The Origin of New Flu Strains
Avian, Swine, and Human Interaction
New influenza strains of influenza virus arise continually as a result of two mechanisms that alter the genetic code in the viral RNA. These mechansims are referred to as genetic drift and genetic shift.
Genetic Drifts— Small changes in genetic material called point-mutations. The antigenic proteins of influenza may be subject to the small mutations and not recognized by antibodies (Hunt, 2010). (Explain this more)
Genetic Shifts result from reassortment of genetic material between vastly different strains of influenza viruses. (This needs to be explained much better.) This can involve the creation of new H and N proteins on the virus coat (Hunt, 2010). If the new viral proteins are sufficiently different from pre-existing viral proteins, there may be limited immune recognition by humans. As a result, the majority, if not all, of the human population may be susceptible to the new viral form. In addition, reassortment may also result in a very virulent new strain. (Hunt, 2010).
All three organisms can be infected with influenza virus. Pigs are susceptible to both human and avian virus strands. They act as an intermediate step in combining avian, human, and swine influenza genetic material. This occurs when pigs become infected with both human and avian virus. The influenza RNA strands reassort while in the pig to form new combinations of avian, swine, and human strands. In turn, these new influenza viruses could possible infect humans and have surface proteins (H and N proteins) not previously seen by the human immune system.
Pandemic Influenza
In the last century there have been four main worldwide pandemics:
- 1918 Spanish Influenza—The worst of the know influenza pandemics was the 1918 pandemic, which was caused by an H1N1 strain. It killed at least 25-50 million people worldwide. More recent studies suggest that these are underestimates because of underreporting in the developing world and the inordinately high mortality in underdeveloped countries. There were at least 675,000 deaths in the United States alone. A disctintive feature of the 1918 pandemic was that mortality rates were very high for young adults between 20-40 years of age.
- 1957 Asian Influenza—The 1957 Asian flu (H2N2), originating in East Asia, was the first in which surveillance and vaccine production were implemented. This epidemic also had a "second wave" of infection in which first children and young adults were infected, followed by another wave of illness in the elderly. Approximately 69,800 people died in the United States.
- 1968 Hong Kong Influenza—The Hong Kong influenza of 1968 (H3N2) first arose in Hong Kong, and slowly spread to the United States, where those over 65 years of age were most likely to become infected. The virus returned again in 1970 and 1972. The mortality rate for the epidemic was 33,800. It is thought that prior immunity to the Asian flu virus and improved medical care and surveillance may have helped reduce the severity of illness in 1968.
- 2009 Swine Influenza—The more recent 2009 flu pandemic, commonly known as the swine flu although it doesn't infect pigs, first appeared in April 2009 in Mexico as a re-assortment of bird, swine and human flu viruses. Over 14,000 people died around the world, including 3,600 in North America. A mass vaccination campaign was introduced in addition to the trivalent seasonal flu vaccines and by November 2009, over 65 million doses of vaccine had been distributed. By June 2010, the World Health Organization declared that the epidemic was over, although the swine flu strain is still present in the seasonality of influenza.
Source: New England Journal of Medicine, http://www.nejm.org/doi/full/10.1056/NEJMp058281
The Threat of Bird Flu
Are we prepared?
This is a 15 minute talk at TED.com given by Laurie Garrett
. This talk initially focuses on the potential threat from H5N1, so called "bird flu," but her remarks then turn to the threat from pandemic flu in general, and she has incisive observations about "preparedness" for pandemic influenza.
Influenza Epidemics
Influenza epidemics are influenced by several factors:
- Population characteristics
- Pathogen characteristics
- Behavioral and lifestyle
- Environment
Population characteristics
How a specific population is characterized by immunization status, age composition, and the number of people in the population at higher risks all affected the factors of an influenza epidemic.
- Immunization Status - The higher amount of a defined population is immunized for the influenza, the potential for an epidemic decreases. A highly vaccinated population protects those individuals who are unable to be vaccinated by decreasing the odds of exposure to the influenza virus. This process of protecting unvaccinated individuals with a larger immunized population is called herd immunity.
- Age Composition - Specific age groups are more at risk for influenza. Adults who are older than fifty years are more at risk (American Lung Association, 2011). Also, children from six months to eighteen years are at an increased risk for influenza (American Lung Association, 2011). For example, a population with high amounts of these two groups could increase the likelihood of an epidemic.
- Higher Risk Populations - Certain individuals have a great potential to be susceptible to influenza, besides looking at age alone. Pregnant women and individuals with chronic illnesses [insert rollover—asthma, chronic obstructive pulmonary disease, cardiovascular disease, and diabetes, ect.] are more susceptible (American Lung Association, 2011). Furthermore, residents in long-term care facilities and those who work in these facilities are at an increased risk (American Lung Association, 2011).
Pathogen Characteristics
The latency period of influenza, the infectious period, mechanisms of transmission, and virulence of the influenza strand are considered the pathogenic [insert rollover—disease causing agent = pathogen] traits which are also determinants.
- Latency period - The latency period is the amount of time from initial infection to illness. Influenza has a latency period of about two days (World Health Organization, 2009).
- Infectious period - The infectious period is the time an infected individual is contagious to other people. For the influenza, a person is contagious one day before symptoms appear and up to five to seven days after becoming symptomatic (Centers for Disease Control and Prevention, 2011).
- Mechanisms of Transmission - This pathogenic trait of influenza plays a vital determinant of infection. Influenza is spread person to person via infected droplets (CDC, 2011). Droplets can come from sneezing, coughing, talking, or touching one's mouth. The infected droplets have the ability to infect others as far as six feet away (CDC, 2011). Influenza can also be transmitted by fomites [insert rollover—transmission by touching inanimate objects such as tables or doorknobs].
- Virulence of influenza - Virulence refers to the severity of the pathogen and is usually measured by case fatality rates [insert rollover—number of deaths divided by the total number of cases of disease]. The higher the virulence, the more influential this determinant is for influenza, and the higher number of influenza cases. The seasonal flu causes on average 36,000 deaths in the United States (CDC, 2011).
Behavioral and Lifestyle
Behavioral and lifestyle characteristics include population density, economic/political factors, and personal behaviors. These factors are more social depending on human interactions.
- Population density— In order for influenza to infect, it must have "something" to infect. Less populated areas would not have a high risk of influenza. However, highly crowded or urbanized locations increase the odds of exposure from person to person.
- Economic/political factors— Trade, travel, and wars are great examples of economic and political factors that influence determinants of influenza. With the world more globalized than ever before, the possibility of influenza strands can be spread across the world with one international flight or shipping company. Also, wars bring many nations together increasing exposures. This was the case in the previously discussion influenza epidemic of 1918.
- Personal behaviors— These include the frequency and commonality of people's hygiene. Influenza can be determined by if people wash their hands properly and cover their mouths while sneezing or coughing. A group of people who practice bad hygiene would be more susceptible to influenza.
Environment
The environment and climate are the final factors that determine influenza infection. In the northern hemisphere "flu season" primarily takes place during the winter months (CDC, 2011). The influenza virus seems to flourish best in the cooler weather. Flu season can start in October and span into April (CDC, 2011). Below is a graph which depicts the frequency of visits to US outpatient facilities for influenza-like illness during the past several years. Note that the frequency generally begins climbing in the fall, peaks in mid-winter, and then declinces sharply. However, there is variability in the pattern, and the H1N1 pandemic that occurred in 2009-2010 was very unusually with a large spike in spring followed by a sharp decline that contiued throughout the winter.
Source: http://www.cdc.gov/flu/weekly/summary.htm/