Streptococcus equi subsp. equi and Streptococcus equi subsp. zooepidemicus : upper respiratory disease in horses and zoonotic transmission to humans

Abstract: The bacterium Streptococcus equi subsp. equi (S. equi) is the causative agent of the highly contagious upper respiratory disease "strangles" in horses. The ancestor of S. equi, Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) is considered an opportunistic commensal of the equine upper respiratory tract but it is also known to cause disease in several animal species and occasionally in humans. Periodically, S. zooepidemicus alone is isolated from suspected strangles cases. This leads to a clinical dilemma of whether the horse has strangles despite failure to recover S. equi or whether S. zooepidemicus is actually the organism responsible for the clinical disease. The current "gold standard" of bacteriological culture for detection of S. equi may fail in as many as 40% of suspected strangles cases. Results presented in this thesis show that it is possible to increase detection of S. equi up to 90% in acute strangles outbreaks by using a nasopharyngeal lavage in combination with a nasal swab sample and analyzing the samples by real-time PCR directly from the sampling material. Using the same techniques, this thesis also demonstrates that in some strangles-like outbreaks S. zooepidemicus alone is responsible for clinical disease. Determining genetic relationships between different strains of S. equi and S. zooepidemicus is important in epidemiological investigations of outbreaks in both horses and humans. Sequencing of the SeM protein gene in S. equi was useful in establishing relationships between strains isolated from Swedish strangles outbreaks. Characterization of human and equine isolates of S. zooepidemicus revealed zoonotic transmission of certain strains of S. zooepidemicus from healthy horses that caused severe disease in humans. A human isolate of S. zooepidemicus was closely related to a S. zooepidemicus strain isolated from a large disease outbreak in horses, suggesting that certain strains of S. zooepidemicus may be disease-causing in both humans and horses. Characterization of a disease-causing strain of S. zooepidemicus (ST-24) in an outbreak of upper respiratory disease in Icelandic horses suggested that certain strains of S. zooepidemicus may not act solely as opportunistic pathogens, but may be more adapted to infect the upper respiratory tract in horses.