Issues Magazine

Social, Economic and Environmental Drivers of Disease Emergence

By Stephen J. Prowse

The emergence of new diseases and re-emergence of known diseases is related to a complex matrix of social, economic and environmental factors. Difficulties predicting the next disease outbreak mean we need to have the skills and agility to respond to the unexpected.

We are now in a period where we are seeing more infectious disease epidemics than ever before, many of them such as sudden acute respiratory syndrome (SARS) completely unexpected. A recent analysis showed that emerging infectious disease outbreaks have been increasing in frequency since 1940. There is no reason to think that this trend will not continue.

SARS was caused by a completely new virus and took everyone by surprise. It emerged in humans but was subsequently found to have moved from bats to animals in Chinese markets, and then to humans. It rapidly spread around the world, carried by travellers.

In addition to completely new diseases, epidemics can also be caused by known diseases that have re-emerged in a new form. For example, we are currently seeing this with H1N1 2009 influenza (swine flu).

Emerging diseases are diseases that have changed in some way. This change may result in more serious disease, such as the disease caused by bluetongue virus in northern Europe. The disease may have moved into a new geographic location, such as the recent spread of rabies to Bali, or may have appeared in a new host, such as the recent outbreak of Ebola-Reston in pigs in the Philippines. These diseases are important because they can cause significant loss of life and serious illness.

Epidemics of influenza have caused a huge loss of life in the past. Forty million people died in the 1918–19 influenza epidemic compared with eight million in World War 1. Three million people died in two influenza epidemics in 1957–58 and in 1968–69. In Australia, between 1000 and 3000 people die every year from seasonal influenza and related complications.

Epidemics can also have large economic impacts through direct loss of agricultural production, the cost of the intervention and the loss of access to markets. Avian influenza is estimated to have cost US$20 billion globally. The recent Australian outbreak of equine influenza cost an estimated A$1 billion. It has been estimated that a 12-month outbreak of foot-and-mouth disease in Australia, of similar magnitude to the 2001 outbreak in the UK, would cost $8–13 billion. We are particularly vulnerable to economic impact due to our reliance on exports in agricultural products.

The other major indirect impact is the social disruption and hardship that these outbreaks cause. Epidemics also place enormous strains on healthcare systems.

It is likely that these outbreaks are caused by a complex matrix of physical, social, economic and geographic issues that are rapidly changing.

Climate change will have a significant impact on epidemic-prone diseases. Increased temperature may change the geographic range of insects that carry diseases. Higher temperatures can also increase the virus load in the insects and enable them to survive over winter. Changing climate appears to have contributed to the spread of bluetongue into northern Europe, which is perhaps the best documented example of the affect of climate change on disease spread.

Changing rainfall patterns allow the movement of diseases into different locations. These changing patterns, together with increases in population density, mean that the way we use water is changing. For example, the large increase in domestic water storage in Australia will change the geographic distribution of mosquitoes that carry dengue virus. The potential spread of dengue virus-carrying mosquitoes into south-eastern Queensland will have a huge impact on personal health and the healthcare system.

Climate change will probably be accompanied by extreme weather events. Changed drought and flooding patterns will alter the incidence and patterns of disease spread in both positive and negative ways. It should be recognised that the complex combination of climate-related factors will increase the risk in some areas and decrease risk in others.

A second major factor in disease spread is the increased mobility of people and products. Both influenza and dengue have entered Australia carried by travellers. In today’s global market, agricultural produce, including live animals, is easily transported around the globe and can carry diseases into (and out of) Australia. For example, equine influenza entered Australia via an infected horse from Japan. Foot-and-mouth disease most likely entered the UK in 2001 through the importation of agricultural produce from a region where foot-and-mouth disease is endemic.

Both prosperity and poverty affect the emergence of disease. Increased prosperity has resulted in changing food consumption patterns and in livestock production systems in developing countries. Increased intensive production of livestock in situations where there is inadequate infrastructure can result in the emergence of disease.

Intensive and industrial-type farming practices can also lead to the emergence of disease in developed countries. In the UK, the practice of feeding animal-derived protein to livestock led to the emergence of mad cow disease.

Poverty also affects the emergence of disease and spread. Poverty leads to poor living conditions and poor infrastructure, including health care. Such poverty is conducive to the emergence and spread of disease. As an example, tuberculosis in countries to our north is increasing, with only a small proportion of cases treated.

Land use is also an important issue. Changing patterns of land use have resulted from changing levels of prosperity and global access to markets. Deforestation for agricultural purposes can affect the emergence of disease by pushing wildlife and people closer together. Changing irrigation practices alter the distribution of insects, which carry disease. As an example, habitat reduction brought fruit bats, orchards and pigs into closer proximity and resulted in the emergence of Nipah virus in pigs in Malaysia in 1998–99. The disease caused significant economic loss and killed 105 people.

Population pressures can also affect the emergence and spread of disease. Overcrowding in urban and peri-urban areas can result in the rapid spread of disease. In the recent outbreak of H1N1 2009 influenza (swine flu) in Mexico, it has been estimated that between 6000 and 30,000 people were infected before the international public health community recognised that the outbreak was caused by a new strain of influenza.

In Australia, the number of people seeking an alternative lifestyle in the semi-rural regions of our cities is increasing. Many, who have little or no understanding of agriculture, keep small numbers of livestock on the urban fringe. This presents a biosecurity risk. For example, many people who keep small numbers of pigs in the greater Sydney area do not understand the risks of foot-and-mouth disease associated with feeding food scraps such as salami to their animals.

Vaccination remains a cornerstone of disease control in both humans and animals. Where public health infrastructure is poor, public health programs are very difficult to establish and sustain. Political or religious interference can make this more complex, as has been seen with the re-emergence of polio following the difficulties associated with vaccination programs in Nigeria. Even in developed countries such as Australia, disease re-emergence is seen when vaccination rates drop.

In addition to the risk factors arising in this changing world, the disease-causing agents are also continually changing. For example, bacteria develop resistance to antibiotics and viruses develop resistance to anti-viral drugs. The relatively high multiplication rates of bacteria and viruses mean that these organisms can rapidly respond to changes in the environment. Natural selection will favour microorganisms that do best in the new environment.

The emergence of extreme drug-resistant tuberculosis is a good example. Some consider extreme drug-resistant tuberculosis as the most significant global disease threat we face. Extreme drug-resistant tuberculosis is present in Asia.

Evidently, a complex matrix of factors contributes to the emergence of infectious disease. These factors affect infectious disease in humans, domestic animals and wildlife. Considerable uncertainties are associated with many of these related and overlapping factors, which reinforces the need to expect the unexpected.

Despite being an island nation, it would be a mistake to think that we are insulated from these factors. There are three main sources of risk to Australia.

Our northern borders are close to our neighbours, and we know that animals and insects that carry disease cross to Australia regularly. For example, midges carrying bluetongue viruses are carried by the monsoon into northern Australia annually. Bats that can carry Hendra and Nipah viruses regularly cross to Australia from East Timor and Papua New Guinea.

The second area of risk is entry of disease through our ports and airports. H1N1 2009 influenza (swine flu) was carried to Australia by passengers on planes and boats.

Dengue virus is carried into northern Queensland by infected travellers who are bitten by competent local mosquitoes that are then able to transmit the virus to other people.

The third area of risk is from “home grown” microorganisms. Bungowannah and Menangle viruses are two examples of microorganisms that have emerged and caused disease in our pig herds. Hendra virus is carried by Australian fruit bats and causes disease in humans and horses. The 2008 and 2009 outbreaks in Queensland caused the death of two veterinarians.

Australia has a response process that works in the face of outbreaks that could be considered conventional or relatively small. This has been the case in outbreaks of equine flu and Hendra virus.

However, we may lack capacity in the face of a large or ongoing outbreak. In 2008–09 the ongoing dengue outbreak in north Queensland, with over 1000 human cases, proved very difficult to control. In late 2009 there was another outbreak of dengue in Townsville.

We may have problems in response to unusual events such as the Tasmanian devil facial tumour, or leishmania (a protozoan parasite formerly considered exotic to Australia) in macropods. These disease events do not fit into an established investigation framework. We often do not investigate or are not always well-equipped to address unexpected or unusual diseases effectively. This highlights the need for good cross-disciplinary and cross-sectoral investigations and operations.

The world of emerging infectious disease with epidemic potential is one of major uncertainty and, to some extent, inevitability. Hence, we must be well-prepared to rapidly deal with unexpected disease outbreaks and, if at all possible, contain them before they become epidemics.

To be well-prepared to respond to disease outbreaks we must develop and retain skilled people in the science of disease research and investigation. As we do not know where and how the next epidemic will emerge, these skills must be transportable and be able to be applied broadly to any disease outbreak. In some disciplines and sectors there is a severe shortage of expertise, such as entomologists to address insect vectors and scientists trained in virology and in veterinary pathology. For these people to work most effectively they need to be supported by the appropriate infrastructure.

Since the outbreaks of foot-and-mouth disease and bovine spongiform encephalopathy (“mad cow” disease) in the UK and the global spread of bird flu, a number of response exercises have been undertaken. One common outcome has been identification of the need for improved information management. We need to more effectively collect, assemble and analyse this information and use it to set priorities and develop policy that aims to mitigate the risks of disease outbreaks and effectively respond to them when they occur. The initiative to establish the Australian Biosecurity Intelligence Network is addressing this need.

Improving our physical infrastructure is also important. Queensland and Victoria have had major initiatives to co-locate agriculture, public health, university and CSIRO research workers, sharing systems and infrastructure. These initiatives make more efficient use of the limited resources and facilitate cross-sector and cross-organisational collaborations.

Other major developments of additional emerging infectious disease research infrastructure have occurred, such as at the Australian Animal Health Laboratory and at the Queensland Institute of Medical Research. One issue associated with these developments has been a lack of consideration of a national strategic approach to analyse the needs and the costs associated with operating and maintaining these facilities.

Harnessing rapidly advancing technologies to better address emerging infectious diseases remains a challenge. The convergence of biotechnology, nanotechnology and information technology offers many opportunities and at the same time many challenges. Evaluation of new technologies and determining the most appropriate applications in a biosecurity operational framework is an ongoing requirement.

The emergence of disease in our region represents a risk to Australia and its political and economic stability. Hence Australia needs to commit human and other resources to improving disease surveillance and risk mitigation, which must involve capacity-building and collaborative research and operations with our northern neighbours. This will require linkages at multiple levels to develop the required political, scientific and technical relationships.

We also need these linkages to better understand and acknowledge that what we see as a disease risk may not be seen as such a risk to some of our neighbours. A good example of this is the H5N1 avian influenza (bird flu) epidemic, where the impact was of lesser importance than a number of other diseases of both humans and livestock.

Along with many other countries, Australia faces significant threats from emerging infectious diseases that emanate from wildlife or involve a wildlife vector. A salient example of such a disease is Hendra virus. The outbreaks of Hendra virus in 2008 highlight the critical need for a “one health” approach to the management of emerging infectious diseases. A “one health” approach involves collaboration, connections and networks across the agricultural, public health and wildlife sectors. However, such approaches are complex and difficult to put in place.

Australia has made substantial progress over the past decade in building disease response capability and capacity. This has been achieved in the face of pressure on resources. The maintenance of this resource capacity and capability will require ongoing attention to ensure that Australia is well-equipped to address the disease challenges of the future.