Ben Sturgeon, BSc, BVMS, Cert EP, MRCVS

The term vaccination comes from the Latin vacca or cow, and was coined when the first innoculations were given with organisms that caused the mild disease cowpox to produce immunity against smallpox. For centuries, the smallpox virus caused a serious, debilitating and fatal disease in people. Once someone became infected, there was nothing that could be done. In the late 1700s, Edward Jenner noticed that milkmaids who developed the mild and temporary cowpox virus, picked up from the dairy cows, did not become infected with smallpox. As an experiment, he intentionally infected people with the cowpox virus, also known as vaccinia. As with the milkmaids, these people did not get smallpox. The procedure of using a similar substance to prevent viral infection then became known as vaccination.
There are many vaccines available now for use in horses - influenza, herpes, tetanus, viral arteritis, and one we will almost certainly be using at some point - encephalitis. Whilst influenza vaccination is under strict governance by the Jockey Club and the F.E.I. the remaining vaccines and their use are largely down to the opinion of the horse owner, cost factors, use of the horse and previous experience. Certainly it is something we commonly take for granted but with the controversy surrounding MMR and now the Swine Flu jab perhaps it is time to actually consider what we are putting into our horses.
Most vaccines contain two essential components, and antigen and an adjuvant. The antigen is the part of the vaccine that specifically mimics a part of the pathogen (virus, bacteria or ricketsia). The antigen can be a protein or other chemical fragments of the pathogen, or they can be whole dead pathogens. Some viral vaccines contain live viral particles that have been altered enough to render them harmless, but they carry enough fragments of the original pathogen that they stimulate the immune system (a modified live vaccine) and we can include the smallpox vaccine and most equine vaccines in this. There are then 3 types of vaccine
.Component vaccines, usually coupled with an adjuvant to enhance their effect
.Killed vaccines which contain killed but whole pathogens, again mixed with an adjuvant
.Modified live vaccines
The immune system recognises and reacts to the antigen contained in the vaccine but only if it has been exposed to it at least once before. This is referred to as an anamnestic response. The immune system has an almost perfect memory, except that it fades, although how long this takes varies from weeks to months to years dependent on the antigen and individual host factors.
Once vaccinated, if a horse is exposed to the pathogen, the immune system is primed to mount a rapid response to the infecting process. For most vaccines, a vaccination program has to entail the administration of at least two doses separated by several weeks. The first dose acts to prime the immune system in the short term, but does not generate much of a response. It's like the first time you hear something in school, the concept is planted in your brain, but you need to go back and study it again or at least in my case the memory fades. Boosters then massively stimulate the immune system, since the system is primed and it remembers the antigen. After the booster, the body produces billions of antibodies which lodge in tissues, on body surfaces and in the blood awaiting the foreign antigen to attack. The concentration of antibodies (the titer) in the blood gradually diminishes over time and so the horse's immunity must be maintained on a regular basis using boosters. Whilst Jockey Club rules state, at least with influenza or flu, that a booster must be given once annually horses that travel frequently or mix with young stock where exposure to viral strains would be common, boosters can be given more frequently.
There is a powerful argument that boosters should not be given where not required, instead titres should be measured to ascertain the level or readiness or necessity of vaccination. However, titers are very hard to interpret.
.Antibodies are not the only defence, so titers do not estimate the whole potential immune response.
.No one knows what a suitable titer level would be in the face of infection. In a mild infection, the horse may need only a few antibodies, whereas in a severe infection huge numbers would be necessary.
.You would need to spend a lot of money testing the titers of all the potential pathogens, and this would cost much more than the vaccines themselves.
.The treatment of the disease in an unvaccinated animal can be significantly more expensive.
.In some cases the unvaccinated animal will almost certainly die from the disease, e.g. tetanus and occasionally even flu.
In these circumstances then it would appear that it would be far simpler just to go ahead and vaccinate. But we should be aware that vaccines are not a silver bullet panacea preventing any and all future illnesses against the vaccinated agent, far from it, there are plenty of reasons why they may not work and the horse becomes ill.
Improper storage - many vaccines are live and are actually quite delicate, dieing when they are exposed to heat or sunlight, refridgeration and strict observance of the use by date are important.
Vaccines are not effective against all forms of a single virus - whilst a flu vaccine will provide protection against the disease it will not completely prevent infection. Viruses alter their genetic composition (and so infectious nature) regularly so although not a brand new virus changed enough to cause a new infection. The vaccine will give some level of protection but not exclusively so, it will halt a severe disease leaving only mild symptoms, it will mean a far shorter length of illness and it will result in significantly lower level of viral shedding so reducing spread. Compare tetanus in vaccinated and unvaccinated animals where the death rate from infection is above 80% and less than 10% respectively.
Not enough time was given for the vaccine to work - proper vaccination relies on a good anamnestic response, studies have shown that many vaccines require more booster doses than even the manufacturers recommendations. The minimum number of booster doses required is two, but many will give a far more effective response if three are given, hence the Jockey Club requirement for flu to be given 3 times (day 1, between days 21-92 for the second booster, and between 150-215 days for the third dose) and the F.E.I requirement for flu to be given every 6 months.
The horse comes across a new strain of virus after vaccination - many pathogens, especially viruses, have become successful by being able to change their antigenicity. This means that they are able to switch the proteins that they produce - the very proteins that your horse is primed to recognise. Influenza is notorious for this and hence the necessity for vaccines to be regularly updated, just look at swine flu.
The horse mounts the wrong type of immune response - sometimes the wrong type of antibody is produced.. for example the antibodies that fight disease at mucosal surfaces, such as the lining of the nose, are called IgA. The antibodies that fight disease at the level of the tissue are called IgG. Most vaccines are given by intramuscular injection and stimulate the production of IgG. If as with strangles and influenza, the disease is transmitted at the mucosal level, a vaccine that stimulates IgA will be more effective. However, presently these vaccines are still in development and we have to use what is available.
Individual inability to mount an immune response - an individual animal contributes greatly to how well a vaccine works. If the horse is sick, is being treated with immunosuppressive drugs such as steroids, or is very young having high levels of maternal antibody, then the vaccines efficaciousness will be impeded.
Perhaps all this is a little academic, we surely must vaccinate against tetanus and eventually also encephalitis (if nothing more because it can be transferred to humans), and if we intend at all to compete then influenza is necessary. However, remember the first rule of medicine - do no harm, and that means we should not stick anything into our horses we do not understand or consider its benefits and shortcomings equally, only then can we really do what we consider best and right for our whole equine population.


 go back to the previous page