Vaccination is an important preventive measure promoted by equine veterinary surgeons to help combat infectious diseases and, ultimately, improve the health and welfare of horses under their care.

Some key questions are posed regarding vaccination – for example:

• What should we be vaccinating against?
• When is the best time to administer vaccines?
• How do we best use those vaccines that are available to us in the UK?

To assist in answering these questions and optimising the benefits from vaccination, a good understanding of the epidemiology of specific diseases is necessary (Panel 1).

Determining what we should be vaccinating against will depend on individual circumstances and be premises-specific to the risks for diseases occurring in that population.

Assessing – and ideally vaccinating – the entire population can be a challenge with the nature of clinical equine veterinary work on equine premises, with premises often using horses for multiple purposes and with numerous veterinary clinicians from multiple practices being involved.

Disease risk analysis for individual premises should consider:

• the age and use of horses on the premises
• the likelihood of exposure to an infectious disease – for example, considering the threats from new arrivals, particularly of unknown health status; personnel moving between premises; horses travelling nationally and internationally; presence of high densities of horses in the vicinity; and whether a biosecurity plan is in place
• a biosecurity plan reflects the premises’ preparedness for, and approach to, an outbreak if one occurs – for example, availability of isolation facilities, owner awareness and knowledge of infectious disease control, and willingness to stop movements on and off the premises

General principles of vaccination remain important, in particular that primary courses should be given well in advance of potential exposures and, wherever possible, administration of subsequent booster vaccinations are strategically timed to optimise protection conferred by vaccination during periods of highest disease risk.

In addition, a need exists to remain realistic regarding expectations from vaccination:

• Vaccines will work to reduce the infection risk, but may not always prevent disease.
• Most vaccines will not impart protective immunity immediately.
• Specific administration protocols outlined by vaccine manufacturers should be followed (see details in Table 1).
• Not all horses in the population will respond equally well to vaccination; some may have better protection and/or a longer duration of immunity compared to others. However, it is generally not possible or practical to determine if a horse has a protective level of immunity following vaccination.

Vaccination should always be seen as one of several weapons in the armoury against infectious diseases and their use will be optimised through the complementary application of other disease control procedures. The most important of these is a biosecurity plan, which should ideally have had veterinary involvement during its design – it should be realistic, specific to the equine premises concerned and be closely adhered to across the site.

Table 1. Protocols for infectious disease vaccines
	Equine influenza	Equine herpesvirus-1,4	Strangles	Equine viral arteritis	West Nile virus
Groups administered to and justification	All equids regardless of use as virus is highly contagious and airborne.	Ideally all equids, to improve herd immunity to infection.	Identify high-risk groups.	Breeding stallions to stop them becoming chronic virus shedders. Routine vaccination of other equids is not recommended.	Horses travelling to, or residing in, infected countries, such as parts of Europe and the US.
Groups at highest risk from infection	Horses on premises with: • high horse movement, including new arrivals of unknown disease history • geographical proximity to unvaccinated animals • no vaccine and biosecurity protocol • young animals	• Pregnant mares. • Yearlings. • Young performance horses. • New arrivals. • Horses travelling nationally and internationally. • Any factor resulting in stress. • Mature horses thought to be at greatest risk of developing neurological disease.	All horses are at risk. Those at highest risk include horses on yards that: • do not adopt a biosecurity plan that includes laboratory screening for Streptococcus equi • contain youngstock • fail to screen for and eradicate carriers during an outbreak • have high horse movement – for example, competition horses	Breeding horses, particularly stallions. Horses travelling to regions where disease is endemic.	Horses in, or travelling to, countries during vector season, where infection is possible.
Age of animals for commencement*	Licensed from five to six months of age. Usually recommended to commence from at least six months old, when maternally derived antibody levels have declined.	Licensed from five months of age.	Licensed from four months of age.	Licensed from nine months of age.	Licensed from six months of age.
Primary course requirements*	V1 at day zero. V2 four to six weeks later.	V1 at day zero. V2 four to six weeks later.	V1 at day zero. V2 four weeks later.	V1 at day zero. V2 three to six weeks later.	V1 at day zero. V2 three to five weeks later.
Onset of immunity*	Two weeks after V2.	Not reported, but likely to be two weeks after V2.	Two weeks after V2.	Three weeks after V2. However, some horses may not demonstrate a response after V2 and require a third primary dose to illicit a response.	Three weeks after V2.
Booster frequency* and timing of administration	First booster (V3) given five months after V2. Licensed for annual administration, but scientific evidence suggests a more frequent administration will provide a greater level of protection. Many benefits from increasing the frequency of boosters to biannually. This is particularly important when mixing of horses is occurring, a region has heightened virus activity, or all horses are the same amount of time from vaccination Pregnant mares should receive a booster four to six weeks before foaling.	Non-breeding horses should receive six-monthly boosters.

Following the primary course, pregnant mares should be vaccinated at five, seven and nine months of pregnancy. Every three months to maintain immunity. A priming response is maintained for up to six months after primary vaccination; therefore, boosters can be given up to six-month intervals. Can also booster horses strategically when risk is increased (must have previously had a primary course) – for example, during an outbreak or during the competition season. Every six months, but usually administered before the breeding season commences. It is reported that a degree of protection should be achieved with annual boosters, but this schedule has not been fully validated. Additional requirements** The highest level of immunity is detected two to four weeks after booster vaccination. Timing boosters for when risk of exposure is highest is advised. It may also be advisable for a population to stagger its vaccination to avoid immunity dips in the population. When using vaccination, a population approach should be applied by ensuring all resident horses are vaccinated on a premises and additional, complementary biosecurity measures are adopted. Be aware that vaccination with the currently available product in the UK will affect interpretation of S equi serology. This is of particular note when conducting post outbreak testing. Equine viral arteritis is a notifiable disease in the UK. Vaccinated horses will become seropositive as the available vaccine does not have differentiation of infected from vaccinated animals (DIVA) capability. Therefore, stallions must be blood tested and confirmed to be seronegative prior to first being vaccinated, and results should be recorded in the passport. West Nile virus is a notifiable disease in the UK. Vaccination does not have DIVA capabilities, so may interfere with serological testing. * Will be dependent on each product’s data sheet; timings of booster vaccination will be determined by the evidence obtained from scientific testing that the vaccine has undergone. ** Additional requirements to improve the efficacy of vaccines administered includes effective vaccine storage. This involves maintaining the vaccine at a constant temperature, as directed by the specific product data sheet.

Important UK endemic infectious diseases with an available vaccine

Equine influenza

Given how contagious equine influenza (EI) is – and its endemic nature – we should endeavour to ensure all horses in the UK are vaccinated against it throughout their lifetime, and irrespective of their use and the population they reside in.

EI vaccination protocols should be commenced from around six months of age, when maternally derived antibody that is known to interfere with vaccine responses has declined (Table 1).

Timing of vaccination is important, with immunity from booster vaccination declining with time (Newton et al, 2000). An improved level of immunity has also been demonstrated in horses that have had a higher number of EI vaccine doses in their lifetime (Barquero et al, 2007; Ryan et al, 2015).

Another factor to consider, especially when vaccinating large populations at the same time, is that this population will all face the dip in immunity over time at similar time points, with this being a risk factor for infection (Gildea et al, 2019). A benefit may, therefore, exist in varying when each horse in the residing population receives its vaccination.

Three EI vaccines are available in the UK, each a specific vaccine type containing different combinations of viral strains. Following completion of the primary course and first booster, subsequent boosters should be administered a minimum of annually and as determined by the manufacturer’s data sheet for the product.

However, research has demonstrated the benefits from more frequent vaccination to prevent large outbreaks occurring. Mathematical modelling has shown the risk of large-scale outbreaks is reduced if the population is largely vaccinated. With time since last vaccination also being important, increasing the booster frequency will ensure avoidance of these immunity dips when more time has passed since last vaccination.

With the risk for new strain incursions, too, ensuring the vaccines used contain the most up‑to‑date strains is advised and manufacturers must continue to update their product in a timely fashion.

Increasing the vaccine frequency is especially important when heightened virus activity has been demonstrated in a region, or when horses are moving and mixing – such as when attending national and international events.

Equine herpesvirus

One vaccine is commercially available for equine herpesvirus (EHV)-1 and EHV-4, with a population-wide approach to EHV-1,4 prevention being applied wherever possible, with biosecurity measures such as isolating new arrivals undertaken alongside whole population vaccination.

High-risk populations for infection are recognised, including pregnant mares, youngstock, and equine premises that have high numbers of horse movements and animals that travel nationally or internationally.

Strangles (Streptococcus equi)

Given the “healthy carrier” state that recovered strangles cases may have, a high risk exists for uninfected horses being inadvertently exposed to Streptococcus equi infection. Vaccination can be used to assist in reducing the number of infected horses if the likelihood of them encountering the bacteria is high.

As strangles vaccination induces a relatively short‑lasting immunity, horses will require frequent boosters.

One vaccine is licensed in the UK, which is based on a live-attenuated strain of the bacteria and is administered in the inside of the upper lip.

The effects of adopting vaccination on the interpretation of laboratory testing must also be considered as the vaccine bacterial strain will give rise to positive PCR results and stimulate detectable antibodies that cannot be differentiated from those arising from natural infection.

Research has led to the design of a new protein‑based vaccine, Strangvac (Intervacc). Strangvac’s differentiation of infected from vaccinated animals (DIVA) capability usefully enables the application of serological screening of a vaccinated population if required – for example, during a strangles outbreak investigation and control programme (Robinson et al, 2018).

When commercially available, it has been suggested that previously primed populations would benefit from booster vaccinations at times of increased risk – for example, prior to mixing with other populations in the competition season, or during an outbreak on a premises.

Important UK emerging infectious diseases or exotic threats with an available vaccine

Equine viral arteritis

Currently in the UK, only breeding stallions are advised to be vaccinated and serological testing must demonstrate freedom from infection prior to initiating vaccination courses.

One vaccine is commercially available – this is an inactivated vaccine administered IM and it does not have DIVA capability.

West Nile virus

Two vaccines are available in the UK, and licensed to reduce viraemia, severity and duration of clinical signs.

This is an important emerging zoonosis in Europe and UK clinicians should be familiar with its epidemiology (Panel 1).

Currently, vaccination is advisable for horses in the UK that travel to regions with endemic West Nile virus (WNV) during the mosquito vector season. This situation could change with WNV cases being confirmed across Europe and there being the potential for a case in the UK.