Science‐in‐brief: Streptococcus zooepidemicus: a versatile opportunistic pathogen that hedges its bets in horses

Respiratory disease affects a large proportion of horses around the world, reducing performance and requiring the withdrawal of Thoroughbred racehorses from training and racing [1,2]. Respiratory disease is particularly prevalent in young horses: an incidence of 8.9 cases per month per 100 horses in British flat racing yards has been reported [3]. Clinical signs of coughing and nasal discharge typically last around 8 weeks and are associated with infection by certain bacteria, including Streptococcus zooepidemicus, which is often identified in high numbers within mucus in the airway [2]. Numbers of S. zooepidemicus decline in parallel with recovery from respiratory disease and the age of the horse, consistent with the development of acquired immunity, which suggests that this bacterium plays a causal role in disease [2]. However, S. zooepidemicus can also be recovered from healthy horses [4] and hence is often referred to as a normal resident of the equine respiratory tract (a commensal) that benefits from residing in the horse without necessarily causing any harm. Sequencing the DNA of S. zooepidemicus strain H70, which was recovered from a healthy Thoroughbred racehorse [5], facilitated the development of a DNA typing scheme called ‘multilocus sequence typing’ (MLST) that produces a unique genetic fingerprint with which the different strains can be identified [4]. The application of this technique in laboratories around the world currently enables the identification of 366 different sequence types (STs) of S. zooepidemicus [6]. MLST has already been used to confirm that two particular strains were responsible for outbreaks of respiratory disease in Sweden and New Caledonia [7,8], and it is likely that this method may also help to resolve the causes of other outbreaks elsewhere. That some strains of S. zooepidemicus cause disease in horses is not unexpected given that Streptococcus equi, which is actually a type of S. zooepidemicus with which it shares >97% of DNA identity [4,9], causes strangles, one of the most long-known, feared and most frequently reported of infectious diseases in horses throughout the world [10–13]. Strangles is characterised by infection and abscessation of the lymph nodes in the head and neck [14]. The rupture of a proportion of these abscesses into the adjacent guttural pouches enables S. equi to persist in the face of the immune response of recovered horses by hiding in dried balls of pus, known as chondroids [15]. The ability to persistently infect healthy horses that have recovered from strangles enables S. equi to benefit from the transportation of horses, which facilitates onward transmission to new susceptible horse populations [16,17]. In a similar fashion, although the development of immunity against a disease-causing strain of S. zooepidemicus may provide resistance to disease in an individual animal [2], it may not be sufficient to eliminate the strain from that particular horse. For example, the particular strain responsible for the outbreak of respiratory disease in Sweden, ST24, was recovered from a healthy horse 8 months after the resolution of its clinical signs of respiratory disease [7]. Therefore, the persistence of ‘commensal’ S. zooepidemicus in Thoroughbred racehorses that have recovered from respiratory disease permits onward transmission to susceptible animals. Young susceptible animals often succumb to a succession of respiratory infections as they gradually build up immunity to the array of S. zooepidemicus strains that persist in a particular yard over time [2,18]. Therefore, the population of S. zooepidemicus that resides in recovered horses provides a snapshot of the history of infection within that individual animal rather than the previously assumed identity of harmless commensals.