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Drivers of emerging and re-emerging diseases
Drivers of Emerging and Re-emerging
Diseases
Author: Prof Gavin Thomson
Licensed under a Creative Commons Attribution license.
INTRODUCTION
‘New’ infectious diseases of people and animals have, in the relatively recent past, appeared on average
once every 7 years. It is presumed that the increased rate at which these diseases seem to be occurring
world-wide is due ‒ apart from improved surveillance and ease of communication in modern times that
could result in the increase being more apparent than real ‒ to a number of factors or drivers. ‘New’
diseases are commonly referred to as ‘emerging’. Additionally, sometimes circumstances arise that result in
known diseases acquiring new-found importance as a consequence of altered geographic distribution or the
disease may present differently in some other way such as affecting a species not previously recognised as
being susceptible; these are referred to as ‘re-emerging’ diseases. Collectively, these two groups of
diseases are commonly known as emerging and re-emerging (ERDs).
Emerging and re-emerging diseases, particularly those that affect people, fascinate professionals and the
general public alike when they occur, giving rise to intense curiosity about their origin and fear ‒ panic
sometimes ‒ that a ‘new’ epidemic will impact on large numbers of people with gruesome consequences.
Such diseases are, however, only a subset of a wider array of emerging and re-emerging infections (ERIs ‒
‘disease’ may not be a consequence of infection in all or even most susceptible species and therefore it is
more correct to refer to ‘infection’ than ‘disease’) that affect plants and animals of all kinds. In the medical
and veterinary worlds infections considered to be emerging or re-emerging obviously differ but a growing
number of zoonoses are common to both groups.
Although this module focuses on ERIs, it is acknowledged that new diseases may also be caused by multicellular parasites, toxic agents or other biologically active substances that animals may encounter in the
environment. These, however, are usually covered by other branches of veterinary medicine and will not be
considered specifically in this module.
The following definition is adopted for ERIs in this discussion: Infections that have newly appeared in a
population or have existed previously but are increasing in incidence, host-range or distribution (after
Morens et al., 2004). The definition used by the World Organisation for Animal health (OIE) is somewhat
different in that both emerging and re-emerging diseases are incorporated into the definition for ‘emerging
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Drivers of emerging and re-emerging diseases
diseases’, viz. ‘a new infection or infestation resulting from the evolution or change of an existing
pathogenic agent or known infection or infestation, to a new geographic area or population, or previously
unrecognised pathogenic agent or disease diagnosed for the first time and which has significant impact on
animal or public health (Terrestrial Animal Health Code, 2013 – glossary; www.oie.int).
As implied above, it is important to appreciate that ERIs of animals are by no means restricted to livestock
or even mammals and birds. Examples of ERI’s in animals other than mammals and birds are ‘honey bee
colony collapse disorder’ (HBCCD – the aetiology of which has not been accurately established ‒
http://www.sciencedaily.com/releases/2012/04/120405224653.htm) and chytridiomycosis, a fungal disease
of amphibians (Dszak et al., 2003). These conditions, HBCCD particularly, have potentially devastating
effects on the global environment and thereby have potential socio-economic consequences on an
enormous scale. The same applies in the plant world but veterinarians are rarely concerned in a
professional sense with diseases of plants (e.g. the recent introduction into southern Africa of an Asian fruit
fly [Bactocera invadens] via East Africa with considerable potential impact on South Africa’s fruit export
industry).
It is arguable that disproportionate emphasis is accorded to ERI’s that are zoonotic, this being symptomatic
of our pre-occupation with the health and well-being of our own species. It has been estimated that about
72% of ‘new’ human infections are derived from animals, mostly free-living animals (Jones et al., 2008;
Hayman, 2011). There is nothing surprising about this from an evolutionary perspective because the animal
kingdom represents an evolutionary continuum, with humans having evolved relatively recently. The same
generally applies to pathogens that adopt humans as hosts, i.e. many infections of humans have evolved
from those that parasitize other animal species as is so eloquently described in the book ‘Guns, Gems &
Steel’ (Diamond, 1997).
Jones et al. (2008) have shown that when it comes to emerging infectious diseases that affect people,
human population density is a common significant predictor and presume that is a hidden cost of economic
development. They also showed that ‘wildlife host species richness’ is a predictor of emerging zoonotic
diseases. Useful global distribution risk maps are provided for various types of emerging infectious
diseases. Unsurprisingly, parts of Africa are indicated as having high risk of zoonotic pathogen occurrence
derived from wildlife and diseases caused by vector-borne pathogens.
As already indicated, irrational fear is frequently fanned by the media in relation to newly identified
zoonoses through publicity accorded to sensational opinions expressed by scientists and other
professionals following agendas that are not always well considered. It has to be admitted, on the other
hand, that official veterinary services and food producers have in the past contributed to public confusion
and apprehension by minimizing potential human health and environmental hazards of newly identified
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infections without adequate information to substantiate their views. This is precisely what happened in the
UK in the early stages of the bovine spongiform encephalopathy (BSE) epidemic of the 1980s. The
consequence is that, particularly in Europe, public scepticism has increased towards assurances regarding
newly identified hazards provided by governments and large commercial organisations.
This module outlines as far as is currently understood, why and how ERIs arise and identify the factors that
precipitate, ‘drive’ or are conducive to their occurrence.
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