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Phylogeny
The first identification of Taylorella equigenitalis was by Taylor et al., in 1978 and was referred to at the time as Haemophilus equigenitalis [1–4]. Due to specific genomic characteristics and DNA hybridization results, Sugimoto et al.,proposed the change from Haemophilus genus to Taylorella in 1983 [1]. There are two identified biotypes of Taylorella equigenitalis, one of which is resistant to streptomycin [2,5]. Taylorella equigenitalis is an infectious organism for horses, a second species, Taylorella asinigenitalis has been isolated from the genital tracts of male donkeys [6]. Gross morphology and growth rates of T. equigenitalis and T. asinigenitalis are too similar to differentiate the two species without the use of molecular identification techniques such as PCR [1,7] Molecular evaluation of the 16S rDNA sequence has confirmed that there is enough of a genetic difference between Taylorella equigenitalis and Taylorella asinigenitalis to separately classify the two species [1]. Taylorella asinigenitalis does not appear to cause disease in equine mares, only in donkeys [8]. Taylorella equigenitalis was previously identified as belonging to the Pasteurellacae family [9,10]. It is now classified as belonging to the Alcaligenaceae family [7,11].

Morphology
Taylorella equigenitalis is a Gram-negative bacterium, which grows best under specific conditions environmental conditions on the reproductive organs, and in the reproductive tract of horses  [2,7,9,12–15]. Taylorella equigenitalis is morphologically identified as coccobacillary [2,4,5,7,9,10,13–15] or bacillus [2,9,15]. Members of the genus Taylorella, including T. equigenitalis are non-motile and microaerophilic [1,2,4,14,15]. Herbert et al., have reported that T. equigenitalis have a capsule and express pili in vivo [7]. Taylorella equigenitalis is pleomorphic on isolation from equine carriers [15], and there is no formation of endospores [4]. Taylorella equigenitalis has scant growth on blood agar, the best growth is observed on chocolate agar at temperatures between 30-42 degrees Celsius [4]. Taylorella equigenitalistests positive on oxidase, catalase, phosphatase and phosphoamidase reactions and negatively on glucosidase, indole, hydrogen sulfide  and production of chemoorganotrophic acid from carbohydrates [4]. T. equigenitalis is sensitive to penicillin type antibiotics [4].

Ecology, Distribution & Habitat
Taylorella equigenitalis is a venereally transmitted disease of horses and donkeys. Experimental reports have noted intrauterine infection of rodents with Taylorella equigenitalis, but not with cattle, sheep, swine or cats [3,5]. Taylorella equigenitalis can be isolated from the urethral fossa, distal urethra, prepuce, free-part of the penis and uncommonly from the pre-ejaculatory fluid [1,5,6]. In mares, the bacteria is best found in the clitoral fossa and sinus. In stallions, the bacteria is best found in the urethral fossa, urethral sinus, urethral and penile sheath, epididymis and seminal vesicles [9]. On the external genitalia of horses, T. equigenitalis is found particularly in the presence of smegma and cellular debris [2].

Taylorella equigenitalis was first reported in thoroughbreds in 1977 in the United Kingdom [1–3,5–8,10,12,16]. Transmission occurred following the first report in the UK to Ireland, France, Australia, Belgium, USA, and Germany[2,5]. Taylorella equigenitalis has been identified throughout the world including in Japan, Australia, North America, South America and Europe [1]. The first isolation of T. equigenitalis in Japan occurred in 1980. The first report of T. equigenitalis in Turkey was reported in 2001 [12]. As of 2006, Samper & Tibary reported that T. equigenitalis was no longer present in North America, but is endemic within Europe [5]. The most recent outbreak in the USA reported by Schulman et al., was in 2008-2010 and also in South Africa in 2011 [2]. There is thought to be underreporting of T. equigenitalis and that the global distribution of the disease is greater than what is currently reported [2]. Bacteriological and serological screening of horses coming from a country with CEM is required for horses entering countries where CEM is not yet reported [5].

Carriers of T. equigenitalis are the most significant contributor to global spread of CEM in mares and stallions [5].Taylorella equigenitalis can persist on equine genitalia for years and still be transmitted to susceptible mares or stallions through natural service or artificial insemination [5]. Mares may be asymptomatic harbouring T. equigenitalis in the clitoral sinuses and fossa, it may also remain for months to years in the uterine or clitoral tissue of previously infected mares  [12]. More significantly, stallions are frequently asymptomatic when carriers, with T. equigenitalis harbouring in the urethral fossa, urethra or penile sheath [5].

Vertical transmission from mother to fetus through the placenta has been reported [5,9]. In particular, colts trans-placentally infected while in-utero may become a source of future T. equigenitalis transmission to susceptible mares [5].

Biosecurity standards including good hygiene, cleaning of fomites and use or sterile and disposable materials should be considered when handling horses for breeding and semen collection [2,5]. Taylorella equigenitalis does not survive long off of horse tissue and is sensitive to disinfectants, UV light, high temperatures and humidity [2].