Streptococcus equi

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A Microbial Biorealm page on the genus Streptococcus equi


Bacteria; Firmicutes; Bacilli; Lactobacillales; Streptococcaceae; Streptococcus; Equi; Equi [Others may be used. Use NCBI link to find]

Description and significance

The bacterium, Streptococcus equi, is what causes the disease in equine species (horses, donkeys, mules) called strangles. It got its name because historically, affected horses were sometimes suffocated from inflamed lymph nodes in their upper airway and trachea. It is a highly contagious disease that is transmitted directly by contact with infected horses or indirectly by contact with water troughs, feed buckets, pastures, stalls, trailers, tack, grooming equipment, etc. Flies can also act as vectors, spreading the bacteria from horse to horse. Strangles commonly affects young horses, but horses of any age can be infected. An infected horse will usually show signs of the disease for three to seven days. Signs include: decreased appetite, lethargy, fever greater than 102 degrees Fahrenheit, inflamed and enlarged lymph nodes that eventually abscess and drain. Definitive diagnosis is made by culture of the bacteria from a sample of nasal discharge, lymph node abscess, or nasal-pharyngeal wash. Polymerase chain reaction (PCR) is another test that detects bacterial DNA but cannot tell the difference between live and dead bacteria. Strangles vaccines are available, but are only about 50% effective. The best way to prevent a strangles outbreak is by quarantining all new animals in the facility, meaning no direct contact with other animals or equipment used by other horses.

Genome structure

S. equi has circular chromosomes and a length of 2253793. It has 2253793 nucleotides, 2001 protein genes and 94 RNA genes. It is believed to have evolved from an ancestral strain of Streptococcus equi subspecies zooepidemicus, which causes diseases in horses as well as other animals including humans. Both S.equi and S. zooepidemicus belong to the same group of streptococci as the human pathogen, Streptococcus pyogenes.

Cell structure, metabolism & life cycle

S. equi is a gram positive bacterium of the Lancefield’s group C. It grows in long chains of irregularly shaped cocci that are non-motile, encapsulated, facultative. It has an incubation period of 3-21 days and infected horses will show clinical signs for 4-14 days. The colonies are amber in color and look like mucus. S. equi also causes hemolysis, the rupture of red blood cells.

Ecology (including pathogenesis)

When a horse ingests S. equi, the bacteria cells attach to the tonsil and lymph nodes. The bacteria release enzymes and toxins that damage cells nearby and cause inflammation. A protein within bacterial capsule aides the infection by inhibiting phagocytosis and opsonization, the process of marking foreign material for phagocytes to recognize and destroy it.

Interesting feature

Antibiotics may help prevent strangles infection but can also prevent the ability to develop immunity to the infection. S. equi is sensitive to antibiotics like Penicillin and if treated with it at the first signs of fever, before lymph node enlargement, infection can be prevented. However, antibiotic treatment that is too early will prevent the horse from developing immunity to the infection, which makes them vulnerable to reinfection. Despite the contagiousness and seriousness of the infection, most animals recover from strangles with no long term after affects.


American Association of Equine Practitioners. 2008. “Strangles (Streptococcus equi)”. 24 Oct 2011. <>

American Association of Equine Practitioners. 22 Jul 2008. “Understanding Equine Strangles”. 24 Oct 2011. <>

Causey, R., Fay, J., Ridky, C., and Weber, J. “Facts of Strangles (Streptococcus equi) Infections in Horses”. The University of Maine. 2003. 24 Oct 2011. <>

Holden MTG, Heather Z, Paillot R, Steward KF, Webb K, et al. (2009) Genomic Evidence for the Evolution of Streptococcus equi: Host Restriction, Increased Virulence, and Genetic Exchange with Human Pathogens. PLoS Pathog 5(3): e1000346. doi:10.1371/journal.ppat.1000346

Lantz, Rachel. “Investigations into the Use of the M-protein for the Production of a Vaccine Against Streptococcus equi Subspecies equi Using Chicken IgY”. Liberty University. 2011