Simply stated, the goal of vaccination is to sensitize or prime the immune system such that it can generate a population of
unique cells capable of mounting an effective immune response subsequent to infection by a pathogenic organism. But this is
a FAR MORE COMPLEX story. In just the past 5 years, remarkable advances have been made in our knowledge of how the immune
system actually achieves the ability to "learn" and "remember" key features of an infectious organism and then act to either
minimize the consequences of infection ("non-sterile immunity") or, in some cases, eliminate the organism altogether and prevent
infection ("sterile immunity")...it's more, much more, than just antibody!
Table 1: FeLV Testing in the Clinical Setting
Feline retrovirus infections, FeLV and FIV, perhaps illustrate this fact best. Clearly among the most complex infections affecting
the cat, a retroviral infection demands an immune response that is robust and sustained if the infected cat is to survive
long-term. Both innate immune responses (neutrophils, macrophages, and antigen presenting cells [APCs]) as well as adaptive
immunity (B-lymphocytes and T-lymphocytes) are critical. Susceptibility to FeLV infection is greatest during the first 6 months
of a cat's life. Natural resistance of adult cats to FeLV infection is well documented. In the immunologically na´ve kitten,
exposure to FeLV is likely to result in life-long infection. At issue, then, is how long will the cat live? Susceptibility
to FIV infection, on the other hand, does not change with age. Risk among kittens is similar to that in adults.
Vaccination Against Feline Leukemia Virus Infection
Today, it does appear that the prevalence of FeLV within the domestic cat population has declined over the last decade. Two
factors are most likely to have played a major role in this decrease: vaccination and in-hospital testing for FeLV antigen
in sick cats and cats presented for routine vaccination. However, results of a recent survey of cats in the US suggest that
the prevalence of FeLV is still as high as 2.3% of all cats (feral and non-feral). Clearly the need for routine testing and
vaccination of susceptible cats still exists.
Of the 4 FeLV vaccines available today, 2 are vaccines are killed, whole-virus, vaccines (Schering-Plough and Fort Dodge),
1 is a subunit vaccine (Pfizer), and, most recently licensed is the recombinant FeLV vaccine administered transdermally (Merial).
We have recently completed a study at NCSU in which ALL 4 (monovalent) leukemia vaccines were tested in cats. At 21 days post
vaccination, NONE of the vaccines caused false + test results for p27 antigen in cats tested by the ELISA method
All FeLV vaccines, however, are NOT the same. The killed and subunit vaccines contain adjuvantand require a 1.0 ml dose administered
parenterally. The immunity conferred by these products is associated with antibody production only. The recombinant FeLV vaccine
is a non-adjuvanted, canarypox vectored vaccine. The total 0.25 ml dose is administered by the transdermal (or, needle-free)
route. The vaccine induces both humoral immunity and cell-mediated immunity.
The recombinant FeLV vaccine immunizes by its ability to deliver 2 genes that express 2 important immunogenic proteins: p27
(gag) and gp70 (env). The canarypox virus is widely recognized as an ideal "vector-virus" for cats, dogs, and humans since
it does not recognize mammalian tissues and therefore does not multiply (replicate) in the vaccinated animal (or person).
Because the virus does not replicate, there is no risk of the canarypox virus being shed from vaccinated cats.
While the concept of transdermal vaccination is new to veterinary medicine, it is far more significant than simply offering
veterinarians an alternative method of administering vaccine to cats. The transdermal route addresses one of the pre-eminent
vaccination strategies in medicine today: to maximize vaccine safety and efficacy by exploiting our knowledge of how immunity
is induced and how an effective immune response is maintained. By administering vaccine into the skin, rather than through
the skin, antigen processing and lymphocytes activation by dendritic (Langerhans) cells, the antigen-processing cells abundant
in the skin, results in an immune response is potent and sustained.
Vaccination of kittens should consist of 2 vaccine doses administered at a 2 to 3 week interval beginning as early as 8 or
9 weeks of age. A 2-dose regimen is required at the time of administration of the first vaccine, regardless of the cat's age.
Cats presented for the second dose of vaccine at more than 4 weeks following the first dose should be given a third dose 2
to 3 weeks later. Administration of an annual booster is recommended. Regardless of the vaccine used during the initial 2-dose
regimen, it is not necessary to use vaccine from the same manufacturer when administering subsequent annual boosters.