Efforts to combar infectious disease are fought at various levels. The ultimate success is considered eradication, which means no infections in humans worldwide, none in a natural animal reservoir or host that could re-infect humans, and no more need for interventions unless there is an accidental or deliberate reintroduction of the infection. Smallpox is the only disease to have achieved this, and polio and guinea worm are currently targeted for eradication.
Another approach is to aim for elimination. This means stopping new infections in a defined geographic area, and then progressing to having the population in that region free of the disease. Continued interventions are required to prevent reestablishment in the area that has eliminated it. This strategy is being pursued against diseases such as elephantiasis, leprosy and river blindness.
For most other disease campaigns, control is the goal. Control programmes aim to reduce infections, illness and death to manageable levels, and this level varies depending on location. Consistent, sustained prevention measures and treatment are needed to keep the disease under control. As knowledge and technology advance, the goal for some diseases that meet the criteria for elimination or eradication can and does move from control to elimination, and with political will and adequate funding, from elimination to eradication.
The Case Of Smallpox
In many ways, smallpox proved to be an ‘ideal’ candidate for disease eradication and, upon successful certification of eradication, provided science with a rigorous set of criteria for identifying future infectious disease eradication candidates. In 1967 the World Health Assembly, the World Health Organization (WHO)’s annual gathering of the health ministers, passed a resolution calling for the eradication of smallpox through vaccination. The disease displayed attributes that would favour a highly successfully campaign. A high death rate clearly defined smallpox as a major global public health threat and instantly attracted the required political support. The presence of easily identifiable disease and scarring in survivors made it severely debilitating, but also enabled outbreak teams to identify clear geographic areas of transmission to effectively target search and containment strategies. Arguably the most important characteristic that helped stamp out smallpox transmission was the lack of a known animal host to harbour it or insect vector to spread it, which meant that interventions only needed to be aimed at humans. The smallpox vaccine was highly effective in providing long-term immunity and could be easily transported and deployed. In 1980, the WHO certified smallpox eradicated.
Current Eradication Efforts: The Final Push
Two further eradication programmes have been established as a result of the lessons learned from past successes and failures, and these have been greatly aided by advances in epidemiology and operational research.
Dracunculiasis, commonly known as guinea worm disease, is caused by a parasitic worm. People become infected by drinking water contaminated with microscopic crustaceans infected with the larva of the worm. After the female worm matures in the human body, it creates a painful ulcer, usually on the leg, from which it ejects its larva when the infected person submerges the limb in water to relieve the burning. Encouraging the use of safe filtered drinking water through the provision of clean sources and simple homemade filters, health education activities, and the targeted destruction of the crustacean in the water, have constituted a comprehensive and highly successful package of interventions on which to base an eradication campaign. A 1991 World Health Assembly resolution to eradicate the disease was a result of the natural progression from intensified efforts for control, beginning in the early 1980s. Efforts to date have seen the disease eliminated from areas traditionally associated with a high disease burden in the Middle East and South Asia. Recent cessation of transmission in Ghana means that just three countries have not been able to interrupt the transmission of guinea worm - South Sudan, Mali and Ethiopia - from an original twenty endemic countries in 1986. The final stages of elimination in the remaining countries have proven problematic; insecurity issues, particularly in South Sudan, which harbours 94 percent of remaining cases, prevent public health teams from delivering interventions in the region. Also, the disease re-emerged in 2010 in Chad, which had been previously free of the disease for a decade. The re-emergence may have resulted from refugee movements from the Darfur region of Sudan to refugee camps based in Chad. Surveillance for new infections will prove critical during the final push for eradication.
The second infectious disease targeted for eradication is polio, selected by a World Health Assembly resolution in 1988. Vaccination campaigns have been the mainstay for the eradication effort using the oral polio vaccine which contains a live but attenuated virus to immunize children. Since eradication efforts began in 1988, when 125 countries were polio-endemic, only Afghanistan, India, Nigeria and Pakistan remain on the list of endemic countries, and transmission of one of the three types of poliovirus has been successfully halted in all countries.
However, polio re-emergence does remain a threat outside of these countries; exportation of the virus from India resulted in outbreaks across many previously polio-free countries in Asia and central Europe in 2008. Prior to that, in 2003, Kano State in Nigeria suspended vaccination campaigns after erroneous rumours suggested that polio vaccine had been contaminated with HIV and/or hormones that sterilised young girls. The suspension in Kano State led to a Nigeriawide cessation of vaccination that in turn led to the reestablishment of polio transmission across Africa and into Saudi Arabia, Yemen and as far away as Indonesia. Pakistan currently accounts for more reported cases than the three other endemic countries combined, and the number of cases in both Afghanistan and Pakistan appear to be on the rise. Insecurity in the conflict-affected northern regions of Pakistan, particularly along the border with Afghanistan, has suffocated vaccination campaigns. In addition to conflict-affected areas, the inaccessibility of communities, due to either the mountainous terrain or recent devastating floods, has made vaccination coverage across the country highly challenging. Although polio transmission appears close to global interruption, a set of important questions should be tackled now in preparation for the next stage. For example, the potential for the virus used in the oral vaccine to revert to a form that can induce paralysis means that surveillance will have to continue for a significant period of time, and concerns regarding destruction of virus stockpiles and/or maintenance in high-security laboratories need to be carefully considered.
Looking To The Future: New Eradication Targets
There have been calls for eradication of other key infectious diseases. Bill Gates famously called for eradication of malaria in 2007, leading to renewed but potentially premature optimism. After the failure of the malaria eradication effort of the 1950s, the fight against the disease was refocused on control and elimination. That approach has slowly chipped away at the global malaria burden, leaving a number of countries either certified as malaria-free or close to elimination. In certain areas where malaria transmission is particularly intense, and/or health systems severely compromised, the most realistic goal will be control. The current global approach uses a regional scale-up strategy in an attempt to increase the rate at which malaria interventions are delivered to the population most at risk. This has delivered dramatic results, with many countries across Africa reporting a 20-25 percent reduction in malaria-related childhood deaths. New challenges arise with these successes; maintaining political and financial commitment particularly at a time when the malaria burden appears to be declining will be central for furthering elimination efforts. It is vitally important that political engagement flows from not only the international and donor community, but also from the heads of states and finance ministers within the affected countries. Whether the goal is to maintain malaria control, or to move from control to elimination of malaria, in-country political support and mobilisation of funding is needed to achieve sustainable programmes. The current successes have been based on reducing death rates. However, preventing spread is the key to eradication. A malaria vaccine would provide the best option for halting transmission, and the progression of the GlaxoSmithKline-based vaccine, currently in the final phase of trials, may be the first vaccine candidate to enable a glimmer of hope for such a goal.
Scientists have also recently started to assess the feasibility of measles eradication. Measles elimination campaigns are ongoing in many parts of the world, and scope for eradication is being intensely scrutinised. Certain barriers will need to be addressed before eradication is attempted, which differ between developed and developing nations. In developed western countries where measles has been eliminated, there is a lack of urgency as the disease is no longer perceived as a serious public health threat and the presence of antivaccination movements makes it difficult to determine whether elimination can be sustained, and eradication attained. Within developing countries, measles transmission is widespread and intense, but in none more so than countries embroiled in conflict. This will raise similar issues currently faced by polio eradication efforts. It will require careful planning if a new campaign is to co-exist sustainably with the existing eradication efforts and other high-priority international health initiatives.
The challenges of achieving eradication are numerous and vary according to the disease in question. Surveillance, particularly in the latter stages of a campaign, combined with continued political and financial support, will be vital elements to any disease eradication programme. Polio and dranculiasis eradication efforts are nearing fruition, and the push from philanthropic foundations for other infectious diseases to be equally stamped out means there is cause for great optimism. However, this hope must be tempered with lessons of caution and vigilance, and eradication campaigns must ensure that realistic targets are set, given the current state of technologies and knowledge.