COVID-19 Report - 10

NEW ZOONOTIC DISEASES ARE EMERGING AT AN ALARMING RATE

1. THE SYMPTOM

The world is currently in the grip of the COVID-19 pandemic, an
unprecedented global health crisis that emerged from animal pathogens.
Between December 2019 and May 2020, COVID-19 infected
more than six million people worldwide, which is more than
the population of New Zealand.1 Tragically, the disease has
killed more than 370,000 people in over 200 countries,
which is just under three times the number of people killed
by armed conflict and terrorism every year.2 The emergence
of COVID-19 has been linked to a disease prevalent in
horseshoe bats.3 Once it was transmitted to humans, the
disease spread rapidly through our globalized society,
reaching individuals in every region of the world.
COVID-19 is the latest of several recent zoonotic
diseases emerging in people and demonstrates how
human health and nature are closely interconnected.
Interactions with nature can expose people to a wide range
of animal diseases. In fact, approximately three to four
new infectious diseases emerge each year, most of which
originate from wildlife. Over the last 30 years, approximately
60-70 per cent of the new diseases that emerged in humans
had a zoonotic origin.4 Animal pathogens can infect
humans directly through contact with the wild animals
that are natural carriers of these diseases, or indirectly by
transmission through intermediate hosts, such as livestock
and domestic or peri-domestic animals that live in proximity
to humans. These intermediate hosts act as "mixing vessels"
that can lead to the genetic variation of diseases, enabling
them to infect humans.5
Healthy ecosystems can help mitigate humanity's
exposure and vulnerability to different health
risks, including zoonotic diseases. Robust natural
ecosystems enable access to necessities such as clean air,
water, medicines and food, which strengthen health and
immune systems as well as reduce vulnerability to all types

of disease. For example, a study found that in the US, trees
and forests removed 17.4 million tonnes of air pollution in
2010, which is equivalent to taking almost 4 million cars off
US roads for a year.6 The improved air quality alone led to
an estimated reduction of more than 670,000 incidences
of acute respiratory symptoms.7 Further, when natural
ecosystems like forests remain intact, interactions between
major human population groups and wild host species are
often more limited.8 As a result, viruses circulate in natural
ecosystems without crossing over into humans. Similarly,
wild host species have fewer interactions with domesticated
animals and livestock, which generally live in close proximity
to humans.9 It is therefore less likely for domestic animals
and livestock to become intermediate hosts of these diseases.
Some studies also suggest that greater biodiversity of species
in a natural ecosystem like a forest may hinder disease
transmission. This may be attributable to what scientists
call the "dilution effect," which makes it more difficult for a
single pathogen to spread rapidly or to dominate.10 Evidence
is not fully conclusive that this effect applies broadly across
diseases, although one study found significant evidence of
this effect in parasite systems and plant-herbivore systems.11
One study reviewing over 200 assessments found significant
evidence of the dilution effect weakening transmission in
parasite systems and plant-herbivore systems.12
However, over the last century, there has been an
alarming increase in the number and frequency
of new zoonotic disease outbreaks. The frequency of
zoonotic disease outbreaks caused by a spillover of pathogens
from animal hosts to people may have more than tripled
in the last decade.13 The diversity of these pathogens has
also increased, with the number of new zoonotic diseases

Zoonotic diseases

Key definitions:

Zoonotic diseases are any
diseases originating from animals
and transmitted to humans. They
are caused by animal pathogens
(e.g. bacteria, viruses, fungi or
animal parasites). The event in
which an animal pathogen infects
a human and overcomes their
immune system is called spillover.

Pathogen: an organism
that causes disease (e.g.
bacteria, viruses, fungi
or animal parasites)

WWF INTERNATIONAL 2020

Reservoir host: an
organism that carries a
pathogen, often without
causing disease for the
organism itself

Intermediate host
or vector: an organism
that carries a pathogen
as a result of crossinfection and that
can be responsible
for transmitting the
pathogen to humans
COVID-19 Report

Table of Contents for the Digital Edition of COVID-19 Report
