Molecular epidemiological investigation of contagious equine metritis in South Africa

Abstract

Contagious equine metritis (CEM) is a non-systemic, venereally-transmitted disease of horses caused by the bacterium, Taylorella equigenitalis. The second member of the genus, Taylorella asinigenitalis, is found in donkeys and is considered non-pathogenic. Contagious equine metritis was initially identified during the 1977 Thoroughbred breeding season in Newmarket, United Kingdom (UK) where, due to its virulence and contagious nature, CEM was estimated to have caused a markedly decreased foaling rate. The disease was speculated to have originated from France and is now considered endemic in non-Thoroughbred populations in mainland Europe. Since then, sporadic outbreaks of seemingly less virulent strains of T. equigenitalis have occurred worldwide in many non-endemic countries, often associated with assisted reproductive techniques (ART).

Contagious equine metritis is a World Organisation for Animal Health (OIE) notifiable disease and is regarded internationally as one of the most highly regulated equine diseases of global importance to equine health and international trade. Stallions affected with T. equigenitalis are unapparent carriers of the organism and the main source of infection with this carrier status persisting for months or even years. Infected mares typically develop a transient endometritis or cervicitis resulting in irregular interoestrus periods and temporary infertility. Most mares develop a short-lived humoral immunological response and rid themselves of infection, however a small population of asymptomatically infected mares may attain carrier status and pose a potential source of infection to stallions.

South Africa was considered free of T. equigenitalis until May 2011 when an outbreak was confirmed. Initially the outbreak was thought to be confined to one property, affecting the index case, a stallion and a mare bred to him via fresh semen artificial insemination. However, institution of a national stallion screening programme and further epidemiological traceback of in-contact animals during the period July 2011 to May 2020, revealed a total of 42 horses on 15 properties that were affected. All affected animals were traced back to the index property.

In the first chapter, the problem statement, hypotheses and objectives that lead to the publication of the work presented in this thesis are considered.

In the second chapter, the broader context of the risk factors contributing to outbreaks of equine infectious diseases are considered and the pertinent literature on both T. equigenitalis and T. asinigenitalis, including the diagnosis, pathogenesis, clinical signs, transmission, treatment, control and prevention are reviewed.

In the third chapter, the materials and diagnostic methods used during the South African T. equigenitalis outbreak and subsequent epidemiological surveillance are described. These include the diagnostic tools used to identify the first incursion of T. asinigenitalis into South Africa. Later in the chapter, the epidemiological investigations of the T. equigenitalis outbreak and T. asinigenitalis incursion are described and the progressive development of an adapted treatment protocol for the South African context is explained.

In the fourth chapter, the results obtained from outbreak investigation and subsequent epidemiological surveillance are presented. The successive findings from the development of the adapted treatment protocol are also presented.

Finally, in the fifth chapter, the findings of these studies are discussed, in particular how the use of newer molecular technologies and the application of comparative phylogenetic analysis definitely traced the origin of the outbreak. These findings are aligned with future directions and in the final chapter, the overall conclusions from these studies are presented.

Overall, this work reported on the use of more current molecular technologies for epidemiological investigation of the origin and circumstances of both the T. equigenitalis outbreak and T. asinigenitalis incursion into South Africa. Experience gained during the investigation and control of these outbreaks informed subsequent legislation and knowledge regarding the diagnosis and treatment of both T. equigenitalis and T. asinigenitalis.