Improving predictions of outbreaks of Ebola, Lassa fever

people who catch the disease from animals show the same symptoms as those who get it from humans. So is this case a spillover or part of a human-to-human chain of transmission? And if members of the same family get the disease, have they caught it from a family member or from the same pot of contaminated rice?

“Sometimes you can be lucky and work this out, as we did in a previous study, but this was possible because information of outbreaks that were known to be pure human-to-human chains was, unusually, available. But we need more general methods.”

Dr. Lo Iacono and colleagues have developed the most coherent and potentially most accurate mathematical model to date for zoonotic diseases, which incorporates spillover and stuttering transmission.

“The pathogen does not care if it jumped from an animal or from another human; the only difference is that in a stuttering transmission an infected person can trigger other chains of human infections. A general, realistic model should capture this mechanism,” adds Dr. Lo Iacono.

Details of the model, including a demonstration applying the framework to Lassa fever, are published today in the open access journal PLOS Neglected Tropical Diseases.

“By modelling potential outbreaks more accurately, we can help inform public health messages,” explains Professor James Wood, Head of the Department of Veterinary Medicine, and senior author. “If you know that most cases of an outbreak of Lassa fever come from spillovers, then the message might be ‘kill the rats’, but if it is now mainly spreading between humans, the messages will be around washing your hands or avoiding contact with bodily fluids.”

The beauty of the model, say the researchers, is that it is simple to implement, so public health officials and non-mathematicians could easily use it. It also allows for the incorporation of data from different disciplines, factoring in socioeconomic, ecological and environmental factors, for example.

“It’s important to understand if and how these other important factors can increase the impact of stuttering chains,” says Professor Wood. “Ebola has always been a very severe disease but previously confined to small, remote regions. Then suddenly, in the last two years it exploded in West Africa. Why? Was it because social patterns changed? Our model could be used to address such questions better.”

The research informing the paper was carried out as part of the Dynamic Drivers of Disease in Africa Consortium, which was funded by Ecosystem Services for Poverty Alleviation (ESPA).

— Read more in G. Lo Iacono et al., “A unified framework for the infection dynamics of zoonotic spillover and spread,” PLOS Neglected Tropical Diseases (2 September 2016) (DOI: 10.1371/journal.pntd.0004957)