One Step Closer to Rabies-Free?

An estimated 99% of human rabies cases originate from dog bites. Parenteral vaccination is currently the primary tool for eliminating canine rabies virus in several countries. However, the World Organisation for Animal Health (OIE) now endorses adjunct use of oral, baited vaccines for dogs in areas where parenteral vaccination alone is insufficient. A recent review published in Veterinary Research provided information on several candidate vaccines for oral vaccination of dogs (OVD).

Candidate Vaccines

The authors assessed 12 OVD candidate vaccines, including modified-live, attenuated, recombinant, and vectored strains. To be endorsed by the OIE and World Health Organization, vaccines are evaluated extensively based on relative safety, efficacy, bait delivery, thermostability, cost, and limitations.

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Currently used OVD agents are self-replicating, in contrast with injected, inactivated vaccines. OVD is less controlled than parenteral vaccination, as a dog may consume several baits and thus receive an abnormally high vaccine dose. Therefore, researchers typically test the safety of candidate vaccines by administering up to 10 doses to puppy and adult dogs and to major endemic species that are most likely to eat the baits, such as cats, primates, and rodents. Studies determined that several rabies vaccine strains successfully induced detectable virus-neutralizing antibodies after oral vaccination and caused lasting protection, according to viral challenges performed up to 2 years later. However, some strains reverted to virulence in nontarget species and thus were deemed unsuitable for OVD use.

The likelihood of passive vaccine transfer to humans is higher from dogs than from wildlife; therefore, candidate vaccines are also evaluated for human safety. Viral excretion is typically tested in canine saliva after vaccination to predict the risk of exposure if a human is licked or bitten. Current recommendations are that humans contacting the vaccine itself should receive postexposure prophylaxis.

The success of OVD greatly depends on several variables, including economics and the role of dogs in society. Also, bait preference varies strongly by region and can significantly influence acceptance rates. Even if a dog accepts the bait, the vaccine blister must be punctured for immunization to occur. Researchers determined that distributing baited vaccines to owners at a central location is more economical than performing door-to-door distribution. Thermostability is less of a concern for OVD than for wildlife, as baiting strategies include hand feeding and placing baits in populated areas for easy recovery.

Global Impact of Oral Vaccines

Thus far, the safety and efficacy of 2 oral vaccine strains (SAG2 and V-RG) have been demonstrated thoroughly. Roughly 250 million V-RG oral rabies vaccine doses have been distributed worldwide in the past 30 years for vaccination of wildlife. SAG2, according to the authors, is the only oral rabies vaccine currently registered for vaccination of both dogs and wildlife.

The authors estimated that at least 70% of a local dog population must be vaccinated to eliminate rabies. They stressed that “the increase in the immunization coverage resulting from the wise application of OVD may be crucial to achieve rabies elimination.”

Dr. Stilwell received her DVM from Auburn University, followed by a MS in fisheries and aquatic sciences and a PhD in veterinary medical sciences from the University of Florida. She provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting.