EPIDEMIOLOGY


Feline Leukemia Virus infects domestic cats.  It is a recognized virus worldwide, but the incidence in cat populations varies with population density.  The overall rate of disease has decreased within the last 20 years (2), due to the ability to better test for FeLV, thereby allowing removal of infected cats from the population.  Vaccination has also helped to lower the incidence of disease.

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).

There are no breed predispositions for infection.  The virus attacks all cats alike.  There is a notable trend in which purebreds show FeLV less often, not because they are more immune to infection, but rather purebreds are usually kept indoors (3).  Likewise, due to the general public becoming more aware of the disease, breeders are testing their cats more frequently for FeLV to keep their cats free from infection.


The age of the cat when infected has a huge influence on successful infection.  Kittens are more susceptible than adult cats.  Young kittens less than 16 weeks are more susceptible than older kittens.  There is an age-related resistance to the virus, but it is NOT absolute (4).  For example, adults cats have an increased chance of infection with continual exposure to a FeLV cat shedding the virus than if they only contact the FeLV cat once.  The age-resistance is based on cellular receptor number.  Young kittens have an increased number of cellular receptors on their cells that the virus attaches to for entry into the cell compared to older cats (3).  Furthermore, older cats have more mature macrophages, which are more functional.


Studies have shown that the prevalence of FeLV antibodies in cats increases with time, indicating that cats receive continual exposure to the virus over their lifetime, but are able to mount a proper immune response to clear it from their systems (4).  Likewise, susceptibility to persistent viremia decreases with age, because an older cat should have more FeLV antibodies and is better able to resist the development of persistent viremia.  Persistent viremia is most common in cats less than 2 years of age (3).



TRANSMISSION

There are several ways for FeLV to spread among the cat population:

Horizontal Transmission:
  Viral spread between cats is called horizontal transmission.  A susceptible cat needs to come into direct contact with an infected cat that is actively shedding the virus.  Saliva and nasal secretions contain millions of virus particles, which are able to infect susceptible cats.  “Social cats” have an increased susceptibility compared to isolated cats.  Social cats partake in activities such as nose-nose contact and mutual grooming.  They share litter boxes and food bowls.  Cat bites are of less importance for transmission (3).

     

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            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.


IATROGENIC Transmission - A type of horizontal transmission between cats that is induced by the veterinarian.  This can occur through contaminated needles, contaminated instruments, and blood transfusions (2).

            INDIRECT Transmission - It requires that another species become infected with FeLV and then pass it on to a susceptible 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.


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            c) Grooming – the queen can infect her kittens by licking them.

d) Latently infected queen – a queen that is FeLV negative according to the FeLV test does not have the virus in her, but can have the provirus in her DNA.  Stress during the pregnancy causes the virus to reactivate, causing the queen to have circulating virus in her body.  Now she can infect her kittens transplacentally.

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