Modern society human populations at risk of disease pandemics
Sandra Mankarios
42460441
Word count: 2033

The qualities in which humans in modern day society live have increased the potential risks of the transmission of pathogens and their vectors. These risks have become limitless compared to the past; the damage created now will greatly effect future generations where the damage may even be irreversible (Washer 2006). Many factors in everyday modern life contribute to the rapid spread of pathogens and their vectors, which include the advances in modern transport via the air, land and sea, the increase in population size and density, global change and climate change. These features of today assist in the introduction of new diseases and also the re-emergence of old diseases including influenza, HIV/AIDS, dengue fever, malaria, severe acute respiratory syndrome (SARS), Cholera and many others (Tatem et al. 2006).

MODERN ADVANCES IN TRANSPORTATION
In the past, populated establishments were isolated due to lack of efficient transport. Now in today’s modern society, the constant advances in resources and the requirement for travel has seen the travelling of individuals dramatically increased over the years. Transport systems via land, air and sea continue to develop allowing swifter mobility of passengers and goods across vast distances within shorter periods of time. Although this is exemplary of an advancing society full of promising travel and freight options, this creates cause for concern (Grais et al. 2003). Utilising this expansion in transport, pathogens and their vectors can travel and spread quicker, easier and in greater numbers than in the past (Tatem et al. 2006). Three major consequences of this are the occurrence of pandemics of infectious diseases, the invasions of vectors and the importing of vector- borne pathogens.

Pandemics of infectious diseases
Air and sea travel have led to the pandemics of diseases such as HIV/AIDS, plagues, cholera, influenza and severe acute respiratory syndrome (SARS) (Tatem et al. 2006).
The most recent HIV/AIDS pandemic started in the mid 1970s, and had spread to between 100,000 and 300,000 people by 1980 through air and sea travel and human migration (Perrin et al. 2003). Certain groups of individuals such as drug users (venous), tourists, truck drivers, army troops and seamen, have shown through studies, the large role they play in the spreading of HIV/AIDS in confined endemics (Salit et al. 2005). An example of the impact of modern transportation on the spread of HIV/AIDS is a documented experience of infection in Europe where a Norwegian seaman had contracted the disease through heterosexual interaction in a seaport in West Africa. Arriving back home he transmitted the disease to his wife, who then passed the infection on to her daughter (Jonassen et al. 1997). This shows how modern and rapid transportation leads to the transmission of disease much more efficiently than ever before from distant countries.

Invasion of vectors
Modern day ships and aircrafts largely cause the expansion of the variety of plants and animals entering countries. Not only are new plants and animals introduced, but there is the underlying problems of the invasion of potential vectors and disease reservoirs. (Perrings et al. 2005). Aedes albopictus (Asian tiger mosquito) was transferred, via transport, from Asia to the United States, the mosquito proficient in transmitting dengue fever (Craven et al. 1988). Another possibly dangerous virus, La Crosse encephalitis was carried by the same newly introduced vector (Francy et al. 1990). Although the usual vector rarely bites humans, its combination with the Asian tiger mosquito leaves cause for concern.

Importation of vector-borne pathogens
One of the quickest spreading diseases of modern day society is dengue fever and dengue haemorrhagic. The emergence and re-emergence of these diseases over the last 3 decades has been larger and more frequent, leading to a more severe disease. Beginning with the expansion of its primary vector Aedes aegypti, the recent expansion of a new vector-borne pathogen Aedes albopictus has been caused by its incubation within air transport by humans (Mackenzie et al. 2004). Not only is the spreading of both dengue vectors increased due to worldwide air travel, but it also increases the importation incidents elsewhere (Tatem et al. 2006).

The secretion of respiratory aerosols that are dismissed into the air by, a person infected, sneezing, coughing or talking, spreads the influenza virus. It is mainly spread directly from person to person (Cox & Subbarao, 2000). The 1990s saw a dramatic increase of over 460 million passengers flying globally since the 1968-1969 pandemics of influenza. The large rise in numbers creates a greater probability of a reoccurrence of a pandemic of influenza (Grais et al. 2003). A study conducted by Rebecca F. Grais, researched the impact of air travel on the global spread of this pandemic if it were to occur in the year 2000. Through the simulations created, results displayed clear and prominent changes in the spatial dynamics and climates of a future pandemic strain of influenza. Initially influenza was known to follow a seasonal pattern, where it peaked in colder temperatures, where the occurrence of influenza fluctuated between the northern and southern hemispheres, approximately every 6 months. The simulation incorporated the seasonal changes by imitation. The results, if a pandemic occurred in 2000, showed that the evidence and spectrum of the influenza disease was not only during the anticipated cooler seasons, but ranged throughout the year and its consistency was un-uniform. More isolated cities with less air travel experienced seasonal hemispheric fluctuations as per patterns in earlier, less developed times (Grais et al. 2003). Though these were the findings, there were many limitations and the susceptibility of someone not infected being infected with the disease varies by age, economy, social and cultural factors and geography.

POPULATION SIZE, CHANGE, DENSITY
Changes in the size, movement and distribution of human population alter the patterns of the spread of disease. Social factors affect the spread of pathogens through the frequency and type of interactive contact, migration patterns, travel, hygiene, pharmaceutical resources, poverty, public health programmes and medical education (Gubler 2002). On average each host that is infected, infects more than one other host. Therefore, in order for a pathogen to acquire a successful rate of reproduction and host population, it must find a large population that can be infected. From the continuous growth of population size of humans, pathogens have been more effective in infecting Homo sapiens in comparison to the past whereby mainly nonhuman animals were affected. For example, measles couldn’t invade human communities until there were masses of approximately 200,000-500,000 persons (Black 1975). Another change where increased populations have played a large role is the transmission of pathogens from other species to humans. Transmission from other species to humans previously occurred rarely, but human population increase makes the transmission between the two more likely (Daily & Ehrlich 1996). Population growth correlating with large families creates more vulnerability due to the fact that the individuals are alike in immunity. This same concept may have assisted in the extinction of native Americans (Roberts 1989).

A great global change in modern times is the urbanization of populations. Although there are several ways urbanization can improve the epidemiological environment, it also employs severe undesirable effects. Due to the large populations, a characteristic of cities, countless persons come into close contact. This assists in the spreading of infectious disease, even if the city is complete with clean water and access to health care. Disease reservoirs and vectors of disease leave cities troubles, whether they may be rich or poor. In urban societies large numbers of people have been exposed to the domestic mosquito Aedes aegypti, the known vector of dengue fever. This mosquito breeds in water in containers, which could be anything in a standard household (Daily & Ehrlich 1996). Urban conditions may also endorse the use of drugs, prostitution and more sexual promiscuity than in general. The sharing of needles and diseases sexually transmitted create long lasting infections. Hosts of such diseases lack in immunity, therefore giving strength to the disease.

With progression of the human population comes the rise in population density. High population density results in less need for vectors for the transmission of disease due to the enormous levels of direct human-to-human contact. In favour of this are parasites causing diseases such as dysentery and influenza. This proposition is supported by the 1918-1919 flu endemic during the war, where troops experienced repugnant trench environments on the western front, increasing the virulence of the flu (Ewald 1994). Another example of the disadvantages on the epidemiological environment of rising human population densities, is the air tightness of structures, the lower rate of airflow in modern commercial aeroplanes and air conditioning, all to save energy. Air-borne pathogens can easily be transmitted due to the small amount of fresh air and the recirculation of air, especially in the case of influenza (Moser et al. 1979).

GLOBAL CHANGE
Human behaviour against the natural environment has large influences on the occurrence and evolution of disease. Emerging viruses can be caused by the migration of human populations into new areas best for agriculture. The likelihood of pathogens affecting and invading human populations increases when large populations come into contact with vectors and reservoirs of diseases such as HIV, Ebola, Marbug virus and Lassa fever (Daily & Ehrlich 1996). As agriculture intensifies, the number of rodents increases, being consumers of cultivated crops. Through the process of eradicating natural enemies of crops, rodent populations increase because of the protection of their food supplies. With the increase in rodent population comes the increase and spread of diseases such as tick-borne encephalitides, typhus, bacterial fevers and many others (Daily & Ehrlich 1996). The clearing of forests is another way in which humans cause change to land-use. This process of unnatural environmental change can lead to animal vectors being left without their natural habitat, finding new homes amongst the human population, increasing the chances of the spread of disease. In Tanzania, the clearing of forests has caused growing temperatures and the creation of sunny locations, a great breeding ground for the vector of a type of malaria, Anopheles gambiae (Epstein & Chickwenhere 1994).

CLIMATE CHANGE
A significant mechanism creating a defence against disease is the nutritional wellbeing of the human population where climate change has been a main influence causing serious threats to the epidemiological environment. Humans in modern day society implement tasks, such as the burning of fossil fuels, assisting in the rise of global warming and climate change. With the Earth continuing to increase in temperature, so does the probability of the transmission of diseases. This is because warming of the Earth changes the global distribution of pathogens and their vectors (Epstein et al. 1998). The geographic range of microbes and vectors are inhibited by temperature, and the timing and intensity of disease pandemics is constrained by the weather (McMichael et al. 2003). With the increase in climate, pathogens are no longer as restricted by cooler temperatures with the large numbers of temperate zones. Temperate locations are where human populations are found to be the largest and densest, thus creating a more rapid way of transmitting pathogens (Gibbons 1992). The re-emergence, in Rwanda, of the 1987 malaria occurrence was analysed with results showing that 80% of the variation of malaria monthly was due to a change in temperature and rainfall (Loevinsohn 1994). For 6 weeks rain and 3 cyclones in the year 2000 overwhelmed Mozambique and increased the occurrence of malaria by 5 times (Epstein 2005). Climate change is therefore evident in playing a role of transmitting diseases.

It is through the advances in modern transport via the air, land and sea, the increase in population size and density, global change and climate change that pathogens and their vectors can easily transfer through populations and spread faster than ever before. The speed, at which transport can now reach other continents and regions, facilitates in the speed of the transmission of a disease to new locations, where in the past they may have never existed (Wilson 1995). Through the increase in human population, not only does the spread of disease increase but also the strength and lifespan of the pathogens. It is also through the activity by humans of unnatural global changes that the introduction of new vector-borne pathogens can emerge as well as the invasion of vectors in new human populations.