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Equine 'Flu: Myth Busting

AHT ‘flu update 13 Feb 2019 AHT ‘flu update 13 Feb 2019

As with many aspects of life there are a variety of urban myths that float around Equine Influenza (EI) and vaccination strategies in particular. Social media turns into a frenzy around this kind of scenario and information is often distorted through sharing. No doubt everyone has their own views but as vets we regularly see a number of statements or opinions being shared that are based on uninformed opinion or misunderstanding. In light of this we've created a myth busting article to help with a most up to date information currently available. More updates will follow during the week to come.

Map showing location of outbreaks as of 13th Feb 2019

Myth: Most horses are vaccinated anyway, so isn't this all scaremongering to make money?

There are two parts to this suggestion: what proportion of the equine population in the UK is actually vaccinated against EI; and then professional integrity and equine welfare. Immunity obtained from vaccinating against a disease is imparted to both the individual but also to the whole population if a large enough majority is vaccinated (herd immunity). Herd immunity is achieved when over 80% of the population is vaccinated. At this level infection is unlikely to be able to spread and disease becomes increasingly rare.

Unfortunately, currently only around 40% of the equine population in the UK is vaccinated against EIV. In the south east, the percentage is closer to 60%, but still nowhere as high as we aspire too. So in fact, most horses remain unvaccinated in the UK. This in turn is evidence against a scaremongering tactic. As vets our priority is the animals' health and welfare and disease prevention is the best strategy to achieve this. Costs incurred from sick animals far outweigh a vaccination. 

Myth: These extra vaccines are just to drum up business and are another example of over vaccinating our animals.

Additional vaccinations have been advised to any horse that hasn't had a booster in the last 6 months; starting courses are recommended if horses are unvaccinated, and finally additional the third vaccines is ideally given to horses five months after receiving their second vaccine. Higher risk animals may have this vaccine recommended even at the four month mark, although the ‘third vaccine’ will still be necessary to comply with regulatory organisations by 215 days from the second. 

Where did these recommendations come from? Aren't they just proof that vaccines don't work in the first place? Not at all. This advice is based on evidence from solid research performed into vaccination responses over decades. Vaccination immune responses are incredibly complex but a very simplistic explanation follows.

The graph below shows the protection imparted when starting the vaccination course. The first vaccine has relatively little effect on immunity, but this improve to the first large spike two weeks after the second vaccination. Note the peak is between the green and red lines on the graph. The green line represents clinical protection - i.e. the level required to give immunity against the horse becoming ill. The red line on the other hand represents virological protection. Above this level viral shedding is unlikely. You can see that this level of immunity isn't achieved until 2 weeks after the third vaccination.

Why should we care about the red line? This level of protection is needed to minimise the spread of disease within the population. The more viral shedding that occurs the farther the virus can spread and the larger the scale of the outbreak. It is obvious by looking at the graph that before the third vaccine much of the protection is afford to the individual but will not reduce spread of disease, i.e. your horse will be much less likely to get ill, or at worst show mild signs, but will be perfectly capable of spreading the virus wherever he goes. After each annual or biannual booster, immunity stays high - the more regularly the vaccination, the closer we are to preventing spread of disease for longer.

How does this justify the extra vaccines we are advising? Well, unvaccinated animals are unprotected, so the sooner they become protected the less likely they are to get disease. They will still be able to shed virus for several months and will be susceptible to EI until a week or so after the second vaccine - but we can't do anything more other than restrict their movement to protect them.

24 weeks after the third vaccination; horses protect drops close to the green line, where clinical protection is achieved. This corresponds to between 5 and 6 months. It is not unreasonable to see why boostering horse if they haven't had a vaccine in the past 6 months is advised. This is basis on which the FEI rules are founded.

What about the additional vaccine between the second and third jabs? Again note the dip on the graph between week 9 and week 32 where antibody levels are below the green line. This is referred to as the 'immunity gap'. Manufacturers recommend the third vaccine is done 5 months after the second; but much of the time this is delayed until nearly seven months. This immunity gap is often even wider than shown here. Vaccinating horses earlier in this time period helps to 'close the gap', stimulating protection. This supports the manufacturers recommendation to vaccinate at five months from the second. The 150-215 day window stipulated by regulatory bodies such as the BHA is under review and may be moved closer to the second vaccine to provide the best protection possible. 

It is important to remember that protection from vaccination is multifactorial and despite the dip suggesting protection is poor at this time; field challenges shows good protection two weeks from the second vaccine until the third. This is highlights the complexity of the immune response to vaccination and how we can be mislead by looking at a single factor such as the antibody levels shown in the graph above.

Myth: This EI virus is a new strain and the vaccines are ineffective against it anyway.

This is completely untrue. The middle image above includes a column called 'clade of virus'. All of the infected cases so far have been FC1 (Florida Clade 1). This is a relatively old strain of EIV and was last seen in the UK back in 2007. All currently available vaccinations in the UK do offer protection to FC1, but they vary in the specific strain of virus they have used to create their vaccine, with some strains being more recent than others. The vaccination we routinely use has the most recent FC1 strains consistent with those causing current outbreak within them: South Africa '03 and Ohio '03. Whilst there have been some subtle changes to the virus these are expected to occur over time (antigenic drift) and vaccines will still provide good protection. Significant changes to a virus that result in vaccination being ineffective are called antigenic shift. This has not happened in this outbreak. All vaccinations commercially available offer protection to this outbreak; however, those containing strains closest to the virus causing disease will perform best.

One of the non-vaccinated animals reported with FC1 infection in Suffolk at the beginning of this week has had to be euthanased after becoming severely sick and despite intensive care and treatment - this surely highlights the severe pathogenicity of this EI virus strain in non-vaccinated animals and consequently the benefit of vaccination in attenuating severity of signs.

Richard Newton, Animal Health Trust, Feb 9th 2019

Myth: Vaccinations don't work. This is true because EI has been diagnosed in vaccinated horses.

Vaccination strategies control disease in several ways. They help to mitigate the likelihood of contracting disease, diminish the severity of disease if infection occurs and restrict the size of an outbreak by reducing viral shedding. It would be incorrect to suggest that EI vaccination will stop disease in 100% of cases; however it is critical in disease prevention and outbreak control. EI is costly, not only to the horse in terms of their wellbeing; but to the entire equestrian industry.

In 2007, the Australian EI outbreak cost the equine industry over £1 billion. Individual owners will not only experience fiscal frustration due to treatment costs; but that of restriction to their hobby or profession. As a rule of thumb EI cases should be rested for a minimum of two weeks following resolution of their cough; or one week off for every day they have a fever. In other words, a horse with a five day fever needs five weeks off. This is because the EIV is incredibly destructive to the lining of the airways, rendering the horse unable to clear respiratory secretions from their lungs effectively, dramatically affecting performance. This is the reality of the time required for the airways to heal successfully.

Infection rates in unvaccinated populations, such as in 2007 when Australia was an EI-free country, are around 100%. At the peak of that outbreak, 1000 horses were infected each week. Outbreaks within other countries have not reached such a scale, because of vaccination. There is no doubt vaccination works - it does exactly what it is supposed to and it is our most important tool, so please let's use it.

Myth: This pony doesn't leave the yard so there's no point vaccinating

Equine Influenza Virus (EIV) is airborne and spreads rapidly over wide areas given the right conditions. Many ponies that no longer venture off site share a premises, or even field, with those who do. Those ponies, even if vaccinated, are capable of not only carrying but shedding EIV, and will bring EI to the unvaccinated animal. Additionally, the equine population forms clusters in many parts of the UK, so spread from infected to non-infected premises is also possible.

Of course, should your premises experience an EI outbreak the unvaccinated animal is likely to bear the brunt of clinical disease. Sadly this has been made abundantly clear this week with the first death occurring in the this outbreak in an unvaccinated horse; whereas the& majority of vaccinated horses which have been diagnosed have had not signs, and the affected few have experienced relatively mild clinical signs. 

Myth: There are only a few cases anyway, it's all a fuss about nothing.

EI is endemic within the UK, which means the virus circulates constantly. So why don't we see disease more often? The reality is EI is probably far more common than we give it credit for, but is not diagnosed. This begs the question why not?

Firstly, if you review the clinical signs associated with EI: cough, nasal discharge, inappetence, temperature; the first thing that comes to mind in most owner's minds is not 'flu. How many times have you heard someone tell you their horse has had 'a bit of a virus'? Most of the time owners will keep an eye on the horse for a few days, only contacting the vet if there is a significant temperature (if it's checked) and if the horse doesn't bounce back to health in a few days. 

Despite the fact that we have access to free EI testing at all times, courtesy of the excellent Animal Health Trust's Influenza Surveillance Scheme, relatively few samples are taken and submitted outside of an outbreak. This limits are ability to recognise EI cases when they occur, so why is this? If you consider the graph above, the best time to identify EIV is 2-5 days after infection, when clinical signs are present. Bearing in mind what we have said above about monitoring the horse, vets often don't get to examine the horse until after the critical time window for virus shedding and so even if we do take a swab, the chance of detection is low. This explains why many cases are suspected but not confirmed. 

So what else can we do? Part of the free testing scheme includes taking two bloods samples, one at the first visit and a second at least two weeks later. Why? In this test we aren't looking for the virus itself, but rather looking for the antibody levels that horse has. If they have contracted EI then the second blood sample will show far higher level antibody levels. Unfortunately, this test requires time, and provides a retrospective diagnosis only. Horses will likely have recovered from illness two weeks earlier and much of the time the second sample is never taken. 

The take home message is this: EI cases do occur, but are rarely diagnosed. This explains our concern as a profession during this outbreak. It is likely far more cases have occurred but were not diagnosed. If we stand any chance of identifying the severity and extent of EI within the UK then we need to see horses at the right time. Take home message two: if your horse shows any of the signs, however mildly, and whether vaccinated or not, ring your vet and encourage sampling.

Equine Influenza and Vaccination: More Myth Bustin...
Flu update

Comments 1

Imogen on Friday, 15 February 2019 00:07

A follow up article is available with more interesting perceptions discussed!

A [url=]follow up article[/url] is available with more interesting perceptions discussed!
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Friday, 24 September 2021

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