What are emerging infectious diseases?

Introduction

The term
emerging infectious diseases
was introduced late in the twentieth century by Nobel laureate Joshua Lederberg. Most emerging infectious diseases are of zoonotic (animal) origin, and a variety of insect vectors such as mosquitoes help to spread the infections. While “new” infectious diseases continue to emerge, many of the old plagues remain, often appearing in more virulent and drug-resistant forms. While some outbreaks inexplicably appear, often specific identifiable ecologic factors such as climate change and agricultural development and demographic changes such as urbanization place individuals at increased risk through exposure to unfamiliar microbes or their natural zoonotic hosts. The rise of megacities, with their high population densities, dearth of potable water, and foodstuffs, makes urbanites particularly vulnerable to emerging infectious diseases. Moreover, modern humans have the capability to spread contagious disease halfway around the world rapidly through air travel. Mostly these are global problems and are viewed as global infectious disease threats.

Background

Throughout history, populations have been afflicted by major outbreaks of emerging infectious diseases such as the bubonic plague

or Black Death, a zoonosis caused by the bacterium Yersinia pestis that is spread by fleas that feed off rodents. The Black Plague emerged in the fourteenth century, obliterating a third of the European population within a few years. More deadly than Y. pestis, however, was the variola virus, the etiologic agent of smallpox, which evolved from poxviruses in cattle and emerged into human populations thousands of years ago. Between the fourteenth and the sixteenth centuries, the Spanish conquered Central America, aided in large part by the deadly smallpox epidemic that arose when the disease, previously unknown in the Americas, entered the indigenous populations. In 1980, the World Health Organization (WHO) declared that smallpox had been eradicated. However, in 2003, as the United States entered into war with Iraq, US president George W. Bush decreed that the armed forces be vaccinated against smallpox in anticipation of a bioterrorism attack. This pronouncement came on the heels of similar attacks in the United States wherein anthrax
infection caused by Bacillus anthracis was intentionally spread in Florida and New York. In 2009, another bacterial infection, methicillin-resistant Staphylococcus aureus (MRSA), continued to emerge due to the apparent overuse of the antibiotic methicillin.

Influenza, known as a human disease for hundreds of years, emerged in the pandemic of 1918 that killed 20 to 40 million persons, more than died in World War I. Other influenza
pandemics included the Asian flu in 1957, the Hong Kong flu in 1968, and swine flu in 1977. Avian influenza, or bird flu, previously rare in humans, emerged in 2004. The H1N1 influenza
(formerly called swine flu) emerged in Mexico in 2009 and rapidly spread throughout the Northern hemisphere, infecting millions in the United States alone and countless others across the globe, sickening and killing a disproportionate number of young children and pregnant women and rarely affecting those over age sixty-five. Human immunodeficiency virus (HIV), a retrovirus, is the causative agent of Acquired immunodeficiency syndrome (AIDS), one the world’s deadliest infectious diseases; according to the WHO, about 2 million people die from AIDS-related diseases each year, more than from non-AIDS-related tuberculosis or malaria. Multidrug-resistant tuberculosis
(MDR-TB) and extensively drug-resistant tuberculosis (XDR-TB) continued to emerge in the early twenty-first century, especially in those living with HIV infection. Malaria, a parasitic scourge, causes between 600,000 and 700,000 deaths every year, with 90 percent of deaths occurring in sub-Saharan Africa. Malaria continues to emerge with strains of its most lethal species, Plasmodium falciparum, which is resistant to antimalarial drugs.

Examples

Three diseases serve as important recent examples of emerging infectious diseases: H1N1 influenza, HIV/AIDS, and resistant tuberculosis.

H1N1 influenza. A zoonotic disease resulting from a mix of swine, avian, and human flu viruses, H1N1 influenza emerged in the United States following the regular 2008–2009 flu season, during which influenza A (H1), A (H3), and B viruses all circulated. In mid-April 2009, the Centers for Disease Control and Prevention (CDC) documented the first two cases of influenza A pandemic (H1N1) in the United States; after September 1, 2009, the CDC characterized the antigens of collected flu viruses: 412 2009 influenza A (H1N1); 4 influenza B; 3 influenza A (H3N2); and 1 seasonal influenza A (H1N1). In December 2009, 99 percent of flu strains were composed of pandemic H1N1 2009. According to the WHO, as of November 22, 2009, there were more than 40,617 “confirmed and probable” cases of pandemic H1N1 2009, and 7,826 deaths worldwide; by May of 2010, the estimated number of deaths had risen to more than 18,000. However, these statistics are significantly lower than the actual morbidity and mortality because they are based on limited data. Moreover, pandemic H1N1 2009 infections in which the virus mutated to a strain more virulent were identified across the globe. In addition, a number of patients developed strains of H1N1 resistant to the antiviral oseltamivir (Tamiflu). In August 2010, the WHO announced that the pandemic period had ended; although outbreaks were expected to continue to occur, the virus was no longer considered a major threat.

HIV and AIDS. Since HIV, the virus that causes AIDS, was first isolated in the early 1980s, the virus has continued to emerge into new populations and new geographic locations while morphing into new strains and variants, becoming resistant to available antiretroviral therapies (ART). Therefore, new drugs and combinations of old and new therapies must continually be produced to help keep alive the more than 34 million people living with HIV and AIDS. Moreover, while HIV infection is now treated like a chronic disease in many developed countries, many developing nations continue to struggle to obtain adequate drugs and preventive programs to treat all those infected with the virus. Despite the advent of highly active antiretroviral therapy (HAART) in 1996, a range of comorbidities continues to plague those living with HIV and AIDS, including liver disease (hepatitis B and C), non-Hodgkin’s lymphoma, neurological illnesses, malignancy, malnutrition, and increased susceptibility to TB and MDR-TB. Nevertheless, according to the Joint United Nations Programme on HIV/AIDS (UNAIDS), the number of HIV/AIDS-related deaths decreased by more than 500,000 between 2005 and 2011, and the number of new HIV infections decreased by 700,000 between 2001 and 2011.

MDR-TB/XDR-TB. Mycobacterium tuberculosis strains resistant to multiple drugs represent an emerging threat to the global control of both tuberculosis and HIV, which often coinfect patients. The WHO estimates that between 220,000 and 400,000 cases of MDR-TB emerged in 2011. MDR-TB is defined as resistance to a minimum of the anti-TB drugs isoniazid and rifampin. While HIV may or may not be directly associated with the risk of developing MDR-TB, nosocomial outbreaks in individuals living with HIV and AIDS have been noted. HIV/AIDS has also been linked to an increased risk for rifampin-monoresistant TB. In addition, new cases of XDR-TB, defined as MDR-TB resistant to a fluoroquinolone and a minimum of one second-line injectable agent, have been widely reported across the globe. Treatment of MDR-TB is complex, sometimes requiring the use of less effective and more toxic drugs that mandate treatment over longer periods of time, thereby lessening the chance of successful outcomes and posing serious problems for developing countries, especially those with a high prevalence of HIV-1 infection. MDR-TB and XDR-TB are also of concern in wealthier countries where massive immigration and global travel is commonplace.

Bibliography:

Garrett, Laurie. Betrayal of Trust: The Collapse of Global Public Health. New York: Hyperion Books, 2001.

Global Alert and Response. "Pandemic and Epidemic Diseases." World Health Organization, 2013.

Global Alert and Response. "Pandemic (H1N1) 2009—Update 103." World Health Organization, June 4, 2010.

Hill, Stuart. Emerging Infectious Diseases. San Francisco: Benjamin Cummings, 2005.

Leslie, T., et al. “Epidemic of Plasmodium falciparum Malaria Involving Substandard Anti-malarial Drugs, Pakistan, 2003.” Emerging Infectious Diseases 15 (2009): 1753–59.

MacPherson, D. W., et al. “Population Mobility, Globalization, and Antimicrobial Drug Resistance.” Emerging Infectious Diseases 15 (2009): 1727–32.

National Center for Emerging and Zoonotic Infectious Diseases. "Fighting Emerging Infectious Diseases." Centers for Disease Control and Prevention, March 12, 2012.

National Institute of Allergy and Infectious Diseases. "Emerging and Re-Emerging Infectious Diseases." National Institutes of Health, August 24, 2010.

UNAIDS. "World AIDS Day Report 2012." Joint United Nations Programme on HIV/AIDS, November 2012.

World Health Organization. "Factsheet on the World Malaria Report 2012." World Health Organization, December 2012.

World Health Organization. "HIV/AIDS." World Health Organization, 2013.

World Health Organization. "Multidrug-Resistant Tuberculosis (MDR-TB) 2012 Update." World Health Organization, November 2012.