No stable or arena manager wants to hear the dreaded word ‘strangles’ in relation to their operation, however this highly contagious disease can show up uninvited on even the most well managed facilities. Strangles, also sometimes referred to as distemper, is essentially an upper respiratory infection with the bacteria Streptococcus equi subspecies equi (Strep. equi for short). Strep. equi causes a highly purulent infection in the guttural pouches and lymph nodes of horses. Horses with strangles often have yellow-white, thick, purulent discharge from their noses and are often listless with high fevers. Most of the time the infection sets up shop in the retropharyngeal or submandibular lymph nodes which lie near the back of the throat. When these lymph nodes become enlarged they can make it difficult for affected horses to eat, or even breath, hence the term ‘strangles’. The most helpful treatment for horses affected in this manner is to facilitate drainage of the pus that builds up in these lymph nodes. Occasionally, Strep. equi will infect lymph nodes and/or organs in other parts of the body causing a condition known as metastatic or ‘bastard’ strangles. Normally infection with Strep. equi is non-life threatening for horses, though in cases where the swelling occludes the airway or the infection spreads to other parts of the body, it can be fatal.
Strep. equi can show up unexpectedly due to a few characteristics of the bacteria that give it an edge over lax preventative strategies. First, Strep. equi is highly contagious. While Strep. equi is often spread from one horse to another by direct contact or by shared feed and water sources, it can also be spread by flies, or even by exposure to contaminated clothing or tack. Second, Strep. equi can persist in the environment for long periods of time. If the bacterium is encapsulated in a protective layer of mucous or immersed in water, it can survive for over a month outside of its host. Even when the bacterium is exposed to less favorable conditions it often survives for up to 3 or 4 days.
Though probably the most problematic characteristic of Strep. equi is its stealth mode. While most horses eventually become immune to Strep. equi and clear the infection, some horses carry and shed Strep. equi even after they have recovered from the clinical disease. Even more concerning is the existence of horses that harbor and shed Strep. equi without ever showing signs of infection. If these carrier horses go undetected, they can initiate, or perpetuate an outbreak when they shed bacteria that finds its way to horses that have never developed an immunity to Strep. equi, or have had their immunity wane. In short, you don’t want these carrier horses around, or at the very least you want to keep them isolated from susceptible horses, however identifying them can be a real challenge.
Fortunately, we have developed some tools that can help us to identify these carriers. Traditionally, veterinary laboratories would rely on cultures to detect and identify pathogenic bacteria. However, Polymerase Chain Reaction (PCR for short) is a newer laboratory test that uses specialized enzymes to detect and amplify strands of DNA from specific organisms. These markers are designed to be unique to the organism that you are trying to identify, but they typically only require a small number of the organism to be present in order for the detection and amplification reaction to occur. This quality makes PCR both a highly sensitive (very few false negatives) and specific (few false positives) test. Furthermore, the test can be run in a few hours, whereas identifying a bacteria from a culture typically takes several days. In short, PCR allows veterinarians to accurately and quickly identify carrier horses for Strep. equi
Knowing the characteristics of Strep. equi, and being able to identify carriers, enables us to develop strategies for preventing an outbreak, or containing an outbreak if it occurs. Because we know that Strep. equi is spread by direct contact, new arrivals should be quarantined for a minimum of 3 weeks and tested prior to quarantine. Buckets and troughs used for feed and water should be cleaned daily, and disinfected prior to being shared between horses. Because Strep. equi can be spread via clothing and tack, riders should always wear a fresh set of clean clothes when they come to the facility, and should either avoid handling horses that aren’t their own, or wear and change protective clothing between groups of horses. Furthermore tack, especially halters and head stalls, should not be shared between horses and should be cleaned and disinfected before and after events. Tack that would be damaged by disinfection (e.g. leather) should still be kept clean and above all, dry.
In the event of an outbreak, infected horses should be quarantined for 6 weeks and tested via PCR prior to leaving quarantine. Quarantined horses should be fed and handled last, and handlers should change clothing, change or disinfect footwear, and wash hands prior to returning to handle unaffected horses. If the cost of a new or recurring outbreak is sufficiently high, performing 3 PCR tests 1 week apart provides the maximum assurance of pathogen clearance prior to a horse leaving quarantine. Ideally, PCR tests should be run on endoscopically guided washings from the guttural pouch. However, this may not be practical in some cases. When guttural pouch lavage and/or 3 serial PCR tests is not an option, a single trans-nasal retropharyngeal lavage with 60 mL of warm saline usually provides a sufficient sample for PCR analysis. As a word of caution, both sample collection methods are somewhat invasive, and with few exceptions should be performed by a veterinarian with the horse under sedation.
Another possible control strategy for Strep. equi is the use of intranasal and/or injectable vaccines. The intranasal vaccine is moderately effective at preventing infection with Strep. equi, with the injectable being less so. Sometimes, the intranasal vaccine will cause clinical signs similar to the disease itself, and on rare occasions may potentiate a possibly fatal condition called purpura hemorrhagica; the injectable also carries some risk of adverse reaction. In areas where strangles has a high prevalence, the benefits of the vaccine may outweigh the risks. However, a strangles outbreak can still occur in a herd of vaccinated horses, so it is no substitute for the preventative measures outlined in this article, and horse owners and stable managers should consult with a veterinarian prior to deciding on whether to use the vaccine.
In summary, the threat of a strangles outbreak is real, and one that should be taken seriously by anyone who works with horses. Thankfully, there are strategies and tools available to mitigate this risk. Some of these strategies may cause some inconvenience, but in the long run the cost of implementing these strategies pales in comparison to the headache and inconvenience that comes with having your horse or facility involved in an outbreak