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A few years ago, West Africa was struck by an epidemic of Ebola, which killed more than ten thousand people in Liberia, Sierra Leone, and Guinea. The first case of Ebola was a child who had been playing in a bat-infested tree. From him, the disease rapidly spread from person to person in a chain reaction of contagion. A quite different type of hemorrhagic disease is Lassa fever, which was first identified in 1969, and named after the town in Nigeria where the first cases were observed.

Lassa fever is spread primarily by the African “multimammate” rat. This small rodent is ubiquitous in West Africa, which explains why Lassa fever is endemic there, with hundreds of thousands of infections, and thousands of deaths, each year. Anyone who has had small rodents in their home or office knows how frustrating it is to try to control an infestation. These rats live in and around homes in West Africa and scavenge on available food. Apart from coming into physical contact with the rats, the virus may be inhaled from air contaminated with rat excretions. These rats are also consumed as a food delicacy.

Nigeria WHO
Geographic distribution of Lassa fever in West African affected countries, 1969-2018. Source: World Health Organization

According to the Nigeria Center for Disease Control (NCDC), from January 1 to March 11, 2018, there have been 365 confirmed cases of Lassa fever, and 114 deaths. By comparison, there were just 143 laboratory confirmed cases in 2017, and 101 in 2016. The question which all risk stakeholders, including capital market investors, are asking is how to explain this sudden outbreak?

A partial explanation is that more cases are being diagnosed. The great majority (80 percent) of Lassa fever infections are mild and remain undiagnosed. But in recent years, Nigeria has invested more in diagnostic tests that can confirm Lassa infections, and media coverage has raised the profile of the disease. Clinicians have also become more effective at distinguishing Lassa fever from other common tropical diseases such as malaria or typhoid. Another factor in the current outbreak is the climate: warmer weather at the end of 2017 might have been particularly favorable for expanding the urban rat population.

About 15 to 20 percent of hospitalized patients die from their illness. Fortunately, there is an effective antiviral drug for Lassa fever: Ribavirin. If administered within a few days after the onset of fever, the case fatality rate can be reduced to about five percent. Ribavirin should help stem the tide of deaths from this outbreak, because the Nigerian government has assured that Ribavirin will be provided free for every case of Lassa fever.

It is a complex risk modeling challenge to forecast the final human death toll in this battle between a large army of hungry rats and a cohort of brave public health workers. Peak incidence of Lassa fever is usually in the dry season, January to March, so the next month should be decisive.

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Gordon Woo
Catastrophist, Moody's RMS

Gordon is a catastrophe-risk expert, with 30 years’ experience in catastrophe science, covering both natural and man-made hazards. Gordon is the chief architect of Moody's RMS terrorism risk model, which he started work on a year after joining RMS in December 2000. For his thought leadership in terrorism risk modeling, he was named by Treasury & Risk magazine as one of the 100 most influential people in finance in 2004. He has since lectured on terrorism at the NATO Center of Excellence for the Defense against Terrorism and testified before the U.S. Congress on terrorism-risk modeling.

As an acknowledged, international expert on catastrophes, Gordon is the author of two acclaimed books: “The Mathematics of Natural Catastrophes” (1999) and “Calculating Catastrophe” (2011). Dr. Woo graduated as the best mathematician of his year at Cambridge University and he completed his doctorate at MIT as a Kennedy Scholar and was a member of the Harvard Society of Fellows. He also has a Master of Science in computer science from Cambridge University.

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