A workshop was held at the Animal Health Trust visitors centre and seminar room, Lanwades Park, Suffolk on the 5th October 2017 to share the latest research news and discuss progress towards the prevention and eradication of this disease.
A brief introduction to strangles and Streptococcus equi
Dr. Andrew Waller (AW), Head of Bacteriology
Strangles, caused by Streptococcus equi, is one of the most frequently identified infectious diseases of horses world-wide. Approximately 600 outbreaks are identified in the UK each year, but AW reported that many more outbreaks are likely to occur where samples are not submitted to diagnostic laboratories for analysis or a veterinarian is not called upon to examine affected horses. AW described the classic clinical signs of disease and presented details of complications that can lead to mortality rates of up to 10% in some outbreaks.
The ability of S. equi to persist in the guttural pouches of horses that have recovered from strangles is critical to its onward transmission and the occurrence of new outbreaks. The horse is a global traveller and AW explained how the AHT has collaborated with researchers across 22 countries to determine if the occurrence of strangles outbreaks is linked to the international movement of horses. DNA was sequenced from 703 isolates of S. equi from around the World. AW showed examples of the same strain-type being shared between Europe and the USA; the USA, Europe, Japan and Israel; Europe and Australia; Australia, New Zealand and the United Arab Emirates (UAE) and Argentina, Europe and the United Arab Emirates. In particular, the UAE shared 14 different strain types with countries from around the World. AW described how the UAE had taken a proactive response to this information, requiring the mandatory screening of horses pre-export. Such screening had identified four new carriers of S. equi in 2016/2017 prior to export, which likely prevented the occurrence of several new outbreaks. Some discussion took place on whether such measures could be introduced more widely in the UK as a pre-purchase/movement check. It was noted by some that they have seen outbreaks of strangles linked to the import of animals to the UK, most notably from Ireland. AW commented that a collaborative project with Vivienne Duggan (University College Dublin) found that 133 of 319 (42%) healthy horses at unregulated events in Ireland were seropositive using the AHT’s strangles blood test, whilst 16 of 162 (10%) of Irish Thoroughbreds also tested seropositive1.
Dealing with strangles
Dr Richard Newton (RN), Director of Epidemiology and Surveillance
Although strangles is often regarded as an inevitable rite of passage for horses, it is now possible to eradicate strangles and prevent future cases. RN’s presentation described routes of transmission utilised by S. equi to spread through horse populations and the value of good biosecurity towards mitigating this risk. He highlighted the importance of identifying and treating outwardly healthy carrier horses, which are frequently overlooked, but can continue to trigger outbreaks for months or years after they themselves have recovered from strangles.
Recommendations for minimising the impact of an outbreak and the eradication of S. equi were described and discussed. The ‘traffic-light’ system of separating infected animals with clinical signs (Red group) from animals they have had contact with, but that do not have clinical signs (Amber group) from horses (Green group) that have no clinical signs and that have had no contact with horses in the Red or Amber groups, remains the gold standard approach to reducing the number of animals affected and the severity of clinical signs. Again the importance of biosecurity and decontamination of equipment and clothing was emphasised. The use of Virkon and Safe4 disinfectants is known to kill S. equi. Post-outbreak clearance methods using serology to identify healthy horses that were exposed to S. equi and then following up with guttural pouch endoscopy and lavage to identify persistently infected carriers were discussed. Redwings Horse Sanctuary highlighted their success on flushing guttural pouches immediately post-acute disease in order to minimise the development of persistently infected carriers and the subsequent need to use antibiotics.
RN then described protocols for preventing strangles through the quarantine and screening of new arrivals. He presented new results highlighting how vaccination in the face of an outbreak of strangles had confounded post-outbreak serology on a European farm. Approximately half (37 of 75) of vaccinated horses seroconverted compared with none of 18 non-vaccinated horses in a separated barn. As a result of vaccine-induced seroconversion, many more horses had to be examined to see if they had become persistently infected during the outbreak than would otherwise have been needed. RN said that vaccines that do not interfere with diagnostic tests are urgently required. He also commended the ‘Speak out on Strangles’ campaign being run by Redwings, which raises awareness of strangles and encourages openness and best practice so that future outbreaks can be prevented.
How does Streptococcus equi cause strangles?
Dr Romain Paillot (RP), Head of Immunology
RP gave a presentation describing S. equi’s journey through the horse, highlighting the many interactions the organism has with the equine immune response (Figure 1). He described how superantigens, which are produced by several disease-causing bacteria, potentially misdirect and interfere with the immune response helping the bugs to cause disease. He explained that superantigens are produced by some human pathogens and how they play a pivotal role in devastating diseases including toxic shock and necrotising fasciitis.
RP explained that in strangles, the production of superantigens is restricted to the site of infection within abscessed lymph nodes, which may explain why strangles rarely triggers toxic shock-like clinical signs in horses. RP went on to show how the production of superantigens by S. equi is important to its ability to cause strangles in horses by potentially assisting S. equi to colonise and invade the lymph nodes of horses and/or generating a strong but unregulated inflammatory response, which may help S. equi to cause lymph node abscesses. In this manner the production of superantigens may be used by S. equi to cause more consistent disease in horses. In agreement with this theory, RP presented new results showing that the infection of ponies with strains of S. equi lacking superantigens led to an unusual disease progression, with some ponies developing an early onset of clinical signs while others developed no clinical signs. Overall there was a significant reduction in the number of lymph node abscesses that were formed by the superantigen-deficient strain. In the light of these results, RP suggested a complex and important role for superantigens in assisting S. equi to cause strangles.
Vaccine design and the launch of our new BBSRC-funded project.
Dr Coral Dominguez-Medina (CD), Post-doctoral scientist
CD described the current array of vaccines that are produced around the World that seek to protect horses from strangles. Cell-free vaccines such as Equivac S (Zoetis), Strepguard (MSD Animal Health) and Strepvax II (Boehringer Ingelheim) are available in Australia and the USA. These vaccines are based on surface extracts of S. equi cells, but little data on the protection they confer is available and so these vaccines are not licensed for use in Europe.
The Pinnacle IN live vaccine (Zoetis) is available in the USA, Canada and New Zealand. The vaccine is given intranasally and provided protection against strangles. However, CD explained how the vaccine strain can cause adverse reactions and even strangles in some horses. As a result this vaccine is not available for use in Europe.
The only currently available strangles vaccine in Europe is Equilis StrepE (MSD Animal Health). CD explained how this vaccine is based on a live strain of S. equi that is administered into the upper lip of horses. The vaccine protects horses from strangles, but has a short-lasting effect and can occasionally cause adverse reactions. Vaccination with Equilis StrepE interferes with the currently available diagnostic tests (see above).
CD presented data from the recent Dorothy Havemeyer meeting on strangles concerning a new protein-based vaccine, Strangvac (Intervacc AB), which is in development. She showed how the vaccine protected 95% of ponies from developing strangles when tested two weeks after the intramuscular administration of the third vaccine dose. The vaccine did not interfere with the diagnostic tests and was safe when given by intramuscular injection. However, protection decreased over a 2-month period and so CD said that, if the vaccine becomes available, it may be necessary to give horses a booster vaccination in the face of an outbreak or before they travel to high-risk events.
CD then went on to describe new work at the AHT, which is funded by the BBSRC, towards improving the level of protection that strangles vaccines provide. The project harnesses the immune-stimulating activity of a modified superantigen (see above), which normally misdirects the immune response, so that it instead strengthens the immune response following vaccination. CD explained how the modified superantigen will be fused to vaccine proteins, which can either be used directly, or be employed as a sticky coat on the surface of live vaccines. This project has the potential to solve a significant problem in the development of safe and effective vaccines that protect against strangles. Furthermore, CD explained that the coupling of the modified superantigen to other vaccines could also be used to help protect other animals including pigs, cattle, sheep and humans.
Discussions then took place about the short duration of protection provided by strangles vaccines and the long-lasting natural protection generated following recovery from strangles. Historically, it has been hard to demonstrate that natural immunity is not enhanced by the presence of persistently infected carriers within populations of recovered horses. However, Redwings spoke about a group of Irish horses that had recovered from strangles and been cleared of carriers before being transported to their premises in Norfolk. This group of horses had been exposed to large levels of S. equi during the outbreak of strangles at Redwings in 2015. However, none of the Irish horses developed clinical signs of strangles, providing evidence that the natural immunity provided post-recovery is indeed long-lasting and effective. It was felt that modification of vaccine preparations, such as in the new BBSRC-funded project has the potential to provide immune responses in vaccinated animals that may be more akin to those produced following natural infection. It might also be possible to study immune cells from recovered horses to better understand how they provide a higher level of protection than is achieved using the currently available vaccines.
The authors would like to thank Nick Angus-Smith (Newmarket Stud Farmers Association), Jill Crook (National Trainers Federation), Samantha Chubbock (World Horse Welfare), Nicolas de Brauwere and Roxane Kirton (Redwings), Philip Ivens (Buckingham Equine Vets), Fleur Varley (Bell Equine Vets) and Penny McCann (Zoetis) for taking part in and contributing to this workshop.
Figure 1: Illustration of the lifecycle of S. equi.
The bacterium is taken up from the environment by a horse via the nose or mouth from which it transitions to the submandibular and retropharyngeal lymph nodes. Here it uses a variety of mechanisms, several of which remain unknown, to establish a site of infection leading to the formation of abscesses. Abscesses burst externally or into the guttural pouch where incomplete drainage of pus can lead to the establishment of persistent infection in some horses. Shedding of S. equi from abscess material from acutely affected horses or from the guttural pouches of persistently infected animals into the environment completes the lifecycle of S. equi.