IN Strangles Vaccine Report

While many practitioners and horse owners have used the Pinnacle IN intranasal strangles vaccine with no problems, there have been some reports of adverse reactions and complications. According to Tom Overbay, DVM, of Fort Dodge Animal Health, producers of Pinnacle IN, "The vast majority of users love the modified live vaccine, even using it in the face of an outbreak or in chronic situations. We sell about 650,000 doses a year. There are problems with a very small percent of horses--maybe 10 calls a month--where the horse gets swollen lymph nodes and nasal discharge."

However, Hagyard-Davidson-McGee, one of the leading equine veterinary firms in Kentucky, has opted not to use or sell the vaccine until the current level of clinical complications is more fully understood in relation to appropriate use and safety.

On the other hand, Bend Equine Medical Center in Oregon, a leading multi-veterinarian clinic in their region, reported that of several hundred doses administered of the intranasal strangles vaccine, they had experienced no serious adverse reactions. One of the veterinarians said that the older, intramuscular vaccine caused very painful injection site reactions in a large percentage of horses treated, and that he very much preferred the intranasal vaccine.

Overbay theorized that horses which got the minor reactions probably were subclinical carriers of S. equi. There is evidence that the organism can be harbored in the guttural pouch of some animals, and when an immune response to that organism is stimulated in a horse (such as from vaccination), the reaction of the horse's immune system is to seek it out and destroy it. One way a horse fights a respiratory invader is to produce a nasal mucosal immune response.

"So, it's possible that we are seeing a normal response to a pre-existing subclinical infection," surmised Overbay. While there is no way to predict which horse will be a subclinical carrier, the carrier state can be detected using sophisticated molecular procedures (a PCR test).

When questioned whether the strain of S. equi in the vaccine was causing disease or mutating (becoming more pathogenic and causing disease), Overbay was skeptical. "If it was a problem with the vaccine causing disease, then we'd see more cases of disease after vaccination," he said. "If the vaccine mutated, then the USDA and our quality control tests of each batch should detect that."

Because it is recommended that modified live vaccines be given last if other vaccines are administered at the same time, and that the veterinarian wear gloves, there is the perception that the bacterium in the vaccine is very contagious. Overbay said if the vaccine comes into contact with a non-vaccinated horse, then it might stimulate an immune response. If the non-vaccinated horse were a subclinical carrier, then it could develop some clinical signs of strangles, as described above. Research has shown that some horses appear to be unusually susceptible to the vaccine strain of S. equi and develop clinical signs of the disease.

The gloves and the suggestion to vaccinate with a modified live vaccine last are to avoid the risk of the organism contaminating another vaccine administered by the intramuscular route because the vaccine strain of S. equi can cause an abscess if introduced into the muscle injection site.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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