Vaccination : principles and good practices

Vaccinating consists in injecting a pathogenic agent (virus or bacteria) rendered harmless, or a part of the pathogenic agent, into the horse’s system. There is therefore no risk of causing the actual disease. On the other hand, the body’s defense system, called the immune system, recognises the agent  thanks to molecules known as antibodies, and practices fighting the agent. The body will then recognise the pathogen more quickly, and will be more efficient in fighting the disease if a real attack should occur. This is known as immune memory.

If we take the example of a foal born to an unvaccinated mare, and subject to his first time vaccination against equine influenza, the chart below can be observed:

• After the first injection of the first time vaccination (V1), weak immunity, lasting a short time is observed (1).
• After the second injection (V2), 4 to 6 weeks later, protective immunity is acheived, but also for a short time (2).
• After the third injection (V3), 5 to 6 months after V2, protective immunity, stable over time (3) until the booster jab 6 months to 1 year later (V4).
• On the other hand, if V3 is omitted, the horse will not be protected until the following booster jab.

Vaccination is an individual protection as it reduces the risk of the individual animal becoming infected and/or also reduces the severity and duration of the clinical symptoms. It does not however enable to completely prevent the disease in any circumstance.

This individual protection is however very important for serious, or even fatal diseases, such as tetanus or  West-Nile fever

With regard to contagious diseases, vaccinating also reduces the excretion of the pathogen by the sick animal, thus limiting transmission to other animals. The greater the number of vaccinated horses against a disease, less the pathogenic agent is likely to spread, and therefore the risk of an epidemeic is greatly reduced. Comparing the situation in France and the UK concerning equine influenza is a good illustration of the importance of herd vaccination. In France, over 80 % of all equids are vccinated against equine influenza, compared to 50 % in the UK. Between 2015 and the end of 2019, France has not reported any cases of equine influenza, whereas the UK  has recorded cases every year.

This « herd » vaccination limits the economic impact of  these diseases on the horse sector and protects the animals most at risk (gestating mares, foals, old or sick horses).

In France a vaccine can only be commercialised if it has obtained an official authorisation (AMM) delivered by the National Veterinary Medication agency (ANMV). Obtaining the AMM requires prior studies to demonstrate the effectiveness and harmlessness of the vaccine.

The main diseases for which equids are vaccinated are equine influenza, Equine Herpes Virus (EHV)  types 1 and 4 (commonly known as rhinopneumonitis) and tetanus. These diseases are either responsible for severe clinical symptoms (tetanus), or have serious economic repercussions (influenza and EHV).

For some diseases, even though of major importance for the horse sector, there is no commercial vaccine in France, this is the case for example for :
•    Rhodococcus equi, a bacteial disease against which auto vaccines can be administered. Biological samples are taken by the veterinarian from the stud farm concerned, the bacteria is then isolated and auto-vaccines are manufactured in an authorized laboratory to innoculate the gestationg mares in the stud.
•    Equine rotavirus, viral disease for which vaccines can be imported from other EU states, following an authorisation by the ANMV (national agency for veterinary medication)

When and how often?

•    The horse is in good health, any underlying disease could interfere with proper immunity. The veterinarian should therefore conduct a clinical examination of any horse he is vaccinating

•    The horse is not undergoing any treatment with cortisone based medication administered systemically, this could have an influence on the immune response after vaccination.

To optimize the effectiveness of the vaccine, the horse should not be innoculated during a stressful period (competition, weaning, transport, high intensity work, change in living conditions…) or prior to any hazardous period (e.g. change of yard, competition season).  Thus, it is recommended to have carried out the last injection in the first time protocol at least 15 days before exposing the horse to a risk of infection (e.g. change of yard, or start of the competition season). In the same way, the annual booster should be given at least two weeks before the risky period. 

Vaccinal consultations enable you to detect any abnormalities which may have gone unnoticed by the horse owner or custodian.

In some cases, an extra booster in addition to the normal protocol  may prove  beneficial.

•    For brood mares a booster jab (equine influenza, tetanus, EHV)  is recommended between 4 to 6 weeks before foaling, so as to protect not only the mare, but also to increase the concentration of antibodies in the mare’s colostrum and thus protect the foal during the first months after birth.

•    A horse taking part in shows or competitions where a large number of horses are present, even more so when the horse is a youngster, can benefit from being vaccinated twice a year to increase his protection against equine influenza and EHV : for example a jab at the beginning of autumn and one at the beginning of spring when the competition season gets into full swing.

•    Horses living on a stud farm or in contact with gestating mares can be vaccinated twice a year against EHV, even more so if they take part in a number of shows.

When should the vaccination protocol be initiated?

•    At birth, the digestive tract of foals is permeable to the passage of antibodies present in the mare’s colostrum. If the mare is properly vaccinated, the foal is protected by the dam’s antibodies thanks to immunity transfer. These maternal antibodies have an inhibiting effect on the immune response of the foal to a vaccine. This is why it is better to wait for the level of maternal antibodies to decrease before vaccinating a foal.

•    A young foal’s immune system is immature and post vaccinal immunity can therefore be restricted.

Should older horses be vaccinated?

For different reasons, even a correctly vaccinated horse may develop a disease:

• Most vaccines limit the clinical symptoms of a disease but do not prevent it entirely.
• Some vaccines do not offer full protection against all of the forms of the disease. The EHV type 1 vaccine only offers limited protection against the neurological form of the disease.
• When the pathogenic agent is present in very large amounts, or in a particularly aggressive form, the immune system can be overwhelmed.
• All horses do not respond in the same way to an injection of vaccine, and are not necessarily protected in the same way, due to their genetic heritage, but also due to an underlying disease (e.g. Cushing’s disease, very frequent in older horses, and which renders the immune system weaker). A study showed that 5 % of all horses presented a weak response to a standard vaccine against equine influenza. For the same reason, some horses show more sensitivity to diseases than others.

As for any medication, side effects can be observed following a vaccine injection. The following may be noted :

• Depending on the vaccine used, a local reaction at the point of injection of varying intensit, may be observed (swelling, pain, abcess, muscular stiffness).
• Rarely (1 to 10 horses out of 10,000) may show a general reaction (hyperthermia and/or transient tiredness).
• Very rarely (less than 1 in 10,000 horses) show a hypersensitivity reaction (urticaria, generalised oedema, anaphylactic shock) which require the rapid intervention of the veterinarian.

It is also highly recommended to inform either the veterinarian who carried out the injection, or the ANMV (national veterinary medication agency) through their website so as to keep a close watch on the risks and benefits of vaccines once they have been approved for commercialisation.

The side effects of the inactive vaccines used in France are, more often than not due to the adjuvant used, rather than the vaccine itself. Consequently, it is advisable to change from one commercial vaccine to another (using a different ajuvant) when side effects are noted.