Viral Outbreaks and Habitat Loss: Defining the Link

How do the pieces all fit?  As I have illustrated in the previous pages there are several examples of zoonotic diseases that are currently afflicting the world population and this only stands to increase as the the population increases and people are exposed to new habitat that has previously been uninhabited.

This is an arial photo of the Kenema region in Sierra Leone.  As you can see the region is expanding deeper and deeper into what was once unihabited jungle.  This area has also been under the watchful eye of the WHO and the CDC because it has been at the epicenter for several Lassa, yellow fever and Dengue outbreaks over the last several decades. 













The following video illustrates how ecological degradation impacts emerging diseases throughout the world, Dr. Kerry Bowman  presented this talk at the Ecology and Global Health International Conference at Long Beach Memorial Medical Center, Long Beach, California, May 13, 2005.  He does a very good job of defining the link between viral outbreaks and ecological degradation. 

The diagrams below illustrate several examples of diseases that are expected to fluctuate with ecological change.  Also, the emergence mechanism and drivers of possible outbreaks are listed.  These tables do a very good job of defining the coorrelation between ecological changes and the diseases these changes will impact.  The impact of ecological change is measured on a scale of 1 to 4, one + being the least impact and ++++ being the highest impact.  It also illustrates the regions that these changes will impact.

As you can see from the data above several viruses and bacterium will be directly influenced by ecological changes.  To name a few examples; Dengue Virus is predicted to increase significantly due to ecological change, influenced by vector expansion due to climate change and urbanization.  Machupo and Nipah viruses are also predicted to increase due to niche invasion, biodiversity loss and reservoir expansion.  The majority of the regions impacted by these changes can be found in tropical regions but other viruses such as H5N1 can have global effects as well.  There are also a number of emerging viruses that are not listed in this table, but with that said, this gives you a fairly good indication in regard to the scope of the problem at hand.

The diagram below illustrates a Convergence Model for the Emergence of Infectious Diseases, Combining Multiple Causes of Disease Emergence or Reemergence (Institute of Medicine 2003):

This diagram illustrates how several factors can influence the spread of disease.  As you can see several factors including social and political issues have an influence on the spread of disease.  As I mentioned previously Machupo fits this example perfectly as the Bolivian Revolution forced the villagers to clearcut and plant large plots of corn thus causing a severe outbreak.  As more developing nations push for expansion this problem will be compounded even further as food resources become more and more scarce.  

Some of the major factors that can lead to outbreaks are as follows:

• Altered habitat leading to changes in the number of vector
• Breeding sites or reservoir host distribution
• Niche invasions or transfer of interspecies hosts
• Biodiversity change 
• Human-induced genetic changes in disease vectors or pathogens 
• Environmental contamination by infectious disease agents 

The information and diagrams listed above can be found here:

Patz et al, Ecosystems and Human Well Being, Vol. 1, 2005; Human Health: Ecosystem Regulation of Infectious Diseases, pp 319-451, chapter 14