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How the Spread of Ebola Differed between Countries

Looking back now, most of us that were unaffected by the Ebola outbreak barely remember how the disease devastated West Africa and how quarantine measures were made with almost no regard for human rights. As nurses and healthcare workers across the world were immediately quarantined after returning from aid operations, the news was ablaze questioning the decisions of political leaders and healthcare organizations for their “disrespectful” actions. At the same time, experts, particularly from the World Health Organization (WHO), continuously warned against complacency and any such thoughts about stopping aid (to prevent outbreaks from workers) or quarantine efforts. Experts pointed out the differences between the outbreak in Liberia and the one in Sierra Leone: in Liberia, the disease spread much slower and was under control much faster, yet in Sierra Leone, it covered a large distance in little time, infecting communities at an alarming rate with no sign of stopping.

Larger numbers in Sierra Leone than Liberia, even though the epidemic hit Liberia first

While the WHO is not fully certain of the exact causes for this difference (as the strain of disease was equivalent in both countries), they predict certain cultural and economic differences had significant impact, which we can analyze with our understanding of the underlying structure of the contact network in each country. As Ebola is not an airborne disease, but one only transferred through direct contact with bodily fluids, a resident of a first-world country may find it surprising that this disease was one of the fastest growing epidemics in all history. For direct contact contamination, the contact network frequently has much fewer edges, as the edges are only with people who had direct contact in each wave, unlike in an airborne disease, where the edges are between any two people in the same vicinity during each in wave. Indeed, in most first-world countries, there were little to no reported cases and an extremely small ratio of deaths; as quarantine measures were much stronger and sanitation infrastructure much better, the disease was unable to make it past a single wave due to a small reproductive number.

On the other hand, quarantine measures were lacking in West Africa, especially in rural parts of the country, and the lack of sanitation infrastructure was predicted to be a large factor in the spread. Particularly, families in Sierra Leone washed their bodies with the same water they used to wash their hands, allowing infectious bodily fluids to spread quickly throughout families and communities through the water resources. The common infected water resources and its use for all tasks made the contact network much larger in Sierra Leone than in Liberia, as well as the probability of infection itself much larger, resulting in significantly higher reproductive numbers. From this example, we can see that even nearby countries with similar health standards can vary differently with respect to the spread of contagious diseases due to small differences in sanitation infrastructure, indeed showing a “knife-edge” dichotomy.

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December 2015