@DiseaseMapper recently tweeted a link to a very interesting paper (which happily is also free to access so you can read it too – the link is here )
Why do I think this paper is so interesting? Firstly, because it is a useful reminder that zoonotic infections (those that pass from animals to man and vice versa) do not just impact on our lives by causing us illness, and in the worst circumstance, death. They have a massive economic impact as well. In fact the paper reports that the estimated economic impact of zoonotic diseases from 1995-2008 was over 120 billion dollars.
There are many reasons for the economic burden of these diseases being so high: impact on tourism; impact on international trade agreements; impact on consumer consumption and behaviour; loss of farmed animals. In many outbreaks the local economy is negatively impacted in multiple ways and obviously in poorer areas this can also secondarily affect people’s health.
The paper also goes on to speculate about why there should be a resurgence of zoonotic infections. The authors split it into ‘Factors associated with human behaviour’; ‘Factors associated with pathogen characteristics’ and ‘Climate change and zoonotic resurgence’. So, pinching their titles…
Factors associated with human behaviour
Here the authors split it down further:
Individual human practices – the authors use the example of ecotourism. “urban citizens of the developed world who visit developing countries or rural areas of the developed world and engage in activities such as forest camping, river rafting, or bat cave exploring, are prone to zoonotic infections such as vector-borne rickettsioses, leptospirosis, and haemorrhagic fevers or lyssavirus-related illness, respectively“. The authors also talk about how pet ownership, especially the increase in ownership of ‘exotic’ pets like reptiles is increasing people’s exposure to infections that previously they would never have been exposed to.
Socio-economic alterations – with an ever increasing global population there is an ever increasing demand for food, including meat. It also means that as urban populations are expanding people are moving into previously uninhabited areas and so are being exposed to disease-causing agents that they had never been previously.
Political alterations - the authors talk about some countries with poorer veterinary surveillance or that have focal areas of zoonotic infections that previously were not having a global impact because they had strictly state-controlled economies but are now having a global impact because they have transitioned to allowing free trade. They also discuss the role that political disruption and upheaval can have on increasing the spread of zoonotic infections.
Scientific impacts – Part of the reason that we are recognising so many zoonotic infections is that we have got better at detecting them. Infectious agents that we couldn’t have characterised decades ago can now be identified and classified. Another scientific impact the authors mention is one that you will recognise if you are a regular reader of this blog: there have been many advances in medicine that allow us to live to an older age, but that have a negative impact on our immune system (for example, chemotherapy drugs can make us immunosuppressed; if you have an organ transplant you have to take immunosuppressing drugs, etc.) This leaves a section of the population at a much higher risk of contracting any disease and so gives rise to some human infections with agents that would otherwise not normally infect humans.
Factors Associated with Pathogen Characteristics
The authors talk about how pathogens (disease causing organisms) that have a high genetic mutation rate (like flu viruses) can help them become zoonotic infections: in the authors’ words “ their enormous mutation rate is essentially a factory producing the species that are most potently pathogenic for humans“
The authors also talk about how biodiversity can impact zoonotic disease transmission in this section (although, personally I’m not sure why it came into this section). They talk about how sometimes wide diversity can reduce the spread of zoonotic agents because (if I am parsing this correctly) if there are many host animals that a vector (like a mosquito) can feed off there is less chance of it coming into contact with an animal that harbours the zoonotic agent – this is called the ‘dilution effect’.
Climate Change and Zoonotic Infection Resurgence
To quote from the paper: “Global warming is an ecological emergency, but its implications for human disease caused by infectious agents remains understudied“. We do know some of the effects it could have – increases in temperature in previously colder countries leads to the spread of insects like mosquitoes – and the diseases they carry – into those countries. Climate change may also affect bird migration patterns and so may affect the exposure of birds to pathogens and also the exposure of us to them via the birds.
Finally the paper finishes with Projections for the Future. The authors point to 4 issues that “need urgent clarification and further attention“.
1) Recognition of the need for pre-emptive studies on the effects of massive or smaller developmental projects on local animal fauna and local zoonotic reservoirs
2) Recognition and enhancement of the health literacy of special populations that are at increased risk for the development of zoonotic infections (meaning that those patients on immunosuppressant drugs or who are immunosuppressed for other reasons should get more information about where they might encounter zoonotic infections and hw to avoid them.
3) Recognition of the major long-term burden induced by certain of these diseases with a chronic phase. There are some diseases that take a long time for any symptoms to show. If a person has migrated from an area where the disease is relatively common to one where the disease is rare, the clinicians may be less likely to recognise the disease (or may recognise it at a later stage than if they were practicing in a country where the disease is common). The authors recommend that clinicians “should be prepared to recognize the long history evolving in such patients and the extreme costs, mentioned in the introductory section, that will be passed on to the host countries”
4) Planning any intervention is difficult, for financial and scientific reasons. The burden of many of these diseases remains unrecognized… any zoonosis imposes a threat to the family as a unit—exposure is likely to be common for members of a household, particularly in agricultural settings, and animal loss (owing to the disease or state regulations for sick animals) may have a significant impact on the economy of the household, which is further worsened by the often observed inadequate access to appropriate medical treatment for the human patients themselves (imagine the scenario in any impoverished or conflict-active region of Africa or Asia). … ambitious eradication campaigns are not always feasible when all of the aforementioned issues have not been taken into account, and neither are successful elimination campaigns, as these may have temporary positive results but subsequent surveillance degeneration, leading to zoonotic resurgences, usually with some twists. (So basically we don’t really fully know the burden of most zoonotic diseases and rushing in there with eradication campaigns without considering all the other factors is not necessarily the best move.)
I think sometimes it can be really easy to think of zoonotic diseases as something one human gets from one animal, but this paper was a good reminder that these diseases can have a much broader impact. It also had a useful discussion about why the number of zoonotic infections seemed to be increasing, but as it said (and as all papers say) there is still more work to be done in this area.
Cascio A, Bosilkovski M, Rodriguez-Morales AJ, & Pappas G (2011). The socio-ecology of zoonotic infections. Clinical microbiology and infection : the official publication of the European Society of Clinical Microbiology and Infectious Diseases, 17 (3), 336-42 PMID: 21175957