Part V: What contributes to zoonotic diseases?

To-Do Date: Mar 31 at 9:30am

Outbreaks of zoonotic diseases have been increasing in recent decades for a combination of factors.  It is  estimated that 75% of emerging pathogens Links to an external site. that infect people are zoonotic. The COVID-19 pandemic has shed light on the potential for global impacts of these diseases.  Will we learn from this pandemic and make necessary changes to reduce the risk zoonotic diseases pose to humans?

On this page and the next , we'll explore some of the factors that contribute to the rise of zoonotic diseases as well as the solutions to these problems.   You will want to use this information for answer the Unit Assessment Problem Set Questions

1. 1. Pick ONE contributor to the increase in zoonotic diseases and explain the potential solutions to that problem.  Is there anything that you can do to contribute to this solution? Either through your individual life choices, political actions, education ,etc. 

 

1. Agricultural Intensification

Many zoonotic diseases originate in or are transmitted to humans by livestock.  

Avian flu, also known as bird flu or H5N1, is linked to poultry.  It was first detected in 1996 in geese in China and in humans in 1997 in Hong Kong, and has since been detected in poultry and wild birds in more than 50 countries in Africa, Asia, Europe, and the Middle East. It has a case fatality rate of 60%.

Nipah virus is most often transmitted to humans by pigs, and can also be carried by horses, goats, sheep, cats and dogs.  It was first detected in 1999 during an outbreak among pig farmers in, Malaysia.  It was then detected in Bangladesh in 2001, and nearly annual outbreaks have occurred in that country since.  It also occurrs in India. Its case fatality rate is case fatality rate is  40% to 75%.

While these may seem like local diseases restricted to certain parts of the world, the COVID-19 pandemic has highlighted the potential for rapid global transmission of disease though the transport of people (and potentially frozen meat).  Likewise, the rapid spread and establishment of avian influenza in previously disease-free countries illustrates the potential for rapid spread posed by the rapid global movement of both people and livestock.

Optional

Research article from 2011 highlighting the link between agricultural intensification and zoonotic disease outbreaks.  Livestock boom risks aggravating animal 'plagues,' poses threat to food security and world's poor Links to an external site.

 

2. Changes in Land Use

When natural habitats of wildlife species are reduced or removed, animals are forced to live closer to people, creating interactions between the two that provide the potential for spillover of viruses from animals to humans.  An example of the link between changes and land use and disease outbreaks can be seen with Ebola.  Ebola outbreaks often follow local forest clearing.

In Africa where Ebola is found, heavily forested areas aren’t generally occupied by dense populations of humans.  Forested areas are often cleared to plant fruit tree crops, and areas cleared for agriculture or villages will contain more people than forested areas.

Bats are attracted to these cleared areas because bats can better move around and look for food in these open spaces, and they are attracted to fruit trees that may be planted.  This increase in the density of people into areas also preferred by bats provides greater opportunity for transmission from bats to people.  A 2017 study found that Ebola outbreaks were significantly associated with forest losses within the previous 2 years

Similarly, malaria in the Amazon is linked to deforestation, where a 10% increase in deforestation leads to a 3.3% increase in cases of malaria. It is also linked to increases in malaria more broadly around the world.  Like in the case of Ebola, deforestation creates conditions favorable to the vector that transmits malaria, the Anopheles mosquito, by exposing pools of water to sunlight and warming them, making them better breeding pools, and by increasing available pools of water by creating ditches, puddles and tree bowls of water that are conducive to breeding by the mosquito.

Optional Links:

• • Human deforestation activities link bats to Ebola outbreaks Links to an external site.
  • Recent loss of closed forests is associated with Ebola virus disease outbreaks Links to an external site.
  • Amazon deforestation drives malaria transmission, and malaria burden reduces forest clearing Links to an external site.
  • Deforestation and malaria: what’s the relationship between the two? Links to an external site.

 

3. Decreases in Biodiversity

Changes in land use also result in losses of biodiversity, as do other human impacts like overhunting, fragmenting of ecosystems, and the introduction of invasive species.  These losses of biodiversity can directly influence the risk of zoonotic diseases to humans.  This can occur through various mechanisms, two of which we will explore here.

First, more biodiverse communities can slow transmission of a zoonotic disease because of the dilution effect.  This is best understood with a case study.  Let's use West Nile Virus, which is a virus carried by mosquitoes that can infect birds and humans.  Not all birds are suitable hosts for the pathogen, while others, especially American Robins and American Crows, are excellent hosts for the virus.  These are also birds that more often persist in communities that have been impacted by human development.  In more diverse bird communities, a mosquito that bites a human has most likely just bitten a bird that is a poor host and thus is likely uninfected with the virus.  In a LESS diverse bird community, it si more likely that a mosquito that bites a human may have just bitten an infected Robin or Crow, and thus can now transmit that disease to the person.  We'll watch a video in a minute  that can help you make more sense of this.  

Second, the loss of particular species can result in changes to a community that increase the prevalence of a zoonotic disease.  For example, the loss of predators like wolves, mountain lions, bobcats and foxes from forests in New England has increased the abundance of deer and mice, which are excellent hosts for deer ticks, which carry and transmit Lyme Disease.  It is likely not a coincidence that California, which still contains abundant mountain lions, bobcats and foxes has a lower prevalence of Lyme Disease.  We'll learn more about Lyme Disease and some other interesting ecological links later this semester, but if you can't wait, check out this optional link: Biodiversity and the Spread of Disease Links to an external site.

 

4. Increases in Global Connectivity

This is probably obvious... we are more connected globally than ever before .  People travel rapidly around the world by air.  Agricultural products like meat and livestock are traded globally, and animals are transported through the legal and illegal exotic pet and wildlife trade.  Recall the pangolin - a potential amplifier host for COVID-19?  This is the most commonly illegally trafficked animal in the world Links to an external site..  Thus, there is more potential for what once might have remained a local outbreak to spread rapidly.  

 

 

 

 

Humans Are Making Pandemics More Likely

(which means we can also choose to make them LESS likely)

As you've seen, human activities are making outbreaks of zoonotic diseases more likely.  Watch the 6-minute video below that does a great job summarizing some of these contributors.  

 

 

Click Next to begin thinking about what we can do to make future outbreaks less likely.