Streptococcus equi subsp. equi

Causes: Strangles

Introduction

Strangles is highly contagious upper respiratory disease of equids worldwide. It is caused by the very pathogenic bacteria Streptococcus equi subsp. equi resulting in rhinitis, pharyngitis, lymphadenitis and myositis. The bacteria are haemolytic streptococci of Lancefield group C and are common inhabitants of the equine nasopharynx. They multiply extra-cellularly.

The disease often occurs where there are high numbers of young horses that are under stress, such as in racing yards. Most animals affected are under 6 years old though the disease is prevalent in all ages.

Transmission is via purulent exudate discharging from upper respiratory tract or from lymph nodes, which is then transmitted to the other horse by direct contact e.g. on stable doors, tack or feed buckets or by inhalation. A chronic carrier state of the disease can occur when bacteria is in the guttural pouch. The organism remains viable in the environment for months.

There will be 100% morbidity; 5% mortality in most cases as the disease is so contagious.

Clinical Signs

Classic Strangles

After infection, there is an incubation period 3 to 6 days followed by pyrexia, depression, anorexia, purulent bilateral nasal discharge, and swelling or abscessation of regional lymph nodes, especially the submandibular nodes. The lymph nodes may rupture. There may also be guttural pouch empyema.

Death can occur from pneumonia, breathing difficulties from swollen lymph nodes or purpura haemorrhagica (an immune-mediated disease).

S. equi may also be involved in cutaneous lesions.

Atypical Strangles

The clinical signs of atypical strangles are milder. There is only mild inflammation of the upper respiratory tract and minimal nasal discharge. A cough and pyrexia are still present but the lymphadenopathy is self-limiting. Whether this form of strangles occurs depends on the strain of bacteria, the existing immunity of the horse and genetics.

Bastard strangles

This may occasionally occur, with abscessation in many organs of the body. This will be difficult to diagnose as clinical signs are systemic and variable, although history of having strangles in the normal form is presumptive.

Diagnosis

History and clinical signs are presumptive. Samples need to be taken from deep nasopharyngeal swabs, abscess content or guttural pouch washings and sent off for culture. If present, you will see mucoid colonies with beta-haemolysis. Sugar fermentation allows differentiation of S. equi from S. zooepidemicus and S. equisimilis as the last two are commensals of the upper respiratory tract. PCR can be used to detect asymptomatic carriers.

Endoscopy of the guttural pouch can be performed and samples taken to detect disease in here as this is more difficult to destroy and therefore stronger antibiotics need to be prescribed.

To diagnose carrier animals, either three swabs from the nasopharynx taken one week apart or a singe guttural pouch washing and needed. This picks up the majority of infected animals, but not all of them.

Treatment and Control

In the case of an outbreak, penicillin should be administered to the affected animal and to in-contact animals. If abcesses are already present antibiotics should not be administered as this is shown to slow the recovery. Instead abcesses should be hot-packed

Isolation of affected animals is required immediately. The yard needs to be shut and no animals are allowed in or out. Strict control needs to be enforced on all personnel in the yard and them made aware they are not to visit another horse or take their vehicle to other yards. Disinfection of all tack, stables, vehicles, boots and any other equipment that has come into contact with infected animals need to be performed. Water troughs should be emptied and disinfected regularly to reduce the expose of in-contact horses to the pathogens. Before horses are taken out of isolation they should have tested for the presence of infection, so that asymptomatic carrier animals are identified.

Control measures to prevent the disease includes quarantine of all in-coming animals. It is important to try to reduce stress on the yard or within groups of horses turned out together and it is necessary to avoid overcrowding and mixing different age groups.

Immunity occurs in animals that have been infected, however it is short-lived.

References

Interview with Professors Josh Slater and Ken Smith providing an interesting insight into the pathogenesis, prevalence and possible prevention of Streptococcus equi infections in horses - listen to Strangles podcast

Brown, C.M, Bertone, J.J. (2002) The 5-Minute Veterinary Consult- Equine, Lippincott, Williams & Wilkin

Knottenbelt, D.C. A Handbook of Equine Medicine for Final Year Students University of Liverpool