undergo mutations (antigenic drift) or reassortment in which a new HA or NA is produced (antigenic shift). Influenza viruses C and D are only capable of antigenic drift whereas Type A undergoes antigenic shift, as well. When either of these processes occur, the antibodies formed by the immune system no longer protect against these altered glycoproteins. Because of this, viruses continually cause infections. Identification Influenza viruses C and D are different from Types A and B in their growth requirements. Because of this, Influenzavirus D is not isolated and identified as frequently. Diagnosis is by virus isolation, serology, and other tests. Hemagglutination inhibition (HI) is one method of serology that detects antibodies for diagnostic purposes. Western blot (immunoblot assay) and enzyme-linked immunosorbent assay (ELISA) are two other methods used to detect proteins (or antigens) in serum. In each of these techniques, the antibodies for the protein of interest are added and the presence of the specific protein is indicated by a color change. ELISA was shown to have higher sensitivity to the HEF than the HI test. Because only Influenza viruses C and D produce esterase, In Situ Esterase Assays provide a quick and inexpensive method of detecting just Types C and D. Vaccination Because influenza virus A has an animal reservoir that contains all the known subtypes and can undergo antigenic shift, this type of influenza virus is capable of producing pandemics. Influenza viruses A and B also cause seasonal epidemics every year due to their ability to antigenic shift. Influenza viruses C and D do not have this capability, and they have not been implicated in any pandemics; thus, there are currently no human vaccines available for Influenza viruses C or D. An inactivated Influenzavirus D vaccine was developed for cattle; however, the vaccine only provided partial protection in challenge experiments. References Further reading ICTVdB database for Influenza viruses External links Influenza Research Database Database of influenza genomic sequences and related information. Viralzone: Influenzavirus D Orthomyxoviridae Influenza de:Influenzavirus#Influenza-D-Subtypen