Introduction

Emerging infectious diseases are diseases that are newly identified in a population, or that have existed but have changed. They show an increase in incidence, currently or in the recent past, or have a potential to increase in the near future. Emerging diseases can be caused by viruses, bacteria, fungi, protozoans, or helminths. Older diseases that have been known for a long time and were previously under control, but are recurring, are called reemerging infectious diseases.

Emerging/Reemerging Infectious Diseases

The increasing international threat of infectious diseases has received special attention in the past few decades. The emergence of infectious diseases generally involves two steps:

1. The introduction of the pathogen into a new host population, and

2. The establishment and further spreading of the pathogen within the new host population, a process referred to as adoption.

The occurrence of a new infectious disease often seems strange; however, there are specific factors that are generally responsible for such emergence. Although some infectious diseases have been successfully controlled by modern technology, new emerging infectious diseases such as HIV/AIDS, severe acute respiratory syndrome (SARS), and West Nile virus and Ebola infections are continually appearing and are on the rise. In addition, other infectious diseases including malaria, tuberculosis, and bacterial pneumonias are reappearing in strains that are resistant to antimicrobial drug treatments.

Factors of Emergence/Reemergence

Factors responsible for the emergence of new infectious diseases include human demographics and behavior, ecological changes and agricultural development, international travel and commerce, technology and industry, microbial adaptation and change, breakdown of public health measures, human susceptibility to infection, climate and weather, economic development and land use, war and starvation, and intent to harm.

Human Demographics and Behavior

Changes in human demographics due to migration or war are often significant factors in the emergence of infectious diseases. Population movements are often due to economic conditions, which encourage workers to move from rural areas into cities. Beginning in the twentieth century a rapid urbanization of the world’s population occurred: whereas in 1950, 29% of the population lived in urban areas, the United Nations estimates that in 2030 approximately 60% will live in urban areas, increasing to more than 69% by the year 2050 (Figure 18.1 and Table 18.1). The population density in urban areas makes disease transmission easier as it allows infections that arise in isolated rural areas that previously remained vague and localized, to reach the larger urban population. Once in the city, the disease not only spreads locally, but can also spread farther by intraurban transport routes, along highways, and by airplane.

TABLE 18.1

Urban and Rural Population Percentages from 1950 to 2050

Year	Percent Urban
Percent Urban Population from 1950 to 2050
1950	29.1
1960	32.9
1970	36.0
1980	39.1
1990	43.0
2000	46.6
2010	50.6
2020	54.9
2030	59.7
2040	64.7
2050	69.6

Year	Percent Rural
Percent Rural Population from 1950 to 2050
1950	70.9
1960	67.1
1970	64.0
1980	60.9
1990	57.0
2000	53.4
2010	49.4
2020	45.1
2030	40.3
2040	35.3
2050	30.4

FIGURE 18.1 Urban and rural world population from 1950 to 2030.

Human behavior, especially in large population centers, can have important effects on disease dissemination. Well-known examples include sexually transmitted infections, and intravenous drug use, which have contributed to the emergence and spread of HIV/AIDS and hepatitis.

Child care centers have become a way of life especially in many civilized countries, including the United States. According to the U.S. Food and Drug Administration (FDA) more than half of all mothers with young children have jobs outside the home and day care centers provide a necessary service. Unfortunately, this also provides a focal point for certain infectious diseases that can easily spread in the center, and also into the community.

Another factor in the emergence and transmission of infectious disease is the increasing age of the human population, largely due to medical advances. Many persons with advanced age or a chronic condition will have a compromised immune system, resulting in greater susceptibility to disease. Opportunistic organisms will then have the chance to develop an infection that normally would be prevented by an intact immune system. Once the disease has developed in an immunocompromised host, it can spread further into the community.

Ecological Changes and Agricultural Development

Changes in the ecosystem are frequently identified as factors increasing the occurrence of emerging infectious diseases. These ecological changes include changes in agricultural practices, economic development, and changes in land uses. Once the ecosystem is disturbed, outbreaks of previously unrecognized diseases occur and these often turn out to be of a zoonotic nature. Ecological changes often place people in contact with a natural reservoir or host that is unfamiliar to the population moving into the changed habitat. The building of dams and deforestation or reforestation are changes in the ecological makeup that will influence waterborne and vector-borne disease transmission. The movement of people into these areas will expose a larger population to wild animals and potential disease vectors. Agricultural development and urban development are the most common ways by which society alters the environment and introduces the population to previously unknown microbial challenges.

International Travel and Commerce

Persons infected with an exotic disease anywhere in the world can readily travel to any major city, including those in the United States. Even in the sixteenth and seventeenth centuries ships bringing slaves from West Africa to the New World also brought along infectious diseases, such as yellow fever and its mosquito vector. Other examples include the import of a filovirus into Marburg, Germany in 1967, with a shipment of African green monkeys used for laboratory purposes. The virus spread from the primate host to some of the human handlers, and subsequently to staff taking care of the infected people. This virus is now called the Marburg virus, which was responsible for seven deaths during that first outbreak. More outbreaks due to this virus occurred in many countries, including Angola in 2005. The mosquito-borne disease malaria is one of the most frequently imported diseases into nonendemic areas. The sporadic Ebola outbreaks in Central Africa could potentially be spread via air travel throughout the world within a matter of days.

Foods from other countries that are routinely imported, and vectors hitchhiking on imported products, also represent a potential for emerging infectious diseases all around the world. Cruise ships are also a great environment for emerging diseases, because the passengers are frequently exposed to new environments, and live in a high population density.

LIFE APPLICATION

Pathogenic Hitchhikers: It’s a Small World After All

In the 1960s the Surgeon General of the United States acknowledged that the time was at hand when we could “close the book on infectious disease”. As a result of improvements of public health programs, hygiene, and advances in the development of new drugs and vaccines to combat infectious diseases, many public health officials stated that, at least in the Western world, death due to infectious diseases had been virtually eliminated. Unfortunately, this has not been the case as recent worldwide problems exist, with diseases such as malaria, West Nile virus, dengue fever, and SARS still spreading and claiming victims. There are a number of factors that contribute to the successful spread of pathogens to new host populations, but one that has changed considerably over the past century is the spread of disease through international travel and commerce. For instance, increases in cases of yellow fever in Africa have raised some serious concern in the international community regarding the potential epidemic should the virus and insect vector come together on a wide scale. There is fear that the virus, which is now confined mostly to the savannah and forest areas, may be transported to more urban areas as a result of migration due to economic development policies. If the virus is introduced to the mosquito vector in the urban environment, officials fear it could give rise to an urban epidemic. Air travel and transport shipping could then bring the virus to the United States, where the mosquito insect vector is already well established in the southern states. Once again, when the virus and vector are brought together, the potential for a devastating epidemic could arise. Although virtually any form of international transportation has the potential to transport a disease vector, the form that has historically proven to be most effective at spreading pathogens worldwide has been ship transport. The shipboard environment and the equipment and containers being transported favor the survival of animal vectors, with the bacterial/viral pathogens safely hitchhiking on or in their bodies.

Technology and Industry

Technical and industrial advancements in both the food and healthcare industries generally have a positive impact on the quality of life; however, they can also bring about the spread of infectious disease. Modern production methods tend to increase efficiency and reduce costs, but unfortunately also increase the chances of accidental contamination. This particular problem is amplified by globalization, because of the opportunity to introduce microbes from different parts of the world. In bulk processing, a microbe present in raw material may end up in a large batch of the final product.

At present animal feed ingredients include rendered animal products, animal wastes, antibiotics, metals, and fats, all of which increase the potential for higher levels of bacteria, antibiotic-resistant bacteria, prions, and dioxin-like compounds in animals. Animal food products are then intended for human consumption. Although the FDA does closely monitor these practices, accidental outbreaks of old or emerging infections do occur.

Many technological advances have been made in healthcare during the twentieth century, yet there has been a dramatic increase in nosocomial infections (see Chapter 9, Infection and Disease), often due to the development of antibiotic resistance in microbes.

Microbial Adaptation and Change

Like other organisms microbes are constantly evolving, but at a much faster rate because of their rapid reproduction/generation cycles. Microbes, especially bacteria and viruses, can also adapt quickly to adverse environments and this change is a major contributing factor to the emergence of “new” pathogens. Almost all RNA viruses, such as influenza, HIV, and hemorrhagic fever viruses, undergo frequent and unpredictable genetic mutations. Microbes have enormous evolutionary potential as they are able to undergo changes in pathogenicity, as well as often being able to avoid the immune system of the host. Their capacity to adapt to new environments is often enhanced by human activities.

Antimicrobial resistance (see Chapter 22, Antimicrobial Drugs) due to the overuse of antimicrobial drugs and the failure to ensure proper diagnosis and adherence to treatment has become a significant public health problem. This widespread use of antibiotics and other antimicrobials has resulted in evolutionary adaptations in microbes, enabling them to survive many powerful drugs. Antimicrobial drug–resistant strains are a continuing source of emerging infections and diseases.

MEDICAL HIGHLIGHTS

Antigenic Drift: “Putting on a New Coat”

RNA viruses such as the influenza viruses can change in two different ways. One way is referred to as antigenic drift, the other as antigenic shift, and one should not be mistaken for the other. An antigenic shift is a process involving at least two different strains of viruses, or even different viruses, combining to form a new virus that may then infect different species (i.e. a new strain of the avian flu virus infecting humans).

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