Herd immunity: what is it and does it protect against infectious diseases?

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Recently there have been several confirmed outbreaks of equine flu in the UK and around Europe. As a result of that, vet Donal Murphy, MRCVS, MVB, is here to confirm what herd immunity is and whether it can help prevent an outbreak on your yard.

Your Horse magazine reader asked: what is herd immunity? Is it something I should raise with the yard owner when taking my horse to a new home? He’s only a baby and hasn’t had all his vaccinations yet.

Donal explained: Herd immunity occurs when a significant portion of a herd has been vaccinated. It allows for individuals who have not developed immunity to have a measure of protection against the disease(s) vaccinated against. 

This minimises the spread of some contagious diseases, such as equine flu. However, for other diseases, such as tetanus, vaccination of one or more horses in the herd has no effect on the susceptibility of an unvaccinated animal. 

If a majority of the horses are vaccinated against these contagious diseases, there will be a reduction of their circulation. If a low number of animals are susceptible, the spread will be stagnated. 

Important factors

Herd immunity is indeed something you could discuss with your new yard and vet. The two important factors influencing herd immunity at one specific location would be the isolation of new animals entering the yard and the vaccination history of animals already present at the yard. 

Ideally, animals should be isolated for around 30 days (and certainly the isolation period must not be less than two weeks). This allows symptoms of systemic disease to arise and appropriate testing to occur.

Horses should be vaccinated according to risk, with influenza and tetanus being the most common diseases vaccinated against. Your vet can advise you further on which vaccinations your particular horse would need.

Donal Murphy is a veterinary surgeon and head of technical and regulatory affairs with the National Office of Animal Health (NOAH). He was previously a vet with the Animal Health Agency. 

Did you know?

·     Flu can travel up to 150ft

·     It can be passed on clothing, tack, tools and hands as well as contact between horses

·     Horses remain infectious for up to 10 days following symptoms

·     Vaccination is best way to prevent your horse becoming a victim 

Read more about equine flu and the recent outbreaks here

Read the full article in issue 446, available here. 


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