Detection of rabies antibodies in wild boars in north-east Romania by a rabies ELISA test

Background

In the last few decades, Romania has been considered one of the European countries most affected by animal rabies, but a combination of oral rabies vaccination (ORV) campaigns in foxes alongside mandatory vaccination of pets has substantially decreased the number of rabies cases in recent years.

The objective of this study was to detect rabies antibodies in wild boar serum and thoracic fluid samples collected during the hunting season after ORV campaigns in north-eastern Romania in order to identify if wild boars are substantial competitors to foxes for ORV baits.

Results

When the 312 wild boar samples were tested by ELISA (BioPro ELISA, Czech Republic), 42.31% (132/312) demonstrated rabies antibodies. In order to compare these wild boar results in terms of the percentage of immunisation, fox samples were also included in the study, and in this case only 28.40% (98/345) demonstrated rabies antibodies by ELISA. To check the diagnostic sensitivity and specificity of this ELISA, those samples with a sufficient volume from both species that had tested either negative or positive with an initial ELISA were then tested with the Fluorescent Antibody Virus Neutralisation (FAVN) assay. The overall concordance between the BioPro ELISA and FAVN test was 74.26% (75/101) in wild boar samples and 65.66% (65/99) in fox samples, 140 out of 200 samples being correlated with the two methods, although no significant statistical difference (p = 0.218) between the two species was registered. We found a good agreement by both tests for the ELISA-positive samples (91.30%), however the situation was different for the ELISA-negative samples, where a low agreement was demonstrated (41.18%).

Conclusions

This study reports for the first time the presence of rabies antibodies in wild boar samples collected during the hunting season in Romania after ORV campaigns in rabies endemic areas. It is also the first study to demonstrate that ELISA BioPro can be used on wild boar samples with satisfactory results compared to the FAVN test for this species.

Use of filter paper blood samples for rabies antibody detection in foxes and raccoon dogs

The effectiveness of oral rabies vaccination in wildlife is usually evaluated by the detection of rabies antibodies. However, the assessment of rabies antibodies has several technical difficulties in the field, such as the collection, storage, transport and titration of blood samples, often of poor quality. The objective of this study was to assess the feasibility of collecting blood on a filter paper (FP) coupled with enzyme-linked immunosorbent assay (ELISA) titration of rabies antibodies in raccoon dogs and red foxes. The FP blood sampling method was found highly specific and repeatable in both species. Overall, results obtained with the FP sampling method were highly concordant with the conventional (venipuncture) sampling methods. Blood eluates from FP samples from foxes and raccoon dogs tested using ELISA showed concordance values of 92% and 95%, respectively, with serum samples tested using the seroneutralisation test and values of 95% and 91%, respectively, when the ELISA was used on both types of sample. The use of FP blood sampling coupled with the titration of rabies antibodies by ELISA provides a reliable alternative to conventional blood sampling and serum testing by seroneutralisation. This simple procedure is particularly attractive and cost-effective for assessing the effectiveness of oral rabies vaccination in field conditions.

Evaluation of a rabies ELISA as an alternative method to seroneutralisation tests in the context of international trade of domestic carnivores

For several years, international movements with pets have greatly increased. Most countries have relaxed their quarantine measures and adopted a scheme combining vaccination of pets against rabies followed by a serological test to check the efficacy of vaccination. This new scheme has been strongly supported by the OIE, WHO and the European Commission to facilitate the free movement of people and pets around the world. Currently, only two reference methods are recognised and prescribed (the FAVN test and the RFFIT) to measure rabies antibody levels in serum samples for international trade. They are reliable and valuable methods of assessing the efficacy of rabies vaccination but they are time-consuming and require well-trained people and specialised laboratory facilities. A few years ago, an ELISA (Platelia™ Rabies II kit ad usum Veterinarium) was developed for domestic carnivores and wildlife. To our knowledge, this ELISA is the only one certified and prescribed by the OIE. Following its marketing, one task of the EURL for rabies serology was to evaluate the performance of laboratories using this new kit. The results revealed that 26% of the participants, which were already approved laboratories for rabies serology, failed the inter-laboratory trial. Such unsatisfactory results have never been observed during any of the previous proficiency tests organised annually since 2000 by the EURL for rabies serology using reference methods. More investigations were undertaken through internal and collaborative studies to assess the performance of this newly marketed ELISA kit. The results of the internal study revealed that even with a specificity of 100%, the sensitivity evaluated on 593 samples of domestic carnivores came to 78.2%. An issue regarding the underestimation of serum titres was also revealed during the study. The results of a collaborative study involving 23 international laboratories reinforced the preliminary conclusions regarding lack of sensitivity. Indeed, only 5 laboratories out of the 23 obtained satisfactory results. We therefore suggest adopting a threshold of 0.3 EU/mL instead of 0.5 EU/mL to increase the sensitivity of the test.