Straight Talk About Strangles

Debunk the four biggest myths about the highly contagious disease strangles. By Eliza R.L. McGraw for EQUUS magazine.
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Debunk the four biggest myths about the highly contagious disease strangles. By Eliza R.L. McGraw for EQUUS magazine.

Maybe it's the name that amplifies the dread horsepeople tend to feel when the grapevine rumors a neighborhood infection.
Certainly, no one who's had to nurse a horse through to recovery wants a repeat experience, and anyone who's read John Steinbeck's The Red Pony about a young boy's first exposure to death and loss can't help but expect the worst of the disease. Yes, strangles has a terrible name and a worse reputation. Horses who come down with a Streptococcus equi infection get an ugly kind of sick, and they seem to be knocked out of training forever.

"People have definitely gotten more panicked about strangles over the years," says George Sengstack, manager of Callithea Farm, a 60-horse boarding stable in Potomac, Maryland. "Fourteen years ago, before we had horses trailering in and out, I had a horse who got strangles. We kept him away from the other horses in a round pen and were really careful [about using separate equipment]. He got better, and it wasn't a big ordeal."

Sengstack's experience with strangles is more the norm than Steinbeck's fatal scenario, but the highly contagious disease can be a real blight on a stable of susceptible horses. Also called equine distemper, the infection typically begins 10 to 12 days after exposure to S. equi.

First the horse experiences a high fever, depression, appetite loss and enlargement of the lymph nodes between the jawbones. Copious amounts of thick, yellow pus begin draining from the nostrils, and before three weeks are up, the abscessed nodes at the throat may burst open to drain. The disease's descriptive name comes from the "strangling" noise produced as severely affected horses struggle to draw breaths into their obstructed airways. Aside from observing the obvious physical signs in diagnosis, veterinarians can run cultures of the nasal drainage to see if it contains the streptococcal organism.

Exposure often occurs when a new horse, who's shedding the S. equi bacterium without visible signs of sickness, is introduced into a herd. The organisms spread from horse to horse through direct contact, such as touching muzzles, environmental contamination and shared equipment, such as feed buckets and bridles. Strangles spreads rapidly, producing large outbreaks in herds not previously exposed or vaccinated. The infection is especially aggressive in populations of foals and young horses. Most horses recover, but fatalities do occur, primarily from secondary pneumonia that takes hold in debilitated or immune-compromised animals. And every now and again, S. equi infect lymph nodes deeper within the body, producing a more dangerous condition called bastard strangles.

Isolation is the single most effective means of controlling the spread of the disease. Infected horses usually show signs within two weeks of exposure, so preventing contact between potential carriers and a healthy herd for at least that long should reduce contagion. Three- to four-week quarantine periods for newcomers or exposed horses are usually enough to slow or stop the spread of the disease.

In textbook terms, then, strangles isn't really a big deal. It hardly ever kills or causes lasting damage to horses, and it's easily controlled by physical separation, the most basic of medical precautions. Yet the fact that strangles isn't effectively prevented or treated by the same means applied to other common equine diseases gives it a rather sinister cast. It must be some sort of super infection, right, if treatment is iffy and vaccines aren't reliable? Well, not exactly. The myths and misconceptions surrounding strangles tend to overplay the current gaps in scientific knowledge about the disease. Given recent genetic advances in characterizing the strangles organism, truly effective prevention is no longer a pipe dream.

Myth #1: S. equi is Invincible
Not true, says longtime strangles researcher John Timoney, PhD, FRCVS, University of Kentucky Keeneland Professor of Infectious Diseases. "That the organism survives and that pastures and farms that have strangles are at risk of recurrences are myths," he says. "It survives for days or for weeks if frozen. It's a very poor survivor in the environment. It competes poorly with other bacteria and lives for hours or days, not weeks or months." Thus, a farm that has gone through a strangles outbreak is not forever sullied. Months later, when all infected horses have recovered and nasal swabs are negative for S. equi, the soil and surroundings are no longer contaminated from that original outbreak.

If strangles does visit your farm, you still want to exercise vigilance about decontamination practices and isolating sick animals because the bacterium is readily carried from horse to horse. Tom Kranz, owner of Longacres Farm in East Aurora, N.Y., took extreme care to contain the disease when a 21-year-old camp pony named Brownie returned from his winter hiatus in Ohio carrying a brewing infection. Brownie was stabled in the isolation barn and cared for by a single staff member, who removed her shoes and donned designated rubber boots before entering the stable. Avoiding all direct contact with Brownie, she fed him, then removed the boots as she exited the stable and put on her shoes. The precaution reduced the likelihood of her tracking the bacteria to other areas of the farm. Finally, Brownie's caretaker disinfected herself, showered and changed clothes before going back to work at the main barn.

When the abscesses at Brownie's throat came to a head and burst, making him especially infectious, Kranz hired an outsider who did not work around any other horses to perform the tasks that required being near the pony. These sorts of precautions help reduce contagion while a horse is shedding bacteria, and over time, whatever bacteria remain in the environment do succumb to natural forces. The idea that a contaminated farm can never recover is a fallacy.

Myth #2: Once Infected, Forever Protected
Immunity to S. equi following a natural infection tends to be strong, but it's not necessarily a lifelong protection. Timoney says that one in four horses may be reinfected within five years of a strangles infection. Bryan M. Waldridge, DVM, associate professor of equine internal medicine at Auburn University, cites the scientific evidence for the variable nature of equine immune reactions to the bacteria. "Immunity after natural exposure has been reported to last anywhere from several years to six months," he says. "If a horse is reexposed, he is more likely to respond like a vaccinated horse and may show only mild to no clinical signs."

Unless you've taken care of a horse from his birth onward, you can't be certain of his strangles status. Though not routine veterinary practice, a serum antibody test could be run to reveal the presence or absence of immune factors targeted to S. equi. If antibodies are present, the horse was either vaccinated or had strangles previously, both of which should reduce his risk of suffering a full-blown case if he is again exposed to the bacterium.

Myth #3: Antibiotics are a Must
Not really, says Waldridge. "Horses who have only lymph node enlargement or drainage with mild respiratory disease and continue to eat normally and are essentially normal otherwise generally do not require treatment." Timoney recommends treating "horses who have difficulty in breathing, because that can progress to blockage of the airway within hours. A very high fever should be treated, but if the temperature's no more than 102.5 Fahrenheit and the animal's doing well, just let nature take its course."

Brownie, the camp pony, fit into the latter category. "Our vet and a couple of other people I respect felt that if you have a secondary infection with strangles or a horse showing signs of a snotty nose, then antibiotics can help," says Kranz. "But letting it run its course is often just as effective. Brownie had a nasty abscess but he was never sick."

Treatment for more seriously affected strangles victims varies case by case, but it typically includes penicillin. If the sick horse has difficulty breathing or swallowing or has large accumulations of pus in the guttural pouches, he will probably receive injectable penicillin along with drainage of affected lymph nodes and lavage or surgical drainage of pus from the guttural pouches. Anti-inflammatory drugs, such as phenylbutazone and flunixin meglumine (Banamine), are indicated to reduce fever and swelling.

Antibiotic treatment that isn't strong enough or doesn't last long enough to kill off all the S. equi may encourage the development of bastard strangles. So-called because the infection doesn't follow the normal disease pattern, bastard strangles affects lymph nodes in the chest and abdomen. As these internal abscesses progress, they can't be drained or effectively treated, and their rupture within the body cavity usually results in the horse's death.

Myth #4: Vaccinate to be Safe
Strangles vaccination has a spotty history of safety and effectiveness, and even with the introduction of better formulated, more targeted products, horse owners are still faced with a risk-benefit decision. Intramuscular (IM) strangles vaccines of decades past frequently caused injection-site reactions and sometimes prompted actual cases of the disease, or they failed entirely to protect the recipient during natural exposure. Hypersensitivity to the S. equi antigens also produces a sometimes-fatal complication called
purpura hemorrhagica, an immune-mediated reaction affecting circulation in the skin and mucous membranes. Twenty years ago, isolation of a particularly antigenic S. equi surface protein--SeM--allowed makers of injectable vaccines to develop products including just that portion rather than the whole organism, thus eliminating the risk horses might develop the disease from the vaccination. Injection-site abscesses and purpura remained possible complications.

More recently, intranasal strangles vaccines have been developed based on the horse's natural immune reaction to S. equi. "The intranasal vaccine theoretically improves immunity against strangles because it stimulates local antibody production within the nasal cavity to prevent the bacteria from establishing themselves in the respiratory tract and producing disease," says Waldridge. "Additionally, the intranasal vaccine is a modified-live bacteria, and generally modified-live vaccines produce good immune responses." Although this latter characteristic improves protection, it poses a new risk during administration. When multiple vaccines are given to a horse in a single session, the modified live S. equi in the intranasal formulation can contaminate the sites of any IM injections given, with abscesses developing wherever the bacteria get under the skin. The simple precaution of always administering the intranasal vaccine following all IM injections during a vaccination session protects against that possible immunization risk.

But immunization with the intranasal product during a nearby strangles outbreak may transmit wild S. equi strains to the animals being vaccinated. It is not known how the live vaccine responds when wild strains are already circulating among horses. The person administering the vaccine may accidentally transmit the wild virulent organism from horse to horse. "I do not recommend using live vaccine in the face of an outbreak," says Timoney. "There are potential problems in using either type of vaccine because you don't get immunity for two to three weeks. It's better just to segregate, use vigilance and treat if necessary than to vaccinate."

Waldridge agrees that it is unwise to vaccinate any horse showing clinical signs of strangles. Consult with your veterinarian about the advisability of vaccinating your horses when an outbreak occurs locally but without any direct contact through common fence lines, shared bridle paths or commingling events. According to Timoney, vaccination can be appropriate for a group of horses that's geographically segregated from the affected herd.

As for including a strangles vaccine in routine preventive programs, that decision depends upon the kind of life the horse leads. Although immunization against S. equi doesn't approach 100 percent protection, vaccination generally reduces the severity of disease if the horse does become infected. For at-risk horses, even partial protection is better than none, particularly as improved vaccine formulations produce fewer adverse reactions. According to Waldridge, horses who are exposed to a shifting population of horses, as at boarding stables, shows, trail rides and racetracks, benefit from vaccination. Breeding stock may also need protection. "Vaccinate weanlings at six to eight months," advises Timoney. "Mares should be vaccinated a month before foaling if there will be a disease risk to the foals."

Annual strangles vaccination is more than enough for most horses. Those kept in closed herds with few outside contacts may be better off without immunization, given their minimal opportunity for infection.

This article originally appeared in the December 2002 issue of EQUUS magazine.