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The application of oral rabies vaccine (ORV) in stray dogs in Thailand has achieved gratifying results

Oral rabies vaccine (ORV) in stray dogs has achieved gratifying results in Thailand.

Researchers from Thailand and Germany published a paper in the professional journal Viruses (see References) on March 29 this year, reporting that they used German-developed and produced oral rabies vaccine (ORV) in stray dogs in Thailand. ) Feasibility and effectiveness studies conducted. The following is a brief introduction to their research results.

1 Introduction

More than 95% of human rabies cases are caused by bites from infected domestic dogs. Therefore, eliminating dog-mediated human rabies may make a significant contribution to reducing the burden of rabies. The only most cost-effective way to achieve this goal is to eliminate rabies at its source through canine vaccination. Mass Dog Vaccination (MDV) campaign can provide herd immunity and successfully block the transmission cycle of rabies in this host species. This method has been successfully adopted in many countries around the world.

2. 　Thailand’s progress in eliminating rabies

Thailand has made significant progress in eliminating rabies. Human rabies cases have been reduced from 200-300 cases per year in the early 1980s to 3 cases in 2019. Animal rabies has also experienced the same development trend, from 3,000 to 4,000 confirmed cases per year from 1997 to 2000, to 380 cases in 2019. However, controlling and eliminating the last remaining rabies foci in dogs is much more difficult than expected. Therefore, the goal of eliminating dog rabies has been adjusted many times.

In 2019, Thailand reported that 87% of animal rabies occurred in dogs. Most of these dogs are stray dogs without or with an owner and have never been vaccinated against rabies. In Thailand, most stray dogs are fed and cared for by people in the community (dog caregivers), but these caregivers have limited ability to catch or control these dogs.

Therefore, in the large-scale canine vaccination (MDV) campaign carried out from April to June each year, these dogs usually cannot be vaccinated by a more standard method, that is, injection. The injection vaccination method usually adopts door-to-door vaccination or centralized vaccination at a specialized vaccination center. The specific method depends on the preference of the relevant local authorities.

3. 　3 stages of research and design

Oral wild animal vaccines have been successfully used to control rabies in different wild animals around the world, and some people have suggested oral vaccination for dogs that cannot be vaccinated by injection. This project studied the feasibility and field effects of using Oral Rabies Vaccine (ORV) for stray dogs as a supplementary tool for rabies control in Thailand. The research design is divided into 3 stages.

The first stage is to determine the most suitable bait for the target dog population in Thailand. This bait is widely accepted and helps to release the vaccine in the mouth. In the second stage, the candidate vaccine must not only meet the minimum safety requirements set by the World Organization for Animal Health (OIE), but also must induce an adequate immune response after eating a single bait. Finally, as recommended by the World Health Organization (WHO), the feasibility of an oral rabies vaccine (ORV) needs to be evaluated during field trials in various local settings.

The first phase of bait acceptance research showed that by providing bait with egg yolk flavor or cooked sausage flavor, a large part of the stray dog population can be attracted, and bait with egg yolk flavor is more suitable for delivering vaccine in the oral cavity. The second phase of serological studies showed that the selected candidate vaccine SPBN GASGAS can induce a sustained and detectable immune response after oral administration in local dogs, which is comparable to dogs receiving commercially available injection vaccines.

This article presents the results of the third phase of the study, that is, the results of the feasibility and effectiveness study. The main purpose is to determine the benefits of dog ORV as a supplementary tool for injection vaccination, and to optimize the integration of possible constraints and solutions in the MDV (Massive Canine Vaccination) campaign in the local environment by identifying this method The application of this method in stray dogs.

4. 　Experimental method and ORV used

Based on previous serological studies, field studies were conducted using SPBN GASGAS vaccine (ORV) produced in Germany. The third-generation vaccine virus SPBN GASGAS is a genetically engineered derivative of the cDNA clone SAD L16 of the first-generation oral rabies virus vaccine (ORV) SAD B19. SPBN GASGAS lacks the pseudogene ψ. For safety purposes, the amino acid codes at positions 333 and 194 of its rabies virus glycoprotein are designed as glutamic acid and serine, respectively.

The modification of the glycoprotein at position 333 eliminates the residual pathogenicity after SAD B19 inoculation in the brain of adult mice, and the modification at position 194 avoids potential compensatory mutations, that is, it does not cause the original residual pathogenicity. recover. All three coding nucleotides at these two positions have been changed to reduce the risk of reverting to the original form. In addition, an extra glycoprotein gene with the same change was inserted repeatedly, which can ensure that the safety of the vaccine virus is further improved.

Fill the small sac with vaccine virus-containing liquid (3ml, 108.2 FFU/mL) and mix it with two different baits (an industrially manufactured egg-flavored bait-egg bait, and a locally produced intestinal bait), These two kinds of bait have previously proved to be easily accepted by local stray dogs in Thailand. The research team observed that the acceptance of egg bait can be further optimized and improved. The liquid snacks for cats that are common in the local market with tuna flavor or chicken liver flavor can be directly used as dog bait (strong egg flavor bait).

The ORV of dogs was tested in 5 study areas in 4 provinces of Thailand. In these areas, with the support of local municipal staff and dog owners, locations with stray dogs have been identified. The ORV team visited each of the five research areas and distributed ORV (SPBN GASGAS) in three forms of bait. These vaccines were provided to dogs in a distribution and recovery model. The three types of bait tested included egg-flavored bait, strong egg-flavored bait made with commercially available cat liquid snacks, and cooked sausage bait. A dog that receives the vaccine bait is considered to be vaccinated. If the discarded vaccine bait is bitten and perforated, or if the dog chews the vaccine bait at least 5 times before swallowing the vaccine bait (including the vaccine bait), it is also considered to be vaccinated.

5.　Results:

A total of 2,444 stray dogs that could not be vaccinated by injection were confirmed in 338 locations. Since not all dogs are accessible, the actual result was that 79.0% of the dogs were provided with bait; of these dogs, 91.6% received the bait, and 83.0% were subsequently identified as successful inoculation.

Overall, 65.6% of stray dogs in these locations were successfully vaccinated via oral route. Such a significant increase in vaccination coverage of stray dogs can block the cycle of rabies transmission and provide a unique opportunity to achieve the goal of eliminating canine-mediated human rabies in Thailand by 2030.

6. Conclusion

The first large-scale field study conducted in Thailand showed that ORV in dogs can significantly increase the vaccination coverage of stray dogs considered to be unable to be vaccinated by injection. The protocols established in these field studies show the potential of integrating dog ORV into existing canine rabies control programs. In order to achieve the target immunity level, the accessibility of stray dogs is crucial. This can only be achieved through the cooperation and direct participation of local authorities, community members and dog watchers. Therefore, before implementing this campaign, they need to ensure their support. The results of this article show that in the next few years, especially in areas with residual rabies foci in Thailand, the use of ORV on stray dogs may help achieve the national goal of eliminating dog-mediated rabies. This method may also be extended to other countries or regions in Asia and Africa where dog-mediated rabies needs to be completely eliminated.

References:
Chanachai, K.; Wongphruksasoong, V.; Vos, A.;et al., Feasibility and Effectiveness Studies with Oral Vaccination of Free-Roaming Dogs against Rabies in Thailand. Viruses 2021, 13, 571. https://doi.org /10.3390/v13040571

(source:internet, reference only)

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