EMERGENT INFECTIOUS DISEASES

Emergent infectious diseases (EIDs) are conditions caused by pathogenic microorganisms or parasites that have recently appeared or reappeared in human or animal populations. Typically, EID agents have begun to change the range of their infection, spread through new vectors or the movement of preexisting vectors, rely on shifts in patterns of host susceptibility, or have only recently been identified as the causes of existing diseases. This includes reemerging disease agents once thought to have been eradicated, but that have returned in resistant strains, or as a result of disintegrating public health infrastructure. Emergent diseases have tremendous impact on human health, and the health of pets and livestock. Furthermore, they pose a threat to biodiversity because many wild animal species are also at risk.

Science and Origins

An emerging infection can be caused by such viral agents as Ebola virus, HIV, or the SARS-associated corona-virus (SARS-CoV) identified as the cause of severe acute respiratory syndrome (SARS); bacteria such as methicillin-resistant Staphylococcus aureus (MRSA); or prions responsible for bovine spongiform encephalopathy (BSE, or "mad cow disease"), scrapie in sheep, chronic wasting disease in wild and domesticated deer and elk, and variant Creutzfeldt-Jakob disease (vCJD) in humans.

Emergence of an infectious agent is a two-step procedure: introduction into a new host species, followed by dissemination into a population. Varied origins include the evolution of a new virus or variant, bacteria, or prion; the introduction from another species (zoonoses); or dissemination from a smaller into a larger population. The latter two are usually the result of some environmental, social, or political disturbance bringing the naive host population into contact with the infectious agent.

Emergence can be illustrated through the case of Ebola virus, a virus of zoonotic origin. In 1995, a Swiss scientist died from Ebola while studying a chimpanzee population in the Côte d'Ivoire. In January of 1996, twenty-nine of thirty-seven confirmed cases of Ebola in a Gabon village were traced to contact with a dead chimpanzee.

Viruses and bacteria often mutate and adapt through the exchange of genetic material that can select for traits such as virulence, adaptability to different host organisms, and resistance to antiviral drugs or antibiotics. Viruses are ephemeral entities that undergo antigenic mutation, and adapt to new ranges of host, or vector.

A viral example that captures many of the characteristics of an EID is influenza caused by the influenza virus. Many influenza epidemics threatened public health throughout the twentieth century. A mutated influenza virus that originated in swine or avian hosts caused the 1918 Spanish flu pandemic, which killed more than twenty million people worldwide. It is thought that mixed variants of human and avian strains of influenza virus caused the 1957 Asian flu and the 1968 Hong Kong flu pandemics. In these cases, a preexisting swine or avian influenza virus either infected human beings directly and became adapted to the new hosts, or else previously-existing human variants of the influenza virus obtained genetic information from animal viruses. In some of these cases, the virulence of the newly adapted influenza virus was great enough to explode in the newly-acquired human host population, expanding throughout the global population.

Between 1998 and 1999, the previously unknown Nipah virus claimed 105 lives and resulted in the slaughter of 1.1 million hogs in Malaysia. Nipah virus exemplifies many of the characteristics of a newly emergent virus. It is carried by flying foxes (Pteropus vampyrus). The emergence of Nipah virus from the flying fox reservoir resulted from environmental changes in the infected hosts' environment. A drought allowed fires, set to clear land, to run out of control, destroying the flying foxes' habitat and food source. Many flying foxes set up new residence in orchards, often run in conjunction with swine husbandry operations. It has been suggested the bats contaminated fruit that was then fed to the pigs, who then infected their caretakers. Changing environmental conditions and agricultural practices create conditions where the hosts of infectious diseases come into contact with new, potential hosts, with sometimes tragic consequences.

Of particular concern to public health is the emergence of infections by bacteria that have developed resistance traits to a variety of antibiotics. Antimicrobials are perceived as essential for combating both human and animal bacterial infections. In 1945, penicillin discoverer Alexander Fleming (1881–1955) warned of the danger of antibiotic resistance when bacteria in his lab developed resistance traits to penicillin through mutations and a process of natural selection. Resistance also develops through the transference of genes from resistant to non-resistant bacteria. An early case of the danger recumbent in the transference of genes for resistance is ampicillin-resistant Haemophilus influenzae and Neisseria gonorrhoeae, which appeared in the early to mid 1970s. Both diseases shared genetic material thought to have been transferred from one species of bacteria to another (Levy 2002).

It is now considered an item of scientific faith that the use of antimicrobials will favor the selection of resistant strains for most bacterial species. Some authors stipulate increasing resistance is the inevitable outcome of antimicrobial use in both human health and agricultural contexts (Levy 2002). Stuart Levy has coined the expression "the antibiotic paradox" to characterize the intertwined promises and threats of antimicrobial use.

Impacts

Antimicrobial resistant bacteria are increasingly ubiquitous, and their costs are immense and growing. One overview of the human health literature on resistance notes MRSA has been reported in community-based infections at rates from twenty to sixty-two percent of cases in the 1990s. This study reports widespread rising resistance to second and third generation cephalosporins in Enterobacter species, suggesting antimicrobial resistance has "become a fact of hospital life and is so common that it often goes unnoted until it is either extreme or epidemic" (Weinstein 1998, p. 215).

A 2001 Center for Disease Control and Prevention (CDC) publication notes "as we enter the 21st century, many important drug options for the treatment of common infections are becoming increasingly limited, expensive, and in some cases, nonexistent."

In a 1969 speech, the U.S. Surgeon General proclaimed that the frontiers of infectious diseases had been reached, remaining problems in the United States were marginal, and that it was the responsibility of the medical establishment to focus on chronic illness. Antibiotics were proclaimed miracle drugs (Levy 2002). They were understood to be an increasingly effective weapon in the armamentarium against bacterial infections and, with the promise of many new viral vaccines, it was believed the technology existed to eradicate disease worldwide.

With a growing awareness of the vastness of epizootic reservoirs of infectious agents, endemic changes in environment and agriculture, increasingly rapid global movement of animal and human populations, and growing problems with antibiotic resistance, this era of optimism is at an end.

Ethics and Policy Issues

One consequence of the emergent character of these diseases is the burden of uncertainty under which policy makers must function—far greater than the uncertainties faced by policy makers dealing with well understood risks such as cigarette smoking or automobile driving. The next EID may be innocuous, or it may be a deadly pandemic. This uncertainty makes it difficult to compare the risks of EIDs to other, more certain public health hazards. Indeed, one reason the media often covers EIDs more closely is because of this uncertainty.

Contemporary efforts to defend against EIDs follow the stages of prevention, detection, and response. Optimal allocation of resources among these stages is a question that has generated much controversy.

Conventional approaches to public health are incapable of preventing many of the factors that are presently increasing the rate of disease emergence. Public health institutions rarely have the resources or the mandates to put a halt to rapid environmental change or to changing patterns of agriculture, let alone to control the increased global movement of human and animal populations.

In the case of resistant bacteria, restrictions and judicious use guidelines on antibiotic use in human and animal health have been suggested as well as a curtailment of their use as growth promotants in the animal industries (Rollin 2001). Rising levels of resistance have fueled a debate over responsibility between representatives of the human and animal medical fields. For example, in 2004 the U.S. Food and Drug Administration was revising its drug approval and labeling procedure for antimicrobials to be used in animal agriculture, and a number of European countries have banned their use as growth promotants under the precautionary principle. The CDC and a variety of private initiatives are instituting educational programs encouraging patients and medical professionals to curtail their use of antibiotics. In the United States as of 2004, there was no legislation to further restrict doctors' prescriptions of antibiotics.

In the United States, responsibility for managing emerging infectious diseases is distributed widely. The CDC often takes the lead, but in instances of food-borne disease, the Food and Drug Administration (FDA) and the Department of Agriculture (USDA) are also involved. In the case of antimicrobial resistance, a U.S. federal government interagency task force initiated in 1999 involved more than eleven agencies and departments.

The implementation of vaccines as a means of prevention is hindered by the contemporary market in pharmaceuticals. Pharmaceutical companies tend to focus research and development monies on profitable repeatable treatments for chronic ailments. Vaccines and antibiotics—which will only be used once or a few times per individual over their lifetime—do not provide the same return on investment. Incentives, regulatory assistance, or an alternative drug research and development system is needed to address these gaps in the preventive armamentarium.

Around the turn of the twenty-first century, greater emphasis has been placed on understanding the role of industrial agri-food practices in the spread of infectious diseases. In the United States, rapid progress in the development of new techniques for managing industrial animal agriculture for food safety concerns have been hindered by the sometimes conflicting mandates of the principal governmental agencies involved in dealing with emerging diseases among food animals, including the CDC, the FDA, and the USDA.

Some policy suggestions have focused on early detection of aberrant syndromes through disease surveillance, on the anticipation of new host and virus interactions brought about by changing ecological and agricultural conditions, and on the control of new diseases through planned response. Similar surveillance tactics have been suggested to deal with antimicrobial-resistant bacteria.

There is limited but growing international coordination of emerging infectious disease surveillance and response. Most surveillance and response systems are national in scope. This includes the CDC, particularly the National Center for Infectious Diseases, which often responds to emerging disease emergencies outside the United States. Another CDC program, jointly run with the USDA, is the National Antimicrobial Resistance Monitoring System (NARMS).

At the international level, there are two institutions of note. The United Nations World Health Organization (UN-WHO) Communicable Disease Surveillance and Response is the principal international organization that identifies, verifies, and responds internationally to epidemics of infectious disease. This organization is overworked and underfunded. Animal diseases are monitored and managed by the Office Internationale des Epizooties (OIE), organized under the World Trade Organization (WTO) to maintain animal health and welfare worldwide. The OIE publishes trade standards on the presence of epizootic diseases, animal health, and food safety that govern the importation of animals and animal products between WTO member countries.

Responses to EIDs are administered by the above agencies and organizations and relevant agencies within a particular nationality's borders. Responses range from quarantine of humans and animals to radical eradication programs such as the slaughter of infected animals, vaccination programs, and mass-treatment with a variety of antiviral and antibiotic drugs.

The development of new antiviral and antibiotic medication suffers from the same market-induced problems as the development of new vaccines. Incentives or the creation of new, perhaps not-for-profit, institutions of drug research and development could alleviate the current dearth in treatment options. The National Institute of Health (NIH) supports research in drug development, but the costs of bringing these new drugs to market are still deemed prohibitive by many pharmaceutical companies.

Quarantines and mass animal slaughter wreak emotional, moral, social, and economic havoc. The 2003 SARS epidemic shut down international trade and travel and damaged the economic lives of cities as far apart as Toronto and Hong Kong. Foot and mouth disease and bovine spongiform encephalopathy (BSE) eradication programs in the United Kingdom resulted in massive animal slaughter, and in movement bans that eventually necessitated the welfare slaughter of even more animals as feed stocks were depleted. This devastated the British rural economy, and seriously affected British agricultural trade with Europe. Quarantines, animal slaughter, and animal movement bans are currently the most effective means of coping with an epidemic, but there is much research that needs to be done on how to lessen the impact of these techniques on the affected populations of humans and animals.

WESLEY DEAN
H. MORGAN SCOTT

BIBLIOGRAPHY

Centers for Disease Control. (1995–). Emerging Infectious Diseases. Unless otherwise cited, all facts in this piece were taken from this flagship journal in the field.

Levy, Stuart. (2002). The Antibiotic Paradox: How the Misuse of Antibiotics Destroys their Curative Powers, 2nd edition. Cambridge, MA: Perseus Press.

Morse, Stephen, ed. (1993). Emerging Viruses. New York: Oxford University Press. Includes three articles of special note: Donald Henderson's "Surveillance Systems and Intergovernmental Cooperation," Morse's "Examining the Origins of Emergent Viruses," and Edwin Kilborne's "After word: A Personal Summary Presented as a Guide for Discussion."

Rollin, Bernie. (2001). "Ethics, Science and Antimicrobial Resistance." Journal of Agricultural and Environmental Ethics 14(1): 29–37.

Weinstein, Robert, and Mary Hayden. (1998). "Multiply Drug-Resistant Pathogens: Epidemiology and Control." In Hospital Infections, 4th edition, ed. John V. Bennett and Philip S. Brachman. Philadelphia, PA: Lippincott-Raven Publishers.

INTERNET RESOURCE

Center for Disease Control. (2001). Preventing Emerging Infectious Diseases: Addressing the Problem of Antimicrobial Resistance, A Strategy for the 21st Century. Available from http://www.cdc.gov.

Emergent Infectious Diseases

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