Strangles is a disease caused by the bacteria streptococcus equi. The name comes from the fact that it enlarges the lymph nodes between the jawbone, causing the horse to make strangled breathing sounds.

The disease begins with high fever, depression and lack of appetite. There is also a thin, watery nasal discharge that quickly turns thick and yellow.

What's happening to your horse? The lymph nodes in the upper respiratory tract become enlarged, the ones between the jawbones being the most noticeable; they can abscess.

Although it usually isn't fatal in horses, it can be. 

Horses of any age are susceptible, but those most disposed are between one and five years. Susceptible horses usually acquire the disease after being exposed to another horse that is shedding the streptococcus equi bacteria that cause the illness. Often, this is a new horse being introduced to the herd.

Although it may no longer be showing signs of the disease, infected newcomers can spread it for about a month. (Shedding the bacteria continues for up to one month after all clinical signs are gone in 20 percent of horses.)

Horse-to-horse contact is the easiest way strangles infects, but it can also be spread through contaminated equipment such as buckets, stalls and tack. Fortunately, the bacteria can't exist in the environment for long periods.

Once horses are exposed, they begin to show signs of the disease in two to six days. Untreated horses will develop abscessed lymph nodes which open and drain within one to two weeks after the onset of the disease. Although most horses recover, about 10 percent of untreated horses die. Death most commonly occurs due to a secondary infection causing pneumonia.

Occasionally, abscesses spread to other parts of the body -- the lungs, liver or even the brain. This is known as "bastard strangles." While uncommon, it is usually fatal.

Treatment depends on the stage of the disease.

To control strangles, if the health history of any horse new to the stable is vague or unknown, it should be isolated, as much as practical, for four or five weeks.

A veterinarian can take nasal swabs to confirm that the horse is not shedding streptococcus equi. However, because the bacteria can be shed sporadically, a total of three nasal swabs over a period of seven days are required to assure that the horse is negative.

Vaccinations are another useful control. Although current vaccines are more effective and cause less reaction than those of the past, they do not always prevent the disease. Still, the severity of the disease is lessened if the horse has received the inoculation.

One myth is that horses can get strangles from the vaccine. This cannot happen since the vaccine is made from only parts of the bacteria.

If strangles is suspected, notify your veterinarian to confirm your suspicions. The sooner strangles is identified, the less "down time" the whole barn will have to endure.

Horses with early signs of strangles should be treated with appropriate antibiotics, which can prevent lymph node abscesses.

Outdated veterinary literature has warned against antibiotic treatment at any stage of the disease because of the suspicion that it could lead to bastard strangles. There is no evidence to support this belief. Usually, horses treated at this stage fail to recover only if antibiotics are not given in the correct dosage or for enough days.

These animals still need to be isolated to stop further spread of the disease.

Once the lymph nodes become enlarged and abscessed, treatment will only prolong the disease. It is better to allow the abscessed lymph node to open and drain or to have your veterinarian lance it. Treatment consists of flushing the drainage site, keeping the area clean and strict isolation of the horse.

What to do if the horse you were stabled next to at the show had strangles?

Since horses usually show signs two to six days after exposure, it makes sense to treat the horse with antibiotics for at least six days after exposure. However, if your horse remains in a barn where strangles is present, this will be of little use.

Introduction
Transmission and Environmental Survival
Clinical Illness
Diagnosis and Treatment
Prevention of Strangles
Immunity
Control of Strangles
References

Horses of all ages are susceptible, though strangles is most common in animals less than 5 years of age and especially in groups of weanling foals or yearlings. Foals under 4 months of age are usually protected by colostrum-derived passive immunity. ⁽¹⁾ S. equi is main-tained in the horse population by carrier horses but does not survive for more than 6–8 weeks in the environment. Although the organism is not very robust, the infection is highly contagious. Transmission is either by direct or indirect contact of susceptible animals with a diseased horse. Direct contact includes contact with a horse that is incubating strangles or has just recovered from the infection, or with an apparently clinically unaffected long-term carrier. Indirect contact occurs when an animal comes in contact with a contaminated stable (buckets, feed, walls, doors) or pasture environment (grass, fences, but almost always the water troughs), or through flies. ⁽²⁾

Diagnosis can be confirmed by culturing pus from the nose, from abscessated lymph nodes or from the throat of clinically affected horses. Although S. equi isolates are thought to be genetically identical, isolates may vary in virulence and atypical isolates occur, which differ in their sugar tests from typical S. equi.

There is argument among veterinarians as to whether or not to treat an animal with strangles with antibiotics. Many veterinarians think that treatment will impair the development of immunity and may predispose an animal to prolonged infection and to bastard strangles. Treatment of a horse in the early stages of strangles is usually effective and is not associated with untoward effects. The causative agent is highly susceptible to penicillin G. If the disease is more advanced, then most veterinarians will not use antibiotics but rather will recommend nursing care and trying to hasten the development of abscesses (which can be drained) by poulticing. Antibiotics may, however, be used if complications arise.

Prevention of Strangles

Detection of carriers

In recent years, work in the United Kingdom has added substantially to the understanding of the carrier state in strangles. ⁽⁵⁾ This work has shown that carriers are usually horses that, following recovery from clinical illness, remain with persistent infection of the guttural pouches. This infection is associated with persistent, purulent inflammation in this site or, in some cases, with the presence of chondroids. These carriers can be detected either by culture or by detection of S. equi DNA using the polymerase chain reaction (PCR) test. PCR is a more sensitive test but also is currently more expensive. The combination of these tests may be even more reliable, but is expensive.

Because the organism is adapted to the horse, a system of control based on detection, isolation and treatment of carriers could potentially be used to eradicate the organism on a continent-wide basis. Horse owners and veterinarians have not yet organized to take advantage of this new understanding. However, vaccination with a live vaccine may interfere with the detection and eradication approach to control.

A series of 3 nasopharyngeal swabs (e.g., swabs introduced through the nose and collecting material from the back of the throat), evenly spaced over 2 or 3 weeks, will result in the detection of about 60% of carriers using isolation and identification of the organism, or of about 90% of carriers using PCR. For the detection of carriers, the laboratory should use a selective medium (Columbia blood agar with nalidixic acid and colistin).

Investigation of carriers should be done either before a new animal is introduced into a stable or herd, or at least 30 days following recovery of a horse from strangles. Animals should be isolated until there have been 3 consecutive negative cultures and/or PCR reactions.

If an animal is positive, endoscopic evaluation of the guttural pouch is recommended, chondroids removed, and guttural pouches treated by flushing and infusion of 5 million units of penicillin G in 3% gelatin. In addition, these horses should be treated with penicillin G intramuscularly for 7 days, isolated for 30 days, and then retested with the 3 consecutive series of nasopharyngeal swabs and culture. PCR is not usually recommended in these animals because it is so sensitive that it may identify dead bacteria and so give a "positive" reaction. Animals that remain positive should go through a repeat treatment and culture cycle.

This system of identification of carriers by culture and/or PCR, while not 100% reliable, is more reliable than the usual recommendation for the control of strangles. These are to isolate or quarantine new arrivals for 2–3 weeks, look for evidence of strangles-like upper respiratory tract infection, and carry out one or more nasal swabs that are used for culture. Your veterinarian will be able to give you the current laboratory costs per test for bacteria isolation and for the PCR test. Owners may not be prepared to take this route to control strangles due to the financial costs.

Vaccination

Both a killed and a live vaccine are available for the control of strangles. The only killed vaccine currently available in Canada is Strepguard^TM by Intervet. Killed vaccines, in general, are administered with an initial series of intramuscular injections followed by an annual booster. There may be adverse reactions at the injection site (marked pain, even frank abscesses). Some animals have even developed purpura haemorrhagica following vaccination. 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.

More recently, a live, attenuated S. equi vaccine (Pinnacle^TM 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 1–2 weeks. This approach to vaccination is intuitively more attractive than a killed, intramuscular vaccine since it produces the local antibodies necessary for protective immunity. Because the vaccine is a live but attenuated (using a low virulence organism) S. equi, take care to avoid contamination of injections elsewhere in the horse. Concurrent injection of other vaccines has resulted in S. equi abscesses at these sites, presumably through inadvertent contamination. Therefore, it is strongly recommended to not administer other vaccines or injections at the same time as administering the intranasal vaccine — or to be very careful about preventing contamination of injection sites. Other adverse reactions have also been reported. According to the manufacturer, adverse reactions occur at a frequency of about 5 per 10,000 doses. These include submandibular and pharyngeal lymph node swellings, with or without abscessation, purpura haemorrhagica, which may be severe, and even bastard strangles. Since the live organism may persist in the nose, approaches to control that involve detection of carriers may not be effective in horses immunized with this vaccine.

Immunity

After developing strangles, most horses eliminate infection fairly rapidly (i.e., within 30 days after recovery). Approximately 75% of horses develop a solid enduring immunity to strangles following recovery from the disease. ⁽²⁾ However, individual animals may remain with infection persistent within the guttural pouch, and may secrete the organism in nasal exudate or saliva for months or years. These carriers show no evidence of clinical disease and are the major source of infection for other horses with which they are mixed.

Control of Strangles

Isolate clinically affected animals or identified carriers immediately in a quarantine area, and clean and disinfect their water buckets or feed containers daily. Bedding can be burned or alternatively composted under a plastic sheet (to prevent spread by flies). Scrub with water and detergent any areas contaminated by infected horses, then disinfect by steam cleaning and/or applying effective disinfectants. Fly control is required to prevent spread during an outbreak.

Under optimal conditions, the bacteria can survive probably 6–8 weeks in the environment. Jorm (1991) has shown that S. equi survived for 63 days on wood at 2°C and for 48 days on glass or wood at 20°C. ⁽⁶⁾ The organism is readily killed by heat (60°C) or disinfectants (particularly povidone iodine, chlorhexidine). Rest contaminated pasture areas for 4 weeks, since the natural antibacterial effects of drying and of ultraviolet light will kill the organism.

Have quarantine area staff change their coveralls and boots before leaving the quarantine area, and wash their arms and hands carefully with chlorhexidine soap.

Approaches used to control strangles will depend on the circumstances of the individual horse or horse farm, but all people involved with horses need to maintain constant vigilance. These approaches involve a combination of knowledge of the history of individual animals and their source of origin, general hygiene, quarantine, and immunization, with appropriate action if an outbreak occurs. For further reading, refer to OMAF Factsheet Biosecurity for Horse Farms, Order No. 00-091, available on the horse section of the OMAF Web site www.gov.on.ca/OMAF/english/livestock.

Farms with large populations and movement of horses, particularly of older foals and yearlings, will want to maintain a routine immunization program of all horses to reduce the incidence and severity of disease. On these farms, depending on the vaccination program including the type of vaccine used, all incoming horses should be isolated for 2 to 3 weeks and, although expensive, a series of nasal or preferably nasopharyngeal swabs taken during this time for demonstration of the organism or its DNA. Only then should these isolated horses join the rest of the group.

Where a few adult horses are kept together and are uncommonly mixed with other horses, immunization may not be required since all immunization carries a slight risk of adverse effects. Incoming animals should be quarantined for 3 weeks, during which time nasal swabs should be assessed for the presence of the organism.

References

1. Timoney JF. Equine strangles:1999. Am. Assoc. Equine Pract. Proceedings 1999; 45:31-37.

2. Timoney JF. Strangles. Vet. Clin. North Am. 1993; 9:365-374.

3. Sisson S, Grossman JD. Anatomy of the Domestic Animals. WB Saunders Co., Philadelphia 1953; p901.

4. Habel RE. Applied Veterinary Anatomy. Robert E. Habel, Ithaca NY 1975; p57.

5. Newton JR, Wood JLN, Dunn KA, DeBrauwere MN, Chanter N. Naturally occurring persistent and symptomatic infection of the guttural pouches of horses with Streptococcus equi. Vet. Rec. 1997; 140:84-90.

6. Jorm LR. Proceedings of the 6th International Conference on Equine Infectious Diseases, Cambridge, 1991; p39.