Current transmission rates from primary cases to secondary cases could cause Ebola epidemic to explode.
The Ebola outbreak in West Africa may only be the tip of the iceberg if the current transmission rate continues unabated.
In a study published online September 11, 2014, in Eurosurveillance, researchers from Arizona State University and the University of Tokyo presented a bleak forecast for the ongoing Ebola epidemic, assuming the current transmission trend continues. From June to July, the rate of transmission from primary cases to secondary cases increased from 1.4 to 1.7 for every existing case in Sierra Leone and Liberia, respectively.
Under a hypothetical worst-case scenario, the current Ebola burden could expand to 277,124 patients by the end of 2014, the researchers found.
“Our analysis of the reproduction numbers of Ebola cases shows continuous growth from June (2014) to August 2014 that signaled a major epidemic,” said Hiroshi Nishiura, of the University of Tokyo, in a press release. “Uncontrolled cross-border transmission could fuel a major epidemic to take off in new geographical areas, as was seen in Liberia.”
The worst-case scenario estimates are significantly greater than the 20,000 cases the World Health Organization (WHO) projected to be possible if the outbreak is not contained. The current epidemic has caused more than 2000 fatalities, with nearly 5000 confirmed and suspected cases in West Africa.
With a number of cases believed to be unreported thus far, the hardest-hit regions are expected to see a surge of thousands of new cases over the next 3 weeks, according to the WHO. The largest outbreaks previously seen in Central Africa generated 315 cases in Congo in 1976 and 425 cases in Uganda in 2000.
According to the study, global and country-specific estimates for the effective reproduction number of secondary cases generated by a typical primary case have remained above 1 since early June 2014, illustrating that the epidemic has been growing steadily. The researchers hypothesized that mitigating the number of secondary transmissions per primary case could control the Ebola outbreak.
“Our findings suggest that control of the Ebola epidemic that has taken so many lives could be attained by preventing more than half of the secondary transmissions for each primary case,” said ASU researcher Gerardo Chowell-Puente. “This could be attained by isolating those with Ebola and tracing each case to its source.”
While the assumption of continued exponential growth under the worst-case scenario is unlikely, the study findings bolstered the fact that the current Ebola outbreak is an international public health emergency. The study also found that efforts to control the virus have been hindered by transitional Ebola spread, which suggests that there is a non-negligible number of secondary cases caused by this spread.
As a result, the epidemic could be significantly exacerbated by uncontrolled cross-border transmission in new geographical areas. The researchers advised unaffected countries to be on high alert for potential Ebola introductions, in order to limit transitional spread through a timely containment response.
“Characterizing the distribution of secondary cases from a single case can help health care workers and officials understand Ebola transmission dynamics over time in affected countries and gauge the effect of interventions to control spread of the disease,” Chowell said.