Detection of immunity in sheep following anti-rabies vaccination

Purpose

Rabies is a fatal but preventable disease with proper pre-exposure anti-rabies vaccination (ARV). Dogs, as household pets and strays, are the reservoir and vector of the disease, and dog bites have been associated with human rabies cases in Sri Lanka over the past few years. However, other susceptible species having frequent contact with humans may be a source of infection. One such species is sheep and immunity following ARV has never been tested in sheep reared in Sri Lanka.

Materials and Methods

We have tested serum samples from sheep reared in the Animal Centre, Medical Research Institute of Sri Lanka for the presence of anti-rabies antibodies following ARV. Sheep serum samples were tested with Bio-Pro Rabies enzyme-linked immunosorbent assay (ELISA) antibody kits used for the first time in Sri Lanka and our results were verified by a seroneutralization method on cells (fluorescent antibody virus neutralization, FAVN test) currently recommended by World Organization for Animal Health and World Health Organization.

Results

Sheep received annual ARV and maintained high neutralizing antibody titers in their serum. No maternal antibodies were detected in lamb around 6 months of age. Agreement between the ELISA and FAVN test, i.e., coefficient concordance was 83.87%.

Conclusion

Annual vaccination in sheep has an effect on maintaining adequate protection against rabies by measurements of anti-rabies antibody response. Lambs need to be vaccinated earlier than 6 months of age to achieve protective levels of neutralizing antibodies in their serum. Introducing this ELISA in Sri Lanka will be a good opportunity to determine the level of anti-rabies antibodies in animal serum samples.

A time-resolved fluoroimmunoassay for assessing rabies antibody titers in the sera of vaccinated human subjects

Several studies have investigated the use of simple in vitro tests for the assessment of rabies antibody titers in serum samples from vaccinated human subjects, which would allow the effectiveness of rabies vaccination to be conveniently evaluated. To this end, a novel time-resolved fluoroimmunoassay (TRFIA) for the assessment of rabies antibody titers was established in this study for evaluating the effectiveness of protection against rabies. The TRFIA had a satisfactory limit of detection value (0.035 IU/mL) under optimal conditions. Additionally, the application of the TRFIA was demonstrated in 68 serum samples with satisfactory results. The coefficient variations (CVs) were all <10%, and the recoveries were in the range of 90-110%. The correlation coefficient of titer values obtained using the present TRFIA and the rapid fluorescent focus inhibition test (RFFIT) was 0.733, with a coincidence rate regarding the evaluation results (protected or not protected by vaccination) of 100%. The preliminary results confirmed that the TRFIA had a higher performance than an enzyme-linked immunosorbent assay (ELISA), and could potentially replace the ELISA. Based on these results, the novel TRFIA appears to be a convenient tool for the evaluation of rabies vaccination results based on serum samples from vaccinated human subjects.

Diagnostic tests for human rabies

The lack of reliable data concerning the number of human deaths from rabies presents one of the principal difficulties in a realistic assessment of the importance of this disease, and this lack of an accurate assessment has led to its underestimation and neglect. Priority should therefore be given to establishing a diagnostic test that can confirm human rabies on the basis of biological results. Indeed, only a laboratory diagnosis can properly identify infection, because clinical diagnosis remains difficult to interpret and is insufficiently specific. Historically, diagnosis has been based solely on post-mortem analysis of a cerebral biopsy using immunofluorescence techniques. Although this remains the standard method, considerable progress has been made with the advent of new molecular techniques and the evaluation of new, less-invasive sampling methods that are more easily accepted by the patient's family. Intra-vitam diagnosis of human rabies is now possible using reliable, robust, validated techniques that can be used everywhere, including in regions with limited resources, using minimally invasive or non-invasive sampling (such as saliva or skin biopsies). In practice, one of the major challenges with the diagnosis of human rabies is still the transfer and accessibility of such validated techniques in centralised reference laboratories located in low-income enzootic countries, in order to achieve the biological confirmation of each suspected case of rabies. At the same time, it is necessary to develop easy, fast and low-cost diagnostic methods that can be used in rural and remote areas in peripheral laboratories, or ideally at the patient's bedside.

First international collaborative study to evaluate rabies antibody detection method for use in monitoring the effectiveness of oral vaccination programmes in fox and raccoon dog in Europe

The most effective and sustainable method to control and eliminate rabies in wildlife is the oral rabies vaccination (ORV) of target species, namely foxes and raccoon dogs in Europe. According to WHO and OIE, the effectiveness of oral vaccination campaigns should be regularly assessed via disease surveillance and ORV antibody monitoring. Rabies antibodies are generally screened for in field animal cadavers, whose body fluids are often of poor quality. Therefore, the use of alternative methods such as the enzyme-linked immunosorbent assay (ELISA) has been proposed to improve reliability of serological results obtained on wildlife samples. We undertook an international collaborative study to determine if the commercial BioPro ELISA Rabies Ab kit is a reliable and reproducible tool for rabies serological testing. Our results reveal that the overall specificity evaluated on naive samples reached 96.7%, and the coefficients of concordance obtained for fox and raccoon dog samples were 97.2% and 97.5%, respectively. The overall agreement values obtained for the four marketed oral vaccines used in Europe were all equal to or greater than 95%. The coefficients of concordance obtained by laboratories ranged from 87.2% to 100%. The results of this collaborative study show good robustness and reproducibility of the BioPro ELISA Rabies Ab kit.