Originally published at PLoS Speaking of Medicine
There is no question that the eradication of smallpox, a devastating illness costing millions of lives, was one of the greatest achievements of 20th-century medicine. The disease was triumphantly declared eradicated by the World Health Assembly in 1980. Smallpox eradication required extremely focused surveillance as well as the use of a strategy called “ring vaccination,” in which anyone who could have been exposed to a smallpox patient was vaccinated immediately. Why was smallpox eradication possible? For one thing, smallpox is easily and quickly recognized because of the hallmark rash associated with the illness. Second, smallpox can be transmitted only by humans. The lack of an animal reservoir makes controlling the illness much simpler.
The success of smallpox eradication campaigns has resulted in persistent calls to eradicate other infectious diseases in the years since 1980. Unfortunately, disease eradication can be difficult and even impossible in the case of many infectious diseases, and it is crucial to consider the features of each illness in order to come to a proper conclusion about whether the pursuit of disease eradication is the best approach. In the first place, it is important to be clear about what “eradication” means. Eradication refers to deliberate efforts to reduce worldwide incidence of an infectious disease to zero. It is not the same as extinction, the complete destruction of all disease pathogens or vectors that transmit the disease. Elimination, a third concept, encapsulates the complete lack of a disease in a certain population at a certain point in time. Disease eradication therefore specifies a particular strategy for dealing with infectious diseases; other options exist that in some circumstances may be more desirable.
Can the pursuit of disease eradication ever be detrimental? It could be in the case of certain diseases that do not lend themselves easily to total eradication. A claim of eradication logically ends prophylactic efforts, reduces efforts to train health workers to recognize and treat the eradicated disease, and halts research on the disease and its causes. When eradication campaigns show some measure of success, financial support for the control of that illness plummets dramatically. Wide dissemination of information about eradication efforts without the certification of success can therefore prove detrimental. In these cases, complacency may prematurely replace much needed vigilance. If there is a reasonable chance of recurrence of the disease or if lifelong immunity against the disease is impossible, then attempting eradication may prove disastrous because infrastructure to control the disease would be lacking in the event of resurgence. Tracking down the remaining cases of an illness on the brink of eradication can be incredibly costly and divert government money in resource-poor nations from more pressing needs.
Another potential problem with disease eradication efforts is that, as a vertical approach, they may drain resources from horizontal approaches, such as capacity building and health system strengthening. Some advocate a more “diagonal” approach that uses disease-specific interventions to drive improvements of the overall health system. Still others have argued that vertical approaches that treat one disease at a time may divert resources from primary healthcare and foster imbalances in local healthcare services. Vertical schemes may also produce disproportional dependence on international NGO’s that can result in the weakening of local healthcare systems.
Malaria offers an excellent example of a case in which debate rages about whether eradication efforts would be successful. There are four species of single-cell parasite that cause malaria, the most common of which are P. falciparum and P. vivax. P. falciparum is the most deadly and P. vivax is the most prevalent. These two species make it difficult to engineer a single, fool-proof vaccine. Further complicating developing a vaccine for malaria are the ability of the parasites to mutate so that even contracting malaria does not confer life-long immunity. Furthermore, malaria involves an animal vector (mosquitoes). It would clearly be a huge challenge and perhaps even impossible to wipe out malaria completely. Beginning in 1955, there was a global attempt to eradicate malaria after it was realized that spraying houses with DDT was a cheap and effective way of killing mosquitoes. The initiative was successful in eliminating malaria in nations with temperate climates and seasonal malaria transmission. Yet some nations, such as India and Sri Lanka, had sharp reductions in malaria cases only to see sharp increases when efforts inevitably ceased. The state of affairs in India and Sri Lanka demonstrates some of the negative effects of eradication campaigns that are not carried to fruition. The project was abandoned in the face of widespread drug resistance, resistance to available insecticides, and unsustainable funding from donor countries. This failure was detrimental because the abandoned vector control efforts led to the emergence of severe, resistant strains that were much harder to treat.
Recently, discussions of malaria eradication have begun again. At the moment, there is considerable political will and funding for malaria eradication efforts from agencies such as the Gates Foundation. The Malaria Eradication Research Agenda Initiative, in part funded by the Gates Foundation, has resulted in substantial progress in identifying what needs to be done to achieve eradication. Even so, proponents of malaria eradication admit that this goal would take at least 40 years to achieve. It is not clear how long current political will and funding will last. There are concerns that political will might wither in the face of the estimated $5 billion annual cost to sustain eradication efforts.
Disease eradication can clearly be an incredibly important public health triumph, as seen in the case of smallpox. But when should the strategy be employed and when is it best to avoid risks associated with eradication efforts that might fail? Numerous scientific, social, and economic factors surrounding the disease in question must be taken into consideration. Can the microbe associated with the disease persist and multiply in nonhuman species? Does natural disease or immunization confer lifelong immunity or could reinfection potentially occur? Is surveillance of the disease relatively straightforward or do long incubation periods and latent infection make it difficult to detect every last case of the illness? Are interventions associated with eradication of the disease, including quarantine, acceptable to communities globally? Does the net benefit of eradication outweigh the costs of eradication efforts? Proposals for disease eradication must be carefully weighed against potential risk. Rather than being presented as visionary, idealistic goals, disease eradication programs must be clearly situated in the context of the biological and economic aspects of the specific disease and the challenges it presents.