New Strangles Vaccine Research in Progress

There are about 150,000 horses in Montana, about one-sixth the number of people living in the state. Whether they are used for pleasure riding, racing, ranching, or hunting, all of these horses are susceptible to strangles.

"Most horses will get strangles eventually, so it is very critical to find a vaccine," said Benfang Lei, PhD, assistant professor in the Department of Veterinary Molecular Biology at Montana State University.

Strangles is a highly contagious disease caused by the bacteria Streptococcus equi, which infects the upper respiratory tract of equine species such as horses, mules, and zebras. While the disease is rarely fatal (about 5-10% in well-managed cases), about 50% of horses that are exposed to strangles get sick.

The infection starts with nasal discharge and fever, and leads to swelling of lymph nodes under the horse's jowls and throat. The swelling can interfere with the horse's ability to breathe--hence the name strangles--and can be very painful.

Graduate student Yanchao Ran from Binzhou, China is working on the project with Lei.

"I have special passion for horse disease research since I was born in the year of the horse (based on Chinese astrology)," she said. "Strangles is one of the most common infectious disease in horses. It causes enormous pain, respiratory difficulty, and even death."

"There is a live bacterium-based vaccine commercially-available for this disease, but this project aims to develop protein-based vaccine to prevent strangles," Ran said.

Lei hopes to develop a vaccine based on protective antigens--proteins produced by the strangles bacterium that are critical for allowing the bacterium to cause strangles. Immunization with these proteins prompts the horse's body to produce antibodies that neutralize the functions of these proteins and thus prevent the infection.

Lei is starting his search for the protective antigen by working with mice, which are easier to wrangle and less expensive than horses. First, a protein-based vaccine is injected into the mice. Then, they are infected with strangles (strangles does not naturally occur in mice, but does cause similar clinical signs when mice are experimentally exposed to it). If the mice are protected from strangles by the vaccine, Lei might have found the protective antigen. The possible vaccine can then be tested on horses.

"Starting this year we will immunize horses and test that they are producing antibodies. If the antibodies are present, the horses shouldn't develop strangles when they are exposed to the disease," Lei said.

There's more to eliminating strangles than finding a protective antigen and developing a vaccine. There also needs to be an immunization method and formulization that is effective in horses. The vaccine will need to be stabilized before the horse community can use it.

"There is a lot of work to be done, but the most important step is to find something that will work, and hopefully we will have a vaccine in a few years," said Lei.

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Montana State University

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