Milroy Lecture: eradication of disease: hype, hope and reality

Elimination and eradication goals for communicable diseases: a systematic review

Objective

To consolidate recent information on elimination and eradication goals for infectious diseases and clarify the definitions and associated terminology for different goals.

Methods

We conducted a systematic search of the World Health Organization's Institutional Repository for Information Sharing (WHO IRIS) and a customized systematic Google advanced search for documents published between 2008 and 2022 on elimination or eradication strategies for infectious conditions authored by WHO or other leading health organizations. We extracted information on names of infectious conditions, the elimination and eradication goals and timelines, definitions of goals, non-standardized terminology, targets and assessment processes.

Findings

We identified nine goals for 27 infectious conditions, ranging from disease control to eradication. In comparison with the hierarchy of disease control, as defined at the Dahlem Workshop in 1997, six goals related to disease control with varying levels of advancement, two related to elimination and one to eradication. Goals progressed along a disease-control continuum, such as end of disease epidemic to pre-elimination to elimination as a public health problem or threat. We identified the use of non-standardized terminology with certain goals, including virtual elimination, elimination of disease epidemics, public health threat and public health concern.

Conclusion

As we approach the 2030 target date to achieve many of the goals related to disease control and for other infections to become candidates for elimination in the future, clarity of definitions and objectives is important for public health professionals and policy-makers to avoid misperceptions and miscommunication.

Rethinking disease eradication: putting countries first

There have been various infectious disease eradication programs implemented in various parts of the world with varying degrees of success since the early 1900s. Of all those programs, the one that achieved monumental success was the Smallpox Eradication Program (SEP). Most of the global health leaders and authorities that came up with the new idea of disease eradication in the 1980s tried to design and shape the new programs based on their experience in the SEP. The SEP had a very effective tool, vaccine, that did not require a cold chain system, and a relatively simple way of administration. The total cost of the eradication program was about US$300 million and the entire campaign took about 10 y. However, the Guinea worm and polio eradication programs that followed in the footsteps of SEP attained varying levels of success, consuming a huge amount of resources and taking a much longer time (>30 y each). This paper reviews the factors that played major roles in hindering the attainment of eradication goals and outlines possible recommendations for the way forward. Among other things, this paper strongly emphasizes that endemic countries should take the lead in all matters pertaining to making decisions for disease elimination and/or eradication initiatives and that 'elimination as a public health problem' is the preferred option rather than going for complete eradication at the expense of other health programs and thereby contributing to weakening of already fragile health systems, mainly in Africa.

Population genomic evidence that human and animal infections in Africa come from the same populations of Dracunculus medinensis

BACKGROUND

Guinea worm-Dracunculus medinensis-was historically one of the major parasites of humans and has been known since antiquity. Now, Guinea worm is on the brink of eradication, as efforts to interrupt transmission have reduced the annual burden of disease from millions of infections per year in the 1980s to only 54 human cases reported globally in 2019. Despite the enormous success of eradication efforts to date, one complication has arisen. Over the last few years, hundreds of dogs have been found infected with this previously apparently anthroponotic parasite, almost all in Chad. Moreover, the relative numbers of infections in humans and dogs suggests that dogs are currently the principal reservoir on infection and key to maintaining transmission in that country.

PRINCIPAL FINDINGS

In an effort to shed light on this peculiar epidemiology of Guinea worm in Chad, we have sequenced and compared the genomes of worms from dog, human and other animal infections. Confirming previous work with other molecular markers, we show that all of these worms are D. medinensis, and that the same population of worms are causing both infections, can confirm the suspected transmission between host species and detect signs of a population bottleneck due to the eradication efforts. The diversity of worms in Chad appears to exclude the possibility that there were no, or very few, worms present in the country during a 10-year absence of reported cases.

CONCLUSIONS

This work reinforces the importance of adequate surveillance of both human and dog populations in the Guinea worm eradication campaign and suggests that control programs aiming to interrupt disease transmission should stay aware of the possible emergence of unusual epidemiology as pathogens approach elimination.

A dynamic Bayesian Markov model for health economic evaluations of interventions in infectious disease

BACKGROUND

Health economic evaluations of interventions in infectious disease are commonly based on the predictions of ordinary differential equation (ODE) systems or Markov models (MMs). Standard MMs are static, whereas ODE systems are usually dynamic and account for herd immunity which is crucial to prevent overestimation of infection prevalence. Complex ODE systems including distributions on model parameters are computationally intensive. Thus, mainly ODE-based models including fixed parameter values are presented in the literature. These do not account for parameter uncertainty. As a consequence, probabilistic sensitivity analysis (PSA), a crucial component of health economic evaluations, cannot be conducted straightforwardly.

METHODS

We present a dynamic MM under a Bayesian framework. We extend a static MM by incorporating the force of infection into the state allocation algorithm. The corresponding output is based on dynamic changes in prevalence and thus accounts for herd immunity. In contrast to deterministic ODE-based models, PSA can be conducted straightforwardly. We introduce a case study of a fictional sexually transmitted infection and compare our dynamic Bayesian MM to a deterministic and a Bayesian ODE system. The models are calibrated to simulated time series data.

RESULTS

By means of the case study, we show that our methodology produces outcome which is comparable to the "gold standard" of the Bayesian ODE system.

CONCLUSIONS

In contrast to ODE systems in the literature, the dynamic MM includes distributions on all model parameters at manageable computational effort (including calibration). The run time of the Bayesian ODE system is 15 times longer.

Challenges to global measles eradication: is it all in the timing?

The case for global eradication of measles was first made in 1982. Since then, technical aspects of measles eradication have concluded that measles satisfied all criteria required for eradication. To date, only smallpox, among human diseases, has been eradicated, with polio, the next eradication candidate. In all previous eradication programmes, the pattern of slow implementation and missed deadlines is similar. Lessons from these past eradication programs should inform development of a time-limited measles eradication program. Notably, no measles eradication resolution is likely until member states are satisfied that polio eradication is accomplished. However, there is an impetus for measles eradication from the western hemisphere, where governments continue to pay the high costs of keeping their region measles free until global measles eradication is achieved. While previous vaccine preventable diseases eradications have depended on supplemental immunizations (SIAs), measles eradication will have to build both on SIAs and routine immunization systems strengthening. This article reviews non-technical considerations that could facilitate the delivery of a time-limited measles eradication initiative. The issues discussed are categorized as a) specificities of measles disease; b) specifics of measles vaccine/vaccination; c) special considerations for endemic countries and d) organization of international partnerships. The disease and vaccine specific issues are not insurmountable. The introduction of routine measles second dose, in the context of EPI systems strengthening, is paramount to endemic developing countries. In the international partnerships, it should be noted that i) Measles eradication will be easier and cheaper; ii) the return on investment is compelling; iii) leverage is feasible on the experiences of the Measles/Rubella initiative; iv) two disease eradication targets in one initiative are feasible and v) for the first time, an eradication investment case will inform the decisions. However, if previous eradication efforts have been marathons, measles eradication will need to be a sprint.

Past and Ongoing Tsetse and Animal Trypanosomiasis Control Operations in Five African Countries: A Systematic Review

BACKGROUND

Control operations targeting Animal African Trypanosomiasis and its primary vector, the tsetse, were covering approximately 128,000 km2 of Africa in 2001, which is a mere 1.3% of the tsetse infested area. Although extensive trypanosomiasis and tsetse (T&T) control operations have been running since the beginning of the 20th century, Animal African Trypanosomiasis is still a major constraint of livestock production in sub-Saharan Africa.

METHODOLOGY/PRINCIPAL FINDINGS

We performed a systematic review of the existing literature describing T&T control programmes conducted in a selection of five African countries, namely Burkina Faso, Cameroon, Ethiopia, Uganda and Zambia, between 1980 and 2015. Sixty-eight documents were eventually selected from those identified by the database search. This was supplemented with information gathered through semi-structured interviews conducted with twelve key informants recruited in the study countries and selected based on their experience and knowledge of T&T control. The combined information from these two sources was used to describe the inputs, processes and outcomes from 23 major T&T control programmes implemented in the study countries. Although there were some data gaps, involvement of the target communities and sustainability of the control activities were identified as the two main issues faced by these programmes. Further, there was a lack of evaluation of these control programmes, as well as a lack of a standardised methodology to conduct such evaluations.

CONCLUSIONS/SIGNIFICANCE

Past experiences demonstrated that coordinated and sustained control activities require careful planning, and evidence of successes, failures and setbacks from past control programmes represent a mine of information. As there is a lack of evaluation of these programmes, these data have not been fully exploited for the design, analyses and justification of future control programmes.

Political, social and technical risks in the last stages of disease eradication campaigns

Eradication of a disease is one of the greatest gifts any generation can give to subsequent ones, but most attempts have failed. The biggest challenges occur in the final stages of eradication and elimination campaigns. These include falling public support as a disease becomes less common; the emergence of groups who do not support eradication; spiralling costs; and the evolution of drug, vaccine or insecticide resistance. Mass campaigns become less effective as the disease fragments and modelling becomes less reliable. Optimism bias is the biggest risk to any eradication campaign and the long endgame must be planned for from the beginning.

The feasibility of eliminating podoconiosis

Podoconiosis is an inflammatory disease caused by prolonged contact with irritant minerals in soil. Major symptoms include swelling of the lower limb (lymphoedema) and acute pain. The disease has major social and economic consequences through stigma and loss of productivity. In the last five years there has been good progress in podoconiosis research and control. Addressing poverty at household level and infrastructure development such as roads, water and urbanization can all help to reduce podoconiosis incidence. Specific control methods include the use of footwear, regular foot hygiene and floor coverings. Secondary and tertiary prevention are based on the management of the lymphoedema-related morbidity and include foot hygiene, foot care, wound care, compression, exercises, elevation of the legs and treatment of acute infections. Certain endemic countries are taking the initiative to include podoconiosis in their national plans for the control of neglected tropical diseases and to scale up interventions against the disease. Advocacy is needed for provision of shoes as a health intervention. We suggest case definitions and elimination targets as a starting point for elimination of the disease.

Measuring elimination of podoconiosis, endemicity classifications, case definition and targets: an international Delphi exercise

Background

Podoconiosis is one of the major causes of lymphoedema in the tropics. Nonetheless, currently there are no endemicity classifications or elimination targets to monitor the effects of interventions. This study aimed at establishing case definitions and indicators that can be used to assess endemicity, elimination and clinical outcomes of podoconiosis.

Methods

This paper describes the result of a Delphi technique used among 28 experts. A questionnaire outlining possible case definitions, endemicity classifications, elimination targets and clinical outcomes was developed. The questionnaire was distributed to experts working on podoconiosis and other neglected tropical diseases in two rounds. The experts rated the importance of case definitions, endemic classifications, elimination targets and the clinical outcome measures. Median and mode were used to describe the central tendency of expert responses. The coefficient of variation was used to describe the dispersals of expert responses.

Results

Consensus on definitions and indicators for assessing endemicity, elimination and clinical outcomes of podoconiosis directed at policy makers and health workers was achieved following the two rounds of Delphi approach among the experts.

Conclusions

Based on the two Delphi rounds we discuss potential indicators and endemicity classification of this disabling disease, and the ongoing challenges to its elimination in countries with the highest prevalence. Consensus will help to increase effectiveness of podoconiosis elimination efforts and ensure comparability of outcome data.