The BBC (and others, of course) report from the journal Science that Seals Pose Influenza Threat. Specifically, Type B influenza virus, previously thought to be an exclusively human ailment. Type C of course lives in birds and humans (is that also the type that lives in pigs, or am I thinking A here?)
Anyway, of the viruses that infect various animals, what percentage can make the leap from one species to another? What (if anything that we know about) prevents all or most viruses from doing this? Do they infect only organisms with specific DNA patterns? (or in other words, why seals and people but not, say, bunny rabbits?)
I guess I have the same questions about bacterial and fungal infections.
Viruses propagate themselves by infecting a host cell and inserting their own genome into that of the host, so that when the host transcribes and translates its chromosome, the virus is replicated as well. Once enough viruses have built up inside the cell, the cell bursts, and the viruses go on their merry way, ready to do it all over again with the next cell. (When a virus infects a host cell, all it does is inject its genetic material. The little vaguely spider-like representation of the virus body that you might have seen then becomes debris). Viral infection therefore depends on some degree of compatibility between the viral and host genetic material/mechanisms of genetic reproduction.
To give an example - the HIV virus. gp 120 (a protein) on the virus’ surface binds to CD4 (another protein) on a T cell’s surface. Once a chemokine receptor (another protein on the T cell surface) binds to the complex formed by gp 120 and CD4, the virus is able to fuse to the membrane of the T cell, injecting its genetic material (RNA) inside. The RNA is then uncoated, reverse-transcribed into cDNA (rather a neat trick, actually), and the viral cDNA is inserted into the T cell’s chromosome. When the infected T cell is activated as part of your normal immune response, it activates transcription and translation of new HIV virus proteins. HIV protease modifies the HIV virus proteins, and they are packaged into virus particles, which bud off the cell, ready to go on their infecting way.
As you can see, HIV infection depends on very specific compatibility between multiple proteins on both the virus and the target cell. Generally speaking, the cells in your body are similar on the basic level to those in a pig or bird, but there are enough distinctions in the way that they operate to keep enterprising viruses from hopping freely from one species to another.
-ellis
Howdy,
here is a bit of info that may be of interest…
Influenza Virus Strains
Differ in forms of internal proteins. Type C infections are mild or asymptomatic. Subtypes based on hemagluttinin (HA) and neuraminidase (NA) amino acid sequences.
Type A Type B
15 HA subtypes 1 HA subtype
9 NA subtypes 1 NA subtype
Infects humans, Infects humans
pigs, horses, seals,
whales, and birds
cause outbreaks causes outbreaks through antigenic drift
through antigenic drift
and antigenic shift
Type A viruses identified according to which HA and NA molecule are displayed; for example, A(H1N3) or A(H5N2)
of course with the report about seals carrying Type B that info will have to be changed.
Patrocles
Diseases that can be transmitted from animals to people are known as zoonotic diseases. They are a real concern if the human is immunocomprimized from AIDS, for example.
As for viruses, the range of cells that a virus can infect is known as its host range. Some cells have a very narrow host range. EBV, the virus that causes mono, only infects one type of human cell. Herpes simplex viruses, however, has a very broad host range. If can infect rabbit skin cells, monkey kidney cells, human fibroblast cells, and many many more.
The architecture of the cell generally determines its ability to be infected by any given virus. Like ellis said, viruses require specfic receptors to infect a cell. Without the specific receptors, viruses can’t infect. But other aspects of the cell also determine its ability to be infected. Retroviruses, for example, can only replicate inside actively dividing cells.
Influenza viruses that infect both pigs and humans, for example, take advantage of characteristics that are common to the cells of both organisms.
That’s the viral story. Bacteria and fungi aren’t quite so demanding.