The importance of the incubation period in cholera and Ebola outbreaks in Sierra Leone

March 10, 2020

Studying how diseases spread in time and space when an outbreak occurs is key to designing a rapid and effective response. Recent epidemics such as the Zika or Ebola outbreaks only serve as a reminder of the need to improve global surveillance and response capacity if we want to effectively contain it and minimize the impact on society, both locally and globally.

A study published in PNAS, in which IFISC (UIB-CSIC) has participated, has characterized the epidemiology of two outbreaks in Sierra Leone, one of cholera in 2012-2013 and another of Ebola in 2014-2015. To do this, the researchers used data collected by the Ministry of Health and Sanitation of Sierra Leone provided by the World Health Organization to study their characteristics, in addition to conducting numerical simulations to verify the conclusions found.

In epidemiology, the time that elapses between an individual being infected and becoming infectious (i.e. capable of infecting others) is known as the latent period. On the other hand, the time it takes for symptoms to develop is called the incubation period. The relationship between these two amounts plays a key role in characterizing the potential for a disease to become an epidemic. A critical difference between the two diseases studied is the incubation period, which is estimated at 8-12 days for Ebola but only 1-2 days for cholera. In the case of study, both outbreaks lasted similar periods of time and spread over more than 75% of the chiefdoms into which the country is divided. Although outbreaks tend to last longer with those diseases that have longer incubation periods, transmission to more distant locations can progress more rapidly, causing a more rapid and discontinuous expansion front. That is, short incubation periods result in spreads similar to the disturbance generated by a stone falling into a lake (relatively circular and uniformly advancing), while long incubation periods lead to long-range outbreaks appearing in small sparks distant from each other. Travel by infected persons in their incubation period is also a key factor in the geographical dispersion of epidemics, as is predictability, especially in populations of individuals who become less mobile when they fall ill.

In addition to characterizing real data from both outbreaks, the researchers carried out numerical simulations with different incubation periods to test how this parameter affects the spatial spread of the diseases. In the case of short incubation periods, a higher peak of infected individuals is found, but in a shorter space of time. In contrast, for longer incubation periods the maximum peak of infections is lower but the outbreak has a much longer duration. The difference in incubation period also influences the predictability of the outbreak, being much more unpredictable when the period is longer.

A better understanding of the spatial-temporal spreading of outbreaks is key to proposing vaccination strategies to slow down their progression by locating those places with greatest risk of infection. This is critical to address known threats, such as a hypothetical case of a cholera or Ebola resurgence in Sierra Leone, but also new outbreaks that may occur in other parts of the world. 

Incubation periods impact the spatial predictability of cholera and Ebola outbreaks in Sierra Leone. Rebecca Kahn, Corey M. Peak, Juan Fernández-Gracia, Alexandra Hill, Amara Jambai, Louisa Ganda, Marcia C. Castro, Caroline O. Buckee Proceedings of the National Academy of Sciences Feb 2020, 201913052; DOI: 10.1073/pnas.1913052117

 Juan Fernández

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