Types of Influenza
As discussed in the previous section, two types of influenza virus, types A and B, commonly infect humans and pass easily from person to person, causing yearly flu epidemics. Some influenza viruses infect animals, including birds and pigs (swine), but these only rarely infect humans and are not easily spread from person to person. Most often, there must be close contact for the virus to be transmitted from animal to human.
Influenza type A viruses are further grouped into numerous subtypes and they are named using the designations H and N, based on two protein antigens on the virus' surface: hemagglutinin (H) and neuraminidase (N). The most common influenza A viruses currently infecting humans have the subtypes H1N1 and H3N2.
In most cases, the specific name of the influenza virus is only relevant to the medical community and those charged with influenza surveillance. However, in recent years there has been news and focus on, first, avian (bird) flu and then H1N1 (swine) flu.
Other parts of the influenza strain's formal name include the type of influenza, where it was first isolated, the strain number, and the year it was first isolated from the host. For instance, the predominant strain during the 2003-2004 flu season was influenza A/Fujian/411/2002 (H3N2).
Avian (Bird) Flu
Strains of influenza A occur in wild aquatic birds like seagulls and ducks worldwide. They can infect domesticated poultry like geese and chickens. Avian influenza A, subtype H5N1 was first found in humans during a Hong Kong poultry outbreak in 1997. It caused an epidemic in birds in Southeast Asia in 2004, spreading to Africa and Europe. It has caused illness and deaths in birds and in people in parts of Asia and Africa. So far, this strain of influenza has remained a bird-to-human infection, but its fatality rate in humans is much higher than the seasonal flu.
A new avian influenza A virus, H7N9 was reported infecting birds and people in spring 2013 in China. It caused 135 infections that spring and 44 deaths. So far, there is no evidence of extensive person-to-person spread of H7N9, though there may have been some isolated cases under rare circumstances. It has also not been detected outside of China, though scientists are worried about it developing the ability to spread easily from person-to-person and creating a pandemic.
Swine flu is a type A influenza virus that causes a respiratory infection in pigs. Occasionally, this type of virus may cause infections in humans. These are called variant viruses and are denoted with a "v" after the subtype name. Some variant viruses that have caused infections in humans include H1N1v, H3N2v, and H1N2v.
In spring of 2009, a new influenza A (H1N1) virus emerged and caused the first influenza pandemic in 40 years, meaning it spread worldwide and affected many people. The 2009 H1N1 flu virus was originally called "swine flu" because it was similar to a virus found in pigs. It is now known to be a combination of human, swine, and avian flu genes and may cause seasonal influenza infections throughout the world. It was first reported in Mexico and the U.S. as a new influenza A, H1N1 virus.
The 2009-2010 flu season was the most lethal pandemic in recent history and it had a greater effect on those younger than 65. Between April and December 2009, about 90% of influenza-related hospitalizations and 88% of estimated deaths were in people younger than 65. The Centers for Disease Control and Prevention (CDC) estimates that between April 2009 and February 13, 2010, between 42 million and 86 million cases of 2009 H1N1 occurred in the U.S., with between 188,000 and 389,000 H1N1-related hospitalizations and between 8,520 and 17,620 2009 H1N1-related deaths.
For comparison, the 1918-1919 H1N1 influenza A pandemic infected about one-third of the world's population (an estimated 500 million people) and killed an estimated 20 to 50 million people, with more than 500,000 deaths in the U.S. alone.
Another swine influenza virus, H3N2v, is a common respiratory infection found in pigs. In 2012, a jump in the number of cases in humans, particularly in the Midwest, prompted the CDC to alert health care providers to the growing number. Most of the cases among humans were in children with direct contact with pigs on farms or at county fairs. The strain that was circulating within communities had acquired a gene called the matrix (M) gene from the H1N1 virus that caused the 2009 pandemic in humans. The presence of this M gene in the genetic material of the influenza A H3N2v virus concerns epidemiologists, according to the CDC, because it may increase transmissibility of the virus from pigs to people and among people.