The Centre for Disease Control website provides the following definitions for infectious disease status:
- "Control: The reduction of disease incidence, prevalence, morbidity or mortality to a locally acceptable level as a result of deliberate efforts; continued intervention measures are required to maintain the reduction. Example: diarrhoeal diseases.
- Elimination of disease: Reduction to zero of the incidence of a specified disease in a defined geographical area as a result of deliberate efforts; continued intervention measures are required. Example: neonatal tetanus.
- Elimination of infections: Reduction to zero of the incidence of infection caused by a specific agent in a defined geographical area as a result of deliberate efforts; continued measures to prevent re-establishment of transmission are required. Example: measles, poliomyelitis.
- Eradication: Permanent reduction to zero of the worldwide incidence of infection caused by a specific agent as a result of deliberate efforts; intervention measures are no longer needed. Example: smallpox.
- Extinction: The specific infectious agent no longer exists in nature or in the laboratory. Example: none."
So why has no other infectious disease managed to reach 'eradication' status? It's not for lack of trying. At roughly the same time that smallpox was being stamped out, a global malaria eradication effort was also underway. In the US, malaria was considered eradicated in 1951, and in 1955 the WHO proposed a plan for a worldwide effort that included anti-malarial drugs, and widespread pesticide use. Success varied wildly - the program never even made it to sub-Saharan Africa - and eventually malaria re-emerged in certain areas. The goal soon shifted from one of eradication to one of control.
Leprosy is another disease targeted for elimination. Like smallpox, the only source of infection is other humans and there is an established multi-drug treatment to prevent transmission making it a good candidate for eradication. However, the long latency period and an inability to diagnose and treat subclinical leprosy make elimination of the disease a more reasonable target.
One disease that is seeing a strong push for eradication is polio. Vaccination programs have eliminated infections in most of the world, and in February 2006 polio was endemic in only 4 countries worldwide: Nigeria, India, Pakistan and Afghanistan (Egypt and Niger were removed from the list in 2006 after 12 months without a new case). From the beginning of the eradication program in 1998 to 2004, the number of reported cases worldwide was reduced from 350 000 to 1189. All signs point to polio being the next infectious disease to be eradicated, though vaccinations and vigilence still need to persist as evidenced by imported cases being reported in countries sugh as Nepal, Angola or Ethiopia.
The success of an eradication program depends on a number of factors. From the disease side, there needs to be a reliable treatment to stop transmission. A vaccine, a drug treatment - something that prevents the spread of an infectious agent. It's also important that humans be essential for the life-cycle of the agent. For example, malaria is difficult to eradicate in part because it's carried by mosquitoes which act a reservoir - a pool that is much more difficult to deal with. Emergence of drug resistance is another barrier to effective elimination. The reemergence of malaria during the eradication efforts was caused, among other factors, by development of resistance to drugs by the parasite and resistance to insecticides by mosquitoes.
There are also social and economic factors at play. The eradication of a disease requires a firm commitment from governments and communities, since lapses can lead to reemergence. At the community level, this means awareness of programs and diligence in vaccination. Anti-vaccination movements are a barrier to eradication programs. Polio may be unlikely to reemerge in North America, but future diseases may require vaccination to be stamped out. (This also highlights that the disease needs to be of importance to the public, with broad appeal - an elimination of HIV would gain more support than, say, Rocky Mountain Spotted Fever.) At the government level, it requires broad co-operation with other countries, a tenable strategy and a financial commitment. This last point is key as developing nations may not have the resources required (or have eradication prioritized in the face of other problems) to invest in vaccination programs. And it is an investment: "A total of $32 million was spent by the United States over a 10-year period in the global campaign to eradicate smallpox. The entire $32 million has been recouped every 2 months since 1971 by saving the costs of the smallpox vaccine, administration, medical care, quarantine and other costs." In the case of polio, an estimated more than $230 million will be saved in vaccine production and administration in the US each year once eradication is achieved, which seems like it's just a matter of time. After polio, what's next? Eradication programs are also underway for other infectious or parasitic diseases such as guinea worm disease, Chagas' disease and lymphatic filariasis. Will they be successful?