Equine Immunology and Vaccination

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by Ben Sturgeon, Bsc, BVM&S, Cert EP, MRCVS

Any organisms’ ability to withstand infection and disease is dependent upon its immune system. The system provides several layers of protection, both physical and biological, increasing in the complexity as we delve quite literally deeper into the body.

The first layer is the “physical barriers”. The skin, mucosal lining of the gastrointestinal tract and cilia “feathers” and mucus of the respiratory tract and good examples of physical barriers where generally only traumatic or overwhelming infection allow pathogen (bacteria, virus, worms etc) entry; a less recognised but important one would also include the normal “commensal” bacterial populations which provide a living barricade to abnormal bacterial populations accumulating on the skin or of bacteria and protozoal populations accumulating within a gastro-intestinal tract.

The second layer of immunity is known as “non-specific immunity”. Saliva, for example, contains antimicrobial secretions (known as “humoral factors” – proteins and lysozymes) that will kill oral pathogens and “non-specific defence cells” within the blood stream such as white blood cell macrophages which will engulf (phagocytose) some foreign bacteria within the body even without previous exposure.

These first two layers are generally known as the “innate immune system” and are the horses first barrier to infection and is relatively species specific (foe example, horses don’t get human influenza or tuberculosis).

However, once a foreign material or pathogen gains entry the “adaptive immune system” provides the next layer of protection. This is based on specific cellular targeting after recognition of a foreign “antigen” or infective particle.

Once recognition is achieved either, or both, an antibody response is mounted (a B cell response and neutralisation) or cell mediated immunity (a T cell response and destruction) is initiated both of which will induce foreign cell death.

A very important aspect in this is that such immunity is “acquired.” Whilst the adaptive response is precise and targeted, it takes time for the immune system to develop its “weaponry” (up to 96 hours and in some cases 2 weeks) but will generally maintain some memory of the infection providing then, protection for as long as the recognition is maintained, usually many years. This is well recognised, in many species, for example a human baby may succumb to a gastro-intestinal infection unless bottles and milk are sterile compared to adults who would have acquired immunity to bugs such as E.coli unless they are exposed to new strains a la “Dehli-Belly.” The same holds true for horses, yearlings and young horses for example are far more susceptible to respiratory infections including influenza, herpes and streptococcus equi (Strangles) than older peers.

The ability to develop acquired immunity was perhaps first and best illustrated by the famous/infamous smallpox protection afforded to milk maids and recognised by Edward Jenner who produced the first vaccine utilising cow pox scrapings and this simple technique has led to the world wide multi-species acceptance of vaccines.

From an equine perspective, the commonest use is in protection against respiratory and abortogenic viruses and for the bacteria Tetanus. A new trial for Grass Sickness has also recently finished although the results are not yet published.

The common respiratory infections include influenza, herpes virus 1 and 4, picornovirus, adenovirus, viral arteritis and strangles. Of these equine influenza is commonest and results in a highly contagious respiratory disease which can result in lengthy recovery times, significant medical complications and even death. As a result vaccination has been made compulsory in effectively all competition horses. The guidelines for the vaccination are based on racing rules requiring 3 vaccines over the first year (most people will start vaccinating at approximately 6 months old).
• First vaccine – day 1
• Second vaccine – between days 21-92 after the first vaccine
• Third vaccine – between days 150-215 after the second vaccine
• With a booster before 365 days.
However, the ruling does not necessarily agree with vaccine manufacturer recommendations and your vet can advise you the most appropriate tmes to undertake the primary course.

Of perhaps less recognition is Equine Herpes Virus, an equally common, although harder to diagnose, respiratory infection which can also be responsible for abortion in pregnant mares, a severe neurological disease, and severe disease in neonatal foals. The important aspect of herpes is that whilst clinical signs “may” be relatively mild (fever, cough and nasal discharge), older and infected horses can pass on the virus initiating severe disease without showing clinical signs themselves. This is because once the horse is infected, the virus can remain latent in the horses’ nerve endings for some time before shedding again (usually because of stress). Vaccination for herpes is not compulsory, may be dependent upon the nature of your horses role &/or your activities, but would be recommended in competition horses, large yards or where breeding is ongoing. The protocol is:
• 2 doses 3-6 weeks apart
• a booster every 6 months.
The reason that a booster is required twice yearly is because immunity is only partial following vaccination (which is typical of the clinical condition where immunity is also poor following infection.) Where abortion due to herpes requires protection the vaccination programme is slightly different requiring doses at 5, 7 and 9 months of pregnancy. Currently no vaccine is licensed specifically against the neurological form of the disease although, thankfully, the condition is very sporadic.

Whilst also not compulsory in competing horses the vaccination against tetanus is essential in any horse of any age. Tetanus can affect people and all domestic animals but horses are most susceptible. The causal bacteria lives in soil and faeces, and the spores survive for tens of years. The bacteria gain entry to the body via potentially innocuous and small wounds including skin cuts, foot abscesses and even oral abrasions when rasping teeth. The bacteria then produce a neurotoxin absorbed into the central nervous system. Whilst no vaccine will “prevent” an infection occurring, all will reduce the severity and length of any infectious shedding. This is best illustrated with the tetanus vaccine where an infected unvaccinated horse has a fatality rate of up to 95% compared to an infected vaccinated horse which has a survival rate of 95%! Because of this stark evidence and because of the environment horses live in and their propensity to injure themselves, it is arguably a moral issue to ignore tetanus vaccination. The typical course for tetanus vaccine is 2 doses 4-6 weeks apart followed by a booster at 12 months. The horse can then receive further boosters once every 2 years.

Whilst you can never completely prevent your horse from infection, it is only fair to consider your horses’ role and activities, your relationship to other horses nearby, and the potential disease status of the yard or premises and then to decide what the most appropriate protocol would be to protect both your horse and those around you, whilst taking into account the “legal obligations” from the various competition rulings. It is important to remind ourselves of the effectivity of vaccines. It is estimated the Jenners discovery has led to the saving of 5 million lives annually, measles vaccine is estimated to save 1.6 million lives annually. Whilst no such figures or examples are available for equid diseases, it serves as a salutary reminder that prevention is far better than cure.

Author: The Editor

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