthy and alive ten days later, then the rabies virus was presumably not in the saliva at the time of the bite.

The most common sign of rabies in cats is neurologic impairment of a limb where a wild animal bite occurred. The virus travels up the nerves to the brain and eventually reaches the salivary glands. Therefore, any neurologic manifestation is possible, including behavioral changes, seizures, cognitive disorders, and difficulty swallowing.

Vaccination is the best method of rabies prevention. Kittens should be vaccinated at three to four months of age (depending on local laws). The first vaccine is good for one year. After that, boosters are usually good for three years. Because killed rabies virus vaccines contain an adjuvant just as the FeLV vaccine does, the same problem of vaccination-induced sarcoma can occur in cats (see FeLV, above). It is more difficult to avoid this with rabies vaccine because of laws requiring the vaccination. A nonadjuvant vaccine for cats is being developed at this time.