Description

A vaccine is a biological preparation containing antigenic material - that is biological material that will induce an immune response in an animal against a specific bacterial, parasitic or viral infection. The response induced by vaccines (except toxoids) is an active one involving the production of antibodies against the infectious agent, by the vaccinated animal. Once sufficient antibodies have been produced, and the animal's immune system has been "primed" against the infectious agent, the vaccinated animal has a degree of protection against infection, and in some cases it may have cross-protection against other infections as well.

Depending upon the type of vaccine and the infectious agent involved, protection may be life-long following a vaccination - or it may reduce with time, requiring repeat vaccination at regular intervals.

Types of Vaccine

There are two main types of vaccine :

These contain live organisms that have been modified in some way so that they do not cause disease (ie are not pathogenic). This is achieved by treating the infectious agent with heat or chemicals, by administering a naturally occurring non-pathogenic strain, or by growing a non-pathogenic (called a modified or attenuated) strain of the organism in a laboratory. Live vaccines replicate in the vaccinated animal's body and they retain the surface antigens of the infectious agent - and so stimulate the production of specific antibodies by the animal. These antibodies can be produced locally near the site of administration, or at remote sites in the body (systemic), or both.

Vaccines that induce local immunity on mucosal surfaces - such as the lining of the respiratory tract - work very quickly in providing protection against infection.

As a general rule live vaccines provide a higher level of protection which lasts longer than that provided by killed vaccines. However, the protection is less than usually follows natural infection.

Passive protection in the animal (such as from maternal antibodies in milk) can inhibit the replication of the non-pathogenic living organism in the vaccinated animal, and so reduces the immune response. If this is the case repeat vaccination will be need to boost the protection.

One disadvantage of using live vaccines is the potential for a non-pathogenic strain to genetically mutate during it's replication in the vaccinated animal, and for it to re-acquire pathogenicity.

Killed (or inactivated) vaccines contain antigenic material and the antigens present in the vaccine stimulate the production of antibodies by the body - but there is no replication of organisms within the vaccinated animal's body. Some of these vaccines contain subunits of the infectious agent which have been manufactured using genetic engineering techniques.

Inactivated vaccines produce a lower immune response and require booster vaccinations (at least 2) to confer enough protection.

Some of these vaccines contain an adjuvant such as alum, aluminium hydroxide, aluminium phosphate, carbomer or a mineral oil to increase the immune response in the vaccinated animal. Unfortunately some adjuvants are irritating to the animal, cause local discomfort and sometimes a local reaction results. It is thought that the adjuvant used in some Feline Leukaemia Virus vaccines may be responsible for the development of sarcomas at the injection site in cats.

Inactivated vaccines usually require annual boosters to maintain adequate protection in the animal.

One advantage of inactivated vaccines is that they are safer to use in pregnant animals than live vaccines.

Other types of vaccine that are used in veterinary medicine include :

Toxoid - this is a toxin derived from a micro-organism which has had it's pathogenic effects removed by treating with heat or chemicals. It retains it's ability to stimulate antibody production, but often such vaccines (e.g. antitetanus toxoid) requires the use of an adjuvant.

Autogenous vaccine - this is when a vaccine is prepared from materials taken from the animal itself.

Emergency vaccine - are prepared from organisms isolated from individual animals (usually on a farm) when a commercial vaccine is not available

Side-effects

Some vaccines have undesirable side-effects and care is needed when administering them.

Local swelling at the vaccination site, and pain during administration are commonest following the use of vaccines containing adjuvants. Some vaccines cause a local change in coat colour - which may be permanent in some cats.

Rarely individuals may develop a hypersensitivity reaction following vaccination which may need to be treated. Signs of this include vomiting, breathing difficulties, a skin rash (urticaria) , excessive salivation and diarrhoea. Full anaphylactic shock can result in some animals with collapse and severe respiratory distress - this must be treated as an emergency.

Live feline panleucopenia virus vaccine can cross the placenta and may cause abortion or developmental abnormalities in fetuses if it is administered to a pregnant cat.

Most recently, the occurrence of localised sarcomas at vaccine sites following the administration of feline leukaemia virus vaccine to cats is causing great concern, and the precise mechanism involved is being investigated. It is thought that the formation of this cancer is possibly being stimulated by the adjuvant in the vaccine.