Brigadier Paul Jepson MRCVS, Chief Executive The Horse Trust
Vaccination is universally recognised as the key to disease prevention. Sometimes called immunisation, it is the administration of an agent (the vaccine) that stimulates the body’s own immune system to protect itself against disease. Typically this is disease caused by infection with viruses and bacteria but there are increasing scope to apply the principle to other types of disease including cancer.
The body’s immune reaction can be divided into the innate response and an adaptive response. The innate system is an inflammatory response to what it identifies as an invasion of foreign material called an antigen. The response is non specific, immediate and requires no previous exposure to that antigen. For example “clean” young foals, 2-3 months old, challenged with Equine Herpes Virus will develop the disease but recover before any specific antibody is produced. The “adaptive” response is specific to a particular antigen, for example the toxin causing tetanus or parts of the bacterium causing strangles. It is slow, taking days to produce circulating antibodies but it does get better each time it is challenged and after the first exposure to a specific antigen it can prevent the disease developing.
The ideal vaccine is effective, long lasting and safe. They are broadly classified as either dead or live. Dead vaccines cannot produce infectious disease but they do require at least 2 doses, and then boosters to remain effective. To enhance the stimulation of the immune system and the efficiency of the dead vaccine it is mixed with an adjuvant and manufacturers use different adjuvants varying from simple irritants (Alum) to the sophisticated ISCOM (Immune stimulating complex) and Carbopol. Live vaccines replicate in the body simulating infection and stimulating immunity but without disease. They have to be modified in some way to make them safe and this is where a lot of development is currently focused, typically tagging DNA from the antigen onto a harmless live carrier virus such as Canary Pox.
Is vaccination safe? The answer to this is almost invariably yes. It is not possible to over vaccinate and potential vaccines are subject to stringent testing before licensing. There are exceptions such as the live African Horse Sickness vaccine which can cause the disease and fatalities but when faced with the alternative it still makes vaccination a sensible choice.
Are vaccines effective? Again the answer is almost invariably yes, but for the vaccine to work and stimulate immunity, the animal has to be in good health to react. It is important that the vaccine is matched to the microorganism that it is designed to protect against. Influenza viruses are notorious for evolving, a process called “antigenic drift”, and the manufacturers constantly struggle to make sure their vaccines are matched to what is perceived as the current threat based on information from the OIE (World Organisation for Animal Health) and our own Animal Health Trust. There are currently no recommended changes to the composition of influenza vaccine.
What of the future? The refining of existing vaccines and the development of new vaccines continues. There are newly licensed, highly effective, vaccines to protect against the threat of West Nile Virus. There is the exciting prospect of vaccines against Grass Sickness, sarcoids, RAO (heaves), and sweet itch. The value of vaccines has never been better illustrated than by our need to have a safe and effective vaccine against African Horse Sickness which could decimate our horse population and the whole equestrian industry. Such is the threat that we have secured the establishment of an EU bank of vaccine for use in controlling an outbreak in Europe. Sadly the existing vaccine is neither very safe nor very effective but we have every reason to believe that a new generation of high tech African Horse Sickness vaccines will be available within a few years.