Large animal - October 2020

We live in a world dominated by human influences, an age now popularly known as the ‘Anthropocene’. Human influences now effect most processes on earth and there is no denying that human activity and profound economic growth have driven significant climate change and biodiversity loss as we rapidly exceed the planetary boundaries. We have engaged in the large-scale conversion of natural habitats to agricultural crops and urban areas while we have simultaneously degraded the integrity of unconverted natural systems through activities like natural resource extraction, logging and farming. There have been many recent attempts to map the level of anthropogenic environmental modification across the land and ocean with some estimates showing that almost 80% of both realms have clear evidence of human interference. There are many who would claim we are acting as Gods with pronouncements that it is us rather than nature ruling the world but we know this to be but a fragile and temporary position if recent events are to teach us anything. 

The Anthropocene has been irrevocably rocked by a coronavirus which has successfully adapted from its evolutionary bat origin and we have been rendered helpless as it has acquired the traits required to relentlessly spread across the globe’s eight billion susceptible humans. Unfortunately, nothing can drive a message home better than a widespread health crisis and devastating economic impacts and this virus has, in no uncertain terms, exposed the fragility of an accumulation-based, capitalist society which ignores ecosystem health and the implications of global change. At the time of writing (early September) the world had surpassed 28 million confirmed cases of severe acute respiratory syndrome coronavirus 2 or SARS-CoV-2 (Covid-19) with almost one million deaths according to Johns Hopkins University (Figure 1). Quarantines and widespread travel restrictions have become the norm as countries grapple to get a grip on the mounting death toll and economic cost of the Covid-19 crisis while two central questions are being asked repeatedly: where did this virus come from and how can we prevent future pandemics like this from happening again?

Emerging infectious diseases and Covid-19
Emerging infectious diseases (EIDs) are variably defined as ‘infections that have newly appeared in a population or have existed previously but are rapidly increasing in incidence or geographic range’. EIDs not only describe the spread of newly evolved pathogens but also take into consideration pathogens that are spreading to new geographic ranges or populations (eg. lyme borreliosis), pathogens appearing in areas undergoing ecological transformation that may have a significant impact on human or animal health (eg. chytridiomycosis) and pathogens that may be reappearing (or re-emerging) after a decline in incidence (eg. measles). EIDs are dominated by zoonoses, approximately 75% of which have their origins in wildlife and their incidence has dramatically increased over the last two decades (Figure 2), largely driven by widespread ecosystem degradation as well as both the legal and illegal wildlife trade.

Ecosystem perturbations are now widespread across the globe because of climatological variation, landscape alterations, anthropogenic factors and globalisation. The resultant ecosystem degradation has increased the incidence of EIDs originating from wildlife and altered the epidemiological patterns of many diseases such that we can clearly recognise that ecological health and the epidemiological features of agent and host interactions are undeniably linked. There are multiple examples in the literature which support the linkage between ecological degradation and the increased risk of zoonotic disease outbreaks originating from wildlife. Australian scientists have shown that declining eucalyptus habitat altered flying fox foraging behaviour and increased spillover risk of Hendra virus to humans while the emergence of Nipah virus in 1998 was linked to the ecology of bats in changing landscapes. Deforestation, forest degradation, fragmentation, habitat encroachment and expansion of human infrastructure are just some examples of the land-use changes that can perpetuate the introduction of pathogens into new species populations while the legal and illegal wildlife trade has connected biodiverse forest areas to markets and susceptible human populations.

EIDs affecting livestock
While disease outbreaks in livestock are not a new phenomenon, modern trends have significantly increased the risks associated with them. The huge demand for livestock products around the world has led to the emergence of increasingly complex livestock systems and value chains in which the animals are selected based on production traits rather than disease resistance. This coupled with the enhanced biosecurity measures on farms has also contributed to the increased vulnerability of livestock to disease threats, as programmes aimed at the national eradication of infectious diseases mean that animals are immunologically naïve and, thus, at increased risk from pathogen incursions. With increased transportation of livestock and livestock products internationally for trade, intensified farming practices, expanding vector ranges as a result of climate change and the evolution and mutation of pathogens themselves, the likelihood of future serious epidemics in livestock populations as a result of emerging and remerging infectious diseases is growing. The principles of stringent biosecurity and surveillance for emerging disease threats are nothing new to the large-animal practitioner as robust systems of surveillance, prevention and control of animal diseases, including the responsible use of veterinary medicines, can serve to minimise risks to both animal and human health. 

Impacts of EIDs
EIDs that affect both humans and animals have an impact on human society, animal production and welfare and conservation of biodiversity. They present unique challenges in terms of public health protection and economic stability, and the impacts of EIDs on both human and animal health over the last 20 years cannot be overstated as illustrated by the following examples:

The impact of the 2002-2004 SARS coronavirus epidemic (774 deaths) on tourism, food and travel in mainland China alone was estimated at US$8.5 billion (Beutels et al, 2009). The total global cost, as a result of lost economic activity, is estimated to have been around $40 billion (Knobler et al, 2004); 

The 2014 Ebola outbreak in West Africa cost an estimated US$2.2 billion in GDP alone, killed over 11,000 people and wiped out many of the development gains in Guinea, Liberia and Sierra Leone (CDC, 2016); and

The unfolding Covid-19 pandemic will have an immense cost to the world. Recent estimates anticipate monetary costs will reach US$1-2 trillion with huge additional costs to human life and wellbeing as well as economic and political systems. 

‘Spillover’ and the occurrence of Covid-19
Bats (order Chiroptera) are known to be reservoirs of a large number of zoonotic viruses that have caused multiple disease outbreaks in human and livestock populations in the past. A large diversity of coronaviruses, including SARS-related coronavirus and Middle-eastern Respiratory virus (MERS), have been discovered in bats and studies of these suggest a high capacity for transmission across the species barriers through close contact or consumption. Concerns of a serious ‘spillover’ event have been voiced for several years with a University in Hong Kong concluding as far back as 2007 that “the presence of a large reservoir of SARS corona-like viruses in horseshoe bats coupled with the culture of exotic mammal consumption in China was a ticking time bomb.” Recombination is widely accepted to play a central role in the evolution of coronaviruses and viral sequencing in the SARS and MERS outbreaks demonstrated that they originated from zoonotic transmission with intermediate hosts between the bat reservoirs and humans – a common pathway leading to coronavirus zoonosis. Although there are still no definitive answers as to the exact origins of Covid-19, specialists in the field of virology and immunology have suggested that the virus occurred naturally, likely through a recombination event among coronaviruses in an area where humans and wildlife were in direct contact for prolonged periods of time. There is significant evidence to suggest that the live animal markets in Wuhan, China were the original source of virus transmission, although the precise location, time and mechanism of the spillover event is still unknown and likely will be for several years. Bats at these markets are routinely transported and stored in very close contact with large groups of illicitly traded wild animals that are eventually consumed by humans. This form of transportation and storage creates an interface between different wild animals and humans which has aptly been described as ‘a cauldron of contagion’, providing a unique opportunity for the transmission and spread of pathogens between several species that would not ordinarily interact in the wild (Figure 4). 

In general, these interfaces may be locations of increased human-wildlife contact such as forest buffer zones and wildlife markets, as well as locations with substantial land-use change such as logging areas. EID transmission at dynamic wildlife-livestock-human interfaces occurs via aerosols, water, shared food, bodily secretions and vector bites and live markets are considered the perfect petri dish for the creation of a novel virus.

It is hypothesised that Covid-19, a betacoronavirus similar to SARS and MERS, originally transmitted from bats to another susceptible intermediary host before being transmitted to humans after a recombination event. Many intermediary hosts have been implicated at this point including pangolins, racoon dogs and porcupines, although it will likely be several years before any definitive conclusions on the origin of the outbreak can be drawn. 

But we must remember that zoonotic pandemics are not a new phenomenon and we have had very clear and loud warning that such a pandemic was on the horizon. Unfortunately, however, none of this was picked up by policy makers or administrations and we have been found sorely unprepared in the face of the biggest global health crises of this century. So, what can we do to reduce the chances of such a pandemic from happening again in the near future?