By Pr. Oswald Jarrett, University of Glasgow, 2007.
Is annual vaccination necessary?
There is general agreement that vaccination of cats and dogs against the major infectious diseases of each species is highly desirable, and that a primary course of vaccination should be given early in the life of the animal followed by a booster dose after one year to ensure that most animals are immunised. However, there is debate about how often cats and dogs should be vaccinated subsequently, and which vaccines should be used. Previously, it was considered necessary to vaccinate at yearly intervals. Over the last few years, the need for annual vaccination has been challenged. Several authorities have suggested that booster inoculations of the common vaccines need be given only every three years rather than annually.
How can we decide which procedure to follow?
To make this change without compromising animal health requires more knowledge of the duration of immunity in vaccinated cats or dogs. Many vaccine manufacturers have attempted to address this issue by undertaking long-term studies of the duration of immunity, in which young animals are vaccinated and then maintained in isolation before being challenged with the relevant infectious agents at intervals thereafter; for example, one, two or three years following primary vaccination. The disadvantages of this approach are the negative welfare aspects of isolating experimental animals for that length of time, as well as the significant costs involved. An alternative strategy is to determine the efficacy of vaccines by testing the sera of vaccinated pet animals for antibodies to the appropriate antigens at intervals after administration. This strategy only works if there is very good evidence that a particular antibody level is positively associated with resistance to challenge by the infectious agent. The most well known example of such an association is rabies vaccination: a serum antibody level of 0.5 international units or greater is known to indicate that a cat or dog is protected against challenge by live rabies virus.
Can serological testing of pets inform us about vaccine protection?
Can this serological approach be applied also to the common vaccines against feline and canine infectious diseases? Certainly, there is very good evidence that this is possible in the dog. A particular antibody titre measured by virus neutralisation in the case of canine distemper virus and canine adenovirus, or haemagglutination-inhibition for canine parvovirus, is associated with protective immunity to each virus. Using serological tests, it has been shown that in dogs a very high proportion of the population maintains protective levels of immunity over long periods of time after vaccination. These studies support the suggestion that re-vaccination at intervals of three years would be sufficient to maintain immunity.
Is this approach possible in the cat?
What about the cat? Unfortunately here the situation is less clear. Antibodies to feline parvovirus (FPV), feline calicivirus (FCV) and feline herpesvirus (FHV) can be measured in cat sera and there is no doubt that vaccination induces antibodies that persist for long periods. However, the question remains: does a particular level of antibody indicate that a cat is protected against challenge? This is true for FPV but may not be true for the two other viruses. For these viruses, all that can be claimed is that a level of antibody is 'consistent with the cat having been vaccinated', which does not necessarily mean that it is protected from challenge by field strains. In spite of these potential problems, recent research indicates that immunity, as well as antibody level, does persist for long periods of time after vaccination. Therefore the suggestion has been made that for cats, as for dogs, following primary vaccination and a booster after one year, re-vaccination should be at intervals of three years.
What about 'non-core' feline vaccines?
Vaccinal immunity to feline leukaemia virus, chlamydia, Bordetella bronchoseptica, feline coronavirus or feline immunodeficiency virus cannot be determined by serological tests.
We may soon have a better understanding of how to vaccinate cats as many studies are in progress to determine the duration of immunity induced by cat vaccines. Another major advance for gathering data about duration of immunity would be for the registration authorities to demand that manufacturers investigate the efficacy of their products in pet cat populations as a condition of gaining a product licence.
Suggested schedule for cat vaccination