The aim of kitten vaccinations is to provide active, vaccine-derived immunity as early as possible. However, maternally derived antibody (MDA), which is essentially colostrally derived in kittens, may interfere with effective vaccination. As MDA declines, kittens may become susceptible to infection before they are responsive to vaccination, leaving an immunity gap (Figure 2.1). Some vaccines are better at overcoming MDA than others; classically, intranasal vaccines are not interfered with by levels of antibodies that would interfere with systemic vaccines. However, with improvements in vaccines including newer types of adjuvant, other vaccines may also induce protection in the presence of low levels of MDA.
In most diseases of cats, MDA declines to non-interfering levels in the majority of animals by 9-12 weeks of age. The standard protocol is therefore to vaccinate kittens at 8 or 9 weeks and then repeat in 3-4 weeks' time. However, the duration of MDA may be variable between different litters of kittens and even between kittens in a litter, depending on the antibody levels in the queen and the intake of colostrum. MDA against feline herpesvirus has been shown to last for 2-10 weeks, and against feline calicivirus for up to 10-14 weeks (Chapter 22). For feline panleucopenia the majority of kittens born to queens with moderate titres will lose their MDA by 8-12 weeks, but for those born to queens with high titres, MDA may persist for 16 weeks or more (Chapters 14 and 21). Serology for antibody level estimation can be carried out where an accurate timing of first vaccination is required, although in practice this is rarely done.
In some situations earlier vaccination schedules may be appropriate, for example where there is a high risk of disease or where kittens are deprived of colostrum and therefore more likely to be at risk. However, vaccines are generally not licensed for use in younger animals, and therefore care should be taken in making a risk-benefit assessment for the animal in terms of safety and efficacy, and in obtaining informed consent from the owner. An alternative strategy to boost kitten MDA is to vaccinate queens before breeding (or in pregnancy, providing this is indicated on the data sheet).
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