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Pathogen Page

Streptococcus equi

I. General Information

1. NCBI Taxonomy ID:

2. Disease:

Strangles (also equine distemper)

3. Introduction

Strangles (also equine distemper) is a contagious, upper respiratory tract infection of horses and other equines caused by a bacterium, Streptococcus equi var equi. The disease is spread when the nasal discharge or material from the draining abscess contaminates pastures, barns, feed troughs, etc. Clinical signs include fever, heavy nasal discharge, and swollen or enlarged lymph nodes in the neck and throatlatch. Affected animals may also stop eating and have a dull affect. Mortality may be up to 1 in 10 horses affected with strangles, especially bastard strangles. For cases without complications, mortality is usually lower. The disease is very contagious and morbidity is high. Precautions to limit the spread of the illness are necessary and those affected are normally isolated. An isolation period of 6 weeks is usually necessary to ensure that the disease is not still incubating before ending the isolation (Wiki: Strangles).

4. Microbial Pathogenesis

S. equi enters via the mouth or nose and attaches to cells in the crypt of the tonsil and adjacent superficial lymphoid nodules. After a few hours, the organism is difficult to detect on the mucosal surface because it is translocated below the mucosa into the local lymphatics where it may be found in one or more of the lymph nodes that drain the pharyngeal/tonsillar region. Complement-derived chemotactic factors generated after interaction of C1 with bacterial peptidoglycan attract large numbers of PMN. The inability of PMN to phagocytose and kill the streptococci appears to be due to a combination of the hyaluronic acid capsule, antiphagocytic M protein, and a leukocidal toxin released by the organism. This culminates in accumulation of many extracellular streptococci in the form of long chains and large numbers of degenerating PMN. Final disposal of these organisms is dependent on lysis of the abscess capsule and evacuation of its contents (Timoney, 2004).

5. Host Ranges and Animal Models

S. equi primarily is a pathogen of equids, but can also cause disease in cattle, rabbits, swine, sheep and humans (MicrobeWiki: S. zooepidemicus). Mice can be used as a model of disease (Flock et al., 2006).

6. Host Protective Immunity

Most horses develop a solid immunity during recovery from strangles which persists for up to five years in about 75% of animals. Acquired resistance appears to be primarily humoral and mediated by antibodies to SeM and other as yet unidentified antigens unique to S. equi (Timoney, 2004).

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