Mankind has caused no end of accidental extinctions, but there's one intentional one that we're rightly proud of: smallpox, which no longer circulates in the human population. But last year, humanity celebrated a similar success, eliminating another virus from the wild. It just didn't get as much attention because the virus in question targeted cattle. But, for those whose livelihoods depend on cattle, this was a major advance. And, according to a review of its eradication published in Science, our ability to wipe it out in a few remaining pockets of Africa may have important lessons for other diseases, including those that infect humans.

The virus in question, a relative of the measles virus, causes a disease called rinderpest. If it isn't familiar, that may be because it was such a problem that control efforts in Europe date back as far as the 1600s. Use of vaccines against it date back to the 1800s (and, the article helpfully points out, rinderpest control was the target of the first rectal thermometers). As a result, the virus has been largely in check in Europe.

But in the late 1800s, it made the jump to Africa, and the results explain why eliminating the virus is such a triumph: the paper cites a 90 percent mortality as the virus spread through the continent. That's also why it's generally been considered the animal disease that has the largest impact on human well-being.

Out of Africa

The vaccinations and control strategies that worked well in many parts of Europe also limited rinderpest in some parts of Africa. By the 1980s, the international groups and the governments of Africa attempted a continent-wide vaccination campaign, which severely limited the presence of the virus, confining it primarily to a remote area of Uganda and unstable areas in the Horn of Africa, such as Sudan and Somalia and their neighbors. By 1990, Ethiopia was using 21 million doses of vaccine in an attempt to wipe out the last pockets of the virus.

At that point, animal health authorities began to worry that "The eradication effort was drifting toward indefinite disease containment of remote endemic areas, with donor fatigue looming on the horizon." A fundamental rethink seemed to be required.

Fortunately, some of the pieces were already in place. In the 1980s, a vaccine that could maintain potency without refrigeration was identified as a research priority. In only four years, the animal health community had one that could go without refrigeration for at least a month.

Meanwhile, studies of the virus itself had provided some measures of the efficiency of its spread, and the size of unvaccinated populations needed for it to maintain itself in the wild. Key measures like the number of infections that result from each infected animal suggested that a herd immunity level of 80 percent immunity would be enough to wipe the virus out.

In areas with limited infrastructure and government, however, getting to that level of vaccination was going to be a challenge. So, the eradication program decided to forgo the usual infrastructure and train the animal equivalent of community health workers. These would both report on outbreaks of the disease, and help get vaccine to the remote regions and administer it there. To gain acceptance from the local communities, the local veterinary experts (both trained and informal) had to be assured that these new workers were not a threat to their livelihood.

With the new plan in place, Ethiopia went from administering 20 million doses down to doing 3 million annually, but targeting them to the areas where they would disrupt transmission of the disease and eradicate reservoirs. In Somalia, total immunity probably didn't get much above half but, with careful targeting, that was enough.

Lessons for other diseases

Two factors were essential for the elimination of rinderpest: the relatively low rate at which infections spread from a single animal, and the fact that African cattle have an average lifespan of five years, which allows a single vaccination to have an extended benefit. The high rate of mortality also ensured that there was a high interest in getting rid of the disease.

These are not likely to be easily met with other diseases. Domestic poultry are too numerous, and have a very short life span on average; goats and sheep are marginal. Meanwhile, foot-and-mouth disease has a very high rate of infection that means that herd immunity needs to be well above 90 percent to prevent transmission. It's also not as lethal, which means that indigenous farmers are more likely to accept it as an occasional hazard.

The success with rinderpest may also have a lesson for those trying to wipe out human diseases. It is possible to run a successful vaccine campaign in areas with limited infrastructure and government. But that success will rely on local cooperation, both in terms of people willing to monitor and report the spread of the disease, and in terms of cooperating with those administering the vaccine. Recruiting local health workers for vaccine administration is also likely to be essential.

All of that assumes a that the local population has bought into the need for the vaccine. And, at least in the case of the human disease closest to eradication, polio, that has become a serious problem, as many local figures in the last remaining pockets have linked the vaccine to various conspiracies.

Science, 2012. DOI: 10.1126/science.1223805 (About DOIs).