FELINE LEUKEMIA VIRUS

VetSuite Veterinarians
Infectious Diseases

Feline leukemia virus (FeLV) is a viral disease of domestic cats that impairs immune system function and causes some types of cancer. The virus is shed in all body fluids, especially saliva. FeLV-associated diseases usually are categorized as neoplastic or non-neoplastic, including impairment of immune function and development of secondary infections.

DIAGNOSIS OF FELINE LEUKEMIA VIRUS

ETIOLOGY AND RISK FACTORS

• Causes - Feline leukemia is caused by a retroviral infection. FeLV is transmitted by bite wounds, shared dishes or litter pans or close contact (grooming). Kittens can be born with FeLV or acquire the virus through their mother's milk.
• Risk factors
• Age - Cats of any age can become infected but the average age of infection is three years.
• Breed/genetics - No known risk
• Sex - Male cats may have a higher prevalence of infection than female cats.
• Geographic/environmental - The rate of FeLV infection is higher in multi-cat households, catteries and urban areas with high cat populations. The FeLV infection rate ranges from 2 to 13 percent in the general cat population.
• Prevention - Feline leukemia is an infectious disease that can be prevented primarily by eliminating interactions with infected cats. Some suggestions include keeping cats indoors, preventing fighting, and neutering males to help decrease roaming. Test other cats in the household and isolate FeLV-negative cats from FeLV-positive cats. Vaccinate "at-risk" cats against FeLV. "At-risk" cats include cats that spend any time outdoors, breeding cats, or cattery cats. Vaccination against other diseases should be discussed with your veterinarian. If yearly vaccinations are given, only killed vaccine products should be used to avoid complications in FeLV-infected cats with potentially weakened immune systems.
  
  Antibiotics should be administered to FeLV-infected cats before preventative dentistry to minimize the risk of secondary bacterial infection.
  
  FeLV is destroyed in the environment within minutes. There is no evidence of transmission of FeLV from cats to humans.

HISTORY AND CLINICAL SIGNS

• Species affected - Cats
• Presenting signs and historical problems - Symptoms of FeLV depend on the type of resulting disease and may include secondary infections, anemia, dysphagia, weight loss, dyspnea, eye problems, coughing, lethargy and poor wound healing.

PHYSICAL EXAMINATION FINDINGS

• General
• Attitude - The mental status can range from normal to comatose, depending on the severity of illness.
• Body condition - Depending on the stage of illness, the body condition can be normal, thin or even overweight.
• Vital signs - Vitals signs are often normal but can be abnormal, depending on the illness caused by the retroviral infection.
• Mucous membranes - Pale mucous membranes are common since anemia is a common finding in cats with feline leukemia.
• Hydration status - Usually normal though some cats may be dehydrated.
• Head and neck - The lymph nodes of the head and neck should be palpated to detect enlargement.
• Eyes - A thorough ophthalmic exam is recommended. Uveitis, conjunctivitis and other disorders may be present.
• Oral cavity - Stomatitis, ulcers and masses may be present.
• Thorax (cardio-pulmonary) - Depending on the stage of illness, tachycardia and dyspnea may be present. Lung sounds may be dull if pleural effusion is present.
• Abdomen (gastrointestinal/urinary) - Abdominal masses, enlarged lymph nodes or ascites may be present.
• Reproductive system - Examination of the reproductive system is often unremarkable, but many cats with feline leukemia are infertile or have stillbirths or abortions.
• Lymph nodes - Peripheral lymph nodes are often enlarged.
• Integumentary system - Abnormalities of the hair coat may be observed in cats with feline leukemia, including an unkempt appearance.
• Neurologic examination - Neurologic signs are common in cats with cerebral disease associated with feline leukemia, especially if tumors occur. These can vary from mild depression to coma or focal neurologic deficits.
• Musculoskeletal examination - Muscle atrophy may be present in some cats.

DIAGNOSTIC STUDIES

• Special examination techniques - Cerebrospinal fluid analysis may be recommended in FeLV-infected cats that have symptoms of neurologic disease. Microscopic and chemical analysis of cerebrospinal fluid may identify infectious agents, inflammatory cells or cancer cells.
• Clinical laboratory tests
• CBC - A complete blood count is recommended to evaluate for anemia, low platelet count, inflammatory or infectious diseases and leukemia.
• Serum biochemical tests - This test is recommended to evaluate the impact of FeLV infection on other organ systems. Biochemical elements usually are normal in cats infected with FeLV.
• Urinalysis - Urinalysis to evaluate kidney function, identify protein loss in the urine and determine if urinary tract infection is present. Urinalysis usually is normal in cats infected with FeLV.
• Serology/immunologic tests - The most commonly performed screening tests are those that look for soluble antigen, such as the ELISA (enzyme-linked immunosorbent assay) test.
  
  The ELISA can be done in the hospital using blood, saliva or tears; however, tests using saliva or tears yield an unacceptably high percentage of inaccurate results and are not recommended as an individual screening test. A positive ELISA on an otherwise healthy cat should be repeated once a month for three months to determine if the infection is transient or persistent. If the ELISA is positive 3 months in row, a persistent infection can be diagnosed. To confirm feline leukemia status, an IFA (immunofluorescent antibody) can be run.
  
  The IFA test is run on blood and must be submitted to an outside laboratory. The IFA is considered the gold standard and definitive test for diagnosing or confirming FeLV infection. A positive IFA correlates to an advanced stage of infection, and false positives are rare. Cats that test positive on the ELISA and positive on the IFA are considered persistently infected and rarely, if ever, revert to negative IFA status.
  
  Discordant (conflicting) results may occur. If the ELISA is negative, the IFA is almost always also negative. Rarely is an IFA recommended if the ELISA screening test is negative. If the ELISA and the IFA test positive, the cat is considered persistent infected. If the ELISA is positive and the IFA is negative, both tests should be repeated in 60 days and then every year until both tests are in agreement. Discordancy makes it difficult to determine a cat's true infection status. Discordant cats should be treated as potential sources of infection to other cats.
  
  Blood tests for other infectious diseases of cats such as feline immunodeficiency virus (FIV) may be completed. Tests for feline infectious peritonitis (FIP), toxoplasmosis and ehrlichiosis also may be necessary.
• Parasitology - A fecal examination is recommended to detect underlying parasitism. Cats with feline leukemia are often immunosuppressed and susceptible to GI parasites.
• Diagnostic imaging
• Radiographs (thoracic/abdominal) - Radiographs of the chest may be recommended in cats with dyspnea. Chest radiographs may show pleural effusion or pneumonia caused by secondary bacterial infection. Abdominal radiographs may reveal abdominal masses or lymph node enlargement.
• Ultrasound (abdominal) - If abdominal masses or organomegaly is palpated or suspected, or survey radiographs are indicated, abdominal ultrasound is recommended.
• Ultrasound (thoracic/ECHO) - If cardiac arrhythmias or murmurs are present, or if survey radiographs indicate, a thoracic ultrasound is recommended.
• Pathology
• Cytology (fluid or tissue) - Microscopic evaluation of bone marrow samples may be indicated in cats with severe anemia, leukopenia or low platelet count. In some instances, FeLV infection can cause decreased production of red cells by the bone marrow and occasionally tumor cells may invade the bone marrow and crowd out the blood cell precursors normally found in the marrow.
  
  Abdominal paracentesis and cytology is recommended if ascites is present. Thoracocentesis and cytology is recommended for pleural effusion.
• Biopsy/histopathology - A biopsy or fine needle aspirate may be performed on enlarged lymph nodes or solid tumors to obtain a sample for microscopic examination.

DIAGNOSIS AND PROGNOSIS

• Differential diagnosis
• Feline ehrlichiosis
• Feline infectious peritonitis (FIP)
• Feline immunodeficiency virus (FIV) infection
• Hemobartonella felis
• Hemolytic anemia
• Histoplasmosis
• Leukemia
• Lymphoid hyperplasia syndrome
• Lymphoma
• Mast cell tumor
• Multiple myeloma
• Myeloproliferative disease
• Cancers of tissues other than lymph nodes and bone marrow
• Sepsis
• Systemic lupus erythematosus
• Thrombocytopenia
• Toxoplasmosis
• Recommended tests - Feline leukemia blood test (ELISA), confirmatory IFA, CBC, biochemical profile.
• Summary of diagnostic criteria - Positive on feline leukemia test. The results of the CBC and biochemical profile may vary.
• Prognosis - Healthy cats should not be euthanized based on the results of one screening blood test.
  
  The course of FeLV infection is variable, and there are 3 main stages of infection. Approximately 33 percent of cats are infected briefly but are able to eliminate the virus within 4 to 6 weeks. Such transiently infected cats do not become ill or develop FeLV-related disorders.
  
  Another 33 percent of cats develop FeLV-related disease with approximately 60 percent mortality within 2 years.
  
  The remaining 33 percent of infected cats cannot eliminate the virus completely, but do not usually develop FeLV-related disease. Such infections cannot be detected with routine blood tests; they require special diagnostic tests such as bone marrow culture of the virus or identification by polymerase chain reaction (PCR). In a small percentage of these cats, reactivation of the virus is possible. Latent infection also may explain development of certain types of cancers that develop in older cats.

TREATMENT OF FELINE LEUKEMIA VIRUS

TREATMENT PRINCIPLES

There is no specific treatment that will eliminate FeLV infection. Treatment of FeLV infection must be individualized based on the severity of the condition.

INITIAL/HOSPITAL THERAPY

• Symptomatic therapy
• Symptomatic treatment depends on the various organ systems affected by the virus.
• Blood transfusions should be administered on an emergency basis as necessary in FeLV-infected cats with severe anemia.
• Suspected concurrent infection by Hemobartonella felis should be treated in FeLV-infected cats suffering from hemolytic anemia. Tetracycline antibiotics are used against this organism and cortisone-like drugs (e.g. prednisone) also may be used if immune-mediated destruction of red blood cells is suspected.
• Manage lymphosarcoma if present in FeLV-infected cats using cancer chemotherapy drugs.
• Antibiotics are used to treat secondary bacterial infections.
• Nutritional support is important in weakened and debilitated cats.
• Fluid therapy is necessary in dehydrated FeLV-infected cats.
• Stomatitis should be treated with topical medications.
• Specific therapy
• Although there is no treatment that eliminates FeLV infection, some drugs have been tried.
• Interferon may be used in an attempt to limit viral replication. It is not licensed for use in cats, but it has been used to treat FeLV-infected cats. It may prevent disease development and prolong survival.
• Other agents that stimulate the immune system could be beneficial in FeLV-infected cats. Examples include diethycarbamazine, Staphylococcal protein A (SPA), Propionbacterium acnes (Immunoregulin) and acemannan (Carrisyn). The effectiveness of these agents is unknown.
• Zidovudine or azidothymidine (AZT) is a nucleoside analog that is used to treat human patients with AIDS. AZT may limit virus replication and may prolong survival of FeLV-infected cats, but it is most effective in experimental cats when administered very soon after infection. AZT has the potential to cause serious adverse effects, including bone marrow suppression, and should only be administered to cats under the supervision of a veterinary oncologist or a veterinarian experienced in the use of anti-viral medications.
• AZT and 9-(2-phosphonylmethoxylethyl) adenine (PMEA) have been reported to reduce the severity of chronic mouth infections in cats with FeLV. These drugs have the potential for serious adverse effects and should only be administered to cats under the supervision of a veterinary oncologist or a veterinarian experienced in the use of anti-viral medications.
• Bone marrow transplantation has been performed in some affected cats and may correct low white cell counts, but cats remain infected with FeLV. This experimental procedure would only be available at a small number of veterinary research institutes.

LONG-TERM/HOME THERAPY

Cats must be fed a high quality nutritious diet to help maintain health. All FeLV-infected cats should remain indoors to decrease exposure to other cats. The goal of long-term therapy is to provide a comfortable life for the cat. Any illness should be investigated and treated aggressively.

FOLLOW-UP CARE

The actual follow-up procedure depends on the severity of the disease and the response to therapy. Cats with feline leukemia should be examined every 4 to 6 months and any illness should be treated promptly.