A step forward in the quality control testing of inactivated rabies vaccines - extensive evaluation of European vaccines by using alternative methods to the in vivo potency tests

The mouse challenge test still remains the reference method for the potency determination of human and animal inactivated rabies vaccines, and it is still widely used throughout the world. This test suffers from many disadvantages - it is expensive and time consuming, uses a large number of mice, causes significant animal distress, and suffers from high variability. Recently, the European Pharmacopoeia has recognised the use of a serological potency assay (SPA) as an alternative method to the challenge test. This new test is based on the determination of rabies neutralising antibody titres in vaccinated mice, by using the modified Rapid Fluorescent Focus Inhibition Test (mRFFIT). With the objective of adopting this new method for the batch release of inactivated rabies vaccines, we evaluated its performance on a large collection of rabies vaccines currently assessed in our laboratory. The Fluorescent Antibody Virus Neutralisation test (FAVNt) was used in parallel with the mRFFIT, and the results were compared to the mouse challenge test. Our results demonstrate that the SPA is capable of estimating the potency of vaccines formulated with a potency margin well above the minimum of 1IU/dose. For low potency vaccines, this new method demonstrated some limitations, due to the recurrent invalidation of the assay. We have also demonstrated the superior sensitivity of the FAVNt when compared to the mRFFIT, and the importance of minimising the risk of detecting non-responders in vaccinated mice.

Experimental infection of foxes with European Bat Lyssaviruses type-1 and 2

Background

Since 1954, there have been in excess of 800 cases of rabies as a result of European Bat Lyssaviruses types 1 and 2 (EBLV-1, EBLV-2) infection, mainly in Serotine and Myotis bats respectively. These viruses have rarely been reported to infect humans and terrestrial mammals, as the only exceptions are sheep in Denmark, a stone marten in Germany and a cat in France. The purpose of this study was to investigate the susceptibility of foxes to EBLVs using silver foxes (Vulpes vulpes) as a model.

Results

Our experimental studies have shown that the susceptibility of foxes to EBLVs is low by the intramuscular (IM) route, however, animals were sensitive to intracranial (IC) inoculation. Mortality was 100% for both EBLV-1 (~4.5 logs) and EBLV-2 (~3.0 logs) delivered by the IC route. Virus dissemination and inflammatory infiltrate in the brain were demonstrated but virus specific neutralising antibody (VNA) was limited (log(ED₅₀) = 0.24–2.23 and 0.95–2.39 respectively for specific EBLV-1 and EBLV-2). Foxes were also susceptible, at a low level, to peripheral (IM) infection (~3.0 logs) with EBLV-1 but not EBLV-2. Three out of 21 (14.3%) foxes developed clinical signs between 14 and 24 days post-EBLV-1 infection. None of the animals given EBLV-2 developed clinical disease.

Conclusion

These data suggest that the chance of a EBLV spill-over from bat to fox is low, but with a greater probability for EBLV-1 than for EBLV-2 and that foxes seem to be able to clear the virus before it reaches the brain and cause a lethal infection.

Experimental infection of foxes with European bat lyssaviruses type-1 and -2

Experimental studies have been undertaken to assess the susceptibility of silver foxes to bat variants of rabies virus, namely European Bat Lyssaviruses (EBLVs). Both EBLV-1 and EBLV-2 have been isolated in European bats since 1954, in Eptesicus serotinus and Myotis species, respectively. Since 2000, the number of reported cases has increased largely due to the improvement of the surveillance of bat rabies virus throughout Europe. Although over >800 EBLVs cases have been reported in bats in Europe, EBLV-1 and -2 viruses are rarely reported to infect humans and terrestrial animals. The study presented here shows that the sensitivity of silver foxes is low when infected with EBLVs via the intramuscular route; in contrast all animals infected via intracranial inoculation succumbed to the experimental challenge. The mortality rate was 100% for both EBLV-1 (approximately 4.5 log) and EBLV-2 (approximately 3.0 log). This data suggests that the susceptibility of foxes to EBLV-1 and EBLV-2 is low and that the transmission (spillover) and adaptation of EBLVs from a bat to a fox may be theoretically possible but unlikely.