Increasing globalisation and continued human induced environmental degradation is enhancing the ability for infectious diseases to emerge and spread. Globalisation, climate change, urbanisation and livestock intensification are all features of the modern human society disrupting the natural ecological system and altering disease transmission (Patz, J.A. et al 2000). Changes in population size, density and distribution, aided by international migration and mobility further emphasises the likelihood emerging diseases will traverse entire continents and rapidly spread in short periods of time. Dengue Fever is a recent successful outbreak directly relating to environmental changes. The history of SARS (Severe Acute Respiratory Syndrome) and HIV/AIDS also reflect the link between increasing modern globalisation and risk of exposure to emerging diseases.
Ecological changes due to human intervention can influence the emergence of infectious diseases (Patz, J.A. et al 1998). A warming and unstable global climate for example, can be linked to the increasing emergence of vector-borne diseases (VBDs) over the past decade, by influencing the survival and transmission of vectors and pathogens within the environment (E.A Gould & S. Higgs). Tests demonstrating the effects of ambient temperatures between host-parasite interactions revealed that lower temperatures reduces the host's susceptibility to infection, but not higher (Murdock, C.C., et al, 2012). Effects of warming temperatures have been suggested to increase egg laying/replication, egg development/maturation and mass gain in parasites, in association with shortening of incubation periods in mosquitoes (Patz, J.A., et al. 1998) (Paaijmans, K.P., et al. 2012). Infected species of mosquitoes, ticks and sand flies are therefore more active and reproduce in warmer climates, and parasites mature in time to cause infection (Shuman, E.K., 2010). Further, dynamic models suggest a warming climate will expand the area range in which infectious transmission is capable of surviving (Martin. P., 1995).
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Increased heating leads to greater evaporation, dryer land surfaces and more intense droughts. Warming causes greater water vapour in the atmosphere, lending to more intense precipitation events (Trenberth, K.E., 2011). Studies demonstrate mosquito outbreaks and vectorial competence coincide with natural droughts and rainy seasons (Chase, J.M. & Knight T.M., 2003). The emergence of infectious diseases may increase in these anomalously wet or dry periods, depending on the type of disaster. For example, Dengue Disease is a mosquito borne disease associated with stagnant stored water in highly populated areas experiencing hot, dry climatic conditions. Severe drought and high temperatures create breeding grounds in water containers for mosquitoes within or near households. (Anyamba, A., et al. 2014). Previously, In the 1990’s Aedes aegypti , the main vector in the Dengue transmission vastly increased in quantity across South and Central America (Oaks Jr, et al, 1992). Currently, more than 2 billion people are at risk.
Rising urbanisation in developed industrialised countries has generally contributed to an overall improvement of health, and allows cities to grow economically. It is in developing low income countries that rising urban populations see the most detrimental consequences. As these cities are becoming more densely populated, whether it be due to migration or natural population, economic growth does not keep pace with urban growth. As a result, many governments do not have enough resources to support these populations and they live in poor conditions. The poor health of urban dwellers, unsanitary living conditions and high rate of contact allows a high risk of infectious diseases to be transmitted. Built up over time, these diseases can potentially circulate to other more cleaner parts of the cities and worldwide (Alirol, E, et al. 2011).
As global transport networks and the ease of migration between countries expand, we are offering new pathogens, previously obscure, the opportunity to infect new host populations (Tatem, A.J., et al, 2006). Prior to recent centuries, pathogen infection remained confined to regional and continental populations, where humans were relatively isolated from one another (Dobson,A.P., and Carper, E.R., 1996). Now, the emergence, efficiency and extensive use of trains, cars, planes in modern transport by air, land and sea can facilitate global pandemics for communicable diseases. In particular, the global air transportation network has grown exceedingly and largely influences local, national and international economies (Guimera, R., et al, 2005). Almost 700 million people fly each year, with passenger numbers growing nearly 9% each year since 1960 (Tatem, A.J., et al, 2006). Aviation is also indirectly responsible for the propagation of diseases such as severe acute respiratory syndrome (SARS). SARS is an animal based coronavirus that was responsible for a pandemic in 2003. Originating in Guangdong Province, China, the virus spread within weeks to infect 8,448 people over 37 countries and 5 continents, killing 774 people (Peiris, J.S.M., et al, 2004). The spread could be aided by tourists entering the country where they were exposed to illnesses to which they have no resistance. Though small in mortality compared to historic pandemics, and mainly damaging the economy, the speed and extent in which SARS traveled in such a short time emphasised the potential for increasing global air transport networks to spread diseases. However, a significant factor facilitating the response and management of the SARS outbreaks was modern technology and humans ability to communicate through globalisation. As fast as the virus spread, the international public health community responded to contain disease spread and impact. Modern interventions enhanced infection control by measures such as finding and isolating patients, implementing ‘social distancing,’ and encouraging better personal hygiene. Domestic and international travelers were issued travel advisories and screened at borders (Bell, D.M., 2004). Globalisation plays a role in both enhancing and reducing the emergence and spread of infectious diseases.
Migration groups such as refugees, immigrants and asylum seekers are part of migratory groups socially disadvantaged and contributing to an increasing global movement. Some migrants undergo harsh transitional periods and experience refugee camps, trafficking or smuggling in order to move to a new geographical location, all greatly affecting their health. Should the health and disease parameters that influence the rate of infection in these transitional periods be different from those at the destination, the process of migration can act as a method for infection transfer between regions. Infectious diseases can manifest in susceptible migrant populations after moving to new destinations. For example, there is concern Mexican migrants into California are at an increased risk for HIV infection (Sanchez, et al. 2004). Migration related factors such as continual mobility, limited access to health care services, poor education and knowledge, poverty, discrimination and low quality housing all increase the liklihood they are to engage in unsafe sexual acts. Use of drugs and alcohol, and exchanging sexual favours for money and food all increase the risk of an HIV/AIDS outbreak in these population settings.
Commercial globalisation and the rising movement and use of livestock greatly enhance the dissemination of exotic pathogens into new populations. The introduction of infected animals otherwise foriegn into a naive geographical area, is an important driver of emerging diseases (Gummow, B., 2010). The United States is amongst the largest importers of wildlife, importing >1 billion individual animals in 2000-2004 (Palvin, B.I., et al, 2009). Limited disease testing is conducted; only wild birds and primates require quarantine, and mandatory tests exist for only a few diseases. Furthermore, the process of importation often involves containing large quantities of animals in unnatural groupings of species. Combined with poor sanitation measures, this provides the opportunity for cross-species contamination and rise of pathogens to be imported into the country. For example, imported animals like hogs, cattle and poultry can be contaminated with Salmonella enteritidis and E. coli microbes when they are crowded in facilities without proper waste disposal systems.
Advances in medical treatments has led to an increasing number of immunosuppressed individuals, such as those undergoing organ transplantation or cancer chemotherapy. Immunosuppression lowers the body's resistance to infections, making people more susceptible to the development of opportunistic infections. Similarly, malnutrition increases the risk of people contracting infectious diseases, seen in socioeconomically disadvantaged countries and especially their children (Ibrahim, M.K., et al, 2017). Immune responses require energy to execute defense functions; energy acquired from micronutrients such as vitamins, fatty acids and amino acids. Malnutrition results from an inadequate intake of energy requirements. An imbalance between nutrient intake, energy consumption, and ability to perform bodily functions persists, impairing immune function, leaving children underweight, weak and vulnerable to infections (Schaible, U.E. and Stefan, H.E., 2007). Families living in low income households increase the likelihood children will be underfed and malnourished. These children are least likely to have access to proper healthcare and sanitation. Mothers who are underweight are more likely to have a child who is stunted. Her child will less likely grow to be strong and healthy, complete school, gain a job and earn economic opportunities. The child is more likely to remain in poverty and get sick. A constant cycle of poverty persists with malnourishment and sickness. The increased susceptibility of the malnourished host to infectious diseases can be supported by studies regarding children under the age of five. For example, a study on children in the Philippines with a median age of 1.8 years and were underweight, were at a higher risk of severe lower respiratory tract infections like Respiratory Syncytial Virus (RSV)(Paynter et al, 2014). Nutrition therefore plays a key role in the development of the immune functions during childhood and their consequential ability to ward infections and disease.
Therefore, there are many factors of the modern human society that may increase the risks of emerging diseases. Population growth in an increasingly mobile and commercial world is allowing infectious diseases like SARS and AIDS the opportunity to spread from person to person more easily than ever. Climate change is causing extreme weather events that influence pathogen population and poor living conditions for an exceedingly large number of low income populations, results in greater exposure to emerging diseases in the future.
References
- PATZ, J. A., GRACZYK, T. K., GELLER, N. & VITTOR, A. Y. 2000. Effects of environmental change on emerging parasitic diseases. International journal for parasitology, 30, 1395-1405.
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