Prof Dunham began by explaining that viruses are very small, much smaller than bacteria for example. Viruses essentially comprise a section of DNA wrapped up in a protein coat - and needs a host cell to replicate.
The Human Rotavirus (which causes diarrhoea in children) can be used to give some examples images of a virus:
|Schematic showing structure of a Virus|
|Computer generated model of Rotavirus|
|Rotavirus in childs faeces|
Some examples of common animal viruses are :
Rinderpest (which has now been eradicated but previously could kill 100% of a cattle herd in a matter of days)
Bird Flu (more on this later)
Cat "Flu" (actually a form of Herpes)
Distemper in Seals
Coronavirus which cause respiratory diseases in animals
|Cattle who have died from Rinderpest, South Africa, 1896|
The reason viruses cause animals to get sick is that they cause cell death and damage or can cause cancer.
Prof Dunham mentioned that, in some cases (Palilloma Virus being one) a human vaccine has been developed on the back of previous work developing a vaccine for animals
One problem in combating viruses is that they can spread in animal communities without the animals showing any symptoms, or may have delayed effects in terms of reduced fertility or later disease
Factors in the effect a virus has on animals include the virus type and load, as well as the age and condition of the host. The environment is also a factor, particularly the degree of overcrowding that the animals are living in (chicken herpes being an example of this, and is a disease that was not seen before the industrial housing of poultry that began in the 1950s)
Animals have a number of barriers and defences to viral infection, including tears/sneezing/saliva, fatty acids(which can attack viruses), Diarrhoea (to expel the virus) and Fever (to overheat the virus).
The response from the immune system is also multi-layered. Withing the first two days it is the Cytokines that are mainly doing the defensive work. Killer Cells then peak at day 3-4, with antibodies and T-cells arriving on the scene at around the one week mark.
Going back to the case of bird flu, there are many of varieties of this virus and most are carried by ducks without any ill effect - but the ducks can pass the virus on to other species who are very vulnerable to the virus. Chickens, for example, can die overnight from bird flu.
The designations given to the strains of bird flu (e.g. H5N1) relate to two proteins, hemagglutinin and neuraminidase, that are on the surface of the virus. Hence the H and the N abbreviation. The numbers that are included in the virus name signal a genetic change in the virus. Some combinations of H and N cause serious illness and death, while others only cause mild symptoms. Flu viruses that begin with H5 or H7 are highly likely to make birds and people sick.
It is the accumulation of random changes in the genetic code of the virus (which is not as stable as the genetic code of animals) cause new strains of viruses to develop. Indeed, only two genetic changes are required to get from bird flu to human flu.
Fortunately, some strains which could be more dangerous to humans lack the ability to spread via airborne droplets, reducing the chance of them causing epidemics.
|Avian Flu - showing H and N protein structures|
Virus Structure rotavirus reconstruction, Rotavirus, Rinderpest, Cat "Flu", Bird Flu