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The duration and characteristics of the current devastating and unprecedented Ebola epidemic highlight the need for global public health surveillance to establish preparedness mechanisms for future outbreaks. Since the discovery of the virus in 1976, at least 25 Ebola outbreaks have been recorded, on average occurring every 1.5 years with case fatality rate (CFR) between 30% and 90%.1 ,2 The largest interval between two outbreaks is 15 years, that is, from the 1979 outbreak in Sudan due to the Sudan Ebola virus and the subsequent 1994 outbreak in Gabon caused by the Zaire Ebola virus.
Remarkably, only six previous outbreaks generated >100 deaths but the approximately 11 222 deaths (as of 30 June 2015) in the ongoing epidemic is already more than seven times the number of deaths reported for all previous outbreaks combined, which is estimated to be about 1580 deaths.3 ,4 Obviously, the present epidemic is the longest, largest and most complex Ebola outbreak since the virus was first discovered about 40 years ago. The outbreak started in December 2013 in Guinea,2 spread across land borders to Sierra Leone and Liberia, and then subsequently, by some infected persons, to seven other countries (Mali, Nigeria, Senegal, Spain, the UK, Germany and the USA). In some of these countries, individuals were only diagnosed as Ebola virus disease (EVD) cases after their arrival (eg, the USA), while other countries received known patients for treatment (eg, Spain). However, in these countries, the disease was rapidly contained, thanks to improved healthcare facilities, timely patient isolation and treatment. Meanwhile, the outbreak in Guinea, Liberia and Sierra Leone continued on an unprecedented scale, with unparalleled severity and complexity against a backdrop of poverty, insufficient health infrastructures and social customs that made contact tracing difficult and also made interrupting human-to-human transmission a big challenge. Over the past months, considerable progress has been made in interrupting transmission in these three countries. On 9 May 2015, the WHO declared Liberia free of Ebola virus transmission. This announcement came 42 days after the last laboratory confirmed case patients were buried, on 28 March 2015, making the duration of the epidemic in Liberia 1 year. The interruption of transmission is considered a monumental achievement for this country, which, to date, has recorded the highest number of deaths (4806) due to Ebola. Tributes certainly need to be extended to the government and the people of Liberia for their concerted determination, and further to the international community for its efforts (though late) to stop Ebola transmission. However, Guinea and Sierra Leone are still affected by the outbreak, but the number of weekly cases has dropped significantly compared with the number of cases during the peak of the epidemic. This trend suggests there is hope that the present epidemic will eventually be stopped. The question on the horizon, however, is, why has the present epidemic taken so long to control, and where and when will Ebola show its ugly face again? Ebola is at least not the most infectious disease to humans. Other diseases such as influenza, for example, are far more infectious because they can be transmitted through the air rather than requiring direct contact, and one infected person can potentially infect several others by merely sneezing or coughing in crowded places such as buses, aeroplanes, markets, etc; yet, control efforts are almost always effective, thanks to the availability and efficacy of the influenza vaccine.
In principle, it should be possible to bring Ebola outbreaks under control in a timely manner through public health measures such as early diagnosis, patient isolation, contact tracing, community sensitisation, adequate treatment, and clinical research and trials of potential vaccines. Interestingly, the CFR range of 30–90%, depending on the virus species, for most previous outbreaks including the present outbreak,2 suggests that a substantial proportion of exposed people are asymptomatic or do not come down with the disease due to protective immunity to the infection.5 These asymptomatic individuals also contribute to boost herd immunity and therefore could potentially dampen the spread of the epidemic. Plasmapheresis is a treatment modality that was not exploited at the beginning of the current epidemic but only later, when it was too late for this intervention to have a significant impact.6
It might be recalled that, in 1995, during an outbreak of Ebola virus in Kikwit, Democratic Republic of Congo, there was a 12.5% CFR among eight patients with Ebola haemorrhagic fever who received blood transfusions from convalescent donors. That is, of the eight patients who received transfusion of whole blood from survivors, seven survived.7 A similar therapeutic potential was observed earlier in 1977, when an infected health worker was treated with blood plasma from a survivor. Although the study design of the earlier work is generally considered to be weak due to the lack of power and the timing of the intervention,8 it set the stage for conducting a properly designed trial with convalescent plasma that could have been exploited in the current epidemic. Plasmapheresis has been attempted for other infectious diseases such as smallpox and Lassa fever (a viral haemorrhagic fever)9 and has a reasonable chance of working for Ebola as well.6 However, while these advanced treatment options for Ebola are important in order to save lives and provide care for victims, they need to be viewed in the larger context of the necessity for both basic resuscitative care (fluid and electrolyte management) and public health containment efforts.
It is also clear that, at the beginning of the current epidemic, many governments of wealthy countries and international organisations assumed that the usual Ebola actors (WHO, CDC (Centers for Disease Control and Prevention), MSF (Médecins Sans Frontières), etc) would take care of the outbreak, until it became evident that these actors were slow to act and consequently delayed global action gave way for the disease to spread within the population.10 I speculate that the reason for the reluctant timely intervention was partly because most previous outbreaks resulted in a few dozen or, at worst, a few hundred cases, therefore, that the outbreak could escalate to such as alarming scale was not imagined. Hopefully, such delayed actions will be avoided in future outbreaks to prevent what could likely become global disasters. Perhaps the lessons from the current outbreak will yield some technological innovations that will help in the future such as clinical trials of an effective vaccine. Hopefully, such a vaccine can help protect people already exposed to the virus but who have not yet developed the disease.
Taken together, and following the trend of the present epidemic, it is likely that future outbreaks may be more serious. Therefore, the need for international public health investment on research and development for Ebola vaccines and antivirals cannot be overemphasised. But, it is difficult to predict with certainty where and when an outbreak is likely to occur. The WHO has listed 14 countries in Africa (Burkina Faso, Benin, Central African Republic, Cameroon, Cote d’Ivoire, Ethiopia, The Gambia, Ghana, Guinea Bissau, Mauritania, Niger, Nigeria, Senegal and Togo) as at-risk countries and suggest the establishment of preparedness plans by each country to ensure that it is ready to effectively detect, investigate and report potential EVD cases, and to mount an appropriate response.11 With the exception of the outbreak in the Democratic Republic of Congo, which occurred in 2014, there is considerable risk that subsequent outbreaks may likely occur within a couple of years and that these may occur in currently unaffected countries. However, with adequate preparedness measures, the introduction of the virus can be contained before it escalates into a larger outbreak. This can ultimately be achieved by, among other measures, a coordinated national and international response, Ebola research and development, improvement on healthcare infrastructure in at-risk countries, disease surveillance and community engagement. The world needs a global response system for outbreaks to complement the activities of the WHO, as an outbreak anywhere is a risk everywhere.
Competing interests None declared.
Provenance and peer review Commissioned; externally peer reviewed.