How Many Dog Lives Is One Human Life Worth?

Health and medicine explained.
Sept. 12 2013 5:25 AM

A Dog’s Life

An infectious disease forces us to consider how many dogs a human life is worth.

Nine-year-old homeless Boxer dog, Spike, sits in his kennel at the RSPCA Animal Rescue Centre in Barnes Hill, Birmingham, England on 4 April 2007.
How many of these guys are worth one human life?

Photo by Christopher Furlong/Getty Images

How much is a dog’s life worth? Less than a human life, obviously, but how much less? Would you kill 1,000 dogs to save one human? A million dogs? Perhaps you’re an absolutist, and you’d sacrifice every dog in the world to save one person. But what if you weren’t sure how many people you’d save? How many dogs would you euthanize for a 10 percent chance of saving one human?

This sounds like chatter at an exceptionally morbid cocktail party, but it’s not the slightest bit hypothetical. For the past 20 years, public health experts in parts of Asia, Africa, South America, and Europe have debated the ethics and efficacy of large-scale dog culling to prevent transmission of a deadly human disease.

Leishmaniasis is a parasitic infection that can affect several parts of the human body. The most dangerous form, visceral leishmaniasis, causes swelling of the internal organs and kills 95 percent victims within two years without treatment. Treatment is somewhat expensive and complicated, as parasites in different regions respond differently to medication, and many of the medications have high toxicity. Drug resistance is also becoming a problem. Officially, the disease is responsible for 50,000 deaths annually, with South Asia, Sudan, and Brazil suffering most of the burden, but that estimate is probably low. Most countries eased off leishmaniasis monitoring decades ago, when they thought the disease was under control. In many parts of the world, however, it is now resurgent.

One of the difficulties in dealing with leishmaniasis is that the protozoan parasites that cause the disease have so many places to hide. In South Asia, humans appear to be the main reservoir. The bite of the sand fly spreads leishmaniasis from person to person, the same way mosquitoes spread malaria. In other places, particularly Brazil, the sand fly spreads the disease from wild or domestic animals to people. Although there is much debate in the scientific literature on this point, many public health experts are convinced that domestic dogs are the primary contributors to the spread of the disease. In some Brazilian neighborhoods, veterinarians report that as many as 50 percent of dogs test positive for the parasite that causes visceral leishmaniasis in humans.

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Brazilian veterinarians, like their colleagues in many other leishmaniasis-affected countries, are required to euthanize these animals. Treating canine leishmaniasis is notoriously difficult. Even when doctors are able to suppress the symptoms, there’s no way to be sure that the parasite is gone. Leishmaniasis screening tests measure antibodies against the disease, and those antibodies remain in the dog’s system long after treatment, making a cured dog indistinguishable from an infected or reinfected dog. Euthanasia is, in theory, the safest option.

Still, many pet owners find the euthanasia mandate outrageous. In the abstract, it’s difficult to sympathize with them. The program is intended to save human lives, and dog lives simply aren’t worth as much. The details, however, are more complex. First, most of the dogs that test positive for leishmaniasis are asymptomatic. No skin lesion, no weight loss, and no renal failure. Although they might become more contagious in the future, asymptomatic dogs are less likely to transmit the disease than those with noticeable suffering. In addition, the leishmaniasis test returns a high number of false positives. Public health experts estimate that about 20 percent of euthanized dogs are, in fact, leishmaniasis-free.

Now put yourself in the position of a Brazilian dog owner. You bring your cocker spaniel in for an ordinary checkup, and the vet tells you she has to euthanize and incinerate your ostensibly healthy dog because it might be carrying a disease that, at present, is unlikely to spread. Suddenly the moral course isn’t so obvious.

There’s one other thing: Public health officials aren’t even sure that mandatory dog culls are effective. Consider the experience of China in the 1950s, a time when more than 500,000 residents carried leishmaniasis. The government determined that it could suppress the disease only by killing 75 percent of the dogs in high-prevalence areas. Animal control personnel killed dogs indiscriminately, whether or not they showed any sign of infection. The cull worked temporarily, but doctors observed rapidly rising prevalence rates in the surviving dogs just four years later. China eventually managed to reduce its burden of leishmaniasis, but, to this day, public health experts argue over how significant a role dog culling played in the process. (Widespread application of the pesticide DDT to control sand flies was likely the more important intervention.)

More recently researchers have turned to mathematical models to determine whether and how dog culling affects transmission rates, but they’ve run into a decidedly human complication. A 2008 study showed that about 40 percent of people whose dogs are euthanized buy a new puppy almost immediately, and many of them buy multiple new dogs. This not only maintains canine numbers, but it makes the dog population more susceptible to the disease. Young dogs are more likely to contract and transmit leishmaniasis than the adults they replaced. A large percentage of the second-generation dogs are themselves euthanized within two years for exactly the same reason.

European countries with a leishmaniasis problem, such as Italy, have so far resisted mandatory euthanasia. Epidemiologists point out that the programs’ efficacy is debatable, and the European public would not accept the killing of apparently healthy animals. Vaccinating dogs and using treated collars to prevent sand fly bites are also more common in the developed world.

Circumstances have shielded Americans entirely from this messy ethical debate. The only leishmaniasis we see in the United States is the cutaneous form, which is far less deadly and only appears sporadically in the Southwest. We’re unlikely to experience a major outbreak of the parasite that causes visceral leishmaniasis anytime soon.

“It’s a numbers game,” says Karen Snowden, a professor in parasitology at Texas A&M’s College of Veterinary Medicine. “Leishmaniasis transmission is a complex interaction between reservoir animals, the sand fly, and humans. It is unlikely that we have enough infected animals, flies, and humans in close proximity to create a serious outbreak in this country.”

In other words, we don’t have visceral leishmaniasis because we haven’t had it in the past. The disease is theoretically capable of spreading here. In fact, the CDC was called in during the early 1990s to investigate a smattering of cases involving hunting dogs imported to East Coast states from Europe. The disease appeared to spread among foxhounds after making landfall but died off before we had to start thinking about euthanizing pets.

Zoonotic diseases, however, appear to be on the rise. Diseases are jumping from birds, bats, monkeys, and other animals to the human population. We may one day have to discuss the relative value of animal and human life, using actual numbers

Brian Palmer writes about science, medicine, and the environment for Slate and the Natural Resources Defense Council. Email him at explainerbrian@gmail.com. Follow him on Twitter.

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