FeLV incidence is most prevalent in high-density populations of cats where the cats have direct contact with each other (3). Such conditions exist in catteries and multi-cat households. As well, cats allowed outdoors become infected more often than cats that remain confined indoors. This is due to the direct contact that is required to spread the virus from cat to cat. The infection rate in free-roaming healthy cats has been calculated to be 1 – 8% (3). These healthy infected cats are viremic and shedding the virus, but then clear it from their system quickly. The infection rate in free-roaming sick cats can reach 21% (4). Sick cats are more “at risk” of becoming infected because their immune system is slightly depressed.Infection between male and female outdoor cats is fairly even. Males may have a slightly higher incidence only because they tend to roam more and therefore may come into contact with more cats (2). But transmission of FeLV is not primarily based on cat scratches (from territorial males getting into cat fights). Although FeLV can infect cats through scratches, this is much more rare than “social” activities, such as sniffing or licking one another (3).
Fecal, urinary, fomite, and aerosol transmission are less likely because the virus cannot survive in the environment for a prolonged period of time. It quickly becomes inactivated (4).
The direct contact requirement for transmission is why animal shelters do not see infection spread. As long as the cats are housed individually and cages are routinely disinfected, horizontal transmission is avoided.
Transmission - It requires that
another species become infected with FeLV and then pass it on to a
cat (6). This cannot happen with FeLV (3). (Eg. a human cannot become infected with the
virus and spread it to susceptible cats.)
Vertical Transmission: This occurs when a queen passes the infection to her kittens. There are four possible methods (3):
a) Transplacental – the virus infects the kittens in utero by crossing the placenta. Reproductive failure results from early embryonic death and resorption of the embryo, or from abortion. 20% of kittens will survive to birth, but they become persistently infected kittens. These kittens are born infected and will never be free from infection. They will eventually die. Some kittens may not show a positive FeLV test immediately after birth because it may take the virus some time to reproduce itself. These kittens will show a positive FeLV test at a later date.
b) Lactation – the virus
enters the queens milk which the kittens drink.
c) Grooming – the queen can infect her kittens by licking them.
Latently infected queen – a queen that is FeLV negative according
FeLV test does not have the virus in her, but can have the provirus in
DNA. Stress during the pregnancy causes
to reactivate, causing the queen to have circulating virus in her body. Now she can infect her kittens