What are epidemics and pandemics?
Epidemics, those caused by old diseases that have been around for centuries or newly identified diseases, break out regularly in the human population. Whether they become full-scale pandemics depends on several factors, including how contagious the pathogen; the number of pathogens needed to initiate a disease; how the pathogen is transmitted; the period during which a person is infectious before and after symptoms appear; how long the pathogen can survive in the environment; if an intermediate or alternate host for the pathogen exists; whether vaccines are available, effective, and have been widely used; whether there are any drugs or medications to treat the disease; and whether the contagion can be isolated and contained. Some examples can illustrate these points.
Ebola, one of the most deadly diseases known, is a hemorrhagic fever first identified in 1976 along the Ebola River in the Democratic Republic of the Congo (formerly Zaire). It is caused by a filovirus, a negative-sense single-stranded RNA virus that, in the electron microscope, looks like string, or thread, or filament. Infection leads to a disruption of connective tissues, including that of the blood vessel wall. This results in hemorrhaging from all orifices of the body and rapid death. The death rate for Ebola hemorrhagic fever is greater than 80 percent. There is no treatment or cure. Fortunately, the virus is not airborne; it is transmitted only by direct contact with contaminated bodily fluids. There is an alternate host, believed to be another mammal, possibly bats. Thus periodic, local outbreaks occur. However, despite it being extremely deadly, Ebola can be contained by isolation. It has never led to a widespread epidemic or pandemic.
Severe acute respiratory syndrome (SARS) is another example of a recent disease that did lead to an epidemic. SARS was first identified in Guangdong Province, China, in 2003. It is caused by a coronavirus, a positive-sense RNA virus that, in the electron microscope has a ring of viral spikes somewhat like a crown, from which this type of virus gets its name. The virus infects the epithelia of the upper respiratory tract. The symptoms are similar to influenza (fever, headache, muscle ache, fatigue, coughing, sneezing). However, in a significant number of cases, the lungs become involved. This causes severe respiratory distress. There is no cure for SARS, and the disease needs to run its course. The body will produce antibodies against the virus. Antivirals can reduce the severity, and when appropriate, antibiotics are used to treat secondary bacterial infections. Nevertheless, the death rate is estimated at about 10 percent, but more than 50 percent in those over sixty-five.
The disease is spread by virus-containing droplets from coughing or sneezing. Unlike influenza, however, fairly close contact is needed, and the virus does not remain viable in the environment for more than a few hours. SARS quickly spread to twenty-nine countries in 2003 because of air travel, but a rapid and effective global public health response with isolation of SARS cases prevented the disease from becoming a severe pandemic. Although SARS was widespread, a total of only 8,096 cases worldwide were reported to the World Health Organization (WHO); the organization declared SARS to be eradicated in 2005. Because the virus can infect mammals and birds, however, some scientists believed that new outbreaks of the disease could occur in the future.
In 2009, a pandemic of the H1N1 influenza virus broke out. Initially believed to have begun in Mexico, later reports indicated that it most likely developed in Asia. The swine flu, as it was called, claimed 18,500 lives, according to the World Health Organization, which declared an official end to the outbreak in August, 2010.
In contrast, a disease becomes a pandemic when it is both widespread and affects a significant percentage of the population. The best example is the seasonal influenza (flu). The well-known symptoms are fever (often elevated), head and general body aches, sore throat, cough, nasal congestion, and fatigue. It may or may not be accompanied by diarrhea and/or vomiting, symptoms more often seen in young children. It usually runs its course in a week to ten days. Influenza is caused by orthomyxoviruses, which in the electron microscope appear as a spherical or ovoid particles, or occasionally as a filament. These viruses have a negative-sense, single-stranded RNA genome, but it is segmented into several RNA molecules. Infection reaches a peak during the winter months (December through March in the Northern Hemisphere, June through September in the Southern Hemisphere). Estimates indicate that 5 to 30 percent of the world population may become infected in any given year. The reasons that flu pandemics are so common is because flu viruses are highly contagious, survive in the environment for long periods, and frequently mutate so that immunity to one strain of influenza virus does not protect against the same but mutated virus in the future. Moreover, influenza viruses can infect other organisms, which act as a reservoir and in which the virus can mutate or recombine with other coinfected influenza viruses, producing a new strain. The mortality rate for seasonal influenza is about 0.1 percent, usually in the very young, the elderly, and those with other underlying medical conditions such as asthma, diabetes, or cardiovascular disease. Flu vaccines are effective in preventing or reducing the severity of the disease, if they have been made against the type of influenza virus circulating in the world that year. Antiviral medications such as Tamiflu reduce severity of the disease.
Treatment and therapies for various contagions will depend on the specific disease, but they usually includes antibiotics for bacterial infections and antiviral medications for viral diseases.
Epidemics and pandemics have plagued humans throughout history. Approximately forty thousand years ago, humans began to domesticate plants and animals for food. Clearing land for crops exposed them to new pathogens, and the close proximity to domesticated animals allowed for transmission of animal pathogens to people. Ancient diseases including smallpox, tuberculosis, measles, and influenza probably arose from animal pathogens adapting to humans. In more recent times, human immunodeficiency virus (HIV) probably adapted to humans in the past fifty years and can be traced to similar simian viruses (SIV) of the chimpanzee and the sooty mangebey monkey. Monkeys are common pets in Africa. Chimpanzees have been used as bush meat, and the transmission of SIV probably occurred during butchering of the animal. The gathering of ancient peoples into larger groups and forming villages with close proximity of inhabitants allowed for pathogens to spread easily. Later, diseases spread along trade routes from Asia and Africa to Europe. Wars have always contributed to the spread of disease as a result of unsanitary conditions, malnutrition, rape, susceptibility of the new population to the pathogen, and lack of sufficient medical care.
History records many epidemics and pandemics. Homer wrote of pestilence devastating the Greeks in the siege of Troy. Thucydides described the Athenian plague of 430-427 BCE Hippocrates, the so-called father of medicine, has one of the earliest descriptions of an influenza epidemic in 412 BCE The Bible records pestilences in the books of Numbers and in Exodus, the latter most likely being an epidemic of Black Plague in Egypt. In Rome, the physician Pliny the Elder described epidemics of smallpox (165–85 CE) and measles (251–66). Probably the best-known pandemic is the Black Plague that devastated Europe in the fourteenth century. In December, 1347, three spice ships from China disembarked at Messina, Italy, carrying plague-infected rats. The plague spread quickly throughout Europe. Ultimately an estimated 25 million died of plague by 1355, amounting to as much as 42 percent of the European population. This pandemic led to great social changes, including the end of the feudal system and the rise of the middle class. Lesser epidemics of plague reappeared in the seventeenth century in Europe and each century since, including one of pneumonic plague in India in the 1990s.
The greatest pandemic in recorded history is that of the influenza pandemic of 1918. Indeed, 80 percent of all Americans who died in Europe during World War I died of influenza, not in direct combat. The 50 to 100 million people worldwide died of what has been called the Spanish flu not because it originated in Spain but because Spain was particularly hard hit by the pandemic. This virus, a mixture of avian and human influenza components, was one hundred times more lethal than the typical seasonal flu. This has caused concern over a reemergence of a highly contagious form of avian flu.
Since about 1980, many new or reemerging diseases, or old pathogens that have developed drug resistance, have been identified. They include Lyme disease; Ebola hemorrhagic fever; Marburg hemorrhagic fever; Lassa fever; Legionnaire’s disease; toxic shock syndrome; Acquired immunodeficiency syndrome (AIDS), caused by HIV; hantavirus pulmonary syndrome; E. coli O157:H7, a Shigella toxin-producing strain of E. coli; variant Creutzfeldt-Jakob disease (vCJD), the human equivalent of mad cow disease; hepatitis C; cryptosporidiosis; cyclospiridosis; Whitewater arroyo virus; enterovirus 71; hendra virus; West Nile virus; Malaysian Nipah virus; human monkeypox; pneumonic plague; SARS; avian flu; methicillin-resistant Staphlococcus aureus (MRSA); vancomycin-resistant Staphlococcus aureus; drug-resistant malaria; and multi-drug resistant tuberculosis.
Finally, there is great concern that many of these highly contagious and deadly agents could be “weaponized” and used as biological agents for bioterrorism. Moreover, using biotechnology, new viruses with the contagion of smallpox or influenza along with the lethality of something like Ebola could be developed.
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