Will cattle be the next evolutionary lab for flu host-switching?

Will cattle be the next evolutionary lab for flu host-switching?

Sub: Science and tech

Sec:  Health

Context:

H5N1 virus, which was first detected in dairy cattle in Texas in late March, has rapidly spread to 37 herds in nine States in the U.S. as of May 7.

About the recent study:

• The U.S. FDA study of pasteurized milk reveals that about one in five of the retail samples tested positive for bird flu viral fragments.
• FAO noted that H5N1 virus was detected in high concentrations in milk from infected dairy cattle and at levels greater than that seen in respiratory samples.

Why does dairy milk contain high concentrations of H5N1 virus?

• The expression of H5N1 receptors in the mammary gland, respiratory tract and cerebrum of cattle, both the human and the duck receptors to be highly expressed in the mammary glands.
• In the mammary gland, the human receptors and the duck receptors were found to be widely distributed in the alveoli but not in the ducts.
• Chicken-type influenza receptors were common in the cow respiratory tract. 
• The high concentration of H5N1 virus fragments in milk from H5N1-infected cows could be due to local viral replication in the mammary glands of cows.
• The chicken receptor was expressed on the surface of the respiratory epithelium in the upper respiratory tract and upper part of the lower respiratory tract.
• Human and duck receptors were either lacking or very limited in expression.

Evolution of H5N1 virus:

• A favorable environment for the development of H5N1 viruses, which can readily transfer from animals to people, was created by the abundance of human and duck receptors in the mammary glands and the significant presence of human, chicken, and duck receptors in the lung alveolar cells of cows.

Pigs as “evolutionary lab for flu host switching”:

• Pigs are called the “evolutionary lab for flu host switching” precisely due to the presence of both the human-flu and avian-flu host cell receptors in their upper-respiratory tract.

What is Host Switching?

• In parasitology and epidemiology, a host switch (or host shift) is an evolutionary change of the host specificity of a parasite or pathogen.
• For example, the human immunodeficiency virus used to infect and circulate in non-human primates in West-central Africa, but switched to humans in the early 20th century.

About H5N1:

• Influenza A virus subtype H5N1 (A/H5N1) is a subtype of the influenza A virus, which causes influenza (flu), predominantly in birds.
• It is an enzootic (maintained in the population) in many bird populations, and also panzootic (affecting animals of many species over a wide area).
• H5N1 virus can also infect mammals (including humans) which have been exposed to infected birds.
•  The H1N1 pandemic of 2009 was due to reassortment of the virus in pig populations.

About hemagglutinin (H) and neuraminidase (N):

• Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: hemagglutinin (H) and neuraminidase (N).
• There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes (H1 through H18 and N1 through N11, respectively).

About Hemagglutinin (H):

• The hemagglutinin(HA) of influenza virus is a major glycoprotein and plays a crucial role in the early stage of virus infection.
• HA is responsible for binding of the virus to cell surface receptors, and it mediates liberation of the viral genome into the cytoplasm through membrane fusion.

About Neuraminidase (N):

• Viral neuraminidase is a type of neuraminidase found on the surface of influenza viruses that enables the virus to be released from the host cell.
• Neuraminidases are enzymes that cleave sialic acid (also called neuraminic acid) groups from glycoproteins.