Strangles (S. equi equi)

In this episode of Current Vet, Dr. Lottie talks about strangles, the complications that can develop, how to manage both complicated and uncomplicated cases, and how best to prevent spread between horses.

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Timeline:
00:00 Intro
00:25 Case
02:05 Aetiology & Pathogenesis
05:15 Clinical Signs
06:41 Diagnosis
09:41 Treatment
14:33 Prevention
17:35 Key Points
18:59 Outro

References

• Boyle, A.G. (2017) ‘Strangles and its complications’, Equine Veterinary Education, 29(3), pp. 149–157. Available at: https://doi.org/10.1111/eve.12568.
• Mallicote, M. (2015) ‘Update on Streptococcus equi subsp equi Infections’, Veterinary Clinics: Equine Practice, 31(1), pp. 27–41. Available at: https://doi.org/10.1016/j.cveq.2014.11.003.
• Chhabra, D. et al. (2023) ‘Strangles in equines: An overview’, Microbial Pathogenesis, 178, p. 106070. Available at: https://doi.org/10.1016/j.micpath.2023.106070.
• Durham, A.E. et al. (2018) ‘A study of the environmental survival of Streptococcus equi subspecies equi’, Equine Veterinary Journal, 50(6), pp. 861–864. Available at: https://doi.org/10.1111/evj.12840.
• Waller, A.S. (2014) ‘New Perspectives for the Diagnosis, Control, Treatment, and Prevention of Strangles in Horses’, Veterinary Clinics: Equine Practice, 30(3), pp. 591–607. Available at: https://doi.org/10.1016/j.cveq.2014.08.007.
• Boyle, A.G. et al. (2018) ‘Streptococcus equi Infections in Horses: Guidelines for Treatment, Control, and Prevention of Strangles—Revised Consensus Statement’, Journal of Veterinary Internal Medicine, 32(2), pp. 633–647. Available at: https://doi.org/10.1111/jvim.15043.

Current Vet is an educational podcast intended for veterinary students, veterinary professionals, and individuals with an interest in veterinary medicine.

All content provided in this podcast and its associated materials is for educational and informational purposes only. It is not intended as, and must not be considered a substitute for, professional veterinary advice, diagnosis, or treatment.

Any clinical cases discussed in this podcast are fictional, and are designed to reflect typical or likely clinical scenarios for educational purposes. They do not represent specific real-life cases, clients, or animals.

While every effort is made to ensure accuracy and alignment with current evidence at the time of publication, veterinary medicine is a rapidly evolving field, and recommendations may change over time.

Creators and Guests

Host

Dr. Lottie Wilkinson

Creator and host of the Current Vet podcast

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Intro

Hello, hello, and welcome back to Current Vet, the podcast that makes vet med simple. Today's episode is all about equine strangles caused by Streptococcus equi subspecies equi. So strangles is definitely one to know for everyone in equine practice, and it is also a favorite for exams. So let's get straight into it.

Case

It is a rainy Tuesday morning and you're covering a call-out for a local riding school with 14 horses. The yard manager, Emma, tells you that one of their ponies called Smokey, a 12-year-old gray gelding, has been off his food and looking a little bit dull for the past day. He's usually used in lessons every day, so she noticed something was wrong straight away.

When you arrive, Smokey is standing quietly in the corner of his stable. He is not particularly bright. He's pyrexic at 39.8 degrees Celsius, and there is bilateral serous nasal discharge that is starting to turn a little bit mucopurulent. , , his submandibular lymph nodes are enlarged, firm, and tender on palpation.
Emma mentions that two of the other ponies , have had snotty noses over the past week, but nothing as bad as this. She's been keeping them in, but she thought it was just a bit of a seasonal cold. As you continue your exam, you feel a fluctuant area in Smokey's left submandibular region.

You ask when he was last vaccinated, and Emma says she's not sure as the yard vet usually does them. Your top differentials are going to include equine influenza, equine herpes virus, but the lymphadenopathy, abscesses, and nasal discharge also point to strangles. If it is strangles, the riding school environment with the shared tack, the feed buckets, the water troughs, the different age groups all mixed in together, movement on and off the yard, make this a really perfect place for an outbreak to start.

Aetiology / Pathogenesis

So let's talk about what's actually going on to cause infection. Streptococcus equi equi is a gram-positive beta-hemolytic bacterium It is a Lancefield group C streptococcal bacteria, , which is really just a classification based on the types of carbohydrates present in the bacterial cell wall.

It's classed as an obligate pathogen of equids, so it doesn't survive well in the environment long term or infect species other than equids. The bacteria enter through the nasal or oral mucosa after inhalation or ingestion, and then migrate to the local lymph nodes from there. Virulence factors are characteristics that pathogens have which help them cause disease, and Strep equi equi has a few.

So these are gonna include the SEM surface proteins and the hyaluronic acid capsule, which are gonna help to inhibit phagocy- phagocytosis of the bacteria. The bacteria also produce superantigens, which trigger massive immune activation and inflammation So these bacteria are gonna continue to replicate within lymph nodes, which form abscesses, and these abscesses can rupture either internally or externally through the skin

So in uncomplicated cases, we generally see signs developing within a couple of weeks of infection. We see clinical signs like pyrexia, lethargy, and abscesses developing. They then can rupture and drain, and the horse can recover well within a couple of weeks. But strangles can also cause several really important complications.

So one, we have guttural pouch empyema, and this can occur if the abscess ruptures internally and pus drains into the guttural pouch. This can lead to persistent purulent nasal discharge, dyspnea, or the horse becoming a chronic carrier of the infection because the pus and the source of infection remains within the guttural pouch. The second complication is bastard strangles, and this is when the bacteria travel through the bloodstream to internal lymph nodes and organs, leading to internal abscessation. We can see chronic weight loss, colic, and various other signs depending on which organs are affected The third complication is purpura haemorrhagica, and it's a form of vasculitis caused by a type III hypersensitivity reaction to the bacterial antigens, and this causes immune complex deposition within the blood vessel walls.

It causes edema, petechiae, ecchymosis, and skin necrosis in severe cases. It's most commonly seen when recently infected horses are vaccinated, which is why we should not be vaccinating horses during or immediately after an outbreak, and that's really important to remember

Clinical Signs

Let's quickly just retouch on the clinical signs that you might see in both classic cases and in horses with complications. So with Smokey, you're seeing the classic presentation of lethargy, pyrexia, that bilateral nasal discharge, and painful, swollen submandibular lymph nodes. If he then developed guttural pouch empyema, you might also see a thick, purulent nasal discharge, which can look quite similar to perhaps a choke presentation, and these cases are the ones which often become chronic carriers, so they can continue to shed bacteria even if they aren't showing clinical signs.

If Smokey had bastard strangles, this causes more chronic or vague clinical signs like weight loss, mild pyrexia, recurrent colic, or dyspnea, and it can be unresponsive to treatment because it is difficult for the antibiotics to penetrate internal abscesses. And then finally, if Smokey had purpura haemorrhagica, we can see typically a severe peripheral edema, petechiae and ecchymosis forming on the skin and mucus membranes

So with Smokey, we're seeing a typical case here, but there may also be other horses on the yard that are asymptomatic but recently infected

Diagnosis

At this point, we've examined Smokey. He is pyrexic, dull, and has mucopurulent nasal discharge, as well as those enlarged fluctuant lymph nodes. You're pretty sure it's strangles, so do you need to bother with diagnostics?

So with strangles, we can often start treatment based just on the clinical presentation, but just the clinical signs are not obviously gonna tell you which horses are asymptomatic but infected, or whether a recovered horse is still infectious, or if there are any carrier horses with guttural pouch infection that are continuing to spread bacteria.

So testing is essential to confirm the disease, screen those in-contact horses, and prevent outbreaks. But before we start any test with Smokey, we're gonna make sure that he's isolated from the rest of the group
so the mainstay of diagnosis is direct detection of the bacteria with a culture or more commonly PCR. You can use nasopharyngeal swabs or guttural pouch lavage and submit these samples for testing. And culture allows for definitive diagnosis, but it can take several days for the results to come back, so it's really not useful when trying to implement immediate outbreak control strategies.

PCR is going to detect the bacterial DNA. It's more sensitive and it gives faster results, so it's often the preferred test for diagnosis and screening

We could also do abscess aspiration and do culture or PCR on this. So if the lymph node is fluctuant and close to rupturing, you can do an aseptic aspirate from the abscess and send the pus for culture and PCR, and this is really great because the bacterial load is typically highest within the abscesses.

We can also do serology, which are gonna detect antibodies specific to S. equi, and these are most often useful for screening horses before they have been vaccinated. It is important to note though that serology isn't gonna confirm active infection or tell you if the horse is still shedding bacteria

The best way of identifying subclinical carriers is gonna be with endoscopic examination of the guttural pouches just to see if we can identify any chondroids, which is basically hardened balls of pus, which is a bit gross, or other signs of infection within the pouches, followed by lavage and sampling for PCR and/or culture.

It is recommended to do this at least three weeks after clinical recovery, and ideally repeat it two to three times, each at a week apart, with negative results each time before ruling that that horse is clear of infection and they are definitely not a chronic carrier

Treatment

Okay, so we've confirmed the diagnosis in Smokey, and you wanna make sure that he makes a full recovery without infecting any other horses at the school. For most cases like Smokey, the focus is gonna be on supportive care, monitoring him really well, and just letting the disease run its course. So this is gonna include keeping the horse rested in the stable and isolated from others, NSAIDs like flunixin or phenylbutazone to reduce fever and release any discomfort , especially from that lymphadenopathy.
Some warm compresses can be applied to abscesses, which helps them to mature and rupture. Feeding a soft palatable food can be really, really beneficial because swollen lymph nodes can make chewing quite painful and also food quite difficult to swallow. We must make sure that adequate water is available and that the horse is drinking, and we should monitor horses for dyspnea or breathing problems as those swollen lymph nodes can sometimes compress the trachea.

And if this occurs, popping those abscesses or in some cases a tracheostomy may be done
Once lymph nodes rupture and drain either spontaneously or with help, most horses start to recover over the next couple of days. case

His abscess is close to rupturing, so you may choose to lance, drain, and flush it if it softens further, as draining can speed up the recovery and reduce the risk of systemic spread. But it must be done under really controlled conditions and careful biosecurity to make sure that we aren't contaminating the environment and spreading infection to other horses

In cases of strangles, antibiotic use can go either way so in general, antibiotics are not routinely recommended once abscesses are formed because they can actually delay abscesses maturing and rupturing, and they reduce the immune response without properly clearing the infection. But there are situations where antibiotics may be appropriate, So potentially if the horse is in really early disease before abscesses have developed, if the horse is systemically unwell, if we're suspecting bastard strangles, or if there are complications like guttural pouch empyema or purpura haemorrhagica.
The go-to antibiotic is gonna be penicillin, but there are other options like cephalosporins and potentiated sulfonamides

Let's briefly touch on how to approach a couple of those complications for treatment, though. So guttural pouch empyema, we're gonna perform that endoscopy and then lavage the pouch to fully clear all of the pus out. Antibiotics can then be instilled into the guttural pouch, and we can also give systemic antibiotics just to make sure the infection has been completely cleared.

Bastard strangles requires quite long-term systemic antibiotics and potentially imaging, like ultrasound, to visualize the internal abscesses. But the response to treatment is quite variable with this form, and it doesn't always have a great prognosis. But purpura haemorrhagica is an emergency, and it should be treated with high-dose corticosteroids like dexamethasone, fluids, and anti-inflammatories like NSAIDs, like flunixin, as well as antibiotics while those corticosteroids are being given. So we must be giving penicillin as well as the corticosteroids Make sure to monitor for things like laminitis and organ dysfunction, even if there is a good response to treatment

Even after clinical signs resolve, horses can continue to shed S. equi for weeks, especially if guttural pouch edema is present. So we should be isolating those affected horses for at least three to six weeks, and then only returning them to the herd once they are clinically normal, not pyrexic, and are getting negative PCR results on guttural pouch washes.

So to quickly summarize that load of information, supportive care is typically sufficient for the majority of uncomplicated cases. Antibiotics are only indicated either very early in the disease process when abscesses have not yet developed or late when systemic disease and/or complications have occurred.
Horses must be isolated from others at the earliest possible point, and then only returned several weeks later after clinical resolution and ideally when they are PCR negative on three consecutive tests.

Prevention

Prevention is absolutely key with strangles because once it's on a yard, containment can be really challenging. So once you confirm or suspect strangles in one horse, your job is gonna be to prevent further transmission. So first, we can split the yard into zones, like a red zone is gonna be infected horses showing clinical signs, a yellow zone is gonna be horses exposed but not yet clinical, and then the green zone is horses with no known contact.

Each zone should have its own dedicated equipment and own dedicated staff if possible. If physical separation isn't possible, we can still make sure we interact with each zone individually, so handle and muck out the green zone first and then the red zone last. Stop all horse movements on or off the premises.
We need to be doing daily temperature checks for all horses as pyrexia is often the first clinical sign to develop

We need to be monitoring, identifying, and then screen testing any horses that show potential clinical signs, even if it is just being a little bit off feed or slightly pyrexic We should be disinfecting and cleaning the premises thoroughly, and then before lifting restrictions, make sure that all are clinically well, they are not pyrexic, and they have two or three consecutive negative PCR tests

We can then next think about vaccinations. So they are available, but they are not part of the standard protocol for horses. So vaccines are generally reserved for yards with a history of outbreaks, any high movement facilities like competition yards or rescues, or they can be given before new arrivals come in or if there's any off-yard travel.

It is an intranasal vaccine that can be sprayed up the nose, and that is gonna stimulate much better mucosal immunity. Or there is also an injectable vaccine as well . But the most important thing is remember, do not vaccinate horses during an active outbreak or for about six to 12 weeks after recovery, as it can trigger purpura haemorrhagica.

If you don't know if a horse has been exposed, consider doing serology first if you need to vaccinate that horse And then finally, we need to think about longer term prevention for the facility. So quarantining new arrivals for several weeks before they join a group, ideally with a guttural pouch screen , before they're integrated, using dedicated tack and feed buckets for all horses, maintaining really good hygiene, prompt investigation of any sick horses, and making sure to educate owners and yard staff so they're confident in recognizing signs and sticking to prevention protocols

Key Points

Okay, so these are the key points that you need to know about strangles. Strangles is caused by Streptococcus equi subspecies equi. The classical signs are going to include pyrexia, nasal discharge, lymphadenopathy, and abscessation While a lot of cases are uncomplicated and quite straightforward, complications can occur, and these main three are gonna be guttural pouch empyema, bastard strangles, and purpura haemorrhagica.

PCR from nasal swabs, abscess material, or guttural pouch lavage is best for diagnosing active infections and identifying carriers. Supportive care is gonna be the mainstay of treatment, so including NSAIDs, soft feed, good hydration, and drainage of abscesses. But using antibiotics should be reserved really only for severe or complicated cases.

Vaccines are useful for high-risk individuals, but should not be used during or immediately after outbreaks, and instead isolating affected or contact animals, good hygiene, and regular monitoring of temperatures and checking PCR tests are gonna be best for preventing further transmission.

Outro

And that is equine strangles.

Thank you so much for listening to this episode. If you found it helpful, follow or subscribe wherever you get your podcasts, and share it with a friend or colleague who needs to hear it too. As always, you'll find the reading and links in the show notes, and if there's a topic you'd like to hear about next, let us know in the comments or come and find us on TikTok and Instagram @veterinaryvista.

We'll see you next time.