Feline Leukaemia Virus

Also known as FeLV

Pathogenesis

• FeLV is the "disease of friends": transmission usually occurs through mutual grooming
• From the oropharynx, virus spreads to most tissues in the body to replicate, notably:
  • Bone marrow
  • Thymus
  • Salivary glands
  • Reproductive tract
• Most kittens but only 30% of adults become viremic for life without producing antibody
  • 20% of viremic cats die of tumors
  • 30% of viremic cats die of FeLV-associated disease
  • 80% die within three years of exposure
• 30% of adults exposed become latently infected and can become viremic when immunosuppressed
• 40% of exposed adults remain healthy and develop Ab and CD8+ Tcells after clearing the virus, without becoming reinfected or silent carriers

Clinical Signs

• Leukemia - a neoplastic increase in blood cell numbers - usually white blood cells
• Multicentric lymphosarcoma: B or T cell tumors, which may be palpable as enlarged lymph nodes (particularly mesenteric)
• Thymic lymphosarcoma: T cell tumors, with only the thymus enlarged
  • Results in dyspnoea
  • Confirmed by radiography
• Alimentary lymphosarcoma: B cell tumors of the Peyer's patches

FeLV-associated disease:

• Immunodepression causing secondary disease
• Reproductive failure: FeLV crosses the placenta, causing fetal resorption or viremic kittens with thymic aplasia

Epidemiology

• Vertical transmission of FeLV-A from mother to kittens either via placenta, grooming, or milk
• Horizontal transmission occurs via saliva during mutual grooming
• Recovery linked to age and presence of maternal antibody
• Particular concern for intensively bred cats because of crowding, etc.

Antigenicity

• Main envelope protein: gp70
• 3 antigenic subgroups:
• Group A
  • is transmitted between cats
  • is monotypic: one vaccine covers all isolates
  • causes lymphosarcoma
• Group B:
  • is recombinant with transmissible FeLV-A
  • increases the chance of thymic tumors
• Group C:
  • is a mutant of A
  • isolates are rare, and occur as A+C mixtures
  • increases chances of anemia

Diagnosis

• FeLV should be suspect in any cat with recurrent bacterial infections, anemia or weight loss
• ELISA for Antigen (capsid protein p27 or envelope protein gp70)
• Immunochromatography is now trusted as ELISA testing can give false positives
• Virus isolation from heparinised blood can now be performed to confirm a positive diagnosis

ELISA

• • Rapid-Immuno-Migration
  • Western Blot
  • Virus Isolation
  • Immunofluorescence
  • PCR

Infection Control

• Antigen positive sick cats should be destroyed
• Healthy positive cats should have diagnosis confirmed
• Vaccination:
  • Should take place once antigen-negative status has been determined
  • Leukogen: Subunit vaccine (using envelope protein gp70) produced in E. coli mixed with Quill-A and alhydrogel
  • Others include inactivated virus and canarypox recombinants, but all MUST include FeLV-A

(FeLV)

FeLV Electron Micrograph Public Health Image Library Image #5610

Kinetics of FeLV - Copyright Dr Brian Catchpole BVetMed PhD MRCVS

• Oncogenic retrovirus
• Causes neoplasia (lymphoma), myelosuppression (anaemia) and immunosuppression (of T cells)
• 2 strains:
  • FeLV-A
    • Natural strain
  • FeLV-B
    • Formed through FeLV-A recombining with endogenous retroviral sequences in the feline genome
    • Increases the risks of lymphoma
  • FeLV-C
    • Formed from the spontaneous mutation of FeLV-A
    • Is more myelosuppressive
• Virus replicates in the oropharyngeal lymphoid tissue causing a viraemia (virus circulating in the bloodstream) which then spreads to the systemic lymphoid tissue
• Shed in saliva
• Passed by oronasal route, e.g. mutual grooming
• Kittens between 6 weeks and 6 months are most susceptible
• 60% of cats will become immune to the disease and recover
• Cats that are persistently viraemic will progress to develop FeLV-associated diseases
• Some cats will become viraemic again if treated with corticosteroids or stressed if the infection lies dormant in the bone marrow

Treatment

• • Antibiotics for secondary infection
  • Anti-retroviral therapy
• For vaccinations see here