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Role of Emerging Infectious Diseases

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    Microbiologist and Nobel Prize winner Joshua Lederberg first coined the phrase “emerging infectious diseases” to describe infections that were newly appearing in a population or having existed but rapidly increasing in incidence or geographic range. 1 Emerging diseases can be attributed to either true emergence (i. e. a newly appearing pathogen that has not been present), increased recognition (i. e. pathogen that was present in a population but has only recently been recognized due to improved diagnostic testscapabilities), and increased incidence of previously recognized diseases mainly due to increased globalization, travel, urbanization, and susceptibility of host population4. When there is a disturbance to the natural environment, for example logging and urbanization, pathogens and reservoirs are disturbed and displaced leading to emergence in the human population and increased infection rates.

    Many emerging viral diseases are caused by some variety of RNA viruses, which compared to DNA viruses, have a higher susceptibility to mutation. For example, influenza is caused by RNA viruses of the family Orthomyxoviridae and known for strain mutation year after year, thus the need for reformulation of the vaccine for what will hopefully be the most common strains to infect people that particular season. Though the factors responsible for disease emergence can often be recognized, intervention and defense against these diseases has proven to be difficult.

    The definition of emerging disease has evolved rapidly over the last few decades. In 1992, the Institute of Medicine released a statement defining “emerging infectious diseases [as] diseases of infectious origin whose incidence in humans has increased within the past two decades or threatens to increase in the near future. 2 The IOM definition was too restrictive, not accounting properly for animal and zoonotic diseases and their impact on the public. Even though specific factors for disease emergence can be identified in most cases, not all causes of emerging or re-emerging disease are due to infection alone.

    There is a perfect trifecta of disease between host, agent, and environment. For a disease to be introduced and maintained in a population, the host has to be susceptible to colonization and infection, the agent has to be infective for the host, and the environment in which host and agent are interacting has to be conducive to providing effective contact between the host and agent. The Black Death of the 14th century killed millions along the Silk Road from Asia to Europe. Roughly one-third of the population of Europe succumbed to Yersinia Pestis carried by fleas that were harbored by rats aboard passenger ships. Crowded ships, emerging trade routes, and increased urbanization created a breeding ground for disease, in this case the perfect vector carried the agent which infected a susceptible host. Emerging diseases are commonly recognized with their regard to impact on populations: high morbidity, mortality, or dramatically high incidence of clinical disease. In the case of the Black Death, Y. pestis devastated the world’s population and made its mark in history. Rarely if ever do emerging diseases occur for no reason.

    There are a multitude of factors responsible for disease emergence: ecological changes, human behavior, increased travel and globalization, and microbial mutations/adaptations to name a few. Antimicrobial resistance is the ability of a microorganism to resist the effects of an antimicrobial agent either due to intrinsic characteristic, acquired selection for mutation, or by acquisition of a resistance gene from other microorganisms. 5 Not only have inappropriate prescribing of broad spectrum antibiotics fostered the growth of resistant organisms, but also widespread misuse of antibiotics by the public.

    The notion that a pill will cure all disease, even viral infections which should be treated symptomatically, introduces bacteria to antibiotics but at concentrations where they can learn to adapt and mutate to become superbugs. MRSA, methicillin-resistant Staphylococcus aureus, evolved from pathogens adapting to beta-lactam antibiotics and becoming resistant to their mechanism. One of the key elements in preventing MRSA infections and outbreaks is simple techniques such as decontaminating surfaces and proper hand hygiene. Many times the strategy to combat disease means going back to the basics.

    Globalization, the flow of people and trade, has been on an exponential rise in recent years. With the advent of modern transportation and technologies, people, products, and resources circle the globe in a matter of minutes. With such interdependency and connectivity, our “new world” has created a huge opportunity for the global spread of disease. Diseases previously endemic to one area are now spreading from place to place as people travel quicker and more frequent to places not previously seen, unknowingly bringing disease back with them.

    In 1999, West Nile Virus, previously endemic to tropical and temperate regions of the world, was found to have infected people in New York City. Though the exact route of transmission is still unknown, a suspected scenario is infected frogs flown into the country were bitten by exotic Asian mosquitoes that had hitchhiked from Asia to New York in a shipload of used tires. The mosquitoes then went on to bite animals and people, who in turn have fallen ill. Global trade in used tires and a shipping process called containerization were bringing mosquitoes that spread tropical diseases to America.

    The current era of globalization is an amplified version of the trends in history. People have always traded commerce; they have migrated from region to region, but never at such an expeditious pace. Despite the efforts of modern medicine to identify and eradicate infectious disease, a significant number of unknown and underestimated pathogens have emerged as a serious health risk to the world’s population. Many pathogens may be traced back to a zoonotic source, traveling via vector and infecting humans causing severe outbreaks.

    The ability of an infectious agent to adapt to changing environmental conditions and variations in human behavior, population development, nutrition, education, social, and health status are relevant factors affecting the correlation between pathogen and host. The 1993 hantavirus outbreak in the Four Corners of the United States was caused by the Sin Nombre virus carried by deer mice. Having antibodies to the virus, mice spread the virus via urine and feces which were inhaled or by biting an unsuspecting intruder to their environment. The initial discovery of hantaviruses began with an outbreak among US soldiers in Vietnam in the 1970.

    Many quickly fell victim to what become known as Hantaan virus while walkingwading through rice fields, breeding grounds for rodents and mosquitos. Each transfer of hantaviruses from their original natural hosts to other incidental hosts was accompanied by a change of ecology, a change of environment, a modulation of numerous factors probably influencing the pathogenicity and virulence of the virus. Increased globalization, travel, and urbanization have led to our world becoming “one nation”. A global approach to disease surveillance is necessary as diseases travel and spread s often as people do. Since the mid-1990s regional networks have been established, however no global surveillance networks have been established. The CDC has worked over the years to help other countries identify outbreaks, pathogens, and implement relief plans. However, the CDC alone cannot monitor the world. If resources could be made available, such as laboratories and personnel, an established global surveillance would be able to guide global health initiatives. Vaccine programs and safe water practices could be better focused and utilized.

    The practice of medicine could hopefully shift from reactive to proactive medicine. Increased travel among people and animals, and therefore pathogens and vectors, has led to the spread of diseases to non-endemic areas. When outbreaks occur, the differential diagnosis needs to be broader as in the case of 1993 hantavirus outbreak in the Four Corners region of the US. Hantavirus previously endemic to China and the Far East and was not thought be in consideration when people started to fall sick and die. It took weeks before the Sin Nombre virus was identified as the culprit.

    In the end, prevention is the key to stopping the emergence and spread of disease. By creating a global surveillance network, there will be greater monitoring of disease and disease patterns and hopefully better communication worldwide so if an outbreak does occur, it can be contained in a timely fashion. “Public healthcare infrastructure is the backbone of any efficient public health activity. ” Some of the key components of the public health system are trained personnel, available and proper diagnostic services, and advanced communication methods to rapidly dispense information.

    Without proper systems to respond to outbreaks and promote infection control, in addition to creating strategies for preventing disease emergence andor spread, diseases would run rampant in the world affecting millions upon millions. Also with advanced modes of communication such as email, texting, etc. we have the capability to disseminate important information in the matter of seconds. A recent article by Microsoft Research highlighted valiant efforts made by a group from University of Melbourne in war-torn Mozambique.

    Though few tools and resources are available in remote areas of Africa, many health care works do carry cell phones. Jim Black and his colleagues developed an inexpensive smartphone-powered “oximeter” that would give health care workers a front-line tool in diagnosing pneumonia vs. malaria vs. other febrile diseases. If public health components could be aligned, national, regional, and global surveillance networks could come to fruition. Bringing together the world’s expertise in different areas and working as one group would benefit all parties involved.

    Not all emerging diseases are caused by novel pathogens. As winter approaches, many fear the dreaded flu – the headache, fever, chills, malaise, and occasionally nausea accompanied by the loss of work (thereby income) and productive time. The 2009 H1N1 outbreak showed not only the medical devastation the virus can create but also financial and emotional. The United States experienced its first wave of 2009 H1N1 activity in the spring of 2009, followed by a second, larger wave of 2009 H1N1 activity in the fall and winter, during typical “flu season’ in the US.

    Since the flu vaccine is dependent on the age and health status of the person getting vaccinated and the similarity between the virus strains in the vaccine and strains which -are circulating and infecting, it became difficult to protect those who were immune-compromised or naive (i. e. children)10. Key factors in decreasing the spread of infection are basic precautionary measures such as hand washing and decreasing sick contacts. Though maybe not as devastating in terms of mortality, the flu has greater impact on the United States as a whole.

    It knows no boundaries in terms of who it infects and severity of disease can range from mild symptoms to death. The ubiquitous nature of influenza and its ability to mutate into a variety of unknown strains makes the virus a constant re-emerging disease in the United States. In my opinion, this constant nature makes influenza a significant infectious disease. Tuberculosis (TB) is defined by the CDC as a disease “caused by a bacterium called Mycobacterium tuberculosis which usually attacks the lung but can attack any part of the body such as the kidney, spine, and brain11”.

    TB ranks as the second leading cause of death from an infectious disease worldwide, after HIV. 13 Tuberculosis has been present in humans since antiquity, though in recent years due to severe drug resistant, it is making a name for itself as re-emerging infectious disease due to its rate of mortality. As an air-borne pathogen, spread of disease is easily aided by over-crowded cities and lack of education in regards to transmission. Approximately 60% of the global burden of MDR tuberculosis cases is in Russia, China, and India12.

    In many cases, these countries are ill-equipped, either with lack of resources and/or finances, leaving people with inadequate treatment options. Increased travel and globalization have led to TB moving around the world at rapid pace. Lack of education as to the spread of disease (like many other diseases) and lack of global surveillance leave tuberculosis as a contender for most significant impact by an emerging disease worldwide. A pandemic is defined as “an epidemic occurring over a very large geographic area”. 4 Looking over the course of history, the common features of most pandemics are easily transmittable (mostly airborne), longer incubation periods giving time for the pathogen to infect other susceptible hosts without symptoms present in index cases, and non-specific symptomology leading to a longer list for differential diagnosis. The next pandemic will likely be caused by a pathogen which will easily mutate (i. e. type of RNA virus) and spread easily between species and among humans. These characteristics will let the disease remain rampant while we search for clues as to the origin and treatment, and then subsequent prevention. If fingers must be pointed, I would say a strain of influenza.


    1. Morse SS, Schluederberg A. Emerging viruses: the evolution of viruses and viral disease. J Infect Dis 1990; 162:1-7. 2. Institute of Medicine. Emerging Infections: Microbial Threats to Health in the United States. Washington, D. C. : National Academy Press, 1992. 3. Rollins S, et al. Yersinia Pestis and the Plague. Am J Clin Pathol 2003; 119(Suppl 1):S78-S85. 4. Morse, S. Factors in the Emergence of Infectious Disease. Emerg Infect Dis 1995; 1(1):7-14. 5. “Antimicrobial resistance. ” medical-dictionary. thefreedictionary. com, 2012. Web. 20 Nov 2012. . Nash D, Mostashari F, et al. The Outbreak of West Nile Virus Infection in the New York City Area in 1999. NEJM 1999; 344(24):1807-18112 7. Sheedy JA, Froeb HF, Batson HA, et al. The clinical course of epidemic hemorrhagic fever. Am J Med. May 1954; 16(5):619-28. 8. Lee HW, Baek LJ, Johnson KM. Isolation of Hantaan virus, the etiologic agent of Korean hemorrhagic fever, from wild urban rats. J Infect Dis. Nov 1982; 146(5):638-44. 9. Center for Disease Control. (2011). Evalution of and Recommendations for the National Notifiable Disease Surveillance System Within the Federal Centers for Disease Control and Prevention.

    Washington DC. Retrieved from http://www. cdc. gov/osels/phsipo/dndhi/docs/pdf/NNDSS-Evaluation-Report-FINAL. pdf 10. http://www. cdc. gov/flu. Accessed 18 Nov 2012. 11. http://www. cdc. gov/tb/topic/basics/default. htm. Accessed 2 Nov 2012 12. Dalton, T. et al. Prevalence of and risk factors for resistance to second-line drugs in people with multidrug-resistant tuberculosis in eight countries: a prospective cohort study. The Lancet; 80 (9851): 1406-1471. 13. http://www. who. int/ tb/publications/factsheet_global. pdf. Retrieved 19 Nov 2012. 14. http://www. cdc. gov/vaccines/about/terms/glossary. htm. Accessed 13 Oct 2012.

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