Influenza or Flu Part II: How flu spreads? What is avian flu? What is H1N1 or swine flu?

SciGuru.com: How flu spreads? The main form of transmission is from person to person through droplets and small particles produced when infected people cough or sneeze. In rare cases, it can spread from other animals such as the case with avian flu. Infection can also spread if one touches materials infected with the flu virus and then touch their mouth or nose. According to CDC "healthy adults may be able to infect others beginning one day before symptoms develop and up to 5-7 days after becoming sick. That means that you may be able to pass on the flu to someone else before you know you are sick, as well as while you are sick." Flu may also spread from animals to humans, such as the recent outbreaks of avian and swine flu.

Courtesy: Wikipedia

What is avian flu?

Avian refers to bird. Avian influenza or avian flu is caused by avian influenza viruses. Usually transmission from the bird to human is very rare as avian flu viruses do not infect humans. But several cases have been reported where the flu viruses have been transmitted from birds to humans. This is very important during flu outbreaks among poultry. Under normal circumstances, wild birds carry the virus in their intestines without themselves getting sick. However avian flu is very contagious and can make poultry sick and kill. Mortality rate among poultry can reach up to 100% in 48 hours demonstrating how potent and harmful these viral strains can be. Infected birds shed influenza virus in their saliva, nasal secretions, and feces.

Avian flu viruses are referred to as Influenza A viruses. There are different subtypes of this virus, depending on the form of two different proteins they carry, hemaglutinin (HA) and neuraminidase (NA). There are 16 known HA subtypes and 9 known NA subtypes of influenza A viruses. Many different combinations of HA and NA proteins are possible. Each combination represents a different subtype. All known subtypes of influenza A viruses can be found in birds. H5N1 is an influenza A virus that is highly contagious among birds with reported outbreaks in the humans. This is one of the very few flu viruses that crossed species barrier and infected humans. More than half of those people reported to be infected with H5N1 virus have died. CDC states: "While there has been some human-to-human spread of H5N1, it has been limited, inefficient and unsustained. For example, in 2004 in Thailand, probable human-to-human spread in a family resulting from prolonged and very close contact between an ill child and her mother was reported. In June 2006, WHO reported evidence of human-to-human spread in Indonesia. In this situation, 8 people in one family were infected. The first family member is thought to have become ill through contact with infected poultry. This person then infected six family members. One of those six people (a child) then infected another family member (his father). No further spread outside of the exposed family was documented or suspected. "It is feared that H5N1 virus may one day mutate, like other flu viruses, and gain human-to-human transmissibility. Considering the deadly virulence of this virus, this is a scary scenario. The H5N1 virus that threatened Asia is resistant to antiviral drugs such as amantadine and rimantadine.

What is H1N1, or swine flu?

Originally called swine flu, H1N1, is a new influenza virus causing illness in the people and have scared the public in the year 2009. This virus was first detected in the United States and is spreading world-wide. Scientists call this virus a "quadruple reassortant" virus because it has two genes from the flu that infects pigs, avian flu gene and human genes. It was initially called swine flu as some genes in this virus were closely matching influenza virus that normally occur in pigs. This is contagious like other flu viruses and spreads like any seasonal flu. Apart the usual flu symptoms described above, a significant number of people who have been infected with this virus also have reported diarrhea and vomiting.

Vaccines are currently being developed to protect against H1N1. However CDC has the following recommendations for those who have flu and those who need protection from flu:

1. Those who have flu may stay home for at least 24 hours after the fever is gone.
2. Wash hands with soap frequently.
3. If a family member is sick with H1N1 and you develop symptoms of flu, consult a physician and stay home till the symptoms disappear.

Influenza A, B and C.

Influenzavirus A belongs to the Orthomyxoviridae family of viruses. Wild aquatic birds are the natural hosts for different strains of influenza A. However, humans can get infected ocassionally. The type A infection in the human can be very severe and have caused pandemic deaths in the past. Influenza B infects mostly human but is less common and Influenza C which may infect human, pigs and dogs is less common than A and B. All the three, influenza A, B an C belong to the same family of viruses, but different genera.

What is the structure of Influenza A virus?

The overall structure of all influenza viruses is the same. In technical terms, Influenza A virus is a single stranded RNA virus. The RNA genome of influenza A virus is anti-sense and segmented. The eight RNA segments in influenza A virus code for eleven proteins. Though all the eleven proteins are important for the survival and propagation of the virus, the two critical proteins that are important in the context of its transmission and nomenclature are HA and NA. HA stands for Hemaglutinin A and NA stands for Neuraminidase.

What is hemaglutinin A and its function?

The function of HA is in determining the infection of the host cells. HA on the surface of the viral particles binds to certain very specific sugar molecules on the surface of target host cells. This allows the virus to stick to the surface of host cells. Now that the virus is attached to host cell, the host cell takes it inside by a process called endocytosis. Once inside the cell, the virus again uses the same HA protein as an anchor to get out of the endocytotic vescicles to get into the cytoplasm.

What is neuraminidase and its function?

Once inside the cell, the viral RNA uses the host cell's machinery to multiply itself. Once again, to exit the host cell, the virus needs to cleave certain specific sugar residues on its own surface as well as on the host cell to detach itself. This is done by the enzyme (an enzyme is a biological catalyst), neuraminidase, or NA.

Naming the virus: basis of Influenza virus A nomenclature.

Hemaglutinin and neurominidase form the outer coat of the viral particles with the core of the particle formed with genomic RNA and other proteins. There are 16 subtypes of HA protein and 9 subtypes of NA proteins. That means there can be 144 different combinations of influenza A viruses. These protein subtypes are named H1-H16 and N1-N9 and viruses carrying the specific combinations of these proteins are named according these subtypes, for example a virus with H1 form of HA and N1 for NA is called H1N1.

What is swine flu, or H1N1 2009?

In the subtypes of H and N proteins, there can be substantial mutations occurring. These mutations make efforts to develop effective vaccines difficult. It is reported that the H1 protein of H1N1 2009 strain is 29% different from H1N1 virus that circulated in 2008; and 18% differerent at the N1 protein (Gallaher, Virology J. 2009, doi:10.1186). This means that the available vaccine for H1N1 strain of 2008 may not work for the 2009 strain. The H1N1 strain that developed into a pandemic in 2009 was initially called swine flu and was suspected of originating from pigs (see below). There was H1N1 strain infections and pandemic prior to 2009. However the virus stain was substantially different.

What are Flu (Influenza) drugs?

The available drugs are possibily limited to Tamiflu (oseltamir and Relenza (zanamivir). Both of them are directed against the viral envelope protein neuraminidase. As mentioned before, this enzyme is required for detaching viral particles from the host cell. This enzyme also helps the virus to move through mucus by breaking down mucus sugar residues and paving the path to infect other cells. Neuramindase inhibitors block the enzyme's function and prevent new virus particles being released from the host cell.The inhibitor may also help in preventing its motility through mucus. This would limit the spread of infection. Unless the virus can enter a new cell, it cannot multipy either. Other drugs that are used to control influenza A infection include amantadine and rimantadine. As these drugs block M2 protein and this protein is not present in influenza B, the drug could be ineffective in such infections. Before taking any of these drugs, one should consult with a registered medical practitioner.

The H1N1 2009, a reassortment virus.

It may be noted that the 1918 flu pandemic (spanish flu) was also caused by H1N1, but a different strain than the 2009 H1N1 strain. The 2009 H1N1 virus, also called swine flu, is considered to be a reassortment of gene segments from swine flu virus genes, avian flu and human influenza virus genes, and therefore called a "assortment" virus. This probably led to the conclusion that eating or handling pork may not cause the infection. There is currently no evidence that the 2009 H1N1 initially called as swine flu, was spread from pigs to people.



Disclaimer: The above material is only for information purpose and some information may change with advancement of knowledge. Do not use this information to treat any medical condition. If you suspect a medical condition, consult a registered medical practitioner.

Part I