Biosecurity and Health
Protocol for the Management of Strangles in Horses
Table of Contents
- Disease Information
- Human Health Risk Data
- Horse Health Risk Data
- Ecology Information
- Regulatory Information
- Committee's Recommendations
- 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
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
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.
- 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.
Jorm LR. Proceedings of the 6th International Conference on Equine
Infectious Diseases, Cambridge. 1991:39.
Guidelines for Vaccination of Horses - American Association of