The Horserace Betting Levy Board codes of practice was set up in 1977 in response to an outbreak of the venereal disease contagious equine metritis on stud farms in Newmarket and in other counties. It was caused by the until then unknown bacterium Taylorella equigenitalis and caused a severe acute, purulent endometritis and cervicitis in mares resulting from mating with carrier stallions and likely lateral spread by iatrogenic contamination (Greenwood and Allen, 2020).
The codes of practice set out pre-breeding swabbing protocols for mares and stallions, which acted as a screening process for the Taylorella organism and had the added benefit of detecting the carrier state of other venereal pathogens, most importantly Klebsiella pneumoniae and Pseudomonas aeroginosa, certain strains of which can also cause venereal disease. As well as laboratory detection of these organisms, the codes of practice set out routine hygiene and biosecurity guidelines to be implemented on stud farms to prevent lateral spread of the pathogens, which brought about a marked improvement in general hygiene on stud farms and an increased awareness of the potential for spread of infection, resulting in a much reduced incidence of venereal infections in mares and stallions (Allen and Wilsher, 2017).
Over the years, the codes of practice have been expanded to include codes for equine viral arteritis, equine herpesvirus, equine coital exanthema, equine infectious anaemia and dourine, as well as guidelines on equine influenza, strangles, piroplasmosis, West Nile fever and on artificial insemination. The basic principles of the codes have remained much the same over the years; however, from time to time there are some additions and amendments, the most recent of which will be summarised in this article.
Changes to the codes for 2022
Clarification regarding the control of contagious equine metritis infection
A section has been added here on preparing mares for covering in response to reports of an increasing number of mares being presented for walk-in covers with polymerase chain reaction (PCR) test certification only, sometimes with inadequate gynaecological preparation. The significance of this point is that although PCR is a useful test for quickly ruling out the contagious endometritis infections caused by the main organisms specified in this code, T. equigenitalis, K. pneumonia and P. aeroginosa and other potential causes of endometritis, both infectious and non-infectious, should not be ignored.
A full genital examination should be carried out in early oestrous when the cervix is sufficiently relaxed to enable swabbing. Where PCR testing of the swab is used, it should always be accompanied by aerobic bacterial culture to rule out endometritis caused by other bacteria, such as equine skin or environmental contaminants.
Examination should also include visual examination of the reproductive tract including speculum examination of the cervix and vagina at the time of swabbing to detect any physical abnormalities, as well as thorough rectal palpation and ultrasound exam to detect other potential indicators of endometritis, such as poor uterine involution and the presence of uterine fluid or air.
Failure to detect any of these abnormalities and the subsequent covering of unsuitable mares is likely to result in failure of conception or early pregnancy loss. This can mean significant economic losses to breeders as a result of the costs of repeat matings, as well as lost time in the breeding season, potentially failing to establish a pregnancy at all or the establishment of pregnancy late in the season. This can also have negative consequences on the reproductive efficiency of busy stallions, particularly in the thoroughbred industry, if much sought after covering slots are being wasted on unsuitable mares.
Under the ‘control of infection’ heading in the codes of practice, a section has been added on the swabbing of at-risk or suspected horses. The main point of this is to highlight the fact that on a farm where infection has been established, but where the risk of infection to certain mares and stallions is believed to be low, for example in stallions that have not had direct contact with infected animals or breeding equipment and mares covered by these stallions, then a single set of clearance swabs will likely be sufficient to rule out any unexpected lateral transmission. However, mares covered by an infected stallion or stallions that have covered an infected mare will always require at least three sets of negative swabs to be considered free from infection.
Revised guidelines for strangles
The main update in this section is regarding the diagnosis of the carrier state of Streptococcus equi and the use of the blood enzyme-linked immunosorbent assay (ELISA) test to detect antibodies to S. equi. Most carrier animals will have detectable antibody levels to S. equi, although recent investigations have shown that a negative blood ELISA result, either a single or paired sample, does not always guarantee absence of the carrier state (Durham and Kemp-Symonds, 2021). Therefore, in situations where an animal is potentially at high risk or of unknown risk then guttural pouch sampling should be carried out and samples tested by PCR alone, or culture and PCR, regardless of blood results.
Another minor change in the diagnosis of the carrier state and the confirmation of freedom from disease concerns the two methods of sampling, which are a series of three nasopharyngeal swabs taken 1 week apart or sampling of the guttural pouches on one occasion, which has a similar sensitivity. In the previous code it was recommended to take a single nasopharyngeal swab at the same time as the guttural pouch washes, but this has been omitted in the updated code. The guttural pouch washes alone are sufficient.
New guidelines on piroplasmosis
New guidelines have been added on piroplasmosis caused by the blood-borne intracellular parasites Babesia caballi and Theileria equi. The UK is currently considered free from endemic disease and there is no formal requirement for pre-import screening in the UK. However, piroplasmosis is widespread globally, particularly in Africa, Asia, South America, the Middle East and Mediterranean countries, as well as eastern and southern Europe (Onyiche et al, 2019) and recent cases have been reported in France and Holland (Coultous et al, 2018), countries that regularly transport horses to the UK.
Piroplasmosis has the potential to become a more significant problem in the UK because of the widespread international movement of horses, the potential for subclinical carriers to go undetected and the fact that a species of tick capable of transmitting the organisms has been demonstrated to be present in the UK (Medlock et al, 2017).
The establishment of the disease in the UK could lead to significant losses, as the acute disease can be severe and occasionally fatal; horses in non-endemic countries are more likely to be susceptible to acute forms of disease owing to lack of immunity. Chronic disease can result in weight loss and poor performance.
The main significance of piroplasmosis in the UK is the complications that arise when positive blood results are found on preexport serological screening tests (Coultous et al, 2018). The diagnosis of piroplasmosis is primarily based on serological testing for detection of antibodies against B. caballi and T. equi. There are three different serological tests available for detection of piroplasmosis: the indirect fluorescent antibody test, the competitive ELISA and the compliment fixation test. However, false positives are common and this should be taken into account when interpreting the results of these tests, and a second test may be needed to confirm the results if a positive or borderline result is obtained. Agent detection tests can also be used, which involve the microscopic examination of blood smears to detect the parasites in the erythrocytes and the use of PCR testing. However, the parasites are not always detectable in the blood, particularly in chronic carriers. A positive test for piroplasmosis can have serious consequences for horses intended for export, especially to non-endemic countries such as the USA, Australia, Canada, New Zealand, Japan and Hong Kong, which have more stringent piroplasmosis control measures in place to prevent entry of the disease. The USA, Australia and Hong Kong in particular represent a large market for the sale of UK-based horses, especially for the thoroughbred industry, so if the disease were to establish itself in the UK, leading to an increase in the number of carrier animals, this could lead to serious economic losses for the industry. As an example. Australia will not accept any horse that has had a positive test, but some countries will accept them if they have been treated and shown to be clear of infection. In recent years, it has become common practice for vendors to have horses tested for piroplasmosis before presenting them for public auction, if they feel they are likely to appeal to international buyers.
Treatment of clinical cases of piroplasmosis, which commonly occur in endemic countries, is carried out with the obvious aim of reducing severity of clinical signs and preventing fatalities. Treatment of non-clinical carrier animals with the ultimate goal of complete clearance of infection is difficult and there is no licensed treatment for piroplasmosis in the UK. The off-licence use of antiprotozoal agents such as Imidocarb dipropionate has been reported (Pelzel-McCluskey and Traub-Dargatz, 2015), with the aim of clearing the infection and subsequently reducing antibody levels sufficiently to allow animals to be exported. This should not be undertaken lightly, as these drugs are not without side effects and have varying degrees of success at clearing infection. Consent needs to be obtained from owners for the use of off-licence drugs and client expectations need to be managed as complete clearance of infection cannot be guaranteed, long periods of time need to be allowed for antibody levels to drop and levels may rise again following transport if complete clearance is not achieved.
Updated vaccination information:
In appendix 8 of the codes, the new equine influenza vaccination requirements introduced by the British Horseracing Authority (2021) are mentioned (Table 1). These requirements lay out new vaccination intervals for the initial primary course and annual boosters. Horses requiring a primary course will now need their second vaccination 21–60 days after the first, and their third vaccination 120–180 days after the second, with booster vaccinations now required every 6 months thereafter. These changes were brought about in response to the 2018–2019 outbreak of equine influenza throughout Europe, including the UK, when it became clear that horses that had been vaccinated within the previous 6 months were less likely to succumb to infection.
Table 1. Updated intervals for equine influenza vaccines
| Current interval | From 1 January 2022 | |
|---|---|---|
| V1 | ||
| V2 | 21–92 days | 21–60 days |
| V3 | 150–215 days | 120–180 days |
| Booster | No more than 1 year apart | No more than 6 months apart |
Adapted from the British Horseracing Authority (2021)
All young stock on stud farms that are intended for racing will need to comply with these regulatory requirements and it is recommended that all other animals including broodmares also receive a booster every 6 months rather than annually. Covering farms require that mares have received a vaccine within 6 months before covering. More information can be found on the British Horse Racing Authority and Newmarket Stud Farmers Association websites. For clients with non-thoroughbred horses, it may be necessary to consult the relevant regulatory bodies for clarification as to whether these new requirements are being enforced across other disciplines.
Clarification regarding the diagnosis of West Nile fever
West Nile fever, a notifiable disease in the UK, is caused by the flavivirus West Nile Virus. It is an insect vector-borne virus spread by various species of mosquito. The virus is maintained in infection cycles between wild birds and mosquitos, but can spill over into other species, including horses and humans, via insect bites. Horses are considered incidental or dead-end hosts and do not act as a sufficient source of virus for feeding mosquitos to infect other animals and there is no direct spread between horses. Clinical signs vary from subclinical to severe neurological disease with a high fatality rate. The guidelines on West Nile fever were added to the codes in 2020 and it is considered a risk to the UK, as the geographical distribution of the disease seems to be spreading and mosquitos capable of spreading the virus are present in the UK (Engler et al, 2013). The disease is considered endemic to Italy and has been detected in southern France, Portugal, Spain and Germany (Browne and Medlock, 2019). Effective vaccines are available for West Nile fever and this should be considered for horses travelling abroad to high risk regions, as well as taking protective measures against mosquito bites.
Suspected cases of West Nile Fever can be tested by the Animal and Plant Health Agency (APHA) under the ‘testing to exclude’ process following discussion with an APHA duty vet. The update to the diagnosis section of the codes guidelines on West Nile Fever highlights the fact that there are now only two serological tests available in the UK to detect West Nile Virus antibodies at the APHA laboratory. These are the IgM ELISA detection test and the total antibody detection ELISA. The plaque reduction neutralisation test is no longer available.
Conclusions
The Horserace Betting Levy Board codes of practice are a vital resource for vets and horse owners of all disciplines and it is worth reviewing them each year to refresh existing knowledge and keep up to date with new developments and amendments. It is particularly important for new and recent graduates entering into equine practice to familiarise themselves with the content of the codes, as it is not uncommon to be faced with a situation such as an equine herpes virus abortion or a strangles case where quick and decisive action can make all the difference in the prevention of further spread of the disease.
Events such as the equine influenza outbreak throughout Europe and the UK in 2018–2019 and the outbreak of the neurological form of equine herpes virus in 2021, which originated at a competition in Spain, serve as reminders of the importance of constant vigilance with regard to infectious diseases, particularly for clients participating in events where a lot of horses are mixing, whether it be in the UK or abroad.
Strict biosecurity protocols should be adhered to in yards and stud farms, with particular attention given to horses returning from competitions and new horses arriving especially if they have come from a public auction.
Climate change and global warming are influencing the geographical distribution of many disease vectors such as ticks and mosquitoes, thereby increasing the potential for spread of diseases such as piroplasmosis, West Nile fever and African horse sickness in new locations. Clients should be made aware of this when travelling abroad for equestrian events and when buying horses from overseas, and vets should keep this in mind when presented with a horse with unusual clinical signs and a history of international travel.
Finally, it is worth mentioning that the protocols and guidelines laid out in the codes of practice are generally considered to be minimum requirements, and it is at the discretion of any equine establishment to request additional laboratory testing before allowing an animal to enter a premises.
Further reading
Other resources which provide useful information regarding infectious disease surveillance and control include the Equine Infectious Disease Surveillance website (https://equinesurveillance.org/landing/), which produces the equine quarterly disease surveillance report as well as sharing information through the International Collating Centre, and Equiflunet. The British Equine Veterinary Assocation's website also has a useful infectious diseases section (https://www.beva.org.uk/Guidance-and-Resources/Infectious-Diseases), and the Equibiosafe app provides easy access to the contents of the Horserace Betting Levy Board codes of practice (https://codes.hblb.org.uk/), as well as the National Trainers Federation codes of practice (https://www.racehorsetrainers.org/publications/pdfs/cop.pdf).
Other useful resources for further reading are listed below:
- Belgrave RL. West Nile virus. In: Sprayberry KA, Robinson NE (eds). Robinson's current therapy in equine medicine. 7th edn. Missouri: Elsevier; 2015:152–154.
- Newmarket Stud Farmers Association. 2022 Breeding regulations. https://nsfa.org.uk/wp-content/uploads/2022
KEY POINTS
- The Horserace Betting Levy Board codes of practice was set up in 1977 in response to an outbreak of contagious equine metritis caused by the bacterium Taylorella equigenitalis.
- Polymerase chain reaction testing for contagious equine metritis should not be solely relied upon in the preparation of mares for covering and all mares should have a full gynaecological examination, including bacteriological testing, before being presented for covering.
- A negative blood enzyme-linked immunosorbent assay test for S. equi antibodies does not always guarantee the absence of the carrier state of strangles. Therefore, when in doubt, guttural pouch wash samples should be tested by polymerase chain reaction test, or polymerase chain reaction and culture testing.
- Piroplasmosis can lead to complications when positive results are obtained on pre-export serological screening tests, as some other non-endemic countries will not accept horses that have had a positive test.
- New vaccine intervals for equine influenza have been introduced by the British Horseracing Authority, meaning that all horses intended for racing will require booster vaccines every 6 months instead of every 12 months.
- West Nile fever is not currently present in the UK, but the geographical distribution of the disease does seem to be spreading in Europe, meaning it could potentially emerge in the UK in the near future.