Leukosis is a serious infectious disease in cats caused by the Feline Leukemogenic Virus (or FeLV). This contagious disease is found all over the world and particularly affects the immune system and can cause lymphomas. Its development can be long and go through several phases, sometimes making the diagnosis difficult. Here are the essential points to remember to understand this disease and if possible prevent it.

The Feline Leukemogenic Virus (FeLV) is the retrovirus that causes leukosis in cats. Present all over the world, in Europe its average prevalence is less than 1% but can reach 20% in some regions.

Be careful, although the virus can affect several wild felids, a human being cannot contract feline leukosis.

It is a contagious disease, which is spread by close contact between individuals and exchanges of secretions (salivary, nasal, urinary, etc.). The main modes of transmission are licking, biting and more rarely sharing a bowl or litter. 

Transmission between an infected mother and her young is also possible. This transmission occurs through the placenta or after the birth of the kittens during lactation or grooming. FeLV is a virus that survives very little in the environment apart from a host, so indirect contamination is rare.

After being introduced into the body, the virus will target cells of the immune system and lymphoid tissues (spleen, thymus, lymph nodes, etc.) and then spread throughout the body.

A strong enough immune response can completely eliminate the virus. This is called an abortive infection. This development is unfortunately rare.

Typically, the infection manifests itself in two forms.

Infection progressive

The infection is said to be progressive when the virus circulates actively in the blood and continues to spread until it affects the spinal cord. The disease will then be expressed by clinical signs. 

Regressive infection 

If the virus remains dormant in the body for a long period of time, it is called a regressive infection. The immune system has a sufficient response to inhibit the multiplication and circulation of the virus, but not sufficient to eliminate it entirely. In this case, the cat carries the virus in the spinal cord but is no longer contagious. The virus can nevertheless be reactivated and switch to a progressive infection.

A cat infected with FeLV may remain healthy for a long time and then show clinical signs after weeks, months or even years of latent infection.

The virus affects the way the body works in several ways. It will create blood disorders such as anemia and decrease the immune system which in turn will promote secondary infections. It also has the particularity of being able to cause cancers of the blood and of the immune system (lymphomas, leukemias, etc.). 

Here are some clinical signs of the disease that can manifest acutely, intermittently or chronically:

• Loss of appetite ;
• Weight loss;
• Pale mucous membranes (gums or other);
• Persistent fever;
• Gingivitis or stomatitis (inflammation of the gums or mouth);
• Skin, urinary or respiratory infections;
• Diarrhea;
• Neurological disorders (convulsions for example);
• Reproductive disorders (abortion, infertility, etc.).

The diagnosis of leukosis can be difficult because of its particular course.

There are rapid tests that can be performed in the clinic that assess the presence of a viral antigen in the blood of the cat. They are very effective and are often used as a first-line treatment. However, if the infection is recent, the test may be negative. It may then be advisable to repeat the test or to use another method. 

Laboratory examinations are also possible to confirm a rapid test or to provide precision in the diagnosis (PCR, Immunofluorescence).

Unfortunately, there is no definitive treatment for FeLV. Care will generally focus on treating secondary infections or managing the cat’s clinical signs. 

However, a cat with leukosis should not be condemned. The prognosis for survival depends on the phase of the disease and the secondary conditions developed by the cat. 

The median survival after diagnosis of the disease is around 3 years, but with proper management of the disease, an indoor cat can live much longer.

Vaccination is an essential tool for the management of FeLV. The vaccine is not 100% effective, but its introduction into regular vaccination programs has reduced the prevalence of the virus in domestic cats. It is therefore recommended to vaccinate cats with access to the outdoors.