Getting to Grips with Strangles: Working Together to Break the Strangles-hold

Andrew S. Waller    Animal Health Trust, Kentford, Newmarket, Suffolk, CB8 7UU, UK      Strangles, caused by a bacteria called Streptococcus equi, is one of the most frequently identified infectious diseases of horses worldwide. More than 600 outbreaks of Strangles are diagnosed in the UK each year. Infected horses typically develop fever followed by abscesses in the lymph nodes of their head and neck. These abscesses are painful and the affected horses will often lose their appetites and become depressed. Some horses can be badly affected during an outbreak and the disease kills around one in a hundred animals. The bacteria can spread quickly through yards via contaminated drinking water, food, tack, equipment and people. Some outbreaks can involve all of the horses on a yard and all outbreaks require movement restrictions that usually remain in force for over two months. Consequently, Strangles is responsible for considerable economic and welfare cost. This article will provide an update on the progress being made towards eradicating Strangles and highlight what we can each do to keep our horses safe.      An age-old problem:    Strangles was first described in 1251 by Jordanus Rufus, a knight of Emperor Fredrick II. The disease was seen as inevitable and better for horses to fall ill sooner rather than later to get the disease over and done with. In 1811 Napoléon, Emperor of France, wrote a letter to request that the 543 horses being sent to his army should be “at least 60 months of age and should already have recovered from Strangles” so that they would be less likely to fall ill from this disease on the battlefront. More than 200 years later, many people still believe that it is inevitable that their horse will suffer from Strangles sooner or later. However, we understand so much more about the disease today and really can significantly reduce the risk of horses falling ill.      The secret to Strangles’ success:    Once Streptococcus equi is drunk or eaten by a horse, it sticks to the surface of the horse’s mouth or nose and then travels through to the lymph nodes in the horse’s head and neck, where it can be detected only two hours later. Although the horse’s immune response detects Streptococcus equi in the lymph node, the Strangles bacteria is very resistant to attack from the horse’s immune system. Indeed, recent evidence shows how Streptococcus equi can exploit this unsuccessful immune response to turn the lymph node into an abscess, which effectively serves as a nutrient soup in which the bacteria thrives. The abscesses burst and enable Streptococcus equi to spread to other susceptible horses. However, the main trick up the sleeves of Streptococcus equi, is its ability to hide away in the guttural pouches of horses after they have recovered from Strangles. These ‘carrier’ horses look completely healthy, but intermittently shed Streptococcus equi triggering new outbreaks. A carrier can shed millions of Strangles bugs into a water trough just from having a quick sip. This number of bugs is sufficient, in theory, to infect and cause Strangles in thousands of horses. Therefore, the identification and treatment of carrier horses is extremely important if new outbreaks of Strangles are to be prevented.      Keeping Strangles out:    Only the geographically-isolated population of 80,000 horses in Iceland remain free of Strangles, a situation that has been maintained through a virtual absence of horse import for over 1,000 years. The absence of Strangles in Iceland highlights a key problem faced by every other country: the national and international movement of horses. The horse is a global traveller, journeying throughout the world for competition, training, breeding, events or sales. Although pre-movement and pre-export veterinary checks can prevent the transport of obviously ill horses, carrier horses look completely healthy and cannot be identified through this standard examination.      An unwanted hitchhiker:    The way in which Streptococcus equi hitchhikes around the world in carrier horses has been highlighted in a recent study at the Animal Health Trust. In this study the DNA of 703 strains of Streptococcus equi from horses in 22 countries across the world was examined in great detail. The results showed that the same specific types of Streptococcus equi were recovered from horses in Europe and the USA; the USA, Europe, Japan and Israel; Europe and Australia; Australia, New Zealand and the United Arab Emirates; and Argentina, Europe and the United Arab Emirates. Essentially, wherever horses were moved, then the Strangles bacteria could travel with them. In particular, horses in the United Arab Emirates shared 14 different types of Streptococcus equi with countries from all around the world.      Breaking the chain:    Realising that current pre-export checks were insufficient; the United Arab Emirates have taken a proactive response and now insist on the mandatory screening of horses pre-export to identify Strangles carriers. Initially, horses are screened using the Strangles blood test to identify those that have been recently exposed to Strangles. Positive horses are then examined more closely to see if they are actually still infected with Streptococcus equi. If they aren’t, then the horse can still be exported. If they are, then the carrier horse can be treated and cleared of Streptococcus equi before it is exported. Screening in this manner identified four new carriers of Streptococcus equi in 2016/2017 prior to export, which likely prevented the occurrence of several new outbreaks. In the future, this type of pre-export or movement screen could also help to prevent outbreaks in the UK and elsewhere in the world. Quarantining new arrivals for three to four weeks also helps to reduce the risk of Strangles, and other diseases, getting in. New horses should be kept away from resident horses and not share drinking water or tack. Taking temperatures each day can flag up horses during the first stages of disease so that vets can stop further spread of disease as quickly as possible.      Dealing with an outbreak:    If a Strangles outbreak does start, then the ‘traffic-light’ system of control is an effective way to minimise its impact. In this system, infected animals with clinical signs (Red group) are separated from healthy animals they had contact with (Amber group) and from horses (Green group) that have no clinical signs and that had no contact with horses in the Red or Amber groups. This method minimises the dose of Streptococcus equi that horses receive and will reduce the severity of disease and enable horses to recover from Strangles as quickly as possible.      It is really important to try and minimise the spread of the Strangles bacteria during outbreaks by washing hands between touching different horses and using different equipment and tack for each horse, without mixing it up. Virkon and Safe4 disinfectants are known to kill Streptococcus equi and can be used to disinfect equipment and stables as required.      Giving Strangles the boot:    Around one horse in ten that recover from Strangles become carriers of Streptococcus equi with the potential to trigger new outbreaks each year. So, around four weeks after they recover, horses can be checked using guttural pouch endoscopy, where a small camera is placed into the guttural pouch, to see if any Streptococcus equi remain hidden away. Carriers can then be treated by physically removing any lumps of bacteria and treating horses with a dose of antibiotics. Flushing the guttural pouches of horses immediately after they recover from Strangles might help to minimise the number that become carriers and need treatment.      Horses in the amber and green groups should be tested using the Strangles blood test to see if any of them were exposed to Streptococcus equi during the outbreak. Horses that test positive with the blood test can then be checked with guttural pouch endoscopy to see if they are carriers and if so, be treated as described above.      Preventing Strangles by vaccination:    Horses will always travel to events where they can potentially be exposed to Streptococcus equi. The risk of exposure can be reduced by taking separate tack and equipment to minimise contact with other horses and avoid them sharing drinking water. However, the use of vaccines could help to minimise the number of horses that develop Strangles even if they are exposed to the bacteria.      Several Strangles vaccines are produced around the world seeking 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 Streptococcus equi cells, but little data on the protection they confer is available and so these vaccines are not available for use in Europe.      The Pinnacle IN live vaccine (Zoetis) is available in the USA, Canada and New Zealand. The vaccine is sprayed into the nose of horses and provides protection against Strangles. However, it was recently found that the vaccine strain was recovered from over 60% of the vaccinated horses that went on to suffer Strangles, suggesting that the vaccine itself was responsible for some outbreaks.      The only currently available Strangles vaccine in Europe is Equilis StrepE (MSD Animal Health). This vaccine is based on a different live strain of Streptococcus equi that is injected into the upper lip of horses. The vaccine protects horses from Strangles, but has a short-lasting effect and horses need to be vaccinated every three to six months. New results from Europe show that vaccination with Equilis StrepE interferes with the Strangles blood test. So vaccinated horses would be likely to trigger further pre-export/-movement checks.      The above vaccines were all developed many years ago, but the completion of the Streptococcus equi genome sequencing project launched new waves of research to make better vaccines against Strangles. The progress towards the launch of one of these new Strangles vaccine was described at the recent Dorothy Havemeyer meeting on Strangles in Montana, USA, during September 2017. The vaccine, called Strangvac, uses eight proteins from Streptococcus equi. Strangvac protected 95% of ponies from developing Strangles when tested two weeks after giving a third vaccine dose. The vaccine did not interfere with the Strangles blood test and was safe when given by intramuscular injection. However, protection decreased over a two month period and so it may be necessary to give horses a booster vaccination in the face of an outbreak or before they travel to high-risk events.      Improving vaccine protection:    New research at the Animal Health Trust, which is funded by the UK government’s    Biotechnology and Biological Sciences Research Council    (BBSRC), is tasked with improving the level of protection that Strangles vaccines provide. The project will modify a protein that normally misdirects the immune response, so that it instead strengthens the immune response following vaccination. This modified protein will be fused to vaccine proteins to help the vaccines trigger stronger immune responses and better levels of protection. The project has the potential to solve a significant problem in the development of safe and effective vaccines that protect against Strangles. Furthermore, this technique, if successful could also be used to help protect other animals including pigs, cattle, sheep and humans.      The new research to improve vaccines against Strangles is finally reaching a point of delivering products that could enhance the health of our horses. We also know ever more about how the Strangles bacteria spreads and causes disease, enabling horse owners to employ simple precautions to minimise the risk of their horse falling ill. Strangles is no longer inevitable and we hope that the combination of better management, diagnostic tests and vaccines will gradually enable us to reduce the number of outbreaks of this disease around the world.      <<Napoleon picture>>    “Napoleon crossing the Alps”, from Jacques-Louis David, 1801, oil on canvas (Château de Malmaison, France). Source is Wikipedia.      <<lymph node abscess picture>>    A typical submandibular lymph node abscess, which is bursting out from under the jaw of the horse releasing billions of Streptococcus equi into the surrounding environment.      <<Treating carriers>>    Removing a dried ball of pus containing millions of Streptococcus equi from a carrier horse.      <<Illustration of the lifecycle of Streptococcus equi>>    The bacterium is taken up from the environment by a horse via the nose or mouth from which it spreads 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 carrier horses. Shedding of Streptococcus equi from abscess material from acutely affected horses or from the guttural pouches of carriers into the environment completes the lifecycle.

Published in European Trainer, January - March 2018, issue 60.

Strangles, caused by a bacteria called Streptococcus equi, is one of the most frequently identified infectious diseases of horses worldwide. More than 600 outbreaks of Strangles are diagnosed in the UK each year. Infected horses typically develop fever followed by abscesses in the lymph nodes of their head and neck.

These abscesses are painful and the affected horses will often lose their appetites and become depressed. Some horses can be badly affected during an outbreak and the disease kills around one in a hundred animals. The bacteria can spread quickly through yards via contaminated drinking water, food, tack, equipment and people. Some outbreaks can involve all of the horses on a yard and all outbreaks require movement restrictions that usually remain in force for over two months. Consequently, Strangles is responsible for considerable economic and welfare cost. This article will provide an update on the progress being made towards eradicating Strangles and highlight what we can each do to keep our horses safe.

An age-old problem:

Strangles was first described in 1251 by Jordanus Rufus, a knight of Emperor Fredrick II. The disease was seen as inevitable and better for horses to fall ill sooner rather than later to get the disease over and done with. In 1811 Napoléon, Emperor of France, wrote a letter to request that the 543 horses being sent to his army should be “at least 60 months of age and should already have recovered from Strangles” so that they would be less likely to fall ill from this disease on the battlefront. More than 200 years later, many people still believe that it is inevitable that their horse will suffer from Strangles sooner or later. However, we understand so much more about the disease today and really can significantly reduce the risk of horses falling ill...

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