Infection is considered to have taken place when a virus, bacterium or parasite enters a pig and starts to multiply. If it is a potentially pathogenic organism it may change the normal structure and function of the pig. A study of these changes is called pathology and organisms causing disease are described as pathogens. Fig.3-2 shows the sequence of events that may take place when an infectious pathogen infects the pig.

Infection creates two scenarios; the first is when there is no disease. The immune mechanism of the pig responds to challenge and the infectious agent is either eliminated or remains within the body in a carrier state. The carrier pig may or may not shed the organism or may shed it intermittently. The second scenario is that of disease which is followed by an immune response and complete recovery and elimination of the pathogen or recovery with a carrier state, or death. Whether the organism causes disease or not is dependent on how virulent (the capability of the organism to produce disease) it is, how many organisms are present, what other concurrent infections are present, what protective mechanisms are available in the pig to prevent disease and what environmental or other factors are present that will lower the pigs immunity. For example, certain strains of Actinobacillus pleuropneumoniae are only mildly pathogenic and do not normally produce disease. However if they are in combination with PRRS virus the presence of both could produce severe pneumonia. A non pregnant gilt exposed to parvovirus infection only develops an immune response but no signs of disease because disease only occurs in the developing foetus. Nevertheless the gilt would become serologically positive in laboratory tests.

A successful and healthy pig farm is one in balance and harmony with the organisms that are present both in the environment and the pig. It is the manipulation of this balance that is so important to health and disease and ultimately to productivity and profitability.

It follows that if the numbers of organisms in the environment can be maintained at a minimum then the threshold level of organisms necessary to produce disease is unlikely to be reached. Examples here would be the excellent growth rates that can be achieved with all-in all-out systems in nurseries or flat decks, when pigs are housed on weld mesh or slatted floors and they are divorced from their faeces and potential enteric organisms. Likewise in respiratory diseases, the more pigs there are in a common air space, and the smaller the cubic capacity of that air space relative to the numbers of pigs, then the greater will be the numbers of aerosol organisms and the more severe disease.

To be a successful pig producer you should have some idea of how pigs resist infectious disease. Unfortunately, it is a complex subject and although here an attempt has been made to simplify it, it will probably still seem a little complicated.

The technical terms used in immunology (i.e. the study of immunity) are a major part of the problem. If you have not had a good grounding in biology they may be like a foreign language. If that is the case, or even if it is not, start by reading through the terminology below.

Terminology

Adjuvant - A substance added to an inactivated vaccine to make it more effective.

Antibodies - Complex large proteins (called gamma-globulins) which are produced by specialised cells in response to invading antigens and which stick specifically to the invading antigen neutralising it or triggering off a destructive reaction.
Antigen - Foreign invading substance (i.e. a substance which is not normally part of the pig's body), usually consisting of protein or part of a protein, which stimulates the body to produce antibodies. Antigens exist on the surfaces of bacteria, viruses and parasites.

Remember:

• Expose your pigs to too many pathogenic organisms and you have sick pigs - poor performers.
• Lower the level of pathogens in the environment and you have healthy pigs - high performers.

Antiserum - Serum with high antibody levels against a specific infection. It has usually been produced experimentally in laboratory animals by injecting the infection into them.
Blood sample - Whole blood sample taken hygienically with a syringe into a bottle or by a pin prick through the skin absorbing the droplet of blood with blotting paper.
Commensal bacteria - Bacteria that live permanently in or on the body without causing disease.
Epithelium - Cellular membrane (e.g. mucous membranes) containing epithelial and other cells.
Humoral immunity - Blood-borne immunity.
Hyperimmune antiserum - The same as antiserum above but emphasising its high titre.
Lymphocytes - Specialised defence cells in lymph nodes, other lymphatic tissue and the blood which produce antibodies or take part in cellular immunity.
Mucous membranes - Cellular membranes (e.g. those lining the gut) which secrete a sticky substance called mucus on to their surfaces.
Mucus - A clear sticky semi-liquid secreted by cells in mucous membranes.
Pathogenic infection - An infectious organism which has the potential to cause disease. This is in contrast to the many organisms that live normally in or on the body which never cause disease and are called commensals.
Phagocytes - Cells of the body whose special task is to engulf bacteria, viruses, or parasites in an attempt to destroy them. They are also called macrophages.
Phagocytosis - The process whereby the specialised cells of the body engulf bacteria, viruses or parasites in an attempt to destroy them.
Plasma sample - A whole blood sample taken hygienically with a syringe and mixed with an anti-clotting agent so that it remains liquid. The sample is spun fast in a centrifuge and the red and white blood cells sediment to a firm pellet at the bottom leaving a clear liquid - the plasma.
Serology - Tests done in the laboratory to detect the level of specific antibodies in serum samples ("ology" means study of - so literally serology means "study of serum").
Serum sample - A whole blood sample taken hygienically with a syringe and allowed to clot. The serum is the clear straw-coloured liquid which can be drawn of with a pipette. It contains the antibodies.
Titre - The concentration of a specific antibody in a serum sample. It is expressed as the amount by which the serum has to be diluted before a serological test goes negative.
Virulence - How pathogenic an organism is. Organisms with a high capability of causing disease are called highly virulent.

The main components that make up the resistance of a pig to infection may also seem complicated but if you refer to Fig.3-3 as you read it should also help you to understand the text better. You do not need to understand all the components but take note of the following:

• What antibodies are.
• What stimulates them to be produced.
• The importance of colostrum and milk in providing immunity.
• Why blood tests are done and how they are interpreted.
• How vaccines work.