Feline Leukemia Virus Disease Complex

The feline leukemia virus (FeLV) is responsible for more cat diseases than any other infectious agent and is second only to trauma as the leading cause of death in household cats. It is the most important cause of cancer in cats and significantly contributes to the severity of other feline diseases. The virus is transmitted from one cat to another by infected saliva. Sharing water bowls or food dishes, cat-to-cat grooming, and cat bites can also spread the disease. The virus can be shed to a lesser extent in urine and feces. Kittens can acquire the virus in utero and through infected mother's milk.

The incidence of active infection varies. About 1 to 2 percent of healthy, free-roaming cats are infected. In multicat households and in catteries, the incidence may be higher, in some cases with 20 to 30 percent of cats showing the presence of FeLV virus in the blood. About 50 percent show neutralizing antibodies, indicating prior infection from which the cat has recovered. Ill feral or free-roaming urban cats may have an incidence as high as 40 percent.

Signs of Illness

The initial illness lasts 2 to 16 weeks. Signs are nonspecific and include fever, apathy, and loss of appetite and weight. Other signs are vomiting and constipation or diarrhea. Some cats develop enlarged lymph nodes, anemia, and pale mucous membranes. Death at this stage is not common and signs may be so mild that they are missed.

Following exposure to the virus, there are four possible outcomes for cats:

  1. About 30 percent of cats do not develop an infection at all-whether due to resistance or inadequate exposure is not known.
  2. About 30 percent of cats develop a transient viremia with infectious virus present in their blood and saliva for less than 12 weeks. This stage is followed by the production of neutralizing antibodies that extinguish the disease. These cats are cured, cannot transmit the disease, have a normal life expectancy, and are at no increased risk of developing FeLV-related diseases.
  3. About 30 percent of cats develop a persistent viremia with infectious virus present in their blood and saliva for more than 12 weeks. Persistently viremic cats do not mount an effective antiviral immune response and are susceptible to a number of diseases that are invariably fatal. About 50 percent die within six months and 80 percent succumb within three and a half years. These cats shed the virus while they are alive.
  4. About 5 to 10 percent of cats develop a latent infection. These cats are able to produce virus-neutralizing antibodies that eliminate the virus from blood and saliva but do not extinguish the virus completely. The virus persists in the bone marrow and in T-cell lymphocytes. Over many months, the majority of latent-infected cats overcome and extinguish the virus, so the incidence of latent infection after three years is quite low. In latent-infected cats, the disease can become activated during periods of stress or concurrent illness, leading to a recurrence of viremia.
  5. About 30 percent of cats with persistent viremia develop a virus-related cancer months or years after exposure. Lymphosarcoma is the most common variety. One or more painless masses may be felt in the abdomen. There may be enlargement of lymph nodes in the groin, armpit, neck, or chest. The cancer may spread to the eyes, brain, skin, kidneys, and other organs, producing a variety of symptoms.