When we were discussing the ‘cities’ theme that’s running across all the SciAm blogs I didn’t want to take the easy route. Connecting cities and disease is pretty easy, I could look at plague or cholera and be totally within the scope of the theme. Instead I remembered a little paper I had read while ago about the role cities play in the emergence of new infectious diseases.

Commonly, we think the regions most likely be affected by the emergence of infectious diseases would be the developing nations but in fact very few studies have looked into the spatial arrangement of emerging infectious disease reporting. This has significant implications for funding of surveillance and research as typically developing nations lack the resources to adequately handle their current health burdens let alone monitoring for and dealing with new issues as they arise. So an emphasis must be placed on understanding how diseases emerge, where they emerge and what we can do about it.

When we think about new infectious disease emergence often images of new diseases coming out of the jungle come to mind. While this is not necessarily wrong (as we shall see) it’s not exactly correct either. It all comes down to your definition of ‘emerging’.

In their paper “Global trends in emerging infectious disease” Jones et al. define an emerging infectious disease as the original case or cluster of cases representing; newly evolved strains of existing human pathogens, completely new pathogens or re-emergent pathogens. In this way a new strain of antibiotic resistant E. coli is considered equal to SARS or the re-emergence of M. tuberculosis in a population that had previously seen the disease disappear.

When viewed like this, human population density was considered the key driving factor for the emergence of infectious disease. This fits with the previous hypotheses that when you pack humans together then the transmission of disease becomes easier thus even weaker emerging diseases can spread and adapt to the new human host. As is pointed out by Jones et al. the connection of human population density and infectious disease emergence unveils a hidden cost of human economic development. But there were other interesting things uncovered.

The emerging infectious diseases were broken down into four broad categories; zoonotic (wildlife), zoonotic (non-wildlife), drug-resistant and vector-borne. The distinction of wildlife vs. non-wildlife zoonotic disease is important as it distinguishes infectious disease emerging from wild animals vs. pets.

While all four groups had human population density as the most important criteria for infectious disease emergence it seems human population growth (measured as a change in the number of persons per km2 between 1990 and 2000) very strongly drives the emergence of non-wildlife zoonotic and drug-resistant infectious diseases.

It would seem that any urbanisation drives up the emergence of infectious diseases in those areas but primarily these emerged diseases are new strains of drug resistant human pathogens or things we pick up from our pets.

Whilst this is scary it could be argued that adaptations of existing pathogens are manageable. An enemy we already know with a different weapon. What about the new diseases we have never seen before?

Entirely new infectious diseases emerge from nature, away from the cities. Again looking at Jones et al.’s data it suggests that amongst the key variables they analysed the two most associated with the emergence of zoonotic diseases from wildlife were human population density and the biodiversity of the region. The more different animals, each with their own suite of diseases, which can come into contact with a large population of humans the more likely some of those diseases will jump the species gap. This data adds to the already established point of view that zoonoses from wildlife are the most significant, growing threat to global health of all the emerging infectious diseases.

There is something else interesting to acknowledge about this data.

This figure shows the map of derived emerging infectious diseases reported between 1940 and 2004. The data is normalised somewhat but essentially the presence and size of the dot indicates the number of emerging infectious diseases detected at the dot's location. What should be noted here is that the detection of emerging infectious disease occurs in the developed world, primarily Western Europe, Eastern US and Eastern Australia with very little detection happening anywhere else. Diseases don’t just appear in these locations, but it is where we look for them.

It is particularly important to consider this as we allocate funding for surveillance and monitoring of emerging infectious disease. Based on the data they collected Jones et al. they developed risk maps of the world for their four categories of emerging infectious disease.

The statistics are presented in the paper and it takes a better statistician than me to pull it all apart but if they are right we are looking for emerging infectious diseases in exactly the wrong places. According to their predictions the greatest risk of infectious disease emergence is in the rapidly urbanising developing nations, in particular India, eastern China and southern Africa.

So what does it all mean? There seems to be a transition between the kinds of diseases that emerge as places are urbanised into cities. The first infectious agents appear to transition from the wildlife populations of the area into the human population as it the human population density increases in that area. As the environment becomes progressively more developed the infectious agents emerge as a result of the activities of humans within the city whether it be medical or agricultural antibiotic use or just domesticating animals and living with them in our homes.

For me the take home message was that we need to reallocate resources to the rapidly urbanising developing world to ensure that newly emerging diseases, particularly zoonotic diseases arising from wildlife, can be detected early and dealt with efficiently. We can probably never prevent infectious diseases from continuing to emerge, although an argument cold be made for halting urbanisation and encroachment on wild habitats as one way to stop zoonotic diseases developing, but understanding the nature of their emergence allows us to plan and predict and ultimately take steps to limit the effect of emergent infectious diseases. In the future this could prevent the spread and subsequent and persisting morbidity and mortality of the next HIV, SARS or multi-drug resistant tuberculosis.

EDIT: Lab Rat has put together a great post on plague and Victorian Europe. A great example of humans living in high densities with animals resulting in the jumping of a zoonotic disease into the human population.

The featured picture with this piece is an old-timey photo of my home city of Adelaide taken facing South down the main thoroughfare King William St taken in 1923. Credit - State Library of South Australia

Permission to use remaining figures sought from Nature through RightsLink. Licence number 2727980135214.

References

Jones, K., Patel, N., Levy, M., Storeygard, A., Balk, D., Gittleman, J., & Daszak, P. (2008). Global trends in emerging infectious diseases Nature, 451 (7181), 990-993 DOI: 10.1038/nature06536

Morens DM, Folkers GK, & Fauci AS (2004). The challenge of emerging and re-emerging infectious diseases. Nature, 430 (6996), 242-9 PMID: 15241422