Streptococcus zooepidemicus - The bug that can place bets

Respiratory disease affects a large proportion of young horses around the world, reducing performance with significant disruption to training and racing schedules. Inflammatory airway disease affects young horses in particular and it generally causes mucus in the trachea. Some estimates suggest that in British Flat racing yards, for every 100 horses, each month there will be nine cases.

Coughing and nasal discharge  can last around eight weeks and some animals are affected again and again. All of which leads to significant cost to the racing industry. As a result, this problem has been a long-standing focus of attention for the Horserace Betting Levy Board’s (HBLB) veterinary research efforts.

Bacterial genetic code

One reason why bacteria from the same species might affect horses differently is that there are different strains within a bacterial species. This is rather like different breeds of horses – a Falabella pony is the same species as a thoroughbred – but it looks and acts very differently. All living things, from human to single-cell algae in the ocean, have a genetic code written in DNA. Understanding this genetic code can reveal how organisms live and function. 

“In the same way as Strangles, a horse that has recovered from Streptococcus zooepidemicatus might no longer be outwardly affected itself but it may still carry the bug”

An HBLB-funded collaborative team working in Dr Andrew Waller’s lab at the Animal Health Trust and Professor Josh Slater’s lab at the Royal Veterinary College have set out to unlock the genetic make-up of different strains of Streptococcus zooepidemicus in order to understand better if some strains of this bacteria cause disease while others are relatively harmless. The researchers also looked at how different strains of Streptococcus interact with the horses’ immune system. The ulitmate goal of this research is to gain the knowledge which will lead to new vaccines. 

A global research effort

The researchers started by developing a technique to produce a unique ‘genetic fingerprint’ with which to identify each different strain of the Streptococcus. They then tested samples from sick horses around the world and so far 318 different types of S. zooepidemicus have been identified with two particular strains being responsible for outbreaks of respiratory disease. 

Insight from an ancient threat: Strangles

The researchers had some clues about what they might find in the samples from horses with Streptococcus zooepidemicus infection because they already had extensive experience in similar research in Strangles. Strangles is one of the oldest known, feared and most frequently reported infectious diseases of horses throughout the world. Typical signs of Strangles include abscessation of the lymph nodes in the head and neck, with swelling to such an extent that some horses are literally suffocated. It is caused by a relative of Streptococcus zooepidemicus, known as Streptotococcus equi. With Strangles, it is very clear that some recovered horses become carriers. Carriers show no outward signs and this hidden infection enables the bacteria to be spread around undetected. 

S. zooepidemicus colonies grown on a blood agar culture plate

Silent carriers lead to infection in youngsters

In the same way as Strangles, a horse that has recovered from Streptococcus zooepidemicatus might no longer be outwardly affected itself but it may still carry the bug. For example, the particular strain responsible for the outbreak of respiratory disease in Sweden was found in a healthy horse eight months after the horse made a full clinical recovery. 

This persistence of S. zooepidemicus in thoroughbred racehorses that have recovered from respiratory disease allows transmission to susceptible animals and can occur when, for example, older recovered horses are mixed with the next year’s intake of young horses. It is likely that immunity to one strain of Streptococcus does not fully protect a horse from all the other strains, so young horses can often succumb to a succession of respiratory infections as they gradually build up immunity to mix of Streptococcus zooepidemicus strains that persist in that particular yard. 

Bacterial balancing acts

In order to be able persist in recovered horses Streptococcus zooepidemicus must be able to survive despite the fact it is being attacked by the horse’s immune response, and at the same time, the bug must be ready to infect a susceptible animal should the opportunity arise. S. zooepidemicus strains have proteins on their surface and some of these proteins inactivate the horse’s immune response. Other proteins enable the bacteria to stick to the internal surfaces of the horse in order to establish the infection, almost like an ice climber clinging to the surface of a glacier with the crampons on his boots. If he loses his crampons, he is in big trouble. 

Thus, these surface proteins play key roles for the bacteria, but they are also a vunerable point and can be targeted by the horse’s immune response to disable the bacteria. Therefore, balancing the array of surface proteins displayed with the particular requirements of the bacteria at any given time is critical if the bacteria are to successfully establish an infection and transmit to a new susceptible animal.

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THIS ARTICLE FIRST APPEARED IN EUROPEAN TRAINER - ISSUE 47

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Author: Celia Marr, Andrew S. Waller & Josh Slater