Local reactions after vaccination detected via magnetic resonance imaging and compared with pathomorphological examination

Local reactions are possible side effects after vaccination in animals. In the case of live viral vaccines, local reactions are mostly small and transient, whereas in the case of inactivated vaccines local reactions are often more pronounced. These effects are due to the use of adjuvants in inactivated products. Depending on the adjuvant type, local inflammatory reactions at the injection site are common but vary in extent. In Europe the safety of veterinary vaccines is assessed in clinical trials, which use pathologic examinations to describe the extent of the local reaction.

In order to reduce the number of animals in clinical trials, this study aimed to assess the local tissue reaction after vaccination by repetitive magnetic resonance imaging (MRI) at the live animal, which offers the possibility of repeated examinations in the same animal over time. The present study evaluated the extent of local tissue reaction at day 1, 3, 8, 15, 22 & 29 after vaccination via MRI and compared these findings with histopathology of the injection site at day 29 after vaccination. A total of 32 piglets, divided into two examination groups (16 piglets each), were injected into marked injection sites with one of two inactivated commercial vaccines (with Mycoplasma hyopneumoniae antigen). All animals were sedated and scanned repeatedly by MRI up to day 29 after vaccination. 50% of the animals of each examination group (8 piglets each) received a contrast agent during all MRI scans. These eight animals per group were euthanized at day 29 after vaccination and underwent a pathomorphological examination.

The repeated MRI examinations showed different extents (e.g. from 0.09 ± 0.04 cm3 for group I to 0.37 ± 0.14 cm3 for group II, for the contrast agent MRI sequence at day 29) of local reaction between both groups. The comparison of the MRI results with the pathomorphological findings at the injection site yielded matching results concerning the sizes of the affected tissue volumes or areas (e.g. R2 = 0.56 between MRI extent and maximum distribution of local reaction using histopathology at day 29).

This study shows that (1) MRI can describe the local reaction after vaccination in the live animal repetitively, (2) different vaccines show different extents of local reactions and (3) it seems that for regulatory safety testing the number of animals can be reduced to 8 animals per examination group. Complementary a final pathomorphological examination allows the identification of the kind and local distribution of the reaction.

Disclosure of Interest: None Declared.