Dr Erin Lafferty offers a concise explanation of pathogens, and outlines UK research conducted to better understand and prevent their proliferation.
We can be certain that emerging and re-emerging pathogens like SARS, MERS, H5N1, H7N9, and Ebola, to name only a few, will continue to appear in our headlines. We are less certain however about where and when the next pathogen will emerge and what its capacity to spread globally may be. Many avenues of research however are striving to reduce this uncertainty.
Where do new pathogens come from?
Over 70% of newly emerging and re-emerging pathogens are zoonoses1, meaning that they originally only existed in an animal population but have been able to jump the species barrier and cause infection in humans. This often occurs when humans come into contact with domesticated and wild animals through farming, exploration and development in new territories, frequenting or working in markets that sell live and deceased animals for food or medicine, and meat preparation and consumption. Bats are a common source, directly or indirectly, of many deadly zoonotic pathogens including Nipah virus, Marburg virus, Ebola, and SARS. Influenza H5N1 and H7N9, both better known as bird flu, are also zoonoses with most cases occurring in individuals in regular contact with farmed or wild birds.
To study pathogen mixing between animals and humans, researchers from the Oxford University Clinical Research Unit in Vietnam began a pathogen surveillance programme there in 20122. They regularly screen animal populations, as well as a cohort of over 800 people that live and work closely with animals, for known and novel pathogens by looking at pathogen genetic sequences. The goal of this programme is to better characterise the diversity of pathogens that are circulating among and between species, acting as an early warning system for global health surveillance.
How do pathogens adapt to humans?
An animal pathogen has to adapt to be able to infect and spread in humans. One way adaptation can happen is through mutations that arise during replication of the pathogen’s genetic material. In viruses in particular mutations can occur quite frequently due to a lack of proofreading mechanism during the replication process. Pathogen adaptation can also occur through reassortment in which the genetic content of two or more pathogens from the same family is exchanged when they come in to contact in a common host.
Genetic reassortment was key to the emergence of the H1N1 influenza virus (better known as Swine Flu) in Mexico in 2009. Through genetic sequencing, this virus was demonstrated to carry genes from human, avian, and swine influenza strains that were able to mix together, or reassort, in a swine host and then transmit to humans3.
How many people will become infected?
If a pathogen acquires the capacity to infect humans and transmit between them, a major concern is how many people could become infected. Researchers from the London School of Hygiene & Tropical Medicine used a mathematical modelling approach in the recent Ebola outbreak to understand and predict infection spread in Sierra Leone. In mathematical modelling, information on the pathogen and population being infected is used to generate a model or representation of how that pathogen spreads through the population which is then used to test different hypotheses on future infection spread.
Knowing the number of Ebola cases each week from August 2014 to January 2015, the researchers sought to predict new Ebola cases through the end of March. They then used this information to determine if the number of treatment beds built to that point in the epidemic would be enough to meet future demand. They demonstrated that the epidemic in most areas of Sierra Leone had reached its peak and was beginning to fade. Based on their estimates of upcoming cases they also showed that the available number of treatment beds should be sufficient to meet anticipated demand in most areas4. This kind of real-time analysis is important in an epidemic as it can allow more effective targeting of limited health resources in a rapidly evolving situation.
How far will it spread?
In our well-connected world, the rapid movement of people, animals, and goods can allow the global spread of pathogens to occur easily. This happened in the 2003 SARS epidemic when a single doctor who had been treating patients for an unknown illness in Guangdong Province in China spent a night in a hotel in Hong Kong. In his short time there he spread SARS to other hotel guests who carried it to destinations ranging from Vietnam to Canada.
Recently, the emergence of MERS in Saudi Arabia provoked questions about its ability to spread globally, particularly following a large international event like the yearly Hajj pilgrimage for Muslims to Mecca. MERS, a zoonotic virus from the same coronavirus family as SARS, has an estimated mortality rate of over 35% and has been linked to camels that are known to carry the virus. To better understand where MERS might spread, researchers studied flight patterns of planes leaving Saudi Arabia, Jordan, Qatar, and the United Arab Emirates. They found that over 50% of travellers to these countries came from just eight countries, one of which was the UK5. Information on global mobility can allow countries to assess their potential risk for new pathogen importation on a general and pathogen-by-pathogen basis and prepare accordingly.
There are too many factors contributing to pathogen emergence, adaptation, and spread to allow us to ever have 100% certainty about when, where, and what the next emerging or re-emerging pathogen will be. Research studies like the ones described here represent powerful tools that can provide us with up-to-date knowledge and understanding of the interactions between pathogens and their animal and human hosts, allowing us to have a better chance of preventing or slowing pathogen emergence and spread.