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February 18, 2019 | by verity.ramus@hooknortonvets.co.uk | Equine

More cases of equine influenza have been identified in several areas of the UK – the Animal Health Trust websitewww.equiflunet.org.uk is being kept updated with information about all cases diagnosed during this outbreak. The direct link to the case information page is
https://www.aht.org.uk/wp-content/uploads/2019/02/Equiflunet-update-14-02-19.pdf. More general information about Equine Influenza can be found at https://www.aht.org.uk/disease-surveillance/equiflunet/equiflunet-for-horse-owners

‘Florida Clade 1’ Explained

(Click the gallery image below to see a larger diagram)

This diagram shows how Equine Influenza viruses have evolved since the 1960s. The different branches of the diagram show where the virus has undergone a significant change in the antigens it displays to the immune system. Viruses are changing all the time – this is why vaccines are updated from time to time, to try and keep up with changes in the viruses that are actively circulating in the population.

Current advice is to use a vaccine containing an influenza strain from the Florida Clade 1 – both the vaccine brands we stock contain a Florida Clade 1 strain.

The reason for the current outbreak is that the strain of Equine Influenza circulating in the UK horse population has become sufficiently different from the strains in the vaccines that even vaccinated horses are only partially protected from the strain currently causing disease. A high antibody count is therefore essential to ensure that your horse is as well protected as possible. This is the reason for our advice to give a booster vaccination if your horse has not had a booster in the last six months.

How does vaccination work?

1. First exposure to antigen – antibody production begins 7 days later.
2. Antibody levels peak 4 weeks later.
3. Antibody levels fall rapidly
4. Re-exposure to antigen e.g. booster vaccination. Immune ‘memory’ is activated with rapid production of antibodies
5. Antibody levels peak much faster and higher than before
6. Long-term production of antibodies provides long-lasting immunity

As you can see from the graph, the antibody count falls over time. Once the antibody count falls below a protective level, the horse becomes susceptible to the disease again. In the current outbreak, the antibody count needed for a horse to be protected from influenza is much higher, because the antibodies can only provide partial protection. An early booster vaccination keeps the antibody count high.

Why can’t we use a newer vaccine?
There isn’t yet a vaccine available which can fully protect against the current strain of Equine Influenza. Developing a new vaccine and testing it to make sure it is both safe and effective takes a long time – more than a year!

Written by Sally Hodgson, BSc (Hons), MA, VetMB, Cert AVP (EM), MRCVS.

Written by verity.ramus@hooknortonvets.co.uk

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