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Infectious disease is still the most important veterinary problem in farm animal medicine. While there are vaccines available for many diseases in man and animals for some diseases it has not yet been possible to produce effective products.

Infectious disease is still the most important veterinary problem in farm animal medicine. As most animals kept for food production are slaughtered before reaching adulthood and with even ewes and dairy cattle seldom living out their natural lifespan the problems of aging, as seen in human and companion animals, do not regularly occur.

Treatment of infectious disease whilst often highly effective is not without problems such as the effects of residues on meat and milk quality, the development of resistance as a consequence of natural selection and of course any damage done before the disease is recognised and treated.

The ability to vaccinate against such diseases enables the animals to produce immunity before exposure which will usually prevent or dramatically reduce clinical signs and prevent spread of the disease.
Vaccines do not necessarily prevent infection but do usually prevent clinical disease in the majority of the treated population. The normal minimum acceptance criteria for registration of a veterinary vaccine are 80% protection in 80% of vaccinates.

The concern with epizootic infections such as foot and mouth disease (FMD) and classical swine fever (CSF) is that vaccination may mask the infection in vaccinated animals but not prevent spread to unvaccinated stock, thus impairing eradication procedures required by international agreement.
Vaccines can also be used to influence the physiology of an animal by raising immunity in the form of antibodies to hormones and other substances used to control growth, reproduction and sexual maturity. This can reduce the necessity for procedures such as castration needed to reduce aggression and to prevent tainting of meat (boar taint).

While there are vaccines available for many diseases in man and animals, for some diseases such as African Swine Fever (ASF) it has not yet been possible to produce effective products.
Vaccines come in many forms but consist basically of an antigen (or antigen precursor), which the body recognises as foreign and raises an immune response to its presence. This immune response can last for years and even when it has fallen to below an effective preventative level can be quickly re-instated from memory cells if re-exposure occurs. They also often contain an adjuvant which is a substance that carries the antigen and exposes it to the immune system in such a way as to maximise the response and a diluent or carrier medium to protect the antigen prior to delivery.

Most antigens are protein like in nature and therefore sensitive to storage conditions. The need for refrigeration is a major issue in hot climates and much work is directed to finding antigens stable at high temperatures.

Of particular concern in farm animals is the ability to be able to differentiate vaccinated animals from those exposed to natural infections. In the case of diseases such as FMD where there are international trade implications this currently means that in the EU, all animals vaccinated to prevent spread in an outbreak, must be slaughtered as soon as possible after control has been established as these animals will subsequently test positive for FMD antibodies.

For some diseases such as CSF, and Herpes viruses such as Bovine Herpes Virus (BHV1) and Aujeszky’s disease marker vaccines have been developed. In these vaccines genes have been deleted from the viral genome which with the use of monoclonal antibodies enables vaccinates to be differentiated from wild infection by testing for the missing protein in the viral envelope. It is hoped that the development of marker vaccines for FMD and bovine Tb will enable animals to be protected without endangering national eradication schemes. Progress to date has been slow and hampered by the need to obtain global acceptance of such techniques in order to avoid trade disputes.
The use of adjuvants, by increasing the efficacy of a vaccine enables a lower load of antigen to be delivered, making the vaccine both cheaper to produce and market, whilst potentially reducing the risk of adver...