What contributes to the increase in zoonotic diseases?

We've seen that zoonotic diseases are becoming more common. New zoonotic diseases are making the jump from animals to people. Why is that the case?

1. Animal Agriculture

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 occurs in India. Its case fatality rate is case fatality rate is  40% to 75%.

There are many trends that are  leading to an increase in transmission from domesticated animals to humans. 

Increased Meat Consumption Worldwide

While there hasn't been a change in the amount of meat consumed in high-income countries ( it was already  high), developing countries have increased meat consumption dramatically; the amount of protein from animal products has doubled since 1960s. This has lead to an increase in

• • • • Meat production ( 260% increase)
      • Milk production ( 90% increase)
      • Egg consumption  (340% increase)

This means more animals/livestock. Which means more contact between humans and livestock. Which means more chances for diseases to jump from animals to humans. 

Intensification of Animal Agriculture 

Increased demand for meat, requires more intense animal agriculture. Specifically, this means

• • • • Breeding animals for higher production (more meat)  at the expense of breeding for disease resistance
      • Breeding animals that are more genetically similar and therefore more susceptible to the same diseases
      • Growing animals in denser conditions- making it easier for diseases to spread between animals and to people. 

Animal husbandry practices are often  not a well regulated in poorer countries. Best practices of  feeding animals, cleaning up after animals, disposing of animal waste and finally butchering animals  makes it easier for diseases to spread. 

(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)

2.  Increased use and exploitation of wildlife

There are several ways that wildlife is used and traded. All of these are on the rise. 

• • • Harvesting wild animals (wild meat, sometimes called “bushmeat”) as a source of protein, micronutrients and money for the poor;
      Recreational hunting and consumption of wildlife as a status symbol;
    • Trade in live animals for recreational use (pets, zoos)and for research and medical testing
      Use of animal parts for decorative, medicinal and other commercial products.
    • Farming wild animals for all of the above purposes.

All of these lead to close contact between animals and people and allow for transmission of diseases not only due to hunting and farming, but through informal markets that sell living or dead wild animals ( called wet markets)

Why is there an increase in wild meat consumption? 

An increasing human population is demanding more protein-rich food and income that cannot be met
with traditional resources

• • • • Global population densities are increasing
      • Local communities have few incentives to conserve wildlife and wildlife habitats
      • Wild meat trade also serves as a safety net in times of hardship, as it generates both protein and income for
        poor households

Wildlife trade

 A mix of animal species are traded in markets—wild, captive bred, farmed and domesticated—in transport vehicles and in market cages. There is a large increase in demand for these types of animals and the ability to trade them across long distances. 

3.  Unsustainable utilization of natural resources and Changes in Land Use

Urbanization/ Settlement of Humans in Previously Wild Areas 

Movement of humans into new  habitats means closer contact with wild species and the ability for disease to jump 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 as people are forced to move and live deeper into the forest systems.  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

Deforestation

Destruction of forest systems, means loss of species and change in habitat. 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.

Extraction Industries

Encroachment into wildlife habitats that are altered for the purpose of extracting their natural resources— e.g., mining, oil and gas extraction, logging but also harvesting bat guano—also encourages new or expanded interactions between people and wildlife. These activities often come with other changes, such as new human settlements, road building and movements of people and products, which further increase human access to wilderness areas and often provoke changes in how local communities acquire and store their food (e.g., via wildlife hunting, introduction of livestock rearing, and keeping food stocks that attract pest animals).

Optional Links:

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

4.  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 is 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.  

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

5. 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.  Thus, there is more potential for what once might have remained a local outbreak to spread rapidly.  

6. Climate Change 

Many diseases are climate sensitive and do better in the warmer, wetter weather that  that is part climate change.  This means that there will be more habitat for these disease organisms as well as the animals that carry them. 

An example is the type of mosquito that transmits malaria needs warm wet conditions. The spread of its habitat will lead to the spread of malaria.