For the past four years, researchers at the Animal Health Trust, have been analysing the genetic structure of Streptococcus equi (S. equi), the bacterium responsible for equine strangles. The study, funded by the Horse Trust, identified two antigens that are produced only by S equi. When horses are exposed to S equi, their immune system targets these antigens and produces antibodies in response. AHT scientists have now developed a blood test that can detect these antibodies.
Strangles is one of the most commonly diagnosed infectious diseases of horses. An important feature of the disease is the occurrence of clinically normal carriers. Once they have recovered, most horses are no longer infectious 4-6 weeks later. But up to 10% of horses may remain as symptomless carriers.
These carriers probably result from pus remaining in the guttural pouches or sinuses. With time, the pus may solidify to form masses known as "chondroids". These can harbour the bacteria and excrete them intermittently in the nasal secretions - acting as a source of infection for other horses. The carrier state can last for months, even years, without any obvious signs of illness in the horse.
Identifying and treating these long-term carriers is crucial for controlling the spread of the disease. Until now, the only way of identifying strangles carriers has been by culturing a series of swabs from the nasopharynx, or by examining a guttural pouch wash for S equi DNA (a PCR test). Although effective, these techniques are time consuming and can be difficult if the horse does not cooperate.
The new test provides a simple way of checking whether a horse is likely to be a carrier. A positive result does not mean the animal is infected. It is purely an indication that the horse has been exposed to S. equi. It takes about two weeks for horses to produce antibodies after becoming infected. The antibodies will persist for months, even years, after the infection has cleared. So horses with antibodies could be incubating the disease, have had strangles and recovered or have had strangles and remained carriers.
The blood test is not intended to replace the culture or PCR tests, but to be an additional tool in controlling the spread of strangles. Scientists at the AHT envisage it being used in two situations:
The last thing anyone wants is to introduce strangles into his or her yard. Ideally all new arrivals should be quarantined before being allowed to mix with the resident horses. AHT scientists recommend taking a blood test on arrival and again three weeks later. If all animals test negative on both tests, it can be assumed they are not carriers.
On the other hand, if any test is positive it is necessary to conduct further examinations, which would include nasopharyngeal swabs or PCR test on guttural pouch washes, to check whether the horse was a carrier.
It is important to make sure that all horses have fully recovered and that none remain carriers. Horses in contact with affected animals that appeared to escape infection should be tested. Blood samples should be taken at least 30 days after the last affected horse has recovered. If any test positive they could be carriers and should be investigated further with swabs or PCR. (There is little to be gained by blood testing horses known to have had strangles, as they would have raised antibody levels anyway. They need a series of three clear swabs to confirm that they are free of infection)