Study Examines Safety and Efficacy of Strangles Vaccines

Owners who keep their horses at boarding facilities or travel to events are likely familiar with their horses' risk of exposure to the respiratory disease strangles. This highly contagious disease is spread by horses shedding the Streptococcus equi bacterium while ill and for up to six weeks after clinical signs dissipate. Some horse owners choose to inoculate their animals for the disease, but are vaccines both a safe choice for the horse and an effective deterrent to strangles?

A research team from North Carolina State University's College of Veterinary Medicine led by Luke Borst, DVM, PhD, Dipl. ACVP, assistant professor in veterinary anatomic pathology, recently tested the safety and efficacy of a commercially available modified-live strangles vaccine.

The team separated nine ponies into two groups: five ponies (age 1.5 to 8 years) in Group 1 and four ponies (age 6 months) in Group 2. Both groups were vaccinated for strangles at the outset of the study followed by a booster within two to three weeks according to the manufacturer's recommendations. At the time of the booster, researchers also administered a tetracycline-resistant wild-type strain of S. equi to study possible recombination and monitored the ponies for 50 days.

Upon reviewing results of the study, the researchers' key findings included:

• At the outset of the study, the ponies in Group 1 showed antibodies to the bacterium despite no known prior exposure to strangles while those in Group 2 did not;
• Both groups showed an increase in antibody titers after the initial vaccination;
• Group 1 ponies did not develop any clinical signs of strangles, while all Group 2 ponies developed lymph node abscesses (one pony from Group 2 became severely ill and was euthanized); and
• The vaccine strain remained in the tissues of the Group 2 ponies for up to 24 days after the vaccine was administered.

These findings indicate that while the vaccine successfully introduced antibodies against strangles, it might also have increased the incidences of disease in young ponies, even to the point of fatality. Also, the vaccine strain remained in the young ponies' tissues for several weeks, which can lead to recombination (the process by which genetic material is broken and joined to other genetic material) of the vaccine strain; however, no recombination was noted in the study.

"This study highlights a recurring theme with strangles vaccines: We need better vaccines for this disease," noted Borst. "In this work we demonstrated the questionable safety of a commercially available modified-live vaccine for strangles in six-month-old ponies. While this vaccine was safe in older horses, I would urge caution when considering using this vaccine in horses in this age group.

"The major hurdle in the control of this disease is that safe and efficacious vaccines are lacking," he continued. "We need to better understand the way this bacterium causes disease so we can more accurately target it for eradication through vaccination."

This study, "Evaluation of a commercially available modified-live Streptococcus equi subsp equi vaccine in ponies," was published in the August 2011 issue of the American Journal of Veterinary Research. The abstract is available on Pubmed.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.