Biosecurity and Health Committee
Protocol for the Management of Strangles in Horses

Table of Contents

1. Disease Information
2. Human Health Risk Data
3. Horse Health Risk Data
4. Ecology Information
5. Prevention
6. Regulatory Information
7. Committee's Recommendations
8. Reference
9. More Information

Section 1: Disease Information

Strangles is a highly contagious and serious infection of horses and other equids caused by the bacterium Streptococcus equi (S. equi). The disease is characterized by severe inflammation of the mucosa of the head and throat, with extensive swelling and, often, rupture of the lymph nodes that produces large amounts of thick, creamy pus.

Section 2: Human Health Risk Data

Humans appear to be resistant to S. equi under normal circumstances.

Section 3: Horse Health Risk Data

Horses of all ages are susceptible, though strangles is most common in animals less than five years of age and especially in groups of weanling foals or yearlings. Animals show typical signs of a generalized infectious process (depression, inappetence, fever of 39° - 39.5°C). Horses develop a nasal discharge (initially mucoid, rapidly thickening and purulent), a soft cough and slight but painful swelling between the mandibles, with swelling of the submandibular lymph nodes. With the progression of the disease, abscesses develop in the submandibular (between the jaw bones) and/or retropharyngeal (at the back of the throat) lymph nodes. The lymph nodes become hard and very painful, and may obstruct breathing ("strangles"). The lymph node abscesses will burst (or can be lanced) in 7 to 14 days, releasing thick pus heavily contaminated with S. equi. The horse will usually rapidly recover once abscesses have ruptured.

Section 4: Ecology Information

S. equi is maintained in the horse population by carrier horses. The infection is highly contagious. Transmission is either by direct or indirect contact of susceptible animals with a diseased horse. The incubation period for strangles is usually 3 to 14 days. 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. Under optimal conditions, the bacteria can survive probably six to eight weeks in the environment.

Section 5: Prevention

Both a killed and a live vaccine are available for the control of strangles. The only killed vaccine currently available in Canada is Strepguard™ by Intervet. Killed vaccines, in general, are administered with an initial series of intramuscular injections followed by an annual booster. The initial series is started at 4 to 6 months of age with a second vaccination 2 to 4 weeks later and then annually. There may be adverse reactions at the injection site (marked pain, even frank abscesses). Some animals have even developed purpura haemorrhagica reddish-purple-coloured spots on the skin and mucous membrane associated with extravasations of blood) associated with the vaccination. The killed vaccines do not provide complete protection against infection because they do not result in the development of local, nasopharyngeal antibodies thought to be important in protection, but they may reduce the severity of clinical illness should it occur.

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. The first dose is given to weanlings at 6 to 9 months of age and the second given 3 weeks later. 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, care should be taken 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. If intranasal vaccination is performed on the same day as intramuscular vaccination for other diseases, the intramuscular vaccines should be given first.

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. The organism is readily killed by heat (60°C) or disinfectants. Quarantine area staff should change their coveralls and boots before leaving the quarantine area, and should wash their arms and hands carefully with antibacterial soap or use an alcohol-based hand disinfectant solution.

Infected horses should be isolated and not allowed to come into contact with other horses until they are no longer shedding S. equi. Horses should not be vaccinated during an outbreak or following a suspected exposure. Personnel working with infected horses should not work with other horses, or should work with infected horses last. Clothing should be changed after working with an infected horse, and hands should be thoroughly washed. Any items coming in contact with an infected horse or its stall (hay nets, water buckets, etc.) should be disinfected before being used for another horse. Infected horses can shed S. equi for weeks. Contaminated pasture areas should be rested for four weeks, since the organism will be killed by the natural antibacterial effects of drying and of ultraviolet light. Once a case of strangles has been identified, all horses that have been in contact with the affected horse should be considered potentially exposed. Their body temperature should be monitored closely to detect infection as early as possible. Ideally, horses should not leave the premises after an infected horse has been identified, unless they have been tested and determined not to be carrying S. equi.

New arrivals to a barn should be quarantined for at least 2 (and ideally 3) weeks. All quarantined horses should be considered a potential source of S. equi, even if they appear healthy. Depending on the situation, screening for S. equi might be recommended. This would consist of testing for the presence of S. equi in the nasopharynx (nose and throat region) and guttural pouches.

Section 6: Regulatory Information

Strangles is not a reportable disease. Individual animal occurrences or outbreaks of this disease are not required to be reported to any government agency.

Committee's Recommendations

• All "pony" horses at tracks shall have completed their vaccination program (initial and booster shots) for strangles at least two weeks prior to arrival.
• It is recommended that all horses be vaccinated with the intranasal vaccine for strangles (initial and booster shots) prior to arrival at the track or event in consultation with your veterinarian.
• Track and barn owners should install wash stations with hand disinfectant at strategic locations along each shed row or barn for personal hygiene when working between horses.
• All personnel should wash their hands after working with each horse under their care.
• Supplies should be available at all facilities to disinfect stalls and equipment. However, dirt floor stalls with wood walls will require removal of infected dirt (upper 2") and scrubbing of the walls.
• New horses coming into barns from sales and sources with unknown health status should be quarantined for 2 - 3 weeks prior to having contact with other horses.
• Horses from farms with cases of strangles on the property, or suspicious of being infected, should not be taken to or admitted to barns, a racetrack, etc., until they have undergone a 2-3-week quarantine and been tested negative for S. equi.

Reference

Jorm LR. Proceedings of the 6th International Conference on Equine Infectious Diseases, Cambridge. 1991:39.

More Information

Guidelines for Vaccination of Horses - American Association of Equine Practitioners

Strangles in Horses