International Codes of Practice on equine disease for 2023

Article by Victoria Colgate and Richard Newton

Horses are one of the most internationally travelled species, second only after humans, and this mobility, both between and within countries, means that the spread of equine infectious diseases is a very real and ever-present threat.

Indeed, infectious disease outbreaks are often related to new arrivals at, or movement of animals on and off premises, such as to competitions and race meetings. As well as having a negative impact on horse health and welfare, disease outbreaks can have further reaching consequences in terms of treatment costs, economic losses due to movement restrictions and an inability to compete, as well as disruption to the breeding schedule, which may have effects in racing in future years. Additionally, restrictions imposed in the face of the diagnosis of certain diseases can prevent the free export of horses between countries, impacting trade and equestrian sport. Infectious diseases are truly trans-boundary, and such a problem necessitates global cooperation and communication, echoing the mantra that ‘prevention is better than cure’. 

The origins and evolution of the Codes of Practice

figure 1

In 1977 in Newmarket, UK, there was widespread development of vulval discharge in thoroughbred mares post-covering that adversely affected their fertility and to an extent that caused temporary closure of stallion barns and stud farms. Initially unknown, the cause of this outbreak was later identified as the bacterium Tayorella equigenitalis, the causative agent of contagious equine metritis (CEM) and often referred to as the CEM organism, or CEMO. 

The impact on the 1977 breeding season was significant enough for the Horserace Betting Levy Board (HBLB) to consider a serious need to control CEM in future years and in reaction to this, the Codes of Practice (CoP) were created. First developed by HBLB committee discussion in the summer of 1977 and then formally published in 1978, the HBLB CoP outlined swabbing protocols in the weeks prior to covering in an attempt to prevent the venereal transmission of CEM. 

Despite the perceived draconian nature of these first codes, compliance was high and overall were highly successful—with CEM cases drastically falling following their introduction. In subsequent years, the CEM CoP was extended to include control measures on reproductive disease caused by the other venereal bacterial pathogens: Klebsiella pneumoniae (capsule types 1, 2 and 5) and Pseudomonas aeruginosa. Codes on the prevention and management of equine herpesvirus-1 (EHV-1) and equine viral arteritis (EVA) were also added following disease outbreaks of significant concern to the thoroughbred breeding industry in subsequent years. 

Today the CoP are referred to as the ‘HBLB International Codes of Practice’, with the 2020 CoP being the inaugural internationally branded edition and representing a comprehensive manual outlining a series of voluntary standards (codes) and advisories (guidelines), with accompanying appendices. They are intended to assist breeders, trainers and horse owners (in collaboration with their veterinary surgeons) to control and prevent a range of important infectious diseases in equids. The CoP have a broad application among thoroughbred breeders; and the UK, Ireland, France, Germany and Italy are all signatories. Additionally, in a translated form, they have formed the basis of written equine infectious disease advice in many other countries. The initial reduction and then sustained maintenance of low levels of infectious disease outbreaks experienced after the CoP’s introduction is testament to the document’s effectiveness and importance.  

The HBLB International Codes of Practice for the 2023 breeding season

The CoP are reviewed annually by a group of international veterinary breeding and infectious disease experts and stakeholder representatives. This review ensures that all advice is as current as possible regarding the latest scientific evidence and global disease situation. The CoP convey practical recommendations gained considering recent experiences with the occurrence and control of relevant outbreaks. 

The 2023 edition of ‘The CoP Manual’ (title cover, above Fig 1 and table of contents, below Fig 2) comprises:

figure 2

• Six Codes on the following diseases: CEM (covering CEMO, Klebsiella pneumoniae and Pseudomonas aeruginosa), EVA, EHV-1, equine coital exanthema (ECE, caused by EHV-3), equine infectious anaemia (EIA) and dourine 

• Five Guidelines on equine influenza (EI), piroplasmosis, strangles, West Nile Fever (WNF) and artificial insemination (AI) 

• Eleven Appendices with a range of  supporting information and guidance 

The CoP are published on the HBLB website (https://codes.hblb.org.uk/); a PDF download  (https://codes.hblb.org.uk/downloads/2023/Codes%20of%20Practice%202023.pdf) is also available. In Great Britain, the Thoroughbred Breeders Association (TBA) produces a spiral-bound printed hard copy for its members. 

The CoP outline each disease sequentially, using a standardised format of sections which include information on notification procedures, clinical signs, transmission, prevention, diagnosis, control, treatment, freedom from disease and export requirements. It is a document that continues to evolve over time in terms of both the diseases included and the expert advice imparted. 

Why ‘codes’ and ‘guidelines’?

Although the logic behind the distinction as to why specific diseases in the CoP are covered by a guideline rather than a code is not necessarily immediately initiative. It is worth remembering that a Code of Practice may be defined as ‘A documented set of recommended or preferred processes, actions or organisational structures to be applied in a given setting’, whereas a guideline is: ‘A general rule, principle or piece of advice’.  Therefore, the diseases applied as codes are those that directly relate to, and have an impact on, breeding and that necessitate particular actions either to prevent or control disease, should they occur. The guidelines, in contrast, are merely advisory measures to those involved in thoroughbred breeding businesses, but cover diseases and practices that are also highly applicable to other populations of horses. The remainder of the article outlines several of the important codes and guidelines.

Strangles

Strangles, caused by the bacterium Streptococcus equi, is a disease affecting the lymph nodes of the upper respiratory tract. Although endemic within the UK horse population, it only occasionally affects thoroughbred stud farms. Accordingly, it was first included in the CoP as an advisory guideline in 2004 and has since been periodically updated in line with developments in diagnostic testing and vaccination. Due to the ability of Streptococcus equi to develop persistent infection, remaining within material (pus/chondroids) in the guttural pouches long after resolution of clinical signs, one of the key elements in prevention and control is identification and treatment of these carrier animals. This involves performing guttural pouch endoscopy and lavage of strangles cases around 30 days after clinical recovery from infection, and also of any new arrivals at a premises during the quarantine period, before they are allowed to mix with the resident herd. 

Equine infectious anaemia (EIA)

figure 3

EIA, also known as swamp fever, is caused by the equine infectious anaemia virus (EIAV) and is transmitted between horses by the transfer of infected blood, either by insect vectors or contaminated veterinary equipment or through administration of infected blood products (e.g., plasma or whole blood transfusion). It is found in thoroughbred and non-thoroughbred populations worldwide, including parts of mainland Europe; but it is currently not present in Great Britain, where any suspicion of disease is notifiable by law to the Animal and Plant Health Agency (APHA) and would result in compulsory slaughter of the affected animal. Following an outbreak in Ireland in 2006, a CoP for EIA was developed by Defra in the UK in August that year and was included as an addendum for the 2007 breeding season (Figure 3) and subsequently as a full code from 2008 onwards. EIA has been included as a full CoP since 2008, and it recommends pre-breeding testing of all mares and stallions prior to commencement of the covering season as the best way to establish and maintain freedom from infection. 

Dourine 

A new code on dourine was added in 2012, following its re-identification in Sicily and the Italian mainland during 2011, which necessitated pre-import screening of horses arriving from this area. A notifiable venereal disease caused by the protozoan parasite Trypansoma equiperdum, once widespread, had largely undergone eradication and of late had only been reported in Asia, Africa, South America, Eastern Europe, Mexico and Russia. There is no cure for dourine, and euthanasia is usually the advised treatment option on the grounds of animal welfare and population health. As investigations into the 2011 Italian outbreak indicated subclinical seropositivity to dourine in many regions of Italy, it was evident that the disease was closer to our shores than anticipated. That led to its addition to the CoP in order to keep all owners/breeders informed and appropriately advised. 

Equine influenza (EI)

EI was added to the CoP in 2020 as an advisory guideline following the 2019 European epidemic, which saw a major outbreak in the UK (Figure 4), including cases of clinical disease in vaccinated thoroughbreds. This led to the cancellation of British horseracing for six days in February 2019 as a pre-emptive control measure, but still at significant economic cost to the industry. 

Figure 4

Although clinical EI is usually fairly mild and self-limiting, the resulting damage to the respiratory epithelium can impact performance for up to six months and leaves the horse vulnerable to secondary bacterial infections and potential pneumonia. Appropriate vaccination, biosecurity protocols and effective quarantining of new arrivals are outlined in the Code as the cornerstone to EI control. With such a highly contagious virus capable of spreading over large distances and with great speed, especially in the immunologically naïve, awareness and prevention are key.

West Nile fever (WNF)

WNF, caused by West Nile virus (WNV), is an infectious but non-contagious disease transmitted through the bite of an infected mosquito. Although the natural host-vector cycle is between birds and mosquitoes, horses and indeed humans can become infected but act as so-called incidental or ‘dead-end’ hosts; importantly, not presenting is a risk for onward transmission as there is insufficient WNV in their bloodstream. Many horses infected with WNV remain without signs, but approximately 10% will develop neurological disease, which can be fatal. Historically confined to Africa and the East, it entered North America in 1999 leading to widespread infection with many equine and human fatalities. Since then it has become endemic in the USA and continues to spread further into northern Europe as climate change alters vector habitats and life cycles. After the development of equine cases in Germany in 2018 and evidence of human and bird cases in the Netherlands in 2020, WNV was considered to pose an increasing threat to the UK horse population, especially animals that travelled overseas for competition and breeding purposes. WNF was therefore added as a guideline to the CoP in 2021. 

Piroplasmosis

The latest disease addition to the CoP was an advisory guideline on piroplasmosis in 2022, following concern that the disease was becoming increasingly important among the international equine population. Piroplasmosis is a tick-borne disease caused by the intracellular parasites Babesia caballi and Theileria equi. Although the UK is currently considered to be free from locally acquired endemic disease (referred to as ‘autochthonous’), cases have occasionally been confirmed in the UK and are endemic in other European countries. With no formal requirements for pre-import screening, infection could re-enter the UK through importation of infected horses. The code gives informative background information to raise awareness among thoroughbred breeders and owners/keepers of other horse populations.  

Improving accessibility and applicability of the CoP

Another way in which the CoP have more recently evolved is through the mechanisms of delivery to stakeholders. In July 2016, the accessibility and reach of the CoP took a further leap with the generation of the smartphone EquiBioSafe app (https://play.google.com/store/apps/details?id=com.veterinaryadvances.android.equibiosafe&hl=en_GB&gl=US&pli=1). With the HBLB Codes of Practice and National Trainers Federation Codes of Practice précised into key elements for the control and prevention of infectious diseases, the app allows interactive and stable-side access to advice, as well as assisting trainers to comply with sporting authority vaccine regulations and disease notification procedures. With the ability to send emergency notifications in the event of heightened disease threats in a particular area, the app provides real-time relevant information to assist with implementation of proactive biosecurity measures. This helps safeguard horse health and the socioeconomic livelihoods of all those involved in equestrian sport. Like the CoP, the EquiBiosafe app has mainly been targeted to a European audience, but downloads recorded from North America, Asia and Latin America demonstrate its international application. 

The HBLB International Codes of Practice act as broad, minimum requirement recommendations for the identification, treatment, prevention and control of a range of important equine infectious diseases—equally relevant across international borders and from pleasure to elite competition horses. They are also dynamic, evolving over time in line with the ever-changing disease situation, and therefore acting as a vital education and reference resource to all those involved in the equestrian industry. They form a user-friendly instruction manual of exactly ‘how (not) to’ allow infectious diseases to fulfil their devastating potential.  

Gerald Leigh Memorial Lectures 2020 - minimising risk from equine infectious disease - how it will hopefully help support education on equine infectious disease

By Celia M Marr

IN ASSOCIATION WITH:

COVID-19 has affected all corners of the thoroughbred world and has changed lives, work patterns and the social activity that underpins racing. One of its minor impacts was that this year, the Gerald Leigh Memorial Lecture series, usually coordinated by Beaufort Cottage Educational Trust at the National Horseracing Museum, Palace House, Newmarket each summer, was cancelled.

This annual lecture series is supported by the Gerald Leigh Charitable Trust in honour of Mr Leigh’s passion for the thoroughbred horse and its health and welfare. Coincidentally, the topic which had been selected for 2020 was Minimising Risk from Equine Infectious Disease. Finding that a meeting was impossible, the trustees organised for presentations to be filmed remotely, and these are now available online.

THE LECTURES SERIES INTRODUCTION BY NICK WINGFIELD DIGBY, CHAIRMAN, BEAUFORT COTTAGE TRUST

Gastrointestinal disease is a common problem in foals and youngstock with potentially serious illnesses involved. Dr Nathan Slovis, director of the McGee Center, Lexington, Kentucky, USA, explained that by six months of age, 20% of foals will have had infectious diarrhoea. Dr Slovis presents a concise and very practical account of how we can minimise risk of infection in this age group. 

The specific causes of gastrointestinal disease vary with age. Foals frequently display mild diarrhoea at around the time of the foal heat, generally a problem that will clear up uneventfully. Major infections in foals include rotavirus, Salmonella, Clostridium perfringens, Clostridium difficile. Infectious disease can be life-threatening if infection leads to shock, so every gastrointestinal case should be assessed carefully; early intervention is critical. 

Vaccines are available to help minimise risk of rotavirus but prevention relies primarily on proper hygiene and appropriate choice of disinfectants, which vary depending on the particular microorganism concerned.

PRESENTATION BY DR NATHAN SLOVIS, DIRECTOR OF THE MCGEE CENTER, LEXINGTON, KENTUCKY

Diagnosis: PCR and ELISA technologies

The speed and availability of laboratory testing have been revolutionised in recent years with the introduction of ELISA and PCR technology. The enzyme-linked immunosorbent assay (ELISA) is an immunological assay commonly used to measure antibodies, antigens or proteins. An ELISA test relies on finding a molecule which is unique to a virus or bacteria and is used to find several equine pathogens, including rotavirus, Clostridium difficile and Clostridium perfringens.  

PCR technology rapidly makes millions to billions of copies of a specific DNA sample, allowing the lab to take a very small sample of DNA and amplify it to a large enough amount to study in detail. Rapid tests are now available, which have revolutionised the diagnostic approach across a huge range of equine infections; relevant to foal diarrhoea is that this technology is used for rapid testing of faecal samples for Salmonella.

In addition to being extremely quick, both PCR and ELISA tests are very sensitive. Dr Slovis emphasised how important this is in early identification of diseases with the potential to spread rapidly in young horses.

Auditing environmental contamination

All the speakers in the webinar series spoke of the importance of robust biosecurity and common themes emerged in all four webinars regardless of the animals’ age or whether respiratory, skin or gastrointestinal infection is involved.

Dr Slovis’ clinical practice includes offering services to identify areas of environmental contamination. This involves a detailed inspection of all areas on the farm together with laboratory testing for the common pathogens. Key benefits of a facility evaluation service are to help support staff education and to highlight areas of weakness in biosecurity practices; and farmers and vets can work together to devise practical solutions to farm-specific problems. In his webinar, Dr Slovis shows some great examples of what not to do, which are drawn from his extensive experience of advising on biosecurity practices in equine facilities.

THE PRESENTATION BY PETER RAMZAN, A MEMBER OF THE RACING TEAM AT ROSSDALES LLP, NEWMARKET, UK:

Infectious challenges in young horses on training yards

Peter Ramzan, member of the Racing Team at Rossdales LLP, Newmarket discussed how to reduce risks when horses move into training. Piet is a fellow of the Royal College of Veterinary Surgeons and has written extensively on a range of disorders affecting horses in training. He summarised three areas relevant to this age group: lower respiratory tract disease, ringworm and the rare but sporadic disease threats such as strangles and neurological herpes.

Lower respiratory tract disease

This problem is mainly responsible for coughing and affects around 80% of two-year-olds and 25% of three-year-olds. Research in Newmarket has shown that for every 100 horses, there are around 10 cases each month, but prevalence varies between yards and seasons with a peak in early spring. Bacteria are believed to be a more common cause than viral infection, but both can cause coughing and can occur simultaneously. 

Prevention is better than cure

Exposure to disease-causing microorganisms is inevitable and cannot be prevented, but risk of clinical disease can be reduced by optimising immunity. It is helpful if exposure occurs prior to or early in training. Ramzan concluded that homebreds that have bypassed public sales and the inevitable mixing with other horses there are at greater risk of interruptions to their training when they do enter yards as two-year-olds. He went on to emphasise that it is not necessarily helpful to aggressively treat respiratory infections in pre-training—better to let infection run through yearlings and young two-year-olds, providing that they remain mildly affected as this helps them build immunity to protect them during their racing careers.

As well as discussing the biosecurity measures which apply across all age groups and disease threats, particular points that Ramzan emphasised for reducing the impact of infectious disease in training yards included the importance of avoiding the introduction of yearlings to the main yards before the end of the season and adoption of a strategic vaccination programme. Vaccines should be given to horses in a year ahead of the influx of yearlings while maintaining immunity throughout the racing season; autumn and spring boosters are most likely to achieve this.

Antimicrobial stewardship

It is increasingly clear that overuse of antimicrobials is promoting resistance to these potentially lifesaving drugs. Vets and trainers should avoid their use, except where bacterial infection is highly likely,  or ideally confirmed with laboratory testing. Ideally the lowest class of antibiotics should be used first, reserving protected classes, such as enrofloxacin and ceftiofur. Ramzan shared data from his practice over the last two decades which showed an alarming increase in resistance to oxytetracycline, which is the commonly used antimicrobial. Conversely, in the same period, oral trimethoprim sulphonamide, which is not used as much as it could be, has had a rise in sensitivity, likely because it is not used as often as it could be.

Herpes virus Type 1: a uniquely challenging foe

Professor Lutz Goehring,  head of Equine Medicine and Reproduction at Ludwig-Maximilian University, in Munich, Germany, has had a distinguished research career focussed on equine herpes type 1 (EHV1). This virus has the potential to cause both abortion storms and outbreaks of neurological disease in all age groups, including horses in training. 

PRESENTATION BY PROF. LUTZ GOEHRING IS HEAD OF EQUINE MEDICINE AND REPRODUCTION AT LUDWIG-MAXIMILIAN UNIVERSITY, IN MUNICH, GERMANY.

Are you aware of the risks of EHV? ==================================

Equine Herpes Virus is a real threat to horse health and performance. Racing yards can be particularly vulnerable to outbreaks, but there are some easy steps you can take to reduce the risks.

Dr Wendy Talbot, vet at Zoetis explains: EHV is a contagious viral infection causing respiratory disease, abortions and neurological disease. Carrier horses show no clinical signs, but the virus can be reactivated at any time and spread to other horses and this is more likely to happen during times of stress. ^1,2,3

EHV can be transmitted by direct horse-to-horse contact and by nasal or ocular discharge, which can spray or travel through the air over short distances. It can also be spread by sharing infected equipment, and via people who have been in contact with infected horses. This is why it’s crucial to have good biosecurity measures in place at yards, races, training and sales events. ⁴

Signs of the virus can be visually obvious or very subtle: horses may have a nasal discharge, weepy eyes, swollen glands and a cough and fever or a less noticeable lethargy, lack of appetite and reduced performance. ^1,5

Vaccination against EHV is important because it helps tip the balance in favour of the horse’s immune system and reduces viral shedding.

Vaccination programmes should run concurrently with rigorous hygiene and isolation protocols to help minimise the risks of EHV spreading.

It’s also important not to mix unvaccinated horses with vaccinated ones to provide the best level of protection. ^6,7

If you think any of your horses may have any symptoms of respiratory disease,isolate them immediately and contact your vet to discuss the next course of action.

FOR FURTHER INFORMATION FOLLOW THIS LINK

1. Slater J (2014) Equine Herpesviruses. In: Equine Infectious Diseases. Eds., D.C. Sellon and M. Long, Saunders, St. Louis. P151-169

2. Allen GP (2002) Respiratory Infections by Equine Herpes Virus Types 1 and 4. International Veterinary Information Service.

3. Slater J. What is Equine Herpes Virus? Accessed August 2019 https://www.horsedialog.co.uk/Health/WhatisEHV.aspx

4. Allen, GP (2002) Epidemic disease caused by equine herpesvirus-1: recommendations for prevention and control. Equine Veterinary Education; 14(3):136-142.

5. Davis, E. (2018) Disorders of the respiratory system. In: Reed SM, Bayly WM, Sellon DC, eds. Equine Internal Medicine, 4th ed. St Louis, MO: Elsevier:313-386.

6. Lunn, DP., et al. (2009) Equine herpesvirus-1 consensus statement..J Vet Intern Med. 23(3). 450-461

7. Equine herpesviruses: a roundtable discussion Philip Ivens, David Rendle, Julia Kydd, James Crabtree, Sarah Moore, Huw Neal, Simon Knapp, Neil Bryant J Richard Newton Published Online:12 Jul 2019 https://doi.org/10.12968/ukve.2019.3.S2.1

EHV1, like its virus relatives that cause cold sores in humans, has the ability to become latent. This means that the virus can sit in an inactive form in certain nerves and lymph node tissues, only to be reactivated and start to spread amongst groups of horses. While latent, the virus is out of reach of the immune system. Latent infection with EHV1 is widespread in horse populations globally.

Reactivation of EHV1 is not a common event, but it is associated with “stressful” situations such as mixing with new horses, transport, above-normal exercise, and in mares with foaling. Understanding the mechanisms involved in reactivation, spread to other horses and subsequent uptake of the virus into tissues—such as the placenta and fetus to cause abortion and to the spinal cord to cause neurological signs and paralysis—has been the main focus of Prof Goehring’s research career.

EHV1 is easy to kill with soaps and disinfectants when it is outside the body, again highlighting the importance of good biosecurity practices in studs and training yards. The virus spreads from horse to horse when there is close contact and droplets breathed out by an infected horse are inhaled by another. Shortly after inhaling the virus, there is a short temperature spike and then a second more intense spike usually occurs 8-10 days later. Neurological signs or abortion will typically come days or weeks after this second temperature spike.

Outbreak mitigation 

Early detection and effective quarantine are the mainstays of EHV1 outbreak prevention. In the face of a potential outbreak, swift action to stop spread is critical. Movement on and off the property must cease. Horses with subtle clinical signs—slight nasal discharge, lymph node enlargement and fever—can now be tested very quickly for EHV1 using a nasal swab PCR. Horses should be tested to identify any that are shedding the virus; and any which are positive must be removed to isolation. Horses housed near these individuals should be quarantined in case they are incubating the disease. Horses in the early stages of infection may benefit from treatment to prevent neurological complications.

Distance is the key to stopping this droplet-aerosol infection; and although the distance does not need to be great, more is always better. Traditionally racehorses exercise in strings. An exercising horse which is shedding virus creates a tail of viral particles trailing behind it. In his webinar, Prof Goehring talked about the advantages of increasing distance between exercising horses and showed the benefits of exercising alongside rather than one behind the other. When there is infection around, consideration should also be given to the order horses go out to exercise, with those least likely to have infection exercising first.

Immunity, current and future vaccines

Following infection or vaccination, horses produce both antibodies and specialised cells with the ability to fight off EHV1 infection. Vaccination can be expected to reduce both the clinical signs and the shedding of virus if they are challenged. However, this immunity gradually wanes with time and with currently available herpes virus vaccines, repeat vaccination every six months is recommended

. 

It is also important to understand that there is a balance between immunity level and infectious dose such that horses which are challenged with a very high dose of virus are more likely to develop fever than those that are exposed to a low dose—this again highlights the importance of effective biosecurity practices on studs and training yards. 

Finally, although not yet available for equine herpes viruses, novel sub-unit vaccines introduced for similar herpes viruses in humans have been shown to cement latent virus into its hidden location and stop reactivation. Prof Goehring suggested this technology may be the light at the end of the tunnel for horses because this novel approach may reduce the likelihood of the outbreak initiation, which begins with reactivation of latent virus. 

PRESENTATION OF DR. RICHARD NEWTON, DIRECTOR OF EPIDEMIOLOGY AND DISEASE CONTROL AT THE ANIMAL HEALTH TRUST, KENTFORD, UK.

Lessons from the European flu epizootic 2019

Although current attention is on COVID-19, it is important to reflect on lessons from the equine influenza outbreak which affected many countries in Europe last year. Dr Richard Newton, epidemiologist and an authority on equine infectious disease, coordinated much of the UK’s surveillance and communication during this outbreak, working at that time at the Animal Health Trust, Newmarket, Suffolk.

Equine influenza is a contagious rapidly-spreading viral respiratory disease. Common signs of infection include fever and coughing; and coughing is an important factor in spread as infected particles are released and can spread over wide distances to affect others. Unlike EHV1, there is no carrier or latent state, and the flu virus needs chains of transmission to persist in a horse population; an infected horse has to pass the virus on to another in order for the infection to perpetuate in a group. Vaccination is used to break these chains of transmission by reducing susceptibility. However, flu virus evolves continuously, constantly producing new strains; and in order to be effective, vaccine strains must keep up with this evolution and be updated periodically. 

The R number: what does it mean?

The R number, or basic reproduction number, is the number of cases on average that one case generates over the course of its infectious period. If the R number is less than one, the chain of transmission will die out, and infection will cease. Vaccination plays a major role in reducing the R number by limiting the number of susceptible animals.

Lessons from 2019

Flu occurred in several countries in Europe last year. In the UK, we saw two waves of this infection whereas other countries, notably Ireland and Holland, had different patterns. The Clade 1 strain of virus involved in the 2019 outbreak had not been seen in Europe for over a decade.

Flu mainly affects non-vaccinated horses but can occur in vaccinated animals, particularly if a new strain challenges a population. Fortunately, prompt action by the British Horseracing Authority last year minimised flu occurrence within our racing population. In January, based on information coming out of other European countries,  the BHA veterinary committee advised six monthly booster vaccinations.  

A six-day stoppage in racing and horse movements after flu was identified in a racing yard in early February. The majority of flu outbreaks occurred in unvaccinated horses, and the second spike seen in the summer of 2019, was associated with horse gatherings at shows and fairs. Nevertheless 18% of flu cases involved appropriately vaccinated animals, some of which might have been vaccinated after contracting infection, while many of the others were nearing the time when a booster was due. 

The UK’s racing populations are highlight connected, and the racing stoppage was prompted by the occurrence of flu in vaccinated animals. This break provided the necessary pause during which the scale of infection could be assessed. A huge number of racehorses were tested, and Dr Newton explained that an important conclusion from this experience was that there is a need to scale up lab testing capacity to support such a response in future, particularly if we were to be challenged by a completely novel strain of flu.

What did we do well?

Racing heeded the earliest warnings with its six-month booster recommendation, applied a lockdown and implemented test and trace and finally, on releasing lockdown, racing applied biosecurity precautions, concepts now familiar to us all in relation to COVID-19. 

Dr Newton acknowledged that these lessons were missed or ignored outside of racing, leading to a second wave in  the non-thoroughbred during the summer; and horse owners and event organisers did not adequately embrace the simple messages regarding the importance of vaccination and isolation. Many of the outbreaks which occurred last summer were associated with the introduction of new animals on a premise. The UK horse population has a low national vaccine coverage, estimated at around 40%—a statistic which puts the UK in a poor light compared to other European countries where uptake in the general horse population is much higher. 

Do we need to improve vaccines and vaccine strategy?

Vaccine strains are continuously reviewed by The World Organisation for Animal Health (OIE)  panel—critical work which in the UK is supported by the Horserace Betting Levy Board. Currently there is insufficient scientific evidence to recommend an equine influenza vaccine strain update, although this might not be far away. On the other hand, reducing booster vaccine intervals is clearly beneficial. Flu vaccines work by stimulating the horse to produce antibodies which decline with time. There is variation in vaccine response between individual horses with some animals less well protected than others. Dr Newton reviewed information from multiple studies and outbreaks and concluded the weight of evidence overwhelmingly supports a six-monthly booster. Increasing vaccine uptake across the national herd will involve improved education in the non-thoroughbred world but is critical to supporting herd immunity. Improved awareness will benefit all horses including thoroughbreds.

Take-home messages

All four speakers highlighted practical biosecurity measures as critical in reducing the risks of infectious disease. Vaccines are essential for both flu and EHV1. They are not infallible, and ongoing research will lead to improved vaccine technology. Most important of all is that education of people working with thoroughbreds, and across the wider equestrian world, will help support early recognition and management of disease when it occurs. This year’s Gerald Leigh Memorial Lectures will hopefully help support this education by making information on equine infectious disease available online.

CLICK HERE FOR USEFUL RESOURCES ON VETERINARY ADVICE RELEVANT TO THE THOROUGHBRED

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