Going Viral: the Human Impact on Emerging Infections

[source; U.S. Department of Agriculture][source; U.S. Department of Agriculture]

In a rapidly changing world, where human modification of the environment has become routine global practice, are we paving the way for emerging viral infections?

In the mid-20th century, a naïve belief began to surface that in the future, infectious disease would be consigned to history; that our improved sanitation, better socioeconomic conditions and medical breakthroughs -such as antibiotics and vaccination- would eliminate the major factors facilitating the spread of infectious disease. In 1963, T. Aiden Cockburn, a respected physician and anthropologist, proclaimed, “We can look forward with confidence to a considerable degree of freedom from infectious diseases at a time not too far in the future. Indeed…it seems reasonable to anticipate that within some measurable time…all the major infections will have disappeared.”The human population welcomed the prospect of a future free of infection.  However, the decades following this statement saw an emergence of a plethora of viral infections, some of the most notable including Human Immunodeficiency Virus (HIV), first isolated in 1983; Hepatitis C, identified in 1989; and Severe Acute Respiratory Syndrome (SARS), which emerged in late 2002 in China’s Guangdong providence. So what happened to Cockburn’s vision of a world free from disease?

To answer this question, we have to understand what causes a viral infection to emerge in the first place. Emerging viral infections aren’t always necessarily new in a population; they might have existed previously, but are increasing in incidence or geographical range. At the molecular level, mutations within the genetic code of the pathogen can alter the virus to facilitate its emergence. Mutation is a fundamental fact of life; and most importantly -in terms of managing a disease outbreak- it is largely out of our control.

However, alongside mutational changes within the pathogen, the responsibility of emerging viral infections can lie with changes in the environment or in the host population. These are factors that, as a human population, we are ultimately responsible for. The media informs us of the negative implications of human behaviour on the planet- from the damage we are doing to the environment and its consequences, to the exponential population growth and the repercussions in relation to depleting resources. But it is rarely highlighted that our anthropogenic social and environmental modifications are facilitating and conceivably even inducing the emergence of novel and pre-existing viral infections.  Environmental modifications, such as deforestation and climate change, alongside ever-changing and modernising human population dynamics, behaviour and density are actually creating new pathways from which viruses can emanate.

Severe Acute Respiratory Syndrome (SARS) emerged in late 2002 in China’s Guangdong providence with devastating effect, becoming a major global health threat within two months. SARS presented itself as an uncharacteristic form of pneumonia, with individuals developing severe respiratory compromise. The symptoms rapidly developed, becoming extremely severe; patients required ventilatory support and in many cases the disease would even manifest into multiple organ failure. By early 2003, SARS had spread from Hong Kong to more than 300 countries worldwide; infecting over 8000 people with a 10% fatality rate. It provoked large-scale quarantine around Asia and finally resulted in the World Health Organisation issuing a global alert for the first time in more than a decade. On March 27th  2003, Hong Kong closed all its schools and quarantined more than 1000 people. In the succeeding two weeks, alarm and caution resonated around the globe: Malaysia refused entry visas to Chinese travellers, and in Thailand all passengers and crew members on aeroplanes were quarantined if anyone on board was found with symptoms. Citizens were told not to travel to China and Hong Kong unless it was absolutely necessary.

The agent responsible for SARS was found to be a novel coronavirus (the cause of the common cold) distinct from those known to be in existence. The SARS coronavirus is passed on via the respiratory route, allowing it to spread with the pronounced momentum witnessed.  The origin of the SARS virus was traced back to the Horseshoe bat, which is its natural host.

There were several factors that contributed to the emergence of SARS in the human population and its rapid global transmission. In China, livestock markets are rife with captive wild bats, usually placed in the same vicinity as another susceptible mammalian species, the Civet cat. Through such livestock markets, humans put animals in unnaturally close proximity, promoting the spread of infections between species. The virus therefore spread from Horseshoe bats to Civet cats, which are consumed as a delicacy in China’s Guangdong Providence. As a result, the virus transferred from Civets to humans in this region. A mutation in the virus supported this transmission to the human population and the mutation allowed it to infect humans more efficiently.

Health officials reported in early 2003 that the majority of SARS cases in Hong Kong could be tracked down to a single doctor that treated patients in Guangdong. The doctor travelled back to Hong Kong on February 21st, where he infected several members of the public staying in the same hotel. Subsequently, SARS began to spread around Hong Kong; then from Hong Kong to the rest of the world. This extensive transmission was caused by frequent and rapid air travel, which was amplified by virtue of Hong Kong being a global travel hub. The time it takes to travel to a city on the other side of the world is significantly shorter than the incubation period of most infectious diseases: the time elapsed between exposure to a pathogen and the initial appearance of symptoms. As a result, people were delivering SARS to uninfected countries without the slightest awareness. Like W.H Auden’s arriving stranger in his haunting poem Gare du Midi, the airborne passengers landed to “infect a city whose terrible future may just have arrived”.

We can therefore attribute the rapid emergence and devastating geographical extent of SARS to humans, and more specifically, to our livestock markets, which caused unnatural and artificial relationships of close proximity between animals; our consumption of food from these markets; and extensive global travel. It has been estimated that at any time there are more than 500,000 people in the air at once. Indeed, this is a key factor for fostering the spread of all infectious disease from contained countries, not just SARS.  It seems fair to conclude then that we are to blame for the SARS virus emergence into the human population.

SARS is far from the only emerging disease that we have facilitated. Dengue Virus is over two-centuries old, but it is currently the most rapidly spreading mosquito-borne viral disease in the world. As a result of anthropogenic social and environmental changes in the modern world, Dengue incidence has increased 30-fold over the last 50 years; specifically influenced by population growth, urbanisation of the tropics and climate change. Each year it produces around 50 million new infections and at least 1200 deaths, mainly amongst children, worldwide. Originally Dengue fever was contained to non-human primate hosts in the jungle and transmitted by the Aedes stegomia mosquito. Though transmission to humans sometimes occurred, it was extremely rare. However, human modification to the natural dengue environment has led to an explosion of the virus into the human population. Destruction and clearing of the jungle for human habitation brought humans into close proximity with the natural hosts of Dengue, facilitating the movement of the virus between the different species. Simultaneously, the virus acquired a mutation causing a more proficient infection of the Aedes aegypti mosquito, which prefers to bite humans to other animal species, thereby increasing the incidence of human infections. Human destruction of habitats, however, is not the sole accountable factor for the rapid spread of Dengue amongst the human population. The virus requires high population densities to carry out efficient transmission, which recent population growth has provided. Improper disposal of artificial refuse by humans has also augmented the outbreaks. Discarded refuse collects stagnant water, which acts as a basin for mosquito eggs and consequently supports mosquito reproduction.

Furthermore, as anthropogenic climate change has increased, the spatial distribution of the Dengue vector, Aedes aegypti, has increased over the last 25 years. Greater rainfall in certain areas and warmer temperatures have provided expanding areas with optimal conditions for the mosquitos, allowing them to broaden their geographical range from its usual confinement of tropical regions.  There have recently been outbreaks in Florida, which hasn’t seen Dengue in over 40 years: a consequence of the increasing prevalence of the Aedes aegypti mosquito across the USA. Additionally, higher temperatures reduce the time that the virus requires to replicate and disseminate within the mosquito; this extrinsic incubation period is essential for the virus to reach the mosquito salivary glands and transmit to humans during mosquito feeding. Increased temperatures subsequently allow the mosquito to be infectious to humans for a longer time period and infect more people in their lifetime. Dr. Felipe Colón-González from the Tyndall Centre for Climate Change Research at UEA predicts that the ‘mean annual Dengue incidence may increase by about 12—18% by 2030, 22—31% by 2050, and 33—42% by 2080 across Mexico showing an increasing effect of climate change on Dengue’in The Effects of Weather and Climate Change on Dengue’. It is evident that this cumulative growth in the geographical range habitable for mosquitos increases, not only the range of Dengue, but also the reach of other mosquito-transmitted diseases, such as Yellow Fever and Malaria. The recent eruption of Dengue incidence acts as a pivotal illustration of how over the past few decades, the environmental changes resulting from human activity and changing social dynamics have greatly facilitated the emanation of viruses.

While man continues to plunder the environment at a rate previously unparalleled, aided by technology and with little regard for the consequences, T. Aiden Cockburn’s statement seems not only premature but also increasingly unrealistic. In a global context, where we are creating attenuating geographical and interspecific boundaries, it appears that new viruses will continue to emerge and challenge the human population for the foreseeable future. Current debates on the impact of climate change and need for sustainability are not the only grave considerations about our continuing disregard of the environment. The underlying anthropogenic contributions to the emergence of infectious diseases need to be addressed and broadcast with more attention and urgency. This is crucial if we are to reduce the role humans play in emerging infections and develop a more defensible civilisation and environment.