Strangles

The pathogen responsible for strangles is  Streptococcus equi subspecies equi, a Gram positive, β-hemolytic  bacteria belonging to the C de Lancefield group, like the  Streptococcus equi subspecies zooepidemicus, which can also be responsible in rare cases for upper respiratory tract illness.
Unlike S. equi subsp. Zooepidemicus which is part of the commensal flora,  S. equi subsp. Equi is a primary pathogen. Its survival in the environment is not very well known, (one study carried out in a laboratory). It would appear to be weak, (a few days) but could increase substantially in favorable temperature and humidity conditions (up to several months), which could explain cases of resurgence.

Strangles usually affects mostly young horses (aged less than 5) but can erupt at any age. In a group of horses having had no prior exposure, morbity (number of animals affected) is extremely high and can reach 100 %, but mortality (percenetage of deaths of affected animals) is fairly low (from 1 % to 5%) and usually occurs in the case of complications, especially in young foals.
Contamination sources are horses who are clinically ill or convalescing, and carrier animals who host S. equi subsp. equi in the guttural pouches. 10 % of all horses infected become chronic carriers of the disease even after clinical signs have disappeared. These asymptomatic carriers are especially important in the contamination process, as the germ can be excreted for several weeks.
It can be transmitted directly through nasal discharge, pus from the abscesses, expectorations, and milk. It can also be indirectly transmitted by the staff or stable equipment.
Highly contagious, the disease frequently appears after a stressful situation such as transport, an intense effort, or a change of environment.
It takes at least three months to totally eradicate the disease from a group of animals.

Classical form or « catarrhal form »

After a short incubation period (3 to 7 days, the first syptoms consisting in  depression, fever (40 °C), anorexia, and severe rhinitis, with nasal dicharge rapidly becoming mucopurulent, then purulent. This then evolves to pharyngitis with dysphagia, and the clinical signs then include hypertrophy of the mandibular and retropharyngeal lymph nodes, often the cause of stiffness in the neck.
When this stage goes untreated, the lymph nodes abscess over the next 3 to 7 days, with a pussy discharge either outwards (mandibular) or into the guttural pouches (retropharyngeal).

When the outcome is favorable, it lasts 2 to 4 weeks, with an average  non-availability time of 20 days per horse. 

« Bastard » strangles (erratic or metastasic form)

Much less frequent, this form occurs at the same time or after the classical form. It is characterised by multiple abscessing and adenitis suppurativa of the skin, nervous system, lungs, joints, genitalia (castration strangles) and other more anecdotic locations. It can also take the form of pneumonia or pleuropneumonia..

Immune mediated disorders

Extremely rare, they represent a complication of strangles. This is purpura hemorrhagica , also known as congestive or hemorrhagic strangles. It occurs in convalescing or overstressed animals two to three weeks after a classical bout of strangles. Vasculitis is usually observed, with subcutaneous oedema, petechiates and bruising of the mucous lining. Other sypmtoms encountered can vary from glomurelonephritis, hair loss due to vesiculo-papular rashes on the hind legs and other areas subject to rubbing, boils, stomatitis, rhinitis and uveitis.
Muscular complications although exceptional can occur in different forms : either acute with rhabomylosis (acute myonecrosis), or 2 to 3 weeks after the strangles infection, a muscular infarctive myopathy (infarctive purpura hemorrhagica), or an immune-mediated polymyopathy (gradual muscular atrophy).

Once it has entered the organism either via the nose or the mouth,  Streptococcus equi subsp. equi will adhere to the oropharyngeal or nasopharyngeal lymph nodes. Certain pyrogenic substances are also involved and contribute to the inflammatory reaction :
•    Nasal excretion of the germ starts between 4 to 14 days after contamination, i.e one to two days after the horse shows signs of a temperature. It is important to be aware of this to implement preventive measures (isolating horses with a fever) and for an etiological diagnosis. Excretion can subsist for up to 6 weeks ;
•    Contamination via the blood or lymphatic fluid, usually at the start of the bastard form, is possble although rare ;
•    Natural post infection immunity develops in 75 % of horses.

Since clinical signs are usually very distinctive, they usually orient a diagnosis, which is supported by a highly contagious epidemiological context.
The veterinarian is then incited to carry out several paraclinical tests to confirm the suspicion :

• Analysis of blood samples and fibrinogen counts as inflammatory markers will reveal neutrophylic leukocytosis and hyperfibrinogenemia, anaemia and frequently thrombocytopenia;
• Looking for non externalised abscesses can be carried out via an endoscopy of the guttural pouches. For diagnosis of the erratic forms of the illness, X-rays or thoracic and abdominal scans, as well as ultrasound guided procedures can be carried out when the imagery techniques reveal hypertrophy of the internal lymph nodes.

The main etiological diagnosis is based on determining the pathogen present in several samples : naso-pharyngeal swabs, nasal lavage, guttural pouch lavage, purulent discharge. Two methods are used :

•  Bacteriology allows culture and isolation of the bacteria on agar from different types of swab. There is to date no specific culture medium for S.equi. This means they can be confused with other β-hemolytic  bacteria such as S. zooepidemicus or Streptococcus equisimilis. Interpreting negative bacteriological results, particularly from horses presenting typical clinical symptoms should be conducted carefully (for example in the case of substantial purulent nasal discharge, the culture may come up negative because pus is often sterile ; even though the clinical sugns are distinctive). Bacterial populations other than S. equi sbsp. Equi, and other contaminants may mask or hinder the development of  S. equi sbsp. equi in a culture environment, thus providing a falsely negative result. For these reasons, culture has been replaced by molecular biological tests;
• PCR (Polymerase Chain Reaction) enables to search for the presence of the bacteria by demonstrating the presence of amplified genomic sequences. In the case of strangles, the numerous tests developed have evolved towards qualitative PCR tests and a real time quantitative PCR test where the genes targeted come from a comprehensive sequence of  S. equi sbsp. Equi. This technique is much faster and more sensitive than bacteriological tests, although it does not differentiate betwen live and dead bacteria.

The most reliable and therefore recommended protocol today is using a combination of bacteriological and PCR tests to detect the bacteria.

• Serology is based on showing the presence of antibodies directed against the  S. equi subsp. equi proteins. They can be detected around 8 days after the onset of the infection, and the serum titres peak between 4 to 5 weeks after being naturally exposed to the bacteria, and they persist for 6 to 8 months. Serological testing like ELISA (Enzyme Linked ImmunoSorbent Assay) have evolved since the S.equi genome has been fully understood, from the test targeting the SeM protein, considered as the major protective antigen (ELISA commercialised by IDVet) towards indirect more specific ELISA tests, targeting two antigens, SeQ 2190 ansd SeM (ELISA developed by AHT). Serology can be very useful in confirming or infirming a cluster of strangles (herd diagnosis), and in helping the veterinarian cope with a group of animals, especially after the disease, to determine which horses were affected, those who were not (identifying asymptomatic carriers), identifying animals with a high risk of developing  purpura hemorrhagica if vaccinated or in the case of renewed contact with the bacteria. Taking into account the kinetics of the appearance of antibodies, serology tests are not the quickest and most adapted for a biological diagnosis of an outbreak of strangles in its early clinical stages. Moreover it does not distinguish between animals who have been vaccinated and those infected naturally.

Despite the availability of multiple different tests, there is to date no clear consensus on which diagnosis test to use with regard to the different stages of the disease (healthy horse, carrier, or sick). A research project is currently being carried out to confront the different tests within the framework of a field test including horses at different stages of the illness.

Treatment

The choice of a treatment depends for the main part on the evolution of the illness.
Administering antibiotics should be thought through and in no case should be a way to get out of strict sanitary measures.
 

Medical prophylaxis

Different vaccines have been assessed worldwide such as inactivated vaccines, or live attenuated vaccines. The vaccine available in Europe is a live vaccine (mutant strain of  S. equi). It is effetive 2 weeks after the first injection, but only protects from infection for three months. It does not suppress bacterial excretion in an outbreak, and it is not possible to distinguish between vaccinated and contaminated animals. It should only be used on healthy horses and some side effects have been noted.

Sanitary prophylaxis

The fight against this very contagious disease starts with testing of horses who shed the bacteria (particularly healthy carriers) and by implementing strict sanitary preventive measures : quarantine and testing of new arrivals in a yard.  
In the case of an epizootic outbreak :
•    Stop all movements of horses  ;
•    Take the horses’ temperature every day ;
•    Isolate sick horses and those with a temperature ;
•    Implement a care circuit according to areas dedicated to different cases : animals presenting clinical signs, animals having been in contact with sick horses, healthy individuals ;
•    Clean and carefully disinfect any material or stable having been in contact with a sick animal ;
•    Wash and disinfect your hands after handling a sick horse or use disposable gloves, a human hand can become a carrier for the disease for a time;
•    Do not use paddocks having accomodated sick animals for at least four weeks ;
•    Use disposable equipment (overshoes, overalls, gloves and hats) in infected areas, and place a boot bath in front of a stable hosting a sick animal.

Any animal leaving a yard where there has ben an outbreak of strangles should ideally be tested (nasopharygeal swab) to ensure that he is not shedding or is not a carrier.