Hotspots for Emerging Disease
Kate - Emerging infectious diseases are diseases that have basically increased in incidence, impact or range, newly-evolved like multi drug-resistant TB or malaria or entered the human population for the first time like SARS or Ebola or HIV/AIDS.
Meera - What exactly are the factors that cause these diseases to emerge in the first place?
|Zoonotic pathogens passed from wildlife to people, from lowest occurrence (green) to highest (red). © Nature|
Kate - It's usually because there's a change in the transgression dynamics between and within host populations. You have populations of humans,wildlife and domestic animals all interacting. Changes which change those dynamics between these populations and within host populations cause these diseases to enter the human population for the first time.
Meera - What are the actual changes that happen?
Kate - It could be increase in intensification of agriculture. You get more contact between domestic animals and humans. It could be going to ranges of wild animals that people didn't do before because of bush meat hunting. You're exposed to a different disease. Diseases and pathogens don't want to kill their host because they're evolving with them. It's when you get a change in these transmission dynamics and suddenly it's new for the pathogen. That's where it causes the problem because pathogens don't actually want you to die because they're living in you.
Meera - When it comes to emerging infectious diseases there are obviously lots of different types. What are the factors that influence these different types?
Kate - We thought originally about emerging infectious diseases all as one thing. Then it because obvious that there were different drivers for the different things. We cut it up into broadly different types: drug-resistant emerging infectious diseases like drug-resistant TB, vector-borne diseases such as malaria and then we thought about zoonotic. This is diseases from wildlife that entered the human population such as Ebola, AIDS or SARS. Those are the broad categories that we use.
Meera - So what were the different drivers for the different disease playing more crucial roles in them?
Kate - Human population density is absolutely critical for predicting the probability of an emerging infectious disease because you've got more people, more chance of infection. For drug resistant ones human growth is actually more important. The rate of population growth is more important or, for example, zoonotic pathogens we were finding that human population density is really important but the other driver that was equally important was the distribution of possible hosts. In this case we used wildlife biodiversity.
Meera - Earlier this year you managed to create this map that basically showed hotspots on a global scale of where these diseases are occurring more often.
Kate - We took a really innovative, multi-disciplinary approach. I study broad-scale, by diversity pattern so I'm not an epidemiologist at all. I was working with people who were but the problem with the field as it stood before we tried to do this is that people were looking at single diseases and not taking a really broad-scale approach to think well, what are the processes behind those patterns?What we did was we tried to think about an emerging infectious disease and not just the disease but the event. The first time in space and time that that pathogen entered the human population. We tried to model the human event because we wanted to work out what were the factors that were affecting that event happening. Maybe we could predict that event in the future.
Meera - Are there particular examples of diseases that you managed to think about the factors for then?
Kate - Yeah so we tried to do this in a general approach. A few examples would be Nipah/Hendra virus in Malaysia. This is a zoonotic, a disease from wildlife. What happened was an intensification of pig farming in the region and so people put pigs natural forest for the first time and there was an interaction between the bats and the pigs in the forest. The pigs were eating fruit that the bats had eaten. Bats have Nipah virus naturally and they cope with it in their populations. Pigs, however, have never been exposed to that. The humans which were handling the pigs had never been exposed either. You had an emerging infectious disease event of this really nasty disease.
Meera - Is it safe to say that we're moving closer and closer to animals, it's causing a big change in the emergence of these diseases?
Kate - That's a really good question. That's what we were trying to try and answer. Our first result coming out of this huge database was that the incidence of emerging infectious diseases has actually increased over time. What we then did was plot it spatially. Normally in my biodiversity studies you find a lot more biodiversity, a lot more things, in the tropics. Now with emerging infectious disease events the opposite is true. You get more in the north and more in the south. We were thinking that possibly that shows you that biology is important but there are other socio-economic factors involved that we needed to model. Quite frighteningly since 1940 the hotspots of diseases have been in the really big capital cities of the world: London, New York and Tokyo. Maybe one possible explanation is that you put more money into antibiotics and you're getting a lot more emergence of these diseases which are drug-resistant. The other interesting factor was that if you look at the zoonotic patterns, they're affected by human population density but also the density of other wildlife in the region which makes perfect sense really. It's not just that you've got high population densities of humans or wildlife. It's the interaction of those two things. In a way it gives added value to preserving wild areas, to keeping them wild. We're preventing horrible emerging infectious diseases coming and entering the human populations.