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)