- Incomplete attenuation of virus or bacterial component. This is uncommon but not unknown, which is why suspected reactions should always be reported to the manufacturer and/or other appropriate bodies.
- Animal particularly susceptible to infection with attenuated pathogen. For example, a live vaccine given to a very young puppy or kitten or an immunosuppressed individual (e.g. owing to pre-existing infection with an immunosuppressive agent such as feline leukaemia virus, distemper or a parvovirus).
- Vaccine given by incorrect route. For example, live cat 'flu vaccines attenuated for subcutaneous injection will often cause disease if given oronasally (e.g. if cat licks vaccine left at injection site or inhales aerosol made when filling syringe).
- Animal infected and incubating disease at time of vaccination. As can be seen in Figure 1, puppies and kittens are often vaccinated just when maternal antibody levels are declining and so the animal is most susceptible to infection. This is probably the single most common reason for apparent biological reactions to vaccination at 8 or 9 weeks old.
- Animal a carrier of clinically inapparent infection. The stress of vaccination may induce shedding with some clinical signs in, for example, cats latently infected with felid herpesvirus-1.
- It is normal for some live vaccines to provoke mild clinical signs. Subcutaneous injection of many live vaccines gives rise to mild systemic clinical signs such as depression, lethargy and inappetance, generally lasting only 24 hours. Intranasal vaccines often cause mild signs of upper respiratory tract disease including sneezing and, sometimes, limited ocular or nasal discharges.
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