STRANGLES, the most frequently diagnosed infectious disease in horses, could become the next major pathogen to be eradicated from the world.
Staff at the Animal Health Trust say they have developed all the necessary tools to eliminate the threat of the bacterium Streptopcoccus equi and despatch it to the history books along with smallpox and rinderpest.
Dr Andrew Waller, head of bacteriology at the AHT, described progress in the development of improved diagnosis and treatments for the disease at a symposium at the Horse Trust headquarters in Buckinghamshire in October.
The project, supported by the Horse Trust and Horserace Betting Levy Board, is based on a collaboration with scientists in Sweden and industry partners. But the key step was taken by researchers at the Sanger Institute near Cambridge, who decoded the complete genome of S. equi and the closely related though less pathogenic S. zooepidemicus.
Strangles is known to occur in all countries with a significant horse population. It causes outbreaks in which there may be morbidity rates of up to 100% and up to 10% mortality, due to the respiratory distress from which it gets its name.
DNA analyses of the causative organism have shown that it has very little genetic variation, with each isolate virtually a clone of any other. “This is very good news – it means that any diagnostic test or vaccine that we develop in the UK will be just as effective in China, India or wherever else it is needed,” he explained.
Dr Waller and his team have identified a specific gene coding for a protein involved in iron uptake, which is an important factor determining the virulence of the bacterium. They have developed a PCR-based test which will identify the pathogen from a sample with a mixed population of different streptococcal strains.
They are now working with the diagnostic technology company Atlas Genetics to develop a simple point-of care device which an equine practitioner can use to give a result within 30 minutes. This will be a multiplex system that can test simultaneously for several different pathogens when examining a horse with a purulent nasal discharge.
Identifying carriers
However, silent carriers which are not actively secreting the bacteria pose a much bigger threat to the healthy herd mates of an infected animal. The AHT team has developed a second diagnostic tool in the form of a blood test for two S. equi specific proteins which has been shown to be reliable in identifying such carriers before they spread the infection to a new holding.
The team has also been working on a new live vaccine for the disease and again the results here look very encouraging, Dr Waller said. They have developed both a live vaccine and a protein-based vaccine using a bacterial strain in which six key pathogenicity genes have been deleted. These cause no adverse reactions in treated horses and are up to 87% effective in preventing infections of the lymph glands.
Further work will be needed to investigate the duration of protection achieved with the vaccine and whether the number of separate allergenic components in the vaccine can be reduced without compromising that protection. But the results to date are “a huge step forward along the way to getting a safe and effective vaccine for strangles”, he said.
Wider implications
Moreover, this work may also have wider implications for both equine and human health.
The basic knowledge gained of streptococcal biology will be useful in the development of diagnostic tests and vaccines for other pathogens in a large and diverse bacterial family. These include the strains responsible for streptococcal pneumonia and S. pyogenes, the cause of the terrifying human infection, necrotising fasciitis.
In another Horse Trust-funded project, parasitologists at the Moredun Research Institute near Edinburgh are developing techniques for analysing populations of small strongyles in horses with a view to prolonging the effectiveness of available anthelmintic drugs by slowing the development of resistant strains.
Targeted treatment
Professor Jackie Matthews, head of the immunobiology department at the institute, described the research which has produced a test for antibodies to nematode proteins which appear to correlate closely to the worm burden in an individual animal.
This should be useful in allowing targeted treatment of the 20% of the horses in any population which harbour 80% of the total worm burden.
The same team has also developed an alternative to the faecal egg count reduction test in giving an indication of the relative proportions of drugresistant and sensitive strains in an infected horse. The test is simple enough to be carried out in a practice laboratory and measures the ability of larval worms to wiggle through the pores in a filter.
The moxidectin/ivermectin group of anthelmintics induce paralysis in the worms and so any sensitive larvae exposed to these agents will have an impaired migration rate. This technique can therefore be used as a screening test before selecting the products to be used for routine worming, she said.
