Oral vaccination of wildlife against rabies: opportunities and challenges in prevention and control. (2004)

Rabies is an acute, progressive, fatal encephalitis caused by viruses in the Family Rhabdoviridae, Genus Lyssavirus. Rabies virus is the representative member of the group. Warm-blooded vertebrates are susceptible to experimental infection, but major primary hosts for disease perpetuation encompass bats and mammalian carnivores. The dog is the global reservoir, and important wild carnivores include foxes, raccoons, skunks, and mongoose, among others. Traditionally, reliance upon long-term, widespread, government-supported programmes aimed at population reduction of animals at risk has been unsuccessful as the sole means of rabies control, based in part upon economical, ecological and ethical grounds. In contrast, immunization of domestic dogs with traditional veterinary vaccines by the parenteral route led to the virtual extinction of canine-transmitted rabies in developed countries. Taken from this basic concept of applied herd immunity, the idea of wildlife vaccination was conceived during the 1960s, and modified-live rabies viruses were used for the experimental oral vaccination of carnivores by the 1970s. The development of safe and effective rabies virus vaccines applied in attractive baits resulted in the first field trials in Switzerland in 1978. Thereafter, technical improvements occurred in vaccine quality and production, including the design of recombinant viruses, as well as in the ease of mass distribution of millions of edible baits over large geographical areas. Over the past few decades, extensive oral vaccination programmes focusing upon the red fox, using hand and aerial distribution of vaccine-laden baits, have resulted in the virtual disappearance of rabies in Western Europe. The same dramatic observation held true for southern Ontario. During the 1990s in the United States, oral vaccination programmes concentrated upon raccoons, grey foxes, and coyotes, with similar success. For example, raccoon rabies has not spread west of the current focus in the eastern states, grey fox rabies is contained in west central Texas, and no recent cases of rabies have been reported from coyotes away from the Mexican border for several years. Despite the progress observed and the absence of substantive adverse environmental or health effects, oral vaccination is not a panacea, and should be viewed as an important adjunct to traditional prevention and control techniques in human and veterinary medicine. Local outbreak suppression of rabies among free-ranging wildlife is documented, and regional elimination of particular virus variants among specific, targeted carnivore hosts is demonstrable, but true disease eradication is not achievable at the present time by current techniques. For example, no practical vaccination methods have been designed for bats. Although lyssaviruses appear in relative compartmentalization between the Chiroptera and Carnivora, major spillover events have been detected from bats to carnivores, and phylogenetic analyses suggest a historical basis for extant viral origins due to interactions between these taxa. Thus, bio-political considerations aside, the possibility for pathogen emergence resulting from transmission by rabid bats with subsequent perpetuation among other animals cannot be discounted easily on any continent, with the possible exception of Antarctica. Clearly, given their biodiversity, distribution, and abundance, novel methods would be necessary to consider meaningful control of rabies in these unique volant mammals. Newer approaches in biotechnology may be envisaged some day for eventual extension to bats, as well as more widespread application to global canine rabies remediation in developing countries.