While some problems have been traced to poorly attenuated batches of vaccine that revert to virulence after injection or to contamination, others reflect the animals' genetic predisposition to adverse reaction. The incidence is low. Part of the difficulty in obtaining accurate estimates comes in the case of delayed reactions when it is sometimes not possible to establish whether the vaccination was causal or coincidental. However, it is also possible that many of these cases will be missed as the relationship to vaccination a month or more earlier will not seem relevant. Unfortunately, predicting which animals will be susceptible is anywhere from difficult to impossible, and if one owns one of those animals which does react the low odds are unimportant.
Anaphylactic collapse is dramatic and life threatening.
Animals have been previously sensitized to the antigens in the vaccine. It results from the release of histamine and other amines which cause blood vessels to dilate and blood to pool in peripheral vessels. Death may occur before epinephine and antihistamines can be administered. Liver and kidney enzyme levels may be elevated, and either organ may collapse. Bone marrow suppression may occur as well.