Vaccination involves exposing the pig to the protein components (called the antigen) of the infectious agent. Some vaccines contain living organisms that have been altered so that they cannot produce disease but still produce an immunity. Most contain killed or inactivated organisms.

The immune system responds by producing antibodies that destroy the infectious agents, usually in co-operation with specialised body cells or by neutralising the toxins that are responsible for the disease. This process of stimulating immunity is called vaccination.

Vaccines contain antigens from viruses, bacteria, bacterial toxins, or parasites. They are given to pigs, usually by injection, to stimulate an immune response which will protect the pigs against later natural infection with the organism from which the vaccine was derived. Most stimulate both a humoral response and a cell-mediated response.

Vaccines can either contain viable organisms that will multiply in the pig, or inactivated ones that will not multiply in the pig.

In live vaccines the organism has usually been attenuated (i.e. its virulence has been reduced) so that although it multiples in the pig it does not normally cause disease. Examples are the PRRS vaccine, aujeszky's disease (pseudorabies) vaccines and classical swine fever vaccines. Live attenuated vaccines have the advantage that because they multiply in the pig they give a bigger antigenic stimulus resulting in stronger longer-lasting immunity. They have the disadvantage that they may become inactivated in wrong storage conditions (e.g. heat) or during dosing, by exposure to antiseptics or disinfectants, and are then useless. It is also important that they are stable and not able to return to full virulence.

Inactivated (dead) vaccines may contain whole organisms, antigenic parts of organisms or antigens which have been synthesised chemically. Synthesised antigen vaccines are still largely in the experimental stage.

The immunity produced by inactivated vaccines can be enhanced by substances called adjuvants such as aluminium hydroxide of certain types of oil. You should take care, however, if you use vaccines with oily adjuvants because they can cause serious local reactions if you accidentally inject them into yourself, e.g. your hand.

Inactivated vaccines may also contain toxins which have been modified so that they still stimulate an immune response but are no longer toxic to the animal. Toxins which have been modified in this way are called toxoids. The classic vaccine of this type is the tetanus toxoid which is used commonly in horses but rarely in pigs. In pigs, some of the E. coli vaccines against piglet diarrhoea and the clostridial vaccines against piglet dysentery contain toxoids.

Research workers in Guelph, Canada are trying to develop edible pig vaccines in plants. Using genetic manipulation they are introducing genes into plants to code for novel proteins which immunise pigs against specific diseases if eaten.

Autogenous Vaccines *
Autogenous vaccines are bacterial vaccines that are manufactured from the specific pathogenic bacteria isolated from the diseased pig. They are usually made under a licence for use only on that farm. You should consult with your veterinarian. These are available from Salus (QP) Ltd. They can be useful when serious disease outbreaks occur and standard commercial vaccines are not available. Such vaccines could be made from most bacteria including :-

• Actinobacillus pleuropneumoniae
• E. coli
• Haemophilus parasuis
• Pasteurella
• Salmonella
• Streptococcus suis
• Staphylococcus hyicus (Greasy pig disease)

One drawback to vaccinating a herd is that you cannot then use blood tests to check whether the organism is present in the herd or not. All the pigs will test positive which has obvious implications for an eradication programme based on blood tests, for example the eradication of swine fever or aujeszky's disease (pseudorabies). To get over this, gene-deleted vaccines have been developed. A part of the organism's gene which codes for an antigen has been removed so that when the organism multiplies in the pig it does not stimulate antibodies against that antigen. Special blood tests can then distinguish between the array of disease antibodies and those stimulated by the vaccine. A new generation of such gene manipulated vaccines, and possibly also synthetic polypeptide vaccines, can be anticipated.

Autogenous vaccines are those prepared with infectious pathogens from the herd which is to be vaccinated. The causal organisms has to be isolated, grown up, killed, and made into a safe vaccine form. Autogenous vaccines may be useful when serious disease outbreaks occur and standard commercial vaccines are not available.

Vaccine usage
Fig.4-14 lists the pig diseases for which vaccines are available. This list is not exhaustive and some vaccines will be available in some countries and not in others. However they are used in most countries both to protect against disease and to assist in eradication programmes. Some examples of commercial vaccines available are shown in chapter 4 these are but a few of the many available.

Vaccines commonly used on pig farms throughout the world include erysipelas, parvovirus infection (SMEDI syndrome), E. coli diarrhoea, clostridial dysentery of piglets, enzootic pneumonia caused by Mycoplasma hyopneumoniae, necrotic pleuropneumonia caused by Actinobacillus pleuropneumoniae and atrophic rhinitis caused by toxigenic Pasteurella multocida. In many countries, vaccines against disease, such as, salmonellosis, PRRS and TGE are also used depending on commercial availability.

In the European Union vaccination against classical swine fever has been stopped in a programme aimed at stamping the disease out. Vaccination against foot-and-mouth disease has also been stopped for a similar reason. Aujeszky's disease (pseudorabies) virus is widespread everywhere in the EU except in the UK and Denmark. With the exception of these two countries vaccination is widely practised. A blanket vaccination regime for all herds is being applied in some countries such as the Netherlands in an attempt to build up a national herd immunity resulting in the eradication of the virus.

North America is free from FMD and CSF so vaccination is not practised but PR vaccines are widely used in conjunction with eradication programmes. Elsewhere in the world, the situation regarding these three diseases varies, so vaccination policies also vary.

The effectiveness of vaccines
This varies, because of the need to stimulate mucosal immunity locally. As mentioned earlier, vaccines given by injection against respiratory and intestinal disease are generally not as effective as those against systemic or generalised diseases. An exception to this is the vaccine for enzootic pneumonia (M. hyopneumoniae) because it stimulates cell-mediated immunity. If, however, they are fed or sprayed into the upper respiratory tract they may produce a stronger local immunity. The vaccine against piglet dysentery is a toxoid and if given routinely to sows in adequate doses is usually reasonably effective in providing passive protection via the colostrum.

Sometimes vaccines do not work particularly well on a farm and in such cases the following possibilities need to be considered:

• The vaccine was contaminated.
• The vaccine was not capable of producing the required immunity.
• The pig was already incubating the disease when it was vaccinated.
• The vaccine had been incorrectly stored. High temperatures reduce the effectiveness. (Always keep vaccines in a refrigerator but do not freeze).
• The vaccine had been exposed to sunlight.
• The vaccine had gone out of date.
• The needle and syringe were dirty or faulty.
• Chemical sterilisation destroyed the vaccine.
• The animal had inadvertently missed being vaccinated. This is particularly common with parvovirus vaccination in the gilt.
• Vaccine response was poor because there was maternal antibody present.
• The vaccine was deposited in fat and was not absorbed. Faulty injection techniques.