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Seminar Nasional Teknologi Peternakan dan Veteriner 2007
EMERGING DISEASES ASSOCIATED WITH
WILDLIFE – A MULTI – DISCIPLINARY CHALLENGE
(Timbulnya Beberapa Penyakit yang Berkaitan dengan Hewan Liar Suatu
Pendekatan Multidisiplin Ilmu)
HUME FIELD1 and JONATHAN H. EPSTEIN2
1
Department of Primary Industries & Fisheries, Queensland, Australia; and the Australian Biosecurity Cooperative
Research Centre for Emerging Infectious Diseases
2
The Consortium for Conservation Medicine, Wildlife Trust, 61st 9W, Palisades, New York, USA
ABSTRAK
Hampir sekitar 75% penyakit infeksius yang timbul dan menjangkiti serta mengancam kesehatan manusia
adalah penyakit zoonosis yang ditularkan dari hewan. Sebagian besar dari penyakit tersebut adalah dari
hewan liar yang bertindak sebagai reservoir. Timbulnya penyakit tersebut kebanyakan dari faktor
predesposisi, misalnya perpindahan penduduk global, perdagangan, perluasan lahan pertanian, penebangan
hutan, dan arus urbanisasi. Dari hal tersebut maka penyidikan penyakit tersebut dilakukan dengan pendekatan
multidisiplin ilmu yang melibatkan para ahli ilmu kedokteran hewan, kesehatan mayarakat, dan ahli
mikrobiologi, ahli yang mendalami ekologi dari spesies hewan liar, ahli yang betul-betul mengerti resiko
penularan penyakit dari kebiasaan kehidupan manusia. Penelitian kehidupan hewan liar meliputi populasi
hewan yang bersangkutan dan kesulitan yang akan dihadapi pada saat pengambilan sample, bermacam jenis
penyakit yang berjangkit pada spesies tersebut yang belum pernah dipelajari sepenuhnya. Hewan liar yang
hidup dalam cagar alam (konservasi alam) memerlukan perlakuan yang khusus. Dari hal tersebut maka
pertemuan yang membicarakan manajemen resiko terhadap penyakit yang akan timbul memerlukan keahlian
yang khusus dari para ahli dalam disiplin tertentu dan kerjasama yang baik untuk mencapai tujuan tersebut.
Kata Kunci: Penyakit Baru, Hewan Liar, Pendekatan Multidisiplin
ABSTRACT
Nearly 75% of all emerging infectious diseases that threaten human health are zoonotic – they have an
animal origin. The majority of these have spilled from wildlife reservoirs. Emergence of many can be
attributed to predisposing factors such as global travel, trade, agricultural expansion, deforestation/habitat
fragmentation, and urbanization. Thus the investigation of emerging diseases associated with wildlife requires
a comprehensive multi-disciplinary approach that includes, in addition to the ‘traditional’ veterinary, public
health, and microbiological expertise, an understanding of the ecology of the wildlife species, and
increasingly, an understanding of human behaviors that increase risk of exposure. However, wildlife studies
involve uncontrolled populations, and difficulties are frequently encountered in capturing, sampling, and
disease-screening species that may not have been previously studied. Ethical considerations such as
conservation status also come into play when working with wildlife populations. Meeting the challenge of
managing the risk of emergence of infectious diseases from wildlife requires skills from a range of disciplines
working collaboratively towards a common goal.
Key Words: Emerging Desease, Wildlife, Multi-Disciplinary Approach
INTRODUCTION
While the current emphasis on emerging
diseases in the scientific literature and in the
popular press suggests otherwise, novel
diseases have occurred throughout history. By
definition, every newly identified disease is
novel. The outcome of investigations of
cholera epidemics by Dr. John Snow in
London in the 1880s illustrated for the first
time how the actions of man could precipitate
the emergence of disease. At the same time, it
also demonstrated the value of an
epidemiological approach in the investigation
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Seminar Nasional Teknologi Peternakan dan Veteriner 2007
of a disease outbreak. As we begin the 2001st
century, nearly 75% of all emerging infectious
diseases (EIDs) that impact or threaten human
health are zoonotic – they have an animal
origin (TAYLOR et al., 2001). The majority of
these have spilled from wildlife reservoirs into
humans either directly or indirectly. Diseases
associated with avian influenza virus, SARS
coronavirus, Nipah virus, West Nile virus and
HIV are examples of emerged zoonoses that
have (have had, or may have) a significant
impact on human health.
THE EMERGENCE ZOONOSES
The emergence of many zoonoses can be
attributed to predisposing factors such as
global travel, trade, agricultural expansion,
deforestation/habitat
fragmentation,
and
urbanization; such factors increase the
interface and/or the rate of contact between
human, domestic animal, and wildlife
populations, thereby creating increased
opportunities for spillover events to occur
(DASZAK et al., 2000; 2001). To illustrate, the
introduction of a "new" infection into a human
or domestic animal population may follow the
incursion of humans (accompanied by their
domestic animals) into previously remote
natural habitats where unknown disease agents
exist in harmony with wild reservoir hosts.
Upon contact with new species, an agent may
jump species barriers, thereby spilling over
into humans or livestock. Unlike the natural
host, the new host may have no natural
immunity or evolved resistance. Additionally,
high population densities and management
practices may facilitate the rapid spread of
pathogens throughout livestock populations.
Table 1 provides several examples of disease
emergence and the putative factors associated
with their emergence. LEDERBERG et al. (1992)
describe these changes as providing an
‘epidemiological bridge’ that facilitates contact
between the agent and naive population.
DASZAK et al. (2000) regard disease
emergence as primarily an ecological process,
with emergence frequently resulting from a
change in the ecology of the host or the agent
or both. They argue that most emerging
diseases exist within a finely balanced hostagent continuum between wildlife, domestic
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animal and human populations (Figure 1). The
emergence of Nipah virus from fruit bats in
northern peninsular Malaysia in 1999 provides
a useful example. It is suggested (FIELD et al.,
2001; DASZAK et al., 2006) that the emergence
of Nipah virus was (in part) associated with the
encroachment of commercial pig farms into
forested areas of high fruit bat activity. Once
the
virus
‘spilled
over’
into
the
immunologically naïve pigs, high pig and farm
densities
then
facilitated
the
rapid
dissemination of the infection within the local
pig population. The movement of pigs for sale
and slaughter in turn led to the rapid spread of
infection to southern peninsular Malaysia and
Singapore, where the high-density, largely
urban pig populations facilitated transmission
of the virus to humans.
Figure 1. The host-agent ecological continuum1
1
From DASZAK et al. (2006)
FACTOR THAT INFLUENCE OF THE
DISEASE
Thus the epidemiologic investigation of
infections associated with wildlife requires a
comprehensive multi-disciplinary approach
that includes, in addition to the ‘traditional’
veterinary, public health, and microbiological
expertise, an understanding of the ecology of
Seminar Nasional Teknologi Peternakan dan Veteriner 2007
the wildlife species, and increasingly, an
understanding of human behaviors that
increase risk of exposure. For example,
identifying the factors associated with the
emergence of SARS in southern China in 2003
requires an understanding of the ecology of
infection both in the natural reservoir and in
secondary market reservoir species. Thus, a
necessary extension of understanding the
ecology of the reservoir is an understanding of
the trade, and of the social and cultural context
of wildlife consumption. In relation to trade,
we know that a wholesale and retail structure
exists in the wildlife trade in southern China,
with multiple wholesalers providing multiple
retailers at a city level. We know that some
wildlife are farmed and some wild-caught.
What about the marketing structure? Are there
dealers who buy and sell from both sources?
How much farm-to-farm trading occurs? Do
farms periodically augment their stock from
the wild? To answer these questions, it is
necessary to understand what drives the
wildlife trade – a complex mix of economic,
social and cultural factors. The demand for,
and consumption of, wildlife in southern China
has increased in recent years, purportedly
associated with improved economic conditions.
An increase in legal and illegal wildlife trade
has paralleled this demand, with animals
reportedly channeled from many and various
locations in south-east Asia. A rich cultural
heritage underlies wildlife consumption in
China - different species and dishes are
favoured for a range of social, business and
health reasons. And in the markets, wildcaught civets still attract a price premium,
because people believe it is more health-giving
(and tastes better) than its grain-fed farmed
counterpart.
Table 1. Putative factors in disease emergence1
Underlying factor
Example of factor
Changes in water ecosystems
Land-use changes
Climatic changes
Schistosomiasis, Rift Valley fever,
hantavirus pulmonary syndrome
Human demographic,
societal or behavioural
changes
Population growth and movement
High-density habitation
Human conflicts
Intravenous drug use
Sexual behaviour
AIDS, SARS, hepatitis C, ebola
haemorrhagic fever
Ease, extent and frequency
of international travel
Worldwide movement of goods
and people
‘Airport’ malaria, dissemination of
mosquito vectors, rat-borne
hantaviruses
Technology and industry
Changes in food processing and
packaging
Globalisation of food industries
Increased frequency of
medical/surgical transplants
Increased use of
immunosuppressant drugs.
BSE, E. coli haemolytic uraemic
syndrome, SARS, transfusionassociated AIDS and hepatitis,
opportunistic infections in
immunosuppressed patients
Microbial adaptation and
change
Microbial evolution.
‘Antigenic drift’ in influenza viruses,
multiple antibiotic resistant bacterial
diseases
Inadequate public health
measures
Inadequate water quality,
sanitation, and vector control
Response to environmental
selection pressures
Reduction of disease prevention
programs
1
Example of disease
Ecological changes
Tuberculosis resurgence in USA,
diptheria resurgence in former USSR,
cholera in refugee camps in Africa
Adapted from LONGBOTTOM (1997); MORSE (1995)
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Seminar Nasional Teknologi Peternakan dan Veteriner 2007
Wildlife studies involve uncontrolled
populations, and many of the complexities that
arise from surveying wildlife are related to the
inherent difficulties of capturing, re-capturing,
sampling, and running diagnostic tests on
species that may not have been previously
studied. Working in remote locations also
makes the collection, storage, and transport of
biological samples difficult, especially when
optimal diagnostic results depend on
maintaining a cold chain. Identifying
appropriate diagnostic tests and facilities to
enable the effective screening of samples
represents another challenge, especially when
laboratory facilities are limited, or when agents
require the highest level of biosecurity.
Serologic tests specifically developed for
diseases of wildlife are limited, and those used
in wildlife studies are commonly transposed
from domestic species. The validity of such
tests and the meaningful interpretation of test
results can therefore be problematic. For
example, even where a test has been validated
in domestic species, test characteristics should
not be assumed to be the same in wildlife
species, given possible differences in pathogen
strains, host responses, and exposure to crossreacting infections in the wildlife species
(GARDNER et al., 1996).
Finally, ethical considerations must come
into play when working with wildlife
populations, and issues such as the
conservation status of the target species may
influence the appropriate study design.
Meeting the challenges and complexities of
understanding and managing the risk of
emergence of infectious diseases from wildlife
requires skills from a range of disciplines (both
‘hard’ and ‘soft’ sciences) working
collaboratively towards a common goal.
ACKNOWLEDGEMENTS
This work is supported in part by an
NIH/NSF “Ecology of Infectious Diseases”
award from the John E. Fogarty International
Center R01-TW05869, and is published in
collaboration with the Australian Biosecurity
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Cooperative Research Center for Emerging
Infectious Diseases (AB-CRC).
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