Contagious equine metritis

Introduction

There are two known species within the genus Taylorella, T. equigenitalis which causes contagious equine metritis (CEM) and T. asinigenitalis, a closely-related species found mainly in donkeys, but which is currently considered non-pathogenic with natural infection.³⁹

Contagious equine metritis is a true venereal disease naturally transmitted to naïve mares at the time of mating by chronically infected, asymptomatic carrier stallions. A hallmark of the South African outbreak in 2011³⁶ and similarly in other recently reported outbreaks from the USA¹⁴ and the UK,⁴⁵ was the horizontal transmission of T. equigenitalis by unknown fomites to other stallions associated with an equine artificial breeding centre. These reports highlight the need for vigilance with assisted reproductive techniques that are currently commonplace in most equine breeds. Metritis is a misnomer for this disease in that it actually causes a transient endometritis in mares, usually resulting in temporary infertility. Most mares clear themselves of infection, however up to 25 per cent of infected mares may become chronic carriers.²⁹ An outbreak of CEM in an industry reliant solely on natural mating, such as the Thoroughbred industry has the potential for severe economic repercussions. The costs associated with CEM are not only related to the direct losses as a result of infertility, but also due to significant indirect costs in terms of quarantine and surveillance measures, those accrued by outbreak management and treatment protocols as well as international trade and movement restrictions.⁴⁷ As a consequence, CEM is possibly the most highly regulated bacterial equine disease in the world being a World Organisation for Animal Health (OIE)-notifiable disease and is considered part of veterinary certification for international trade purposes.⁴⁷

The causative agent was first isolated in Newmarket, United Kingdom in May 1977,^{8, 44} suspected to have been introduced via Ireland from two Thoroughbred mares originating from France.^{13, 29} The organism was first described as Haemophilis equigenitalis⁵² but was later renamed Taylorella equigenitalis.⁵⁰ By the end of the 1977 breeding season, T. equigenitalis had been diagnosed on 29 Thoroughbred studs in the Newmarket area, with 23 stallions and approximately 200 mares affected.^{13, 29} This resulted in closure of the National Stud and other stud farms in the Newmarket area, resulting in massive losses in both stud fees and foal sales and a reported decrease in the foaling rate to 42 per cent from the 72 per cent recorded in the previous year.²⁹ Despite rapid imposition of a ban on the importation of horses from the UK and Europe, T. equigenitalis was confirmed in Kentucky (USA) in 1978,⁵¹ and subsequently from Missouri and Canada in 1979.¹³  The 1978 outbreak was estimated to have cost the Kentucky Thoroughbred breeding industry a million dollars a day with an estimated cost of 13.5 million dollars for eradicating T. equigenitalis from the USA.^{26, 55}

Following the first reported cases of CEM in the UK and Ireland, the disease has been found to have a worldwide distribution and has been confirmed in various countries including several within Europe, the USA, Australia, Japan and South Korea^{6, 19, 21, 26, 29, 43, 49, 51, 55} with the most recent outbreaks being reported from the USA¹⁴ and the UK.⁴⁵

South Africa was considered free of CEM until May 2011 when an outbreak was confirmed and reported to the OIE.³⁶ Epidemiological evidence initially suggested that T. equigenitalis had entered South Africa via importation of a Warmblood stallion from Germany. A nationwide stallion-screening programme, traceback of the exposed mares and their resultant offspring and further investigation of a sub-population focus in the South African Lipizzaner population, however, revealed that T. equigenitalis was most probably first introduced into South Africa at least as early as 1996 by importation of Lipizzaner stallions from Europe. Thereafter the organism had unwittingly been maintained within this closed population. The later introduction of assisted reproductive techniques by the South African Lipizzaner Centre’s breeding management, allowed for dissemination of this organism by fomite transmission to the wider Sport horse population where it was maintained for several years until the identification in 2011 of the index case.

The current gold standard recognized by the OIE for identification of T. equigenitalis is bacterial culture. Complicating this however, is the organism being characterized during bacterial culture as fastidious, slow growing and rapidly overgrown by other bacteria present in the reproductive tract of horses.^{5, 12, 34} These characteristics decreases the sensitivity of this method. Real time polymerase chain reactions (RT- PCR) methods have subsequently proven to be more robust assays with increased sensitivity and specificity and RT-qPCR is...