Strangles: Horses at Risk

Strangles is a malady that has afflicted horses for hundreds of years. In fact, it was first described in a veterinary publication back in 1614. During the ensuing years, many horses have suffered from strangles. Most have recovered, but some have not. Along the way, the troublesome disease has cost the horse industry millions of dollars.

In those years, strangles has faced a formidable opponent in the form of scientific research. During the early going, very little progress was made, but more recently, thanks in part to the efforts of British scientists, some rather dramatic findings have opened the door to avoiding the malady.

One scientist, John Prescott, MA, VetMB, PhD, a professor in the Pathobiology Department at the University of Guelph in Ontario, takes it a step further. He believes that we can eradicate strangles if the horse industry organizes itself for the effort.

What the British scientists at the Animal Health Trust in Newmarket have learned through research conducted during the past several years is that horses which have recovered from the disease can be carriers, even though they appear outwardly healthy. What this means is that they can inadvertently infect healthy populations just by showing up in their midst.

The good news is that there is a way, albeit a bit expensive, to identify and treat these carriers before they can infect others.

Before we launch into a discussion of prevention and cure, it would be helpful to understand the disease. Simply put by Prescott and Bob Wright, DVM, a veterinary scientist with the Ontario Ministry of Agriculture who co-authored a paper on the subject, strangles is a highly contagious and serious infection of horses and other equids caused by the bacterium Streptococcus equi.

What Is Strangles?

"The disease," they explain, "is characterized by severe inflammation of the mucosa (lining which covers any mucous- secreting glands) of the head and throat, with extensive swelling and often rupture of the lymph nodes, which produces large amounts of creamy pus."

Horses of all ages are susceptible, but strangles seems to rear its head most commonly in equids less than five years of age. Often hit the hardest are groups of weanlings or yearlings. Foals under four months of age normally are protected by antibodies passed on from their mothers via colostrum.

Clinical signs of strangles include lack of appetite, fever, depression, nasal discharge, a soft cough, and painful swelling between the jawbones. Susceptible horses develop strangles anywhere between three and 14 days after exposure. As the disease progresses, abscesses usually develop in the lymph nodes of the throat.

The lymph nodes, report Prescott and Wright, become hard and very painful and can obstruct breathing, thus the name "strangles." If untreated, the lymph nodes normally will burst in seven to 14 days, releasing thick pus that is heavily contaminated with the causative agent, S. equi. Generally speaking, the horses will recover rapidly once the lymph nodes have burst and emptied themselves of the pus.

The recovery process, Prescott says, can be hastened by lancing the lymph nodes or aspirating pus from them. Applying a poultice that produces heat or using hot cloths can also hasten the maturation and bursting of the abscesses. Most horses recover from the malady without serious complications and develop a strong immunity, insuring that they won't contract the disease again.

However, there are exceptions. According to Prescott and Wright, the main and often fatal complications of strangles are:

Bastard Strangles--This condition occurs when there is spreading of the infection to sites other than the lymph nodes draining the throat. Abdominal and lung lymph nodes can develop abscesses and rupture, sometimes weeks or longer after the infection seems to have run its course. A brain abscess can rupture, causing sudden death. Horses also might breathe in (aspirate) pus coming from the guttural pouches, causing respiratory problems.

Purpura Hemorrhagica--This involves acute inflammation of peripheral blood vessels within four weeks of normal strangles signs while an animal is convalescing. It results from the formation of immune complexes between the horse's antibodies and bacterial components. These immune complexes become trapped in capillaries (tiny blood vessels connecting arteries and veins), where they cause inflammation that is visible in the mucous membranes as pinpoint hemorrhages. Fluid escapes from the damaged blood vessels, leading to widespread severe swelling of the head, limbs, and other parts of the body.

While these two complications are the most severe, there are four non-fatal complications that can occur as well, according to Prescott and Wright. They include:

  • Post-strangles myocarditis, or inflammation of the heart muscle. This might follow strangles in a small number of horses. An electrocardiogram (ECG) can be used to determine whether a horse can return to heavy work or training after an episode of strangles.
  • Strep myopathy is an immune-mediated muscle disorder in horses exposed to S. equi, which manifests itself as both an acute and chronic form.
  • Purulent cellulitis, or inflammation of the subcutaneous tissue. This is an unusual occurrence and involves the infection spreading locally in the subcutaneous (beneath the skin) tissue of the head.
  • Laryngeal hemiplegia. This involves permanent paralysis of the throat muscles (and thus the arytenoid cartilages) and is commonly referred to as "roaring." The condition can follow abscessation of cervical (neck) lymph nodes.
  • Anemia or low red blood cell count. During convalescence there often is a lysis or decomposition of red blood cells.

As stated before, some horses can recover from strangles and become carriers of the disease. However, these carriers aren't free of problems from the S. equi bacteria they carry. In these cases, the guttural pouches fill with pus as the result of strangles.

The guttural pouches are unique to horses and tapirs--no other mammals have them. They are large mucous sacs that comprise part of the eustachian tube. This channel establishes communication between the tympanic cavity of the ear and the nasopharynx, and it serves to adjust the pressure of air in the cavity to the external pressure. This adjustment is much like your ears popping when you ride an elevator up several floors or fly in an airplane.

The guttural pouches are situated between the base of the cranium (upper part of the head) and the pharynx, and they open into the nasopharynx. On each side of the pharyngeal wall is a flap-like valve that opens wide with each respiration, admitting air into the eustachian tube and the guttural pouches.

Guttural pouches
DR. ROBIN PETERSON ILLUSTRATIONS

The guttural pouches (shown here in yellow) can be the source of ongoing problems when they fill with pus as a result of strangles.

Unfortunately, the guttural pouches can be the source of ongoing problems when they fill with pus as a result of strangles. Persistent infection of the guttural pouches can lead to a drying up of the pus and, in some cases, the formation of a solid, stone-like substance that is called a chondroid. Horses with persistent infection of the guttural pouches are the ones who become carriers.

In the Face of an Outbreak

When strangles rears its ugly head on a farm, the first thought is not about carriers, but rather about devising a strategy to get rid of the highly contagious malady.

Step number one is diagnosis, which can be confirmed by culturing pus from the nose, from abscessed lymph nodes, or from the throat.

If the disease is diagnosed early, says Prescott, penicillin can effectively combat it. However, if abscesses have already formed, antibiotics might not be the right choice. The danger is that they will hold the disease in abeyance, but won't kill off the causative bacteria.

"The kindest thing one can do for a horse when an abscess forms," says Prescott, "is to lance or aspirate it." Lancing, he says, should occur when the abscess is "pointing." That means the abscess has stretched the skin to thinness at a specific "point." Allowing the pus to escape, he says, provides enormous relief to the horse.

However, combating the disease in an afflicted horse is only part of the overall battle. Preventing its spread to surrounding animals is almost as important. The disease, as mentioned, is highly contagious and is spread from animal to animal through close contact. For example, an afflicted horse can spread the disease to a whole herd if they share a communal water trough. The bacteria also thrive in bedding and can survive on the clothing of handlers.

Here are recommendations from Prescott and Wright for battling the spread of strangles once it arrives on a farm or stable:

"Clinically affected animals or identified carriers should be isolated immediately in a quarantine area and their water buckets or feed containers cleaned and disinfected daily. Bedding can be burned or alternatively composted under a plastic sheet (to prevent spread by flies). Any areas contaminated by infected horses should be scrubbed with water and detergent, then disinfected by steam cleaning and/or application of effective disinfectants. Fly control is required to prevent spread during an outbreak.

Although "under optimal conditions in the laboratory the bacteria can survive six to eight weeks," they survive only a day or two in soil or manure. "The organism is readily killed by heat (60°C or 140°F) or disinfectants (particularly povidone iodine and chlorhexidine). Contaminated pastures should be rested for four weeks, since the organism will be killed by the natural antibacterial effects of the soil microflora, drying, and ultraviolet light.

"Quarantine area staff should change their coveralls and boots before leaving the quarantine area and should wash their arms and hands carefully with chlorhexidine soap," they say.

Carriers At Work

It becomes instantly obvious that strangles is a disease where an ounce of prevention is worth many pounds of cure. However, until recently, it wasn't known that seemingly healthy horses could be carriers. The British scientists at Newmarket changed that.

Their studies revealed that carriers usually are horses which, following recovery from clinical illness, continue to have persistent infection in the guttural pouches.

According to Prescott and Wright, "This infection is associated with persistent, purulent inflammation of this site, or, in some cases, with the presence of chondroids."

The good news is that there are tests to detect carriers. The bad news is that testing can be expensive.

These carriers, according to Prescott and Wright, can be detected either by culture or by detection of S. equi DNA using the polymerase chain reaction (PCR) test developed at the Gluck Center in Lexington, Ky., in 1997. PCR is a more sensitive test, but it also is currently more expensive. The combination of these tests might be even more reliable, but expensive.

(The costs of the tests to detect strangles carriers will vary by area and testing facility. Horse owners interested in using the swab and PCR tests should contact a local veterinarian for price.)

"A series of three nasopharyngeal swab samples (swabs introduced through the nose to collect material from the back of the throat), evenly spaced over two or three weeks, will result in detection of about 60% of the carriers using isolation and identification of the organism, or about 90% of the carriers using PCR," the researchers say.

The Newmarket research team, headed by Richard Newton, BVSc, MSc, PhD, FRCVS, and Neil Chanter, BSc, PhD, reported that in 15 of 22 outbreaks (68%), at least one afflicted horse at each site remained a carrier four weeks after the disappearance of clinical signs. The researchers concluded that horses could remain carriers for more than four weeks. One of their studies found that some horses continued to harbor S. equi many months after clinical signs had disappeared. One horse remained infected for more than three years.

Making use of this capability to identify carriers, Prescott says, the disease could be eradicated if horse owners and veterinarians organize their efforts. Horses should be tested for carrier status before being introduced to a stable or herd. Horses which have had strangles should be tested 30 days following recovery to determine whether they are carriers.

If a horse is a carrier, he says, then it should be isolated and treatment procedures carried out. Recommended treatments might include using an endoscope or catheter to remove infected material--including chondroids--from the guttural pouches. In addition, flushing the pouch with an antibiotic solution might help.

Prescott concedes that getting horse owners and veterinarians to organize in an effort to eradicate strangles is an ambitious undertaking, but remains convinced that it could be accomplished.

Prevention

While a great deal of progress has been made in understanding strangles, such as discovering the carrier status of horses and learning how to treat it, the "perfect" vaccine to prevent strangles has not yet been developed.

At present, there are two types of vaccines available. One is a killed-bacterium type and the other uses a modified live bacterial vaccine.

Prescott and Wright describe the killed bacterium vaccine thusly: "The killed vaccines do not provide complete protection because they do not result in the local, nasopharyngeal antibodies thought to be
important in protection, but they do reduce the severity of clinical illness should it occur."

Of a live bacterial vaccine, they say, "More recently, a live, attenuated S. equi vaccine (Pinnacle I.N. by Fort Dodge) has been introduced as an intranasal vaccine for the prevention of strangles. The vaccine is administered twice, at an interval of one to two weeks. This approach to vaccination is intuitively more attractive than a killed, intramuscular vaccine since it produces the local antibodies necessary for protective immunity."

However, they point out, both types of vaccines can produce negative reactions, including pain and inflammation at the injection site with the killed-bacterial vaccine and swollen and abscessed lymph nodes and even bastard strangles with the live-bacterial vaccine.

Thus, while a great deal of progress has been made in the fight against strangles, some uncertainties still remain. For the horse owner, it is imperative that veterinary assistance be sought if strangles should strike.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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