Ask The Doctor

Cause: The bacterial organism Streptococcus equi.

Transmission: Transmission may be direct from one horse to another through nasal discharge or by drainage from either active or recovering cases. Transmission may also be indirect via contaminated housing, water sources, and feeders or from handler’s clothing and equipment.

It is increasingly recognized that transmission may originate from outwardly healthy animals. These may be animals that are incubating the disease and who later develop symptoms or from recovering animals that appear to be fully recovered but may still be shedding the organism. A recovered horse may be a potential source of infection for at least 6 weeks after clinical signs have resolved.

A chronic carrier state may develop in up to 10% of horses affected. These horses appear normal clinically but harbor the organism primarily in the guttural pouches. The guttural pouches are large air-filled cavities on either side of the horse’s pharynx. Roughly 50% of the horses with these low-grade guttural pouch infections will have an occasional cough and intermittent unilateral nasal discharge.

No good studies have been conducted to determine the length of time the organism can persist in the environment. The organism is unlikely to persist more than 4 weeks on pasture. Feeding, watering utensils and horse trailers should be thoroughly cleaned and disinfected.

Symptoms: Some horses may show no outward signs of the disease or develop only a single abscess with no other symptoms of illness. A typical case will develop firm, painful swelling of the intermandibular lymph nodes which eventually burst and drain white pus. Lymph nodes also break into the guttural pouches and throat which then drain out the nose. Bastard strangles is the term used to refer to cases where lymph nodes in the chest or abdominal cavity may develop abscesses. These animals have rather vague symptoms of illness that when remain undetected until they culminate in colic, peritonitis or sudden death when an abscess ruptures inside a body cavity.

Some horse immune response to Strep infections is exaggerated resulting in an inflammation of the blood vessels know as vasculitis. This condition, also knows as purpura hemorrhagica, is manifested by swelling of the extremities and occasionally rupture of small blood vessels on the mucous membranes.

Diagnosis: The classical form of Strangles can be diagnoses based on symptoms alone. Veterinarians do face a diagnostic challenge when presented with an otherwise healthy horse with a single abscess in the throat or between the jaws. In these cases the veterinarian will most likely refrain from a diagnosis but assume it I until a culture of the organism can be obtained. Cultures of nasal swabs, nasal washes, or aspirated pus from abscesses remain the gold standard for detection of S. equi. The organism is normally not present on the mucous membranes until 24-48 hours after the onset of fever. Twice daily monitoring rectal temperatures may be used to detect and isolate new cases early in an outbreak.

Nasal or guttural pouch washes are used to:

1. detect asymptomatic carriers
2. establish S. equi status before transport or reintroduction with susceptible horses either on the premise or the general public
3. establish the S. equi status of horses after transport and before commingling
4. determine the success of eliminating S. equi from the guttural pouch of chronic carriers

A blood test that detects a specific antibody for S. equi in the horse is used to:

1. detect recent infection

2. determine need for vaccination

3. ID animals with existing high levels of the antibody that may predispose them to purpura

4. support a diagnosis of purpura associated with S. equi infection; and

5. support diagnosis of bastard strangles

Vaccination: Over 75% of horses that recover from strangles develop a solid immunity for 5 years or more. However, attempts to stimulate solid immunity through vaccination have been disappointing.

Earlier intramuscular vaccines had a high rate of soreness and occasional abscess formation at the injection site. These same vaccines were only protective in 50% of horses challenged a few weeks after vaccination.

A new intranasal vaccine contains a live, attenuated, and non-encapsulated strain of Strep equi. This vaccine has provided a high level of resistance against experimental challenge. Safety issues include residual virulence with formation of slowly developing abscesses in a small percentage of vaccinates nasal discharge, and occasional cases of purpura. Since this is a live vaccine contamination of remote injection sites will result in abscess formation at these locations.

Control of Outbreaks:

1. All movement on and off the affected premise should be stopped
2. Segregate clinical cases and their immediate contacts from unexposed horses
3. Monitor rectal temperatures twice daily and promptly remove any horse from the “clean” population that spikes a fever.
4. Maintain high hygiene standards throughout the premises. Affected horses should be handled last and all clothing worn while working with infected horses changed before contacting other horses.
5. Screen convalescing cases after clinical recovery by nasopharyngeal swab or nasal wash to insure animals are no longer shedding the organism
6. Treat carrier animals and retest until the organism is no longer present.

Treatment:

Appropriate treatment of horses with strangles depends on the stage and severity of the disease. Veterinary opinion on the use of antibiotics is markedly divided. In my opinion (Erfle) horses that have evidence of abscess formation should not be treated with antibiotics unless the animal is off feed, depressed and/or manifesting upper airway obstruction. When close monitoring of horses at risk detects a fever before an abscess has started treatment with antibiotics is a means of preventing localized abscesses. Beware however that animals treated in this manner may not develop an immune response and could still break with the disease if they have ongoing exposure to infected horses or environment.

Disinfection

Bacteria are killed by sunlight and drying. Phenolic disinfectants are recommended in facilities. Bleach and quaternary compounds are effective but are easily inactivated by organic matter. Regardless of the product used physically cleaning all surfaces before applying disinfectants is essential.