Vaccination Against Feline Immunodeficiency Virus (FIV)
In July 2002, the first licensed vaccine (Fort Dodge) against feline immunodeficiency virus (FIV) was introduced in the United
States. Although few articles have addressed FIV in the clinical literature over the past few years, most clinicians would
agree that the consequences of FIV infection in the individual cat are significant and justify the need for an FIV vaccine.
Characterized by a long latent period, infected cats gradually experience deterioration of immune function associated with
declining numbers of T helper lymphocytes (CD4+). [REF: Levy, 2000] The consequences are manifest as a wide spectrum of vague
clinical features, none of which are diagnostically distinctive.
The principle serological test for FIV infection used throughout the world is the determination of FIV antibody in serum.
The enzyme-linked immunsorbent assay (ELISA) and immunoblot (Western Blot) methods used to detect FIV antibody have become
the mainstay for diagnosing infected cats and conducting surveys among populations of cats at risk for infection. Epidemiological
studies using these tests have provided good evidence for horizontal transmission of FIV among cats and have identified adult
male cats living outdoors as those at greatest risk of infection. Since the virus can be recovered from the saliva of infected
cats, bite wounds sustained during fighting are believed to be a principle means of virus transmission. On the other hand,
casual contact among infected and non-infected cats is an unlikely means of transmission. Although it appears possible that
FIV can be sexually transmitted, as the virus has been recovered from the semen of infected cats, this mode of transmission
appears to be uncommon in nature. Likewise, transmission from infected queen to fetus (vertical transmission) is possible,
but rare. On the other hand, it is more likely that infected queens will transfer FIV antibody, not virus, via colostrum to
nursing kittens. Since maternal FIV antibody may persist in kittens for several months, it is customary to disregard a "positive"
FIV antibody test result in healthy kittens under 6 months of age.
The introduction and use of the killed FIV vaccine substantially changed the approach clinicians use to assess potentially
infected cats. Of particular importance is the fact vaccination is known to be associated with development of FIV antibody
that interferes with all FIV tests on the market today. In addition, it has recently been demonstrated that a vaccinated,
seropositive queen will pass antibody to kittens (presumably through clostrum). FIV testing of kittens that nursed from FIV
seropositive cats will cause a false positive test result. Until an alternative, reliable, and accessible laboratory test
for FIV infection is made available, or an alternative (recombinant) vaccine is introduced, veterinarians have lost the ability
to distinguish between a vaccinated cat and an infected cat.
The current FIV vaccine is a killed, whole virus vaccine containing two virus subtypes, or clades, of FIV called clade A (Petaluma
strain) and clade D (Shizuoka strain). The manufacturer recommends a vaccination schedule that entails administration of 3
doses initially, 2-3 weeks apart, followed by annual revaccination. Each 1 ml dose is administered subcutaneously to cats
8 weeks of age or older. The vaccine is adjuvanted.
Vaccine Efficacy and Safety
Based on experimental studies submitted to the USDA, the FIV vaccine has a good efficacy and safety profile. Efficacy (challenge)
studies showed that 4 of 24 (16%) vaccinated cats were infected following challenge while 17 of 19 (90%) unvaccinated (control)
cats were infected following challenge. This represents an 82% preventable fraction. In clinical safety studies, over 2000
doses were administered to 689 cats (299 were less than 12 weeks of age). Reactions such as pain at the injection site, lethargy,
and fever, were reported in 1.1%. There were no fatalities reported.