New model improves prediction of Ebola, Lassa fever outbreaks
Potential outbreaks of diseases such as Ebola and Lassa fever may be more accurately predicted thanks to a new mathematical model developed by researchers at the University of Cambridge, United Kingdom (U.K.). This could in turn help inform public health messages to prevent outbreaks spreading more widely.
Details of the model, including a demonstration applying the framework to Lassa fever, are published Monday in the open access journal PLOS Neglected Tropical Diseases.
Many of the major new outbreaks of disease, particularly in Africa, are so-called zoonotic infections, diseases that are transmitted to humans from animals. The Ebola virus, for example, which recently killed over 11,000 people across Africa, was most likely transmitted to humans from fruit bats.
Modelling how outbreaks arise and whether they will take hold or quickly die out has proved challenging, with two factors in particular being difficult to quantify. The first is ‘spillover’, where the pathogen — a virus or parasite, for example — passes from an animal to a person. This can be through direct transmission, for example by being bitten or by eating ‘bush meat’ (wild animals such as fruit bats or monkeys that are caught and consumed), or indirectly, such as through contact with faeces or disease-carrying mosquitoes.
In many cases, a spillover will go no further. When a human is bitten by a rabid dog, they may become infected, but as the disease cannot transmit from human-to-human, the disease hits a dead end.
However, in some cases the infected person goes on to infect other humans. This is the case for diseases such as Ebola, Lassa fever (spread from rodents) and Crimean Congo haemorrhagic fever (spread from ticks). But in many cases, unless there are additional spillover events, the disease eventually fades out. This is referred to as a ‘stuttering chain’, and even though the disease is transmitted from human-to-human, they are still considered to be zoonotic infections.
Diseases such as Human Immuno-deficiency Virus (HIV), however, which almost certainly began as a spillover from chimpanzees, are no longer considered to be zoonotic as the chain of transmission from humans to other humans is continuous and no longer relies on spillover to sustain transmission.
“Modelling spillovers is a real challenge,” says Dr. Gianni Lo Iacono from the Department of Veterinary Medicine at the University of Cambridge. “We don’t have particularly good data on wildlife numbers, such as fruit bats in Sierra Leone, and only a crude idea of their geographic distribution and how many are infected. Even in the UK, we don’t really know how many deer we have, which would be really useful to estimate the risk of Lyme disease.”
In addition, measuring the likelihood of contact with the infected animals is also extremely difficult as it involves understanding human and animal behaviour.
Stuttering transmission, too, can be difficult to model, says Dr. Lo Iacono. “In the case of 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.”