Taking the Bite Out of Rabies, Putting Teeth Into One Health

Rabies is a scourge to humans and animals that was first mentioned in medical texts as early as the sixth century BCE. The development of rabies vaccines by Pasteur in the late 19th century provided a potent tool for protection of both animals and humans (1). Canine rabies has been eliminated from many high-income countries, and human rabies cases are vanishingly rare in Canada and the United States, largely because of requirements for vaccination and the availability of postexposure immunization protection when exposure (to rabid feral animals) does occur. The situation is different in many low- and middle-income countries, where rabies remains an important cause of illness and death in humans and animals. In low- and middle-income countries, risk for human rabies derives largely from interaction with domestic or stray dogs, which may be infected by feral animals (jackals, foxes, and coyotes, for example) that serve as reservoirs for the disease. Human rabies claims tens of thousands of lives annually in less affluent countries, particularly those in Africa and Asia (2). Global economic costs attributable to rabies run into the billions of dollars (2), and countries with rabies endemics may be perceived to lack resources to support effective programs for humane control of animal populations and vaccination of domestic animals. Timely postexposure immunization may not be available in rural areas, causing the bite of a rabid animal to become a de facto death sentence. In some resource-poor countries, culling of dogs is advocated as a means of decreasing rabies risk. However, best estimates suggest that rabies dynamics in dogs are little affected by reductions in dog density that would be achieved with culling (3). Furthermore, in areas where subsistence agriculture and animal husbandry are necessary for survival, the economic well-being of persons may be greatly enhanced by dog ownership because dogs protect herds (and their owners) from carnivores. Consequently, attempts to decrease rabies risk through culling are likely to fail because dogs are simply replaced, ensuring that the number of susceptible animals remains high with ongoing transmission of rabies. From the point of view of disease dynamics, the reproductive number of an infectious disease (the number of secondary cases of infection produced by a primary infection) is a function of both infectiousness (the basic reproductive number) and the fraction of the population susceptible to disease. When reproductive numbers decrease to less than 1, disease incidence decreases. As such, it can be seen that culling would not be expected to be particularly effective, but vaccination of dogs may be highly effective, resulting in direct effects (on vaccinated animals), indirect protection of other animals via herd immunity, and ultimately protection of humans through reduction of disease risk in animals. Efforts to replace culling with dog vaccination programs have been led by nongovernmental organizations, such as the World Society for the Protection of Animals, which has recently embarked on projects in Bali, Bangladesh, and Zanzibar (4). These programs seem to be successful because they introduce immune dogs as a firewall that prevents rabies in nonimmunized dogs, and that prevents dogs from serving as intermediaries in the transmission of rabies from wild animal reservoirs to humans. Governments may regard culling as a cheaper alternative to immunization, but that is not the case. In this issue, Fitzpatrick and colleagues (5) explore the health and economic attractiveness of vaccination programs providing annual rabies vaccination for dogs in 2 rural regions of Tanzania. Their analysis suggests that annual rabies vaccination is highly cost-effective in both regions and is likely to provide net cost savings relative to no vaccination in the more densely populated Serengeti region. Because these authors appropriately used a dynamic transmission model that captured the feedback loops inherent in communicable disease systems (6), the economic attractiveness of different levels of coverage changes in a nonlinear manner because the creation of herd effects results in a steep decrease in human rabies risk. In the Serengeti region, rabies vaccine coverage greater than 25% generates cost savings and saves lives as a result of large herd effects in dogs; however, with coverage greater than 70%, vaccine costs increase with much smaller marginal increases in health because herd immunity has already been attained. The magnitude of rabies as a global public health problem and the innovative approach these authors have taken to evaluate the economic attractiveness of vaccination make this article an important contribution, but it also has implications beyond the realm of rabies control. These authors have used a dynamic systems model to quantify human health benefits associated with a specifically veterinary intervention. Lloyd-Smith and colleagues (7) have previously noted that many diseases of public health importance, and emerging infectious diseases in particular, occur as a result of interactions between humans and animals. However, tools (such as mathematical modeling), which are ideally suited to the exploration and simulation of such complex systems, have been infrequently applied for purposes of optimizing public health policy. Fitzpatrick and colleagues article is also important because it highlights animal immunization as an important area of focus for advocates of the emerging One Health paradigm. The One Health movement regards human and animal health as 2 sides of the same coin, with implications for infectious disease surveillance, clinical practice in both veterinary and human health, environmental health, and teaching and research in the health sciences (8). Vaccination of animals for provision of both animal and human health benefit represents an excellent example of One Health in action and highlights the importance of close interaction among veterinary, medical, and public health professionals in controlling the effects of zoonotic disease on human health. A One Health approach to vaccination may be challenging and may ultimately necessitate explicit linkage of human health outcomes to animal vaccination by animal regulatory agencies and public health authorities. However, rabies, Escherichia coli O1H7:57 (9), and Hendra viruses (10) may all be best controlled in humans by vaccinating animals. Enhancing the health and welfare of our domestic animals (including food animals) is the moral thing to do, but it seems to be in our best self-interest as well.