Feline Leukaemia Virus

Also known as FeLV

Introduction

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

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

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.

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

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

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