An Unusual Reaction to Equine Strangles Vaccination

A rare complication from a routine equine strangles vaccine causes an injection abscess that requires careful treatment.

I’ve been around horses long enough to know that I don’t want to mess with equine strangles. Because my two horses travel often and far, I regularly vaccinate them against this highly infectious disease, which is caused by the Streptococcus equi bacterium. After all, if we come across just one horse with equine strangles in our travels, I might be in for weeks of nursing my horses through the fever, nasal discharge and abscessed, draining lymph nodes produced by the disease. Although it’s not typically deadly, strangles can make a horse and everyone who cares for him miserable for some time. That’s enough incentive for me to do whatever I can to avoid it, including regular vaccination.

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About a year ago, however, my efforts to protect my horses from equine strangles set into motion a very different, yet equally miserable, chain of events centered on an unusual reaction to the strangles vaccine.

Last fall, I left my two Arabian-Quarter Horse crosses, Remy and Monty, in the care of friends in South Dakota while I took a backcountry wilderness medicine class in Missoula, Montana. The plan was that, once my class ended in early November, I would pick up my horses as I drove home to New York for college.

In mid-October I called my friend and asked her to schedule an appointment for Remy and Monty’s fall shots well in advance of their early November departure. I gave her a list of my horses’ typical vaccinations and, knowing we would be staying at unfamiliar barns while driving back east, I added the strangles vaccine. I usually have my horses vaccinated against strangles in the spring, before the show season, but this was a “why not?” moment. I had never had any problems with the strangles vaccine, so what could it hurt? I thanked my friend, hung up the phone and didn’t give it another thought.

Injection reaction
Weeks later, just days before I was supposed to leave Missoula, I received a phone call from my friend. She told me that Remy had suddenly developed a swollen mass on the right side of his neck a few inches above his shoulder. She thought that Monty might have kicked him, and she told me she had given him phenylbutazone and would keep an eye on him. The next day, the swelling had gotten worse, and he was off his feed because it was too painful for him to reach down to the hay feeder.

I called my veterinarian back in New York, Emily Harrison, DVM, to ask her opinion. She was not terribly alarmed until I mentioned that Remy had been given his vaccines three weeks earlier. She asked which specific vaccines were given to my horses, and when I mentioned strangles, she quickly followed up with another question: “Was it given intranasally?” Yes, I told her. “On the same day as the intramuscular shots?” Yes, it was.

Harrison explained why that mattered. The strangles vaccine is available in two forms: an intramuscular injection that uses a killed vaccine, and an intranasal spray that delivers modified live bacteria directly onto the surfaces of the upper respiratory tract.

The intranasal version of the vaccine confers a higher level of immunity against strangles and is generally safe but carries a risk. If the horse objects to having the vaccine squirted up his nostril and pulls away as it’s administered, or if he snorts immediately afterward, some of the live bacteria can be ejected back out into the environment. If those microorganisms contaminate the horse’s skin or the veterinarian’s hands, or even if they just remain suspended in the air, and then if that horse or any other in the barn receives an intramuscular injection, the strangles bacteria can “hitch a ride” on the needle and be inserted directly into the neck muscles.

Unlike typical systemic strangles, which enters through the nose and causes signs such as high fever, depression, swollen lymph nodes and a snotty nose, an S. equi infection that enters the body via an intramuscular injection is walled off into an abscess—an internal pocket of pus that will continue to grow in size. If Remy had localized strangles, he wasn’t currently contagious, but if the abscess reached the skin and ruptured through the surface, it could drain a massive amount of pus filled with live bacteria that could infect any horse in the vicinity.

This complication is rare, but it is a very real threat. In fact, Harrison had seen a case just a few months earlier. To reduce the risk, it is recommended that the intranasal vaccine be administered on a different day than any intramuscular injections. If that’s not an option, then the intranasal vaccine needs to be done last, only after every horse in the barn has already received his injections. Because I wasn’t present when my horses were vaccinated, I’m not sure exactly what happened.

Of course, without actually seeing my horse, Harrison couldn’t know for sure that he had a strangles abscess. “I don’t want to cause any unnecessary hysteria,” she said, “but I am very concerned. We need to get the veteri-narian back out there to ultrasound the area and see if there is a deeply buried pocket of pus. If there is, she’ll need to draw it out with a needle and send it off for a culture before that horse goes anywhere.”

Still in Missoula, I arranged for the original veterinarian to return to the farm and investigate. Unfortunately, she did not have a portable ultrasound machine, but she did insert a hypodermic needle into the site to see what she’d retrieve. I waited anxiously on the other end of the phone as the procedure was done.

I was told the needle drew back only blood, a finding which suggested that it was not an abscess after all but just a nasty hematoma, a blood-filled pocket that could have formed after a trauma such as a kick. Hematomas may look alarming, but they generally do little harm, and veterinarians prefer to leave them alone. Eventually, the body resorbs the blood and the swelling goes down. Opening them to drain creates an opportunity for infection to enter.

Relieved to hear that news, I made the decision to pick up my horses as planned and head back across the country. If Remy’s problems continued, I’d feel better addressing them with Harrison at home in New York.

Heading east
When I arrived in South Dakota a week later, I was surprised at how uncomfortable Remy was. The swelling, which looked like half a cantaloupe stuck to the side of his neck, was clearly causing him pain when he moved or ate. Still he was a total champion traveling. It was obviously painful for him to step in and out of the trailer, despite the bute I was giving him, but he did so without resisting or complaining. The swelling worsened a bit over the four-day trip, but otherwise we made the journey without incident.

The day after we were safely back at our boarding barn Harrison came out armed with her ultrasound machine. In the grainy images she found what she was looking for: a pocket of pus, a good two inches deep under the muscle. She and I were both amazed at how deep the abscess was. Using a syringe with a long needle, Harrison drew a sample of pus from the abscess and sent it to the lab for a PCR, a test that determines the presence of a disease-causing organism by identifying its DNA. The results came back the next day: they were positive for strangles.

Remy couldn’t stay at the boarding barn. If the abscess were to break open he could infect dozens of other horses and subject everyone to weeks of strict quarantine. But now we had to race to find a barn that would let us open and drain the abscess on their property before it ruptured on its own.

Harrison called around and found a farm in upstate New York that specializes in rehabilitating sick and injured horses. They were familiar with quarantine protocols and were willing to take Remy in for the procedure. We arranged to haul him there the next day.

Relief at last
A little more than a month after the initial vaccinations, Harrison cut a slice down the middle of the abscess, and pus poured out for 15 minutes in the safety of a sterile and isolated environment. The release of the pressure within Remy’s neck gave him obvious and immediate relief from the pain, and I simultaneously felt the tension and stress that had been building in my own neck for the past month slowly start to dissipate.

Harrison inserted a catheter in Remy’s neck for the heavy doses of antibiotics he would receive twice daily, and she left the incision open to drain and ultimately heal from the inside out. She explained that if she sewed it shut, any lingering bacteria would just be trapped again, and the hole would refill with pus. The wound would have to be flushed with saline daily, and the drainage would be carefully contained to avoid infecting other horses. Fortunately, the staff at the facility were very skilled at all of this.

Remy remained at the rehab farm for six and a half weeks, until PCR tests confirmed that the tissue and fluids in his wound no longer contained any traces of the strangles bacteria, and the incision was allowed to close. During that time he remained in complete isolation from other horses, with a friendly cow named Peanut and some sheep as his only companions on the other side of his stall door.

When we brought Remy home in January, a scar on his neck was the only lingering sign of his medical ordeal. He was thrilled be to reunited with his human and horse family, and as I watched him race around in his field with Monty, I pondered just how many lucky breaks we’d gotten. If I’d known about the pus in his abscess back in South Dakota, I would have had to quarantine him there and deal with his care from the East Coast. We’re also extremely fortunate that the abscess didn’t open on its own, which would have exposed Monty, my friend’s horses, and those on every farm we stopped at on our trip across the country to strangles.

Of course, I also have a little nagging guilt that all of this could have been prevented, and we might have spared Remy months of discomfort. But in the end I realized that, as much as we try to protect our horses from every little risk, sometimes the freak events will still happen. All we can do is be prepared, as best as we can, to deal with them when they do.

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