- The main route of virus infection is by ingestion although transplacental transmission also occurs.
- Virus is excreted in saliva, urine, faeces and milk, and close contact and licking are the usual means of spread.
- FeLV replicates in the oropharynx, particularly the tonsils, from which it is spread to other lymphoid tissues, especially bone marrow. Many cats mount an immune response which eliminates the virus at this stage although latent infection of bone marrow can still occur.
- More extensive virus replication in the bone marrow can give rise to viraemia and widespread infection, especially in lymphoid tissues and epithelial cells of the oropharynx, salivary glands and upper respiratory tract, with consequent virus excretion and transmission to other cats. At this stage an effective immune response can still eliminate active infection, giving rise to a transient viraemia lasting between 2 days and 8 weeks.
- Some cats, however, will not eliminate virus but develop persistent infection. It is persistently infected cats that will go on to develop clinical disease and which are the main source of infection to other cats.
- The extent and clinical outcome of infection vary according to the age of the cat, its pre-existing immunity and the dose of virus received.
- The susceptibility of kittens to infection decreases markedly with age such that persistent infection develops in only about one in five cats over 16 weeks
- Virus neutralising antibody in colostrum may protect kittens for the first 4 weeks of life.
- The dose of virus received depends largely on the kittens' environment
- In free-range cats the amount of contact between excreting and susceptible cats is small, the dose is therefore small. Although most cats will be infected very few develop persistent infection.
- In multi-cat households, however, the degree of contact and the exposure to virus is high, and in this situation up to 30% of kittens become persistently infected.
- A persistently infected queen will usually result in persistent infection in all the kittens in her litter.
Up to half of those cats which apparently recover from FeLV infection undergo latent infection in bone marrow. Virus release from latent infections is generally too low to be detected or to infect epithelial cells, and latently infected cats are therefore rarely sources of infection to other cats. Latent virus can, however, be induced to replicate by, for example, corticosteroid treatment. Even latent infections are usually eliminated, although around 10% of cats can remain latently infected for at least three years.
Back to menu