Streptococcus equi var equi

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{Uncurated}} A Microbial Biorealm page on the genus Streptococcus equi var equi

Classification

 Monera; Firmicutes; Bacilli; Lactobacillales; Streptococcaceae; Streptococcus; equi

Description and significance

Streptococcus equi is a bacterium that causes the disease strangles, also known as equine distemper, in horses and other equines such as donkey, mule and zebra. This disease is most common in young and old equines, since their immune systems are not as strong because of either lack of prior exposure or aging. Strangles is an upper respiratory tract infection that is highly contagious, especially in domesticated horses. An equine with strangles will spike an extremely high fever, up to 102°F – 106°F, become lethargic and display a significantly decreased appetite. The animal will also develop swollen lymph nodes around their neck and throat, which is where the name strangles originates. The disease is spread through nasal discharge or from puss or ooze that is coming from exposed abscesses. It is easy for the disease to be transferred from equine to equine as the bacterium can live on feed troughs, water buckets or be transmitted via direct contact between animals. However, the mortality rate for strangles is less than 1%, although there are three rare exceptions of complications that are fatal. These complications are bastard strangles, which is when the disease spreads to all areas of the body, purpora haemorrhagica, which is bleeding from small blood vessels under the skin and chronic carrier status, in which a horse is a life-long carrier of the disease. It is fairly easy to recognize strangles based on these characterizations and an equine that display symptoms should be immediately isolated from any other equines, especially equines that may have just arrived to a barn for example. New comers should be isolated for two weeks and infected equines should be isolated for a six-week period. Isolation is the best solution for prevention as there is no completely trustworthy and effective vaccine for strangles. It is up to the veterinarian whether or not to give available vaccines, so this may vary from case to case. However, hygiene and intensive cleaning is very important for preventing the spread of the disease. Once infected anti-inflammatory medications, such as Bute, are prescribed to lower fevers, iodine is used to cleanse draining abscesses and antibiotics like penicillin can be administered to kill infection.

Genome structure

The most common strain of S. equi is the subspecies var equi 4047. This strain has a complete genome that is 2,253,793 base pairs long and contains circular DNA. It has evolved from the zoonotic pathogen Streptococcus equi subspecies zooepidemicus and shares more than 98% of DNA homology with this pathogen, and therefore expresses many of the same proteins and virulence factors. Both of these pathogens share approximately 80% genome sequence identity with the human pathogen Streptococcus pyogenes, which suggest a common phage pool that enhances cross-species pathogen evolution.

Cell structure, metabolism & life cycle

Steptococcus equi is an aerobic Gram-positive bacterium. It is cocci in shape and often is found in pairs and long clusters. The incubation period for the bacterium is anywhere between three to twenty-one days and clinical signs develop anywhere between one and two weeks duration. The bacteria can survive in the environment for long periods of time, up to eight weeks on equipment and in the wood of stables. Bacteria can also survive for at least four weeks on water troughs.

Ecology (including pathogenesis)

S. equi is a host-restricted pathogen that enters an equine from either the mouth or the nose and attaches itself to the cells of tonsils near lymph nodes in the upper respiratory tract. The organism resides in the lymph nodes that are located in this throat region and the inability for the equine to fight off the bacteria is due to chemicals released by S. equi that inhibit immune responses from occurring. This results in the accumulation of a large number of bacteria that cluster together causing greater infection. Strangles is therefore recognized because of external swelling that develops in this region. The disposal of the bacterium from the body of an equine does not occur until the abscess capsule lyses and the infected contents are drained.

Interesting feature

S. equi is a host-restricted pathogen that enters an equine from either the mouth or the nose and attaches itself to the cells of tonsils near lymph nodes in the upper respiratory tract. The organism resides in the lymph nodes that are located in this throat region and the inability for the equine to fight off the bacteria is due to chemicals released by S. equi that inhibit immune responses from occurring. This results in the accumulation of a large number of bacteria that cluster together causing greater infection. Strangles is therefore recognized because of external swelling that develops in this region. The disposal of the bacterium from the body of an equine does not occur until the abscess capsule lyses and the infected contents are drained.

References

“Bacteria Classification: Gram-Positive.” Web. 24 Oct. 2011. <http://www.nsgs.it/indexab000.htm>.

Harrington DJ., Sutcliffe IC. “The molecular basis of Streptococcus equi infection and disease.” Microbes and Infection. 2002. Volume 4. p. 501-510.

Holden, MTG., Heather, Z., Paillot R., Steward, KF. “Genomic Evidence for the Evolution of Streptococcus equi: Host Restriction, Increased Virulence, and Genetic Exchange with Human Pathogens.” Plos Pathogens. 2009.

NCBI. Taxonomy. Web. 24. Oct. 2011. <http://www.ncbi.nlm.nih.gov/nuccore/FM204883.1>.

“Stable Close Equine Practice: STRANGLES.” Stable Close Equine Practice: HORSE VETS IN HAMPSHIRE. Web. 24 Oct. 2011. <http://www.horsevet.co.uk/strangles.php>.

Timoney JF. 2004. The pathogenic equine streptococci. Veterinary research. 2004; 35(4); 397-409. [PubMed].