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Usually the term endemic is applied to a discrete geographical unit, sometimes a country or other defined area. For example, malaria is endemic in many of the hotter regions of the world. Hence, in medical terms it means a disease that is unique to that place or region, not found naturally elsewhere. Some emerging infectious diseases such as Lyme disease have recently become endemic in the USA,
Epidemics occur when there are sudden increases in frequency above endemic levels. The term outbreak is another word for epidemic. Epidemics may be restricted to one locale or may even be global. An outbreak of a disease is defined as being epidemic, however, not by how many members or what proportion of the population it infects, but by the speed that it is spreading.
When each infected individual is infecting more than one other individual, so that the number of infected individuals is growing exponentially, the disease is in an epidemic level. Thus, even if the number of people affected is small, the phenomenon may still be called an epidemic.
A pandemic occurs over large geographical areas (usually worldwide) and affects a high proportion of the population whereas an epidemic, although it may be serious, usually refers to what is happening in one country or region. A pandemic may be thought of as an “epidemic of epidemics.”
Yes. Technically it was considered to be a mini pandemic because it peaked between May and July 2003. It was more of an epidemic on the cusp of being a pandemic. This is because there were a total of 8096 probable cases and 774 deaths. Sixty-six per cent of the cases occurred in China, 22% in Hong Kong, 4% in Taiwan and 3% in both Singapore and Canada. Twenty-one per cent of all cases occurred in healthcare workers. (pubmed, The chronology of the 2002-2003 SARS mini pandemic, Cherry JD, Nov. 5, 2004)
Throughout history, there have been a number of significant pandemics. There have been a number of particularly significant epidemics that deserve mention for more than the destruction of cities:
During the Peloponnesian war, 430 BC an unknown agent killed a quarter of the Athenian troops and a quarter of the population over four years. This disease fatally weakened the people of Athens but the sheer virulence of the disease prevented its wider spread; i.e., it killed off its hosts at a rate faster than they could spread it.
In 212 BC during Roman wartime, when those tending to the ill also caught a disease, funerals traditions were abandoned, and the dead left where they were.
The Antonine plague occurred 165-180 BC. Disease, possibly smallpox, brought back from the Near East killed a quarter of those infected — up to a total of five million in all. At the height of a second outbreak (251-266) 5,000 Roman people a day were said to be dying.
The plague of Justinian 541 AD started in Egypt and spread to Constantinople. It killed (according to the Byzantine chronicler Procopius) 10,000 people a day at its height and perhaps 40 percent of the city’s inhabitants. It continued until it had destroyed up to a quarter of the human population of the eastern Mediterranean.
Bubonic plague (the Black Death) of the 1300’s occurred eight hundred years after the last outbreak with Europe experiencing the return of the Bubonic plague. The disease reached Mediterranean and Western Europe in 1348 and killed twenty million Europeans in six years, a quarter of the total population and up to half of the population in the worst affected urban areas.
Cholera 1816-1961 (seven pandemics) evolved into El Tor (the dominant strain) in the seventh global pandemic. It first appeared in Bengal and then spread across India by 1820. It extended as far as China and the Caspian Sea. The second wave of the pandemic (1829-1851) reached London, Europe, and New York in the same year, 1832, and the Pacific coast of North America by 1834. The third through the sixth pandemic waves occurred 1852-1923 and caused more than a million deaths spreading throughout Europe and Africa. The seventh pandemic began in 1961, came to Indonesia, India and Russia in 1966. It is distinguished from the classic strain at a genetic level first identified in 1905 at a camp in El Tor, Egypt.
In the final year of World War 1, 1918-1919 the Spanish flu began spreading on both sides of the European front. By the end of the flu more than 50 million people had died. More people died from the influenza than those who died in the war.
It is thought to have originated in the United States in February 1918. The second wave (Autumn 1918) was much more lethal, with unusually high mortality rates, especially among young otherwise healthy adults. It was a vicious influenza. Within a few hours after symptoms appeared cyanosis developed and, after another few hours, death occurred. Ordinances made it illegal to spit, cough, or sneeze in public — with threat of $500 fines in New York City. When people went out they wore gauze masks over their nose and mouth, often soaked in camphor or other medicinal substances. It vanished within 18 months and the actual strain has yet to be determined.
A 1957-58 pandemic, identified as “The Asian Flu,” originated in China in February 1957. It spread across the globe in 6 months. A second wave emerged across multiple countries in early 1958, with severity differing by region.
H2N2 1968-69 pandemic of “The Hong Kong Flu” mortality figures appear to be above the 1 million mark (H3N2) according to the WHO (World Health Organization). The H2N2 and H3N2 denote different subtypes of influenza A. (See discussion on types and strains to see what H and N mean in relation to each other.) In brief, H and N are subtypes, and are the basis for flu vaccine. The normal flu that circulates yearly has the same H and N as previous years. A flu pandemic happens when a new influenza A virus circulates with a different H and/or N from previous years.
In 1976 an outbreak of a Swine related strain of flu at Fort Dix in February led to plans for a massive vaccination program in the USA. Over 40 million people were vaccinated, starting in October. The program was suspended on the 16th of December following reports of Guillain-Barré Syndrome (GBS) in recipients.
1977 pandemic of “Russian Flu” was first detected in November 1977, and spread across Siberia and Europe. (Thought to be the A/H1N1).
1997 Avian Flu Scare (A/H5N1) — see discussion on types and strains.
1999 Avian Flu Scare (A/H9N2) — see discussion on types and strains.
The World Health Organization has developed a plan for influenza preparedness that defines the stages of a pandemic and makes recommendations for preparations. The stages are as follows:
Phase 1: No new influenza virus subtypes have been detected in humans.
Phase 2: No new influenza virus subtypes have been detected in humans. However, a circulating animal influenza virus subtype poses a substantial risk of human disease.
Phase 3: Human infection(s) with a new subtype, but no human-to-human spread, or at most rare instances of spread to a close contact.
Phase 4: Small cluster(s) with limited human-to-human transmission but spread is highly localized, suggesting that the virus is not well adapted to humans.
Phase 5: Larger cluster(s) but human-to-human spread still localized, suggesting that the virus is becoming increasingly better adapted to humans, but may not yet be fully transmissible (substantial pandemic risk).
Phase 6: Increased and sustained transmission in general population.
Note, as of this writing, we are in the influenza pandemic alert phase at phase 3.
Pandemics have happened throughout history as seen in this summary.
Viruses change in two ways. One way is “antigenic drift” which occurs by small, but continuous, changes in the virus. Antigenic drift produces new virus strains that may not be recognized by antibodies of earlier influenza strains.
Antigenic drift is due to mutations in the RNA that lead to changes in the antigenic character of the H and N molecules (discussed in greater detail later). Antigenic drift involves subtle changes that may cause epidemics, but not pandemics. Antigenic drift is a natural mutation over time to evade the immune system. Antigenic drift occurs in all types of influenza including influenza A, B and C.
Gradual changes in an already circulating virus typify antigenic drift. Eventually the virus changes enough so that most of the population is susceptible to re-infection. There is then an influenza epidemic. Size and severity of the epidemic is dependent on the degree to which the virus is different from those already experienced by the population.
An example might be: a person infected with a particular flu virus strain develops antibody against that virus. As newer virus strains appear, the antibodies against the older strains no longer recognize the ‘newer’ virus, and infection with a new strain occurs. This is one of the main reasons why people can get the flu more than one time.
The other type of change is called “antigenic shift”. Antigenic shift is the process by which two different strains of influenza combine to form a new subtype having a mixture of the surface antigens of the two original strains. Antigenic shift occurs only in influenza A because it infects more than just humans.
Antigenic shift is an abrupt, major change in the influenza A viruses that results in a new influenza virus that can infect humans and has a hemagglutinin protein or hemagglutinin and neuraminidase protein combination that has not been seen in humans for many years. Antigenic shift can happen in three ways:
Antigenic Shift 1. A duck or bird passes a bird strain of influenza A to a new host such as a chicken or pig. A person passes a human strain of influenza A to the same chicken or pig. When the viruses infect the same cell, the genes from the bird strain mix with genes from the human strain to yield a new strain. The new strain can spread from the intermediate host to humans.
Antigenic Shift 2 and Shift 3. Without undergoing genetic change, a bird strain of influenza A can jump directly from a duck or other aquatic bird to humans. Influenza A can also jump directly from a duck or aquatic bird to an intermediate animal host and then to humans.
Antigenic shift is due to rearrangement of different segments of the viral genome that produces major changes in the antigenic character of the H and N molecules. Antigenic shift usually occurs in animal hosts and is responsible for producing both epidemics and pandemics. The host population has no immunity against a new sub-type and an influenza pandemic follow. Antigenic shift is of public health concern as subtypes from different species may then be able to infect humans.
* Graph created by Anne Gordon based on written information
Influenza is a viral infection of the lungs. Most often it is characterized by fever, cough, and muscle aches. Influenza is not the gastrointestinal upset often referred to as “stomach flu”. The name influenza has its origin in early fifteenth century Italy and was adopted in Europe to explain the sudden appearance of an epidemic disease thought to be under the influence of the stars.
Flu strains are named after their types of hemagglutinin and neuraminidase surface proteins, and therefore are called H and N’s. For example, H5N1 would be a type 5 hemagglutinin, and type 1 neuraminidase. If two different strains of influenza infect the same cell simultaneously, their protein capsids and lipid envelopes are removed, exposing the RNA that is then transcribed to DNA. The host cell then forms new viruses that combine antigens. An example of this would be if H3N2 and H5N1 combined; they could form H5N2. The immune system would not recognize the new strain. That is what happened historically with other pandemics.
Influenza A viruses are found in many different animals, including birds, ducks, chickens, pigs, whales, horses, and seals. Influenza B viruses circulate widely only among humans. There are 15 different hemagglutinins subtypes to date, and 9 different neuraminidase subtypes that have been found among influenza A viruses in wild birds.
Wild birds are the primary natural reservoir for all subtypes of influenza A viruses and are thought to be the source of influenza A viruses in all other animals. Most influenza viruses are asymptomatic or cause mild infection in birds; however, the range of symptoms in birds varies greatly depending on the strain of virus.
Infection with certain avian influenza A viruses (for example, some strains of H5 and H7 viruses) can cause widespread disease and death among some species of avians, especially domestic birds like chickens and turkeys.
Pigs can be infected with both human and avian influenza viruses in addition to swine influenza viruses. Infected pigs get symptoms similar to humans, such as cough, fever, and runny nose. Because pigs are susceptible to avian, human, and swine influenza viruses, they potentially may be infected with influenza viruses from different species (e.g., ducks and humans) at the same time. If this happens, it is possible for the genes of these viruses to mix and create a new virus.
Influenza Type A. viruses can infect people, birds, pigs, horses, seals, whales, and other animals and wild birds. All are the natural hosts for these viruses. As previously stated, Influenza type A viruses are divided into subtypes based on two proteins on the surface of the virus. These proteins are called hemagglutinin (HA) and neuraminidase (NA). There are 15 different HA subtypes and 9 different NA subtypes.
Influenza Type B. viruses are normally found only in humans. Unlike influenza A viruses, these viruses are not classified according to subtype. Although influenza type B viruses can cause human epidemics, they have not caused pandemics.
Influenza Type C. viruses cause mild illness in humans and do not cause epidemics or pandemics. These viruses are not classified according to subtype.
Influenza B viruses and subtypes of influenza A virus are further characterized into strains. There are many different strains of influenza B viruses and of influenza A subtypes. New strains of influenza viruses appear and replace older strains. This process occurs through the change antigenic “drift” (see discussion).
When a new strain of human influenza virus emerges, the already existing antibody protection may not provide protection against the new strain. Thus, the influenza vaccine is updated on a yearly basis to keep up with the changes in influenza viruses.
Humans can be infected with influenza types A, B, and C. However, the only subtypes of Influenza A virus that normally infect people are influenza A subtypes H1N1, H1N2, and H3N2. Between 1957 and 1968, H2N2 viruses also circulated among people, but currently do not.
Only influenza A viruses infect birds. Typically wild birds do not get sick when they are infected with influenza virus. However, domestic poultry, such as turkeys and chickens, can get very sick and die from avian influenza, and some avian viruses also can cause serious disease and death in wild birds.
The H5 and H7 subtype of avian influenza A viruses can be further classified as either highly pathogenic avian influenza or low pathogenic avian influenza. This distinction is made on the basis of genetic features of the virus.
The high pathogenic type is usually associated with high mortality in poultry. It is not certain how the distinction between “low pathogenic” and “highly pathogenic” is related to the risk of disease in people. HPAI (highly pathogenic Avian Influenza) viruses can kill 90 to 100% of infected chickens, whereas LPAI (low pathogenic Avian Influenza) viruses cause less severe or no illness if they infect chickens. Because LPAI viruses can evolve into HPAI viruses, animal health officials closely monitor outbreaks of H5 and H7 LPAI.
We can see that there are many different combinations of HA (Hemagglutinin) and NA (neuraminidase) proteins are possible. Only some influenza A subtypes (i.e., H1N1, H1N2, and H3N2) are currently in general circulation among people. Other subtypes are found most commonly in other animal species. For example, H7N7 and H3N8 viruses cause illness in horses.
Subtypes of influenza A virus are named according to their unique HA and NA surface proteins. For example, an “H7N2 virus” designates an influenza A subtype that has an HA 7 protein and an NA 2 protein.
Similarly an “H5N1” virus has an HA 5 protein and an NA 1 protein.
(Rather than go into the minute details of how the receptor and budding of fresh virus particles spread the virus, let it suffice to say that there are 8 RNA molecules within a gene of influenza A. and we will look at the hemagglutinin and neuraminidase only briefly for this study, so that the nurse can appreciate the distinction of H5N1). They are:
The term endemic is applied to a discrete geographical unit, sometimes a country or other defined area. Epidemics occur when there are sudden increases in frequency above endemic levels. A pandemic occurs over large geographical areas (usually worldwide) and affects a high proportion of the population whereas an epidemic, although it may be serious, usually refers to what is happening in one country or region.
The World Health Organization has developed a plan for influenza preparedness that defines the stages of a pandemic and makes recommendations for preparations. The stages are as follows: interpandemic period, pandemic alert period, pandemic period. We are in a pandemic alert period now.
Viruses change in two ways. One way is “antigenic drift” and “antigenic shift.” Antigenic drift is due to mutations in the RNA that lead to changes in the antigenic character of the H and N molecules. Antigenic drift involves subtle changes that may cause epidemics, but not pandemics.
Antigenic shift is the process by which two different strains of influenza combine to form a new subtype having a mixture of the surface antigens of the two original strains. Antigenic shift occurs only in influenza A because it infects more than just humans.
Antigenic shifts are more dramatic changes, and the virus strains appear antigenically different from previously seen strains. Shifts are catastrophic for immunity. Hence, Antigenic Shift is a major mutation that is dangerous to the human population. Antigenic drift by contrast is not.
How they change is not quite known, but it is most likely a result of some rare events in which two different viruses infect a cell at the same time. Unfortunately, about every ten to twelve years a major shift can occur with a significant protein coat change that is so dramatic that the human body regards it as an entirely new virus. The new virus then sweeps through the human population with great vengeance.
For example, if pigs were infected with a human influenza virus and an avian influenza virus at the same time, the viruses could mix (reassort) and produce a new virus that had most of the genes from the human virus, but a hemagglutinin and/or neuraminidase from an avian virus. This is antigenic shift.
The worst pandemic, (‘Spanish’ flu of 1918) was a subtype of H1N1. The pandemic of the Asian flu of 1957 was H2N2. The pandemic of Hong Kong Flu of 1968 was H3N2. It appears that pandemics occur when the virus acquires a new hemagglutinin or neuraminidase. For example, H3 was found in birds and horses five years before it appeared on the Hong Kong Virus.
In most years, one or two of the three virus strains in the influenza vaccine are updated to keep up with the changes in the circulating flu viruses. For this reason, people who want to be immunized against influenza need to receive a flu vaccination every year.
Influenza is a viral infection of the lungs. Flu strains are named after their types of hemagglutinin and neuraminidase surface proteins, and therefore are called H and N’s. For example, H5N1 would be a type 5 hemagglutinin, and type 1 neuraminidase.
Humans can be infected with influenza types A, B, and C. However, the only subtypes of Influenza A virus that normally infect people are influenza A subtypes H1N1, H1N2, and H3N2. H5N1 is not a normal influenza, and perhaps is the reason there is so much concern.