America's Most Unwanted:
Strangles
(cue ominous music) It follows you home. It sneaks into your barn. It attacks your horses. It’s strangles! Da da dum!! The gossip starts flying. What barn? How many horses? The local grapevine is abuzz with talk and dread about a possible neighborhood strangles outbreak. You know how it goes. Every so often, a horse comes down with a possible strangles infection and the horse world goes crazy with rumors and suspicions. Just mentioning the disease strangles is enough to strike fear into the heart of many a horse owner. Yes, your horse can get very sick and it can be scary. Yes, there are rare, but potentially fatal complications. Hundreds of articles have been written about this disease, its diagnosis, its treatment and its possible complications. I could write books on the subject, but I’d probably bore you all to tears. I would just like to focus on two things - how to best prevent this disease and the current recommendations for quarantine and testing if it does come to your barn. But I suppose I should start off with a brief review of the disease to get us all on the same page, right? Here we go.
Strangles is caused by the bacterium Streptococcus equi and it is spread primarily through respiratory secretions (aka ‘snot’). Transmission occurs with either direct contact like mutual grooming between horses or indirect contact through contaminated housing, bedding, water or feed sources, equipment or handlers. Clinical signs include sudden onset of fever, depression, anorexia, mucopurulent nasal discharge, and lymphadenopathy (enlarged lymph nodes). Lymph nodes often abscess and rupture, draining thick creamy pus. This disease is called ‘strangles’ in reference to the possibility of severe lymph node enlargement causing obstruction of the airway and esophagus. In serious cases, a tracheostomy may need to be preformed to establish an airway. That’s one reason why this disease can be scary.
Treatment varies on the stage and severity of this disease. The majority of horses simply require rest and supportive care (palatable food, easy access to water, etc) while the disease runs its course. Horses with early clinical signs (fever and depression) may benefit from immediate antibiotic therapy to prevent lymph node abscessation. If external lymph nodes are already enlarged, antibiotics are usually contraindicated and treatment consists of enhancing maturation and drainage of the abscesses. Other treatments will depend on the individual (nonsteroidal anti-inflammatory medications, intravenous fluids, feeding tubes, etc) and can vary tremendously depending on the needs of the patient. So, please remember that every case is different, and what may work for your friend’s horse may not work for your horse. The good news is that most horses that recover from strangles develop a good solid immunity that persists in over 75% of horses for 5 years or longer.
Just to be complete, I’ll mention the complications since they can be a little scary. The overall complication rate of this disease is approximately 20%. In one study, 74 horses on one farm had strangles and 20.3% (15 horses) developed complications. Of these 15 horses, 40% died or were euthanized due to these complications. Overall, approximately 8.1% of the cases resulted in death or euthanasia. So, what are these complications? They can generally be grouped in three categories – those associated with the spread of infection to other areas of the body (bastard strangles), immune-mediated processes (purpura hemorrhagica) and agalactia (decreased milk production). Since this is turning out to be a not-so-brief review, I’ll spare you the gory details of all these complications and move on.
Obviously, the best way to prevent the disease is to practice good barn hygiene and proper vaccinations. There are currently two types of vaccines available – extract vaccines and attenuated live intranasal vaccines. Extract vaccines are given intramuscularly or subcutaneously. This vaccine needs a booster schedule of 2 or 3 doses at two weeks apart. Since this is not a live vaccine, pregnant mares can be boostered a month before foaling. Horses that have had strangles within the last year or have signs of strangles should not be vaccinated.
The attenuated live vaccine is given intranasally (ie. in the nose) and it is designed to mimic the immunity from natural stimulation. It has a similar booster schedule to the extract vaccine. Because this vaccine contains live but altered S. equi, some consideration should be taken with administration. It should only be given to healthy, non-febrile horses free of nasal discharge. There is also the risk for localized abscess formation at remote injection sites due to accidental contamination. This means if your vet vaccinates your horse with the intranasal vaccine and then proceeds to give a SQ or IM injection, there is a risk of introducing the S. equi bacterium into those injection sites and causing a localized abscess. Therefore, it should not be given concurrently with other vaccines or it should be given after the administration of other vaccines. Neither of these vaccines should be used in an outbreak except on horses with any known contact with infected or exposed horses. If you remember from a few paragraphs above, virtually anything can act to transmit the disease, so do not vaccinate your horse unless you are positive it has not been exposed.
Prevention is accomplished through quarantine and screening tests. Ideally, new horses to a barn should be quarantined for three weeks away from other horses and be screened for S. equi by weekly nasopharyngeal swabs or lavages. This is often difficult for a number of reasons. First, traffic in some barns is difficult to control. New people and horses can come and go all the time. In addition, many owners take their horses to shows, thus risking exposure to a variety of diseases which they can bring back to their resident barn. Competition can also increases a horse’s stress level, which can suppress the immune system and make them more susceptible to disease.
Previously infected horses can also serve as a source of infection. A moderate number of horses recovering from strangles can continue to shed the bacterium for up to six weeks following the resolution of any clinical signs. So, any recovered horse should be considered as a potential source of infection for six weeks and be isolated accordingly during that time. But a few horses can continue to periodically shed the bacterium for months or years following infection. These horses are called long-term subclinical S. equi carriers and can be sources of new outbreaks in otherwise well managed barns. With these horses, the bacterium is usually harbored in the guttural pouch (a hollow structure within the skull that connects to the back of the throat). These otherwise normal horses may cough occasionally or have intermittent nasal discharge. The best way to diagnose these sub-clinical carriers is to do an endoscopy of the guttural pouches to look and/or test for infection.
Diagnosis itself can be difficult. There are currently three ways to test for strangles – culture, PCR testing or serology. Culturing the organism is the “gold standard” for detection. Nasal swabs, nasal washes or pus aspirated from abscessed lymph nodes can be used for culture. However, cultures may be unsuccessful during the incubation and early clinical phases of the disease as the bacterium is not normally present on surface mucosa to be sampled.
The polymerase chain reaction (PCR) test detects part of the DNA of the S. equi and samples can be obtained through nasal swabs or washes. This is a much faster test and results can be available the same day in some cases. It is also approximately three times more sensitive than a culture. However, this test does not differentiate between live and dead organisms, so a positive PCR is presumptive unless confirmed by a positive culture.
Serology involves drawing blood and looking for antibody levels to a particular antigen of S. equi. It can be a little difficult to interpret the results and does not distinguish between vaccine and infection response. Often, one must compare a series of serology results to determine between exposure and actual infection. It is useful to assess the need for booster vaccinations and can aid in the diagnosis of purpura hemorrhagica and metastatic abscesses (both rare complications of S. equi infections).
Confused enough yet? There has been a lot of research done on S. equi infections, so we veterinarians have a lot of good clinical knowledge to rely on. And the research is on-going, so in the future, we may have even better ways to prevent and treat this disease. Strangles isn’t going away any time soon. It will continue to be a threat to our horses for a number of reasons. The more you all know about this disease and how best to protect your horse, the happier I will be. Hopefully, some of the above made sense to you. If not, you can always quiz your friendly neighborhood veterinarian!
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