Stephen Graham, DVM

June 2006

Many of you have probably have at least heard of strangles.  Some may have heard it called distemper (the disease in the horse is not related to the same named disease in dogs).  Strangles is an illness caused by a bacteria (Streptococcus equi).  This infection rarely causes death, but if exposed, many horses become sick.  Strangles is so named because swollen lymph nodes can in some cases put pressure on the horse's trachea causing it to have difficulty breathing (strangling it).  This is a very contagious disease, easily spreading from horse to horse at a show or boarding facility.  If your horse or barn becomes affected by this bacteria, it can shut you down for months.

What does this disease look like?

The first sign of illness that your horse will exhibit is a fever of up to 103°F or higher.  This is followed by inflammation of the horse's upper respiratory tract and often the formation of abscesses in the lymph nodes in the head.  These lymph nodes become painful and swollen about one week after infection.  Most often the lymph nodes between the lower jaw of the horse (submandibular) and the lymph nodes just behind the jaw (retropharyngeal) are those affected.  Unfortunately, in some instances, the lymph node swelling may not be appreciated by just looking at the horse.

Due to the inflammation and enlarged lymph nodes, horses can have difficulty swallowing.  The discomfort may cause them to stop eating and you may also notice them standing with their neck extended.  Nerve damage can also occur in some horses due to the swollen lymph nodes.  This nerve damage can be another cause of the horse having a difficult time breathing.  Some horses, after attempts to swallow food and water, may have this same food and water come out of their nose due to their inability to swallow properly. Besides the swollen lymph nodes, many horses have a thick, pus-filled nasal discharge. Some horses may get upper respiratory snuffling or rattling. Coughing is not a significant feature in many cases, although some horse develop a soft moist cough that becomes more productive and increasingly severe as the disease progresses.

The swollen and abscessed lymph nodes “mature” and then rupture to drain a tenacious creamy pus, which does not have a foul odor. Other lymph nodes of the neck are also frequently involved and may abscess.  In some instances, the retropharyngeal lymph nodes may drain into and cause pus to accumulate into the guttural pouch. Expulsion of large quantities of pus from the nose or mouth with coughing usually indicates pus is in the guttural pouch.  Pus in these pouches can later lead to a horse being a chronic shedder of the disease, potentially infecting many other horses.  Another major concern with this disease is that it can move to other parts of the body in the chest and abdomen.  This is commonly called bastard strangles.

The severity of this disease varies greatly depending on the immune status of the horse.  Older horses often exhibit a mild form of the disease with mild signs and rapid resolution whereas younger horses are more likely to develop severe lymph node abscessation that subsequently opens and drains.

How do horses get this?

Horses become infected by coming into contact with the bacteria via the mouth or nose.  These bacteria can come from drainage containing the bacteria from other currently infected horses, from shared water troughs or buckets, feed or feeding utensils, twitches, tack, veterinarians, farriers, or even from discharge on stalls or elsewhere in the environment. A recovered horse may be a potential source of infection for at least six weeks after its clinical signs of strangles have resolved.  Apparently healthy horses recovering from recent disease might continue to harbor the organism after full clinical recovery.  There is evidence that a moderate proportion of horses continue to harbor S. equi for several weeks after clinical signs have disappeared, even though the organism is no longer detectable in the majority four to six weeks after total recovery. 

A persistent source of infection is a horse with infection in the guttural pouch.  Horses with guttural pouch infection may persist asymptomatically for months or even years.  Approximately 50 percent of horses with guttural pouch infection cough sporadically and some may have an intermittent unilateral nasal discharge with infective bacteria being spread. Some animals never shed the bacteria despite being infected.

It is unclear how long S. equi persists in the environment, as field trials have not been performed.  In the laboratory, scientists have discovered that the bacteria can live on wood at 2 degrees Celsius (just above freezing) for 63 days and 48 days on glass or wood at 20 degrees Celsius (about 65 degrees F.  The moral here is that it is unclear exactly how long it can live, but it is a long time after an infected horse has recovered or left the premises.

How do I know if my horse has this?

Unless you know that your horse has come into contact with a potential source of infection, the first sign you may notice is swollen lymph nodes or nasal discharge.  If you notice this from your horse, the first thing to do is to isolate them from other horses as well as the horses they have come into contact with.  The second thing to do is to call your veterinarian.

If you know that your horse may have come into contact with a potential source of infection something that you can do is to take the horses temperature.  This sign will show before any of the others - and horses don’t shed the bacteria until a few days after the onset of fever, so other horses won’t have been exposed if isolated from the feverish horse.  If you notice an increased temperature after a suspected exposure to the infection, you should isolate your horse and call your vet. 

There are a few different techniques that can be used to detect the bacteria in your horse.  The vet may try and swab some nasal discharge or wash some bacteria by rinsing the nose of your horse and collecting the rinses, or may place a needle in a swollen lymph node to try and draw some bacteria out. Attempting to isolate the bacteria by culture may be unsuccessful until 24 to 48 hours after the onset of fever, so therefore monitor temperatures and isolate all horses with temperatures.

What do we do now?

As strange as it sounds, in many cases the best thing to do for your horse is to treat the horse symptomatically without giving it antibiotics.  Placing hot packs over swollen lymph nodes and giving anti-inflammatory and pain medications to keep the horses as comfortable as possible is often the best course of action.  Studies have shown that approximately seventy-five percent of horses develop a solid, enduring immunity to strangles for five years or longer after recovery from the disease if they recover on their own.  Veterinarians often have different thoughts about if and when to treat a horse with antibiotics, so discuss the treatment options with your veterinarian.  Another thing to discuss with your veterinarian is whether or not to vaccinate your horses in the face of an outbreak.  In many cases it is best to vaccinate those horses not yet exposed to help prevent infection or to lessen the effects of the infection if they do become sick.

How do I keep my horses from getting this?

In an ideal world we would never expose our horses to this infection.  Unfortunately there are horses that carry this and shed the bacteria without us knowing.  This leaves other horses at risk.  The next best thing is to vaccinate.  There are two different types of vaccine available.  There is one that we give in the muscle and another that we squirt in the nose.  Studies have shown that administering the type of vaccine in the nose conveys good protection and if the horse is exposed to the bacteria and becomes infected, they will have fewer and less severe effects from the disease. 

Foals are often more severely affected by strangles, so a positive note for those mares that have recovered from strangles infection is that their milk contains antibodies that protect suckling foals until they are weaned.  At this point they can be vaccinated and are better able to protect themselves.

What do I do if I have an outbreak of strangles at my farm?

What exactly to do in the face of an outbreak is not clear cut,  as every farm has different things to consider.  Important things to note would be to identify possibly exposed horses and to appropriately deal with those horses, as well as to make sure those horses not yet exposed remain unexposed.  The first thing to do, and possibly most important, is to stop the movement of horses on and off your farm.  Those horses that are infected should be kept in an isolated “dirty” area. Rectal temperatures should be taken at lease once daily during an outbreak to detect, promptly segregate and possibly treat new cases.  Every possible precaution should be taken to ensure very high hygiene standards throughout the premises for the duration of the outbreak.  Swabs or lavaged fluids from washing out the nasal passages should be collected at weekly intervals over at least three weeks following recovery.  These fluids should be tested for the bacteria before a horse is considered to be no longer shedding.  As mentioned above, vaccinating horses is an option and should be considered and discussed with your veterinarian.

With the show season upon us, this disease is a very real concern.  Make sure to do your best to protect you and your horse from this very serious infection.  If you suspect this condition in your horse, isolate it from other horses and contact your vet as soon as possible.