Rabies has the highest case fatality rate of any infectious disease; without treatment or with treatment delayed, an individual, once infected, has a 99.9% chance of dying. Dogs, throughout human history, have been the primary vectors for infecting humans. Even today domestic dogs cause over 99% of around 60,000 deaths in humans each year from rabies—about half of those who die are children.
The US began campaigns to vaccinate companion pets in the 1940s; mass vaccination of dogs has resulted in essentially the elimination of canine rabies in the US, Canada, Western Europe and Japan. The herd immunity threshold, or the percentage of dogs that must be vaccinated to achieve protection for a community against rabies, has been found to be around seventy percent. A recent example of success from this approach was demonstrated in Bali, which like many Pacific Islands had no history of rabies. In 2008, rabies was introduced to Bali, but an island-inclusive vaccination campaign for dogs launched in 2010 reduced the number of human cases of around 90% by 2012.
The demonstrated success in these countries serves as a model for developing nations where rabies is endemic–about half the world’s population faces the risk each day of receiving a bite from a rabid dog. The Global Alliance for Rabies Control recently has called for an international effort to eliminate this threat of virus from dogs through mass vaccination. The treatment for a human, once bitten, requires an injection of antibodies against the rabies virus and four doses of vaccine on the day of exposure, with three more doses of vaccine applied typically during the next two to four weeks. In the US, post-exposure treatment costs around $1,000; in developing nations, post-exposure treatment can cost a family a month’s worth of wages. For many bite-victims in developing nations, post-exposure treatment is unavailable or unreliable. Mass vaccination represents a cost effective alternative—in developing nations, a single dose of vaccine to prevent rabies in a dog for about a year costs about twenty to forty US cents. One study determined that of the total economic burden globally from rabies, only 1.5% is due to costs for dog vaccination.
While health officials in the US should applaud and support efforts to control canine rabies in the developing world, they also should take a more vigorous approach against a growing rabies problem within our borders. Unfortunately, before rabies was controlled in the canine population of the US, the virus crossed over to wild animal populations, in which it continues to fester today:
- racoons across sixteen states from Maine to Florida
- skunks in thirteen states stretching across Montana, Minnesota and south to Texas
- gray foxes from west Texas into New Mexico and Arizona
- skunks in California and Arizona
- arctic and red foxes in northern and western coastal Alaska and the Aleutians
Death from rabies in the US is rare—only a handful of humans die each year. So, shouldn’t we consider this disease a problem of the past? Not exactly. About 40,000 people in the US receive treatment for rabies in the US annually, and over 90% of these cases arise from exposure to wild animals.
The good news is that vaccination of wildlife has been almost as effective as vaccination of domestic dogs. In the 1980s, rabies in foxes infiltrated Europe from the east but was halted at France and northern Italy with the use of oral rabies vaccine. After a prodigious spread of raccoon rabies across eastern states (noted above) in the 70s and 80s, a campaign in the 90s deployed oral rabies vaccine to check the epidemic from entering unaffected states. Oral vaccines typically contain live, weakened (non-infectious) virus within a blister pack that is incorporated into a food bait appropriate for the target animal. Since these vaccine-bait units are widely distributed, sometimes by aerial methods, precautions must be taken to prevent exposure of non-target species to viral components. Skunks, unlike other animals such as raccoons, reluctantly consume baits containing vaccine, and when they do, their immune systems produce antibodies poorly; researchers recommend a ‘trap-vaccinate-release’ strategy using killed virus vaccine for skunks.
We in the US must move beyond our current wildlife management strategy to contain rabies in broad geographic regions, where it remains unchecked and endemic. We should strive for elimination; we can refer to many examples, including in Texas where rabies was eliminated in coyotes, and in Ontario with rabies among arctic foxes.
We have another incentive for eliminating rabies from reservoir wild animal populations—this virus has the potential to re-emerge in our domestic animal population. However, in the future, the domestic host won’t be the dog, but the cat. The most recently available data show that infection with rabies is over three times higher in cats than dogs in the US. Americans own over 70 million cats and almost as many dogs. However, cat owners are far less likely to get necessary vaccinations for their pets—in 2006, 36% of cat owners did not seek veterinary services. The homeless population of cats in the US, which include feral animals and domestic strays, ranges up to 70 million—even with the efforts of feral cat rescue groups and humane societies, at best, ten percent of these animals receive veterinary treatment of any kind. Considering that cats serve as a ‘bridge’ species between wild animals and humans (and other domestic pets), the exploding homeless cat population, exposed to wild animal populations in which rabies is increasingly concentrated, presents a looming risk for human infection in American communities.
Our experience with euthanization, a method that the government of Bali has unfortunately implemented to control flare-ups of rabies, has provided ample examples that it is counterproductive to rabies control—concerned citizens have sheltered animals from authorities, and animal populations have been able to outbreed control efforts. We can complement vaccination for wild animal species with a strategy of ‘trap-vaccinate/sterilize-release’ for domestic homeless animals to approach an effective herd immunity in these populations. Eradication of rabies is not likely possible, especially since bats can harbour the virus and little research is available to inform us how to vaccinate them; however, bats account for a miniscule percentage of human cases. We can reduce the footprint of rabies in the US to small, isolated pockets that we can better manage; we will be able to control flare-ups with ring-vaccination campaigns, and we will have time to develop new approaches for bat rabies. As the developing world makes a commitment to take a significant step forward with rabies, the US needs to show the way to finally putting this virus behind us.
Steven Smith, M.Sc. is an Infectious diseases epidemiologist