Viruses – How do they cause disease?

Background	Viruses are very small and the word means ‘filterable’. Once bacteria were recognised down the microscope by Pasteur many diseases became associated with particular bacteria such as Pasteurella or Salmonella (discovered by Dr Salmon). It soon became apparent that some diseases were not associated with the ‘new’ bacteria and the disease could still be transmitted after all bacteria were removed by filtration. These diseases were then called viral. Foot and Mouth was the first virus recognised.
	Viruses cannot replicate on their own, they require other living organisms (some even parasitize bacteria) to replicate.
	Viruses are classified into two main types depending on the type of genetic material: DNA viruses have their genetic material similar to our chromosomes (the genetic material needs to be transformed into RNA to make proteins) and RNA viruses which have their genetic material in the form from which proteins are made.
	After genetic material classification viruses may have an envelope of lipid (fat) materials. Those viruses with an envelope are more susceptible to disinfectants as the disinfectant removes the envelope rending the virus incapable of survival
	The other characteristics of viruses are determined by their size or shape. Viruses that infect bacteria are called bacteriophages Parvovirus are very small and round (Parvo means small in Greek) Influenza viruses are very long Coronaviruses have protein projections that make them look like a crown or corona
	It is important to note however, that the vast majority of viruses live quite harmlessly in our bodies cells and do not cause any significant or recognised problem.
Treatment	Because viruses do not replicate on their own there is no specific treatment thorough antibiotics. Antibiotics work against bacteria because they will either kill the bacteria or affect their replication ability. There are a few antiviral medicines but they are very expensive and are not really available to pigs.
How to treat viral diseases – the basics	Keep the disease agent out through good biosecurity
	Use the bodies own difference mechanisms through vaccines and antibodies
	Reduce the stress on the animal by good management – pig flow and management of the water, food, floor and air systems.
	Treat any sick pig promptly and appropriately – classically through a hospital pen
Are there things smaller than viruses?
	The prion diseases (CJD of man and BSE in cattle for example) are diseases where the infectious agent is without any nuclear material and appear composed for lipid and proteins only. These diseases have not been recognised anywhere in the world in farmed pigs.
The major virus diseases of pigs
Virus name		Family	Genetic	Envelope	Comments
Adenovirus		Adenoviridae	DNA D	+ ve
African Swine Fever		Un-named	DNA D		Insect borne
Aujeszky’s Disease		Herpes Virus	DNA	+ ve	Pseudorabies
Blue Eye Virus		Paramyoxoviridae	RNA	+ ve
Circovirus		Circoviridae	DNA S	- ve	Two types I and II
Classical Swine Fever		Flaviviridae	RNA	+ ve	CSF Hog Cholera Note BVD and Border’s can infect. Pestivirus
Coronavirus		Coronaviridae	RNA	+ve	Several types, TGE, PRC, ED, HEV
Cytomegalovirus		Herpesviridae	DNA	+ ve
Eastern Equine Encephalomyelitis		Togaviridae	RNA S	- ve	Also similar West Nile Virus
Encephalomyocarditis virus		Picornaviridae	RNA	- ve
Enterovirus		Picornaviridae	RNA	- ve	Numerous types – Talfan, Teshen, SMEDI
Foot and mouth		Picornaviridae	RNA S	- ve	FMD
Hepatitis E virus		Caliciviridae?	RNA S	- ve
Japanese Encephalomyelitis virus		Flaviviridae	RNA	+ ve
Nipah virus		Handra virus	RNA S	+ ve	Paramyoxoviridae
Menangle Virus infection		Paramyxovirus	RNA	+ ve
Parvovirus		Parvovirus	DNA S	- ve
Porcine Mycocarditis virus		Bungowannah	RNA	+ve	Related to Pestivirus
Porcine Reproductive and Respiratory Syndrome Virus		Arterividae	RNA	+ ve	PRRSv
Rabies		Rhabdoviridae	RNA	± ve
Reovirus		Reoviridae	RNA	- ve
Rinderpest		Paramyxovirus	RNA	+ve
Rotovirus		Reoviridae	RNA	- ve	Mainly type A
Swine Influenza		Orthomyxoviridae	RNA Segmented	+ ve	SIV Several types based on H and N antigens.
Swine Vesicular Disease		Picornaviridae	RNA S	- ve	An enterovirus
Swine Pox		Poxviridae	DNA D	+ ve
Torovirus		Torovirus	RNA	+ve	Related to Coronavirus
Torque Teno Virus		Anellovirus	DNA	- ve	Related to Circovirus
Vesicular exanthema		Caliciviridae	RNA S	- ve
Vesicular stomatitis		Rhadboviridae	RNA S	± ve	Affects horses as well
West Nile Virus		Flaviviridae	RNA	+ ve

Pig Viruses General

The major sites where viruses of the pig can be found

Some electronmicroscope pictures of virus

Influenza virus	Coronavirus
Parvovirus	Picornovirus
SVD - Rhabdovirus	Herpes virus
Intranuclear inclusion bodies – DNA virus Adenovirus	Intracytoplasmic inclusion bodies – RNA typically (PCV2 botyroid bodies demonstrated)