While widespread vaccination led to eradication of smallpox by 1979, a changed world and changing health priorities make it tougher to totally eliminate other infectious diseases, experts said at a AAAS meeting.
“Eradication of smallpox gave rise to a glow of optimism,” said Joshua Rosenthal, acting deputy director of the Fogarty International Center at the National Institutes of Health. If smallpox was gone, could measles, polio, and other diseases be far behind?
But as time progressed, Rosenthal said, it did not seem that simple. Challenges in developing effective vaccines, rising resistance to antibiotics, pesticide resistance by disease-transmitting insects, quicker transmission pathways due to global air travel, and other factors make elimination of common communicable diseases much more challenging.
And health officials must weigh the benefits of an all-out eradication campaign for a specific infectious disease against spending for broader public health goals such as better sanitation, improved nutrition, and attention to chronic illnesses such as heart disease and diabetes.
“2011 is not the same as 1979,” said Dennis Carroll, director of the Avian Influenza & Other Emerging Threats Unit at the U.S. Agency for International Development. The smallpox eradication campaign was carried out during a time of global stability when health workers were able to get ready access to remote villages in many parts of the world.
“We don’t see that now,” Carroll said. Many areas where infectious diseases are endemic are torn by strife.
Joshua Michaud, principal policy analyst at the Kaiser Family Foundation, noted that the last indigenous case of naturally occurring smallpox was diagnosed in Somalia, a country now ruled by warlords and considered a failed state. “Can you imagine trying that [a disease eradication campaign] in Somalia today?” Michaud asked.
He and the others spoke at a salon-style discussion on “Infectious Disease: Challenges to Eradication” at AAAS on 7 November. It was the third in a fall series of discussions titled “Science and Society: Global Challenges.” The series is sponsored by the AAAS Office of Government Relations, the association’s Center for Science, Technology and Security Policy, the American Chemical Society, and the Georgetown University Program on Science in the Public Interest. Richard Harris, a science correspondent for NPR, moderated the discussion.
The debate on how to balance available public health resources when considering eradication of specific diseases versus the needs of the broader health system has been around for decades, Carroll said. The United States is able to do both, he said, but often that is not the case in the developing world. He argued that focusing on the eradication of a single infectious disease often undercuts a nation’s ability to address more systemic problems that contribute to the overall burden of death and disability.
“I don’t believe in eradication,” Carroll admitted. As a general rule, he said, “I think it’s fool’s gold.” He said dramatic successes in reducing childhood mortality rates in Africa over the past two decades highlight the effectiveness of strategies to promote integrated management of childhood diseases and improved maternal health rather than focusing on elimination of any specific infectious disease.
Rosenthal said there is value in having major goals—such as eradication of a disease such as polio—in order to help mobilize health resources. Such campaigns can bring lots of local leaders and community members on board, he said. But Rosenthal, too, said he is “not a big advocate of eradication.”
“Control of a disease is an important and achievable element,” he said. “And we have 1300 pathogens to choose from.” At least two new pathogens emerge each year that are identified as causes of human disease, Rosenthal said, “so there is a lot to focus on.”
As a practical matter, the speakers agreed, total eradication of common infectious diseases may not be feasible. In part that is due to technical problems such as developing vaccines with a high level of efficacy. “Very few vaccines give you life-long immunity,” Rosenthal said. Even the polio and measles vaccines, which he described as “better than most,” require booster shots. To achieve an effective vaccine for malaria, he said, is “still a very high hurdle.” The same is true for a vaccine against the virus that causes AIDS.
Social barriers also play a role. There was a great deal of progress initially against polio, thanks to the development of vaccines, Michaud said. But misinformation and fear have disrupted some ongoing eradication efforts. In 2003 in the northern states of Nigeria, there were rumors of possible sterility from vaccination and the levels of vaccination dropped sharply. An outbreak of the disease in Nigeria led to cases in other African countries, Michaud said, and the effort to recover from the setback cost an estimated $500 million. There also are persistent problems with polio in Pakistan and Afghanistan. Even in parts of developed nations such as the United States, “vaccine hesitancy” by often well-educated parents has led to outbreaks of vaccine-preventable childhood diseases such as whooping cough.
Rosenthal also noted that the current polio vaccine, although it has helped bring down the number of polio cases worldwide from the hundreds of thousands to the thousands, is far from perfect. It uses an attenuated form of the polio virus that can mutate to a disease-causing state in a small percentage of those vaccinated, Rosenthal said.
Even when effective measures are available, harmful pathogens can persist. Michaud noted that yellow fever, long considered amenable to eradication through use of an effective vaccine and control of disease-carrying mosquitoes, continues to circulate in non-human primates and can re-emerge. “It’s almost impossible to eradicate,” Michaud said.
Harris asked the panelists to discuss some diseases that are candidates for effective control if not outright eradication. They mentioned Guinea worm, a waterborne parasitic worm that grows and matures beneath the skin. It can be controlled through aggressive education campaigns, Rosenthal said. Residents must be convinced to use only water that has been filtered or obtained from a safe source. Those with Guinea worm wounds must stay away from ponds or wells used for drinking water. The emerging worms release larvae that can infect others. If the cycle can be interrupted for a season, Carroll said, the worm can be essentially eradicated from a locality.
Rosenthal said such efforts have been “remarkably effective.” The disease is now limited largely to three or four African countries, he said, and one of them—Ghana—is close to being certified as free of Guinea worm. More than 90% of the cases are now in South Sudan, Michaud said, and the political instability in the region has made eradication efforts more difficult.
Carroll said there also are good prospects for better control in Central America of river blindness, a parasitic disease caused by a roundworm. The disease is also widespread in sub-Saharan Africa. In the Americas, the cases tend to be geographically localized, Carroll said. The combination of inefficient transmission and availability of an effective drug, ivermectin, makes regional elimination of the disease possible, he said.
Carroll said there are a mix of factors that can affect how well people cope with any infectious disease. Even when a vaccine is available, nutritional status can affect how well a vaccination triggers the body’s immune defenses. The measles vaccine does not take well, Rosenthal said, if the recipient is anemic or otherwise compromised by poor diet.
The speakers were asked how climate change might affect infectious disease control and eradication efforts in the future. As temperatures warm in Africa, for example, mosquitoes that carry the malaria parasite appear to be moving to East African highlands formerly free of the disease. Despite such concerns, Michaud predicted that sound public health measures will continue to have much more impact on the distribution and incidence of diseases like malaria than the effects of climate change.
Carroll cautioned, however, that climate change does bring habitat changes with unpredictable consequences. Already, he said, human and animal populations have been “moving in places and beginning to interact in ways that we’ve not seen before.” Those changing interactions are driven partly by climate factors, he said, but also in a major way by population growth and increased food demands.
“We’re seeing the frequency of new diseases emerging at rates unprecedented in human history,” Carroll said. Any talk of controlling or eradicating diseases, he said, must take into account the fact that “we are in an extraordinarily dynamic environment where diseases are equally dynamic in both their presentation and in how they play themselves out in our lives.”