It was described in veterinary literature as far back as 1251, more than 750 years ago, yet it remains one of the most commonly diagnosed infectious diseases in horses worldwide.
It's called strangles because symptomatic lymph-node enlargement can compress the pharynx, larynx or trachea of infected horses, causing them literally to strangle and die from asphyxia.
In today's environment, careful attention and proper management of the disease can help more horses than ever to survive. Farm operators who pay special attention to testing and isolating asymptomatic carriers eventually can reintroduce those horses back into the general population.
Meanwhile, progress continues to be made toward a vaccination for strangles.
"There's certainly reason to be optimistic," says Corrine R. Sweeney, DVM, Dipl. ACVIM, professor of medicine, associate dean and chief operating officer and executive director of the George D. Widener Hospital for Large Animals at the University of Pennsylvania School of Veterinary Medicine's New Bolton Center.
As data on the bacterial genome sequence evolve, along with other findings, a vaccine seems to be on the horizon. Additional progress is being made on detecting asymptomatic carriers by means other than culture, washes, etc. The goal is a simple blood test that could determine whether a horse is a carrier.
"Are those things done and available yet? No, but those are the directions so that we can prevent the disease. Until it's prevented, we can have a simpler, effective method to detect carriers to prevent the spread," Sweeney says.
Caused by Streptococcus equi, strangles is characterized by a sudden fever (>103° F), followed by upper respiratory catarrh, mucropurulent nasal discharge and acute swelling with subsequent abscess formation in submandibular and retropharyngeal lymph nodes.
Fever persists as the disease progresses. Lymph glands swell and the abscess matures. Infected horses often become depressed, listless and anorexic. Lymphadenopathy is a major clinical sign. The lymph glands of the jaw and neck become painful and swollen within one week after infection. Edema accompanied by serum oozing at the skin surface eventually ruptures and drains to the outside of the neck and into the guttural pouch. Often discharge is noted periorbitally, with severe swelling of the eyelids.
"Nasal shedding of S. equi usually begins two to three days after onset of fever and persists for two to three weeks in most animals," Sweeney says.
"Purulent discharges from horses with active and recovering strangles are an important and easily recognizable source of new S. equi infections among susceptible horses."
Not only is horse-to-horse contact a concern, but also indirect transmission via contaminated equipment and personnel who handle infected animals.
"S. equi can remain viable in frozen mucoid discharges, but survival in the environment requires water, protection from sunlight and protection from other organisms," says Robert Holland, DVM, PhD, a Lexington, Ky., practitioner.
"Bacteria survive in the environment in dirt or on a fence for up to a week, and in water (troughs, etc.) for up to 30 days," Holland says.
"A recovered horse may be a potential source of infection for at least six weeks after its clinical signs of strangles have been resolved," Sweeney explains.
Horses recently recovered from the disease may shed bacteria for several weeks after clinical signs have disappeared, even though the organism is no longer detectable.
Asymptomatic subclinical S. equi carrier horses are potentially infectious for prolonged periods via periodic shedding.
"The majority of those horses at one time had some symptoms, but they are probably why strangles has continued to be perpetuated," Sweeney says.
The carrier state may result either from incomplete drainage of exudates from the guttural pouches, empyema, and/or sinuses following rupture of abscesses. In some affected horses there is no shedding, while in others, especially those with contamination of the guttural pouches, shedding may persist for some time. Those with guttural-pouch chondroid formation may act as asymptomatic carriers for months or even years.
Silent carriers may have only mild purulent debris in the guttural pouches. Chondroid formation is not required to establish the silent-carrier state.
Farms with recurrent disease can harbor asymptomatic carriers that make up 16 percent to as much as 69 percent of the general population, Holland says.
"The old myths about farms that have strangles being stuck with it for a long period have recently been proven wrong," Holland adds. "Research shows that horses can carry S. equi in their guttural pouches for years and show no clinical signs. Farms that have had recurrent problems most likely have asymptomatic carriers in their herd."
"When you look at the environment — the fields, the farm structures, the wood, the troughs — that's rarely the source of the continuation of the disease unless there's an affected animal out there," Sweeney explains.
"Eventually, over a matter of weeks, the fields, the stall walls, the water buckets, etc., are fine, but as long as there's that one horse that is not completely disease-free, although it might look clinically recovered, it is asymptomatic and available to continue to spread disease to other previously unaffected horses," Sweeney says.
"Persistence of strangles on a farm has long been blamed on contamination of the soil with S. equi," says Phoebe Smith, DVM, Dipl. ACVIM, assistant professor of equine internal medicine in the Department of Veterinary Clinical Sciences at The Ohio State University College of Veterinary Medicine.
"This is rarely the case, except when pus or contaminated discharge freeze before natural soil bacteria kill the Strep equi," Smith explains.
Only these frozen bacteria can persist in the environment to infect horses when the ground thaws. Strangles bacteria can survive three to four weeks in water troughs.
"In the absence of clinical disease, persistence of S. equi may be due to asymptomatic or 'silent' carriers," says Smith.
The most common site of bacterial persistence, or carriage, is the guttural pouches; carriers may intermittently shed the bacteria for months to years. That scenario can result in endemic infections on farms, causing frequent outbreaks and substantial economic losses.
Chondroids (hard bodies, formed of dry exudates) may remain in the guttural pouches for years and be a source of transmission in otherwise healthy-looking animals.
Methods of control
Control of strangles traditionally was based on either a four-week quarantine without bacteriological screening, or on the collection of three consecutive negative nasopharyngeal swabs.
But carriers that shed S. equi intermittently still might culture negatively for weeks or months.
The current gold standard that can detect even these carriers is culture of lavage samples collected endoscopically from the guttural pouches.
Another current method of carrier detection is the PCR (polymerase chain reaction) designed to detect DNA from the gene of S. equi. According to Sweeney, this assay may be used in conjunction with guttural-pouch endoscopy and is useful to detect asymptomatic carriers, determine infection status prior to or after transportation to another facility or prior to commingling with healthy animals or determine the success of elimination of S. equi from the guttural pouch.
Smith adds, Screening for silent carriers of Strep equi is useful in varying scenarios, including:
A thorough look at methodology for screening horses for silent carriers of S. equi was initiated on three farms, and the results presented at the 2006 convention of the American Association of Equine Practitioners (AAEP).
Farms 1 and 3 had endured severe strangles outbreaks for several years. Farm 2 had only one previous outbreak. All horses over the age of 5 months were screened on Farms 2 and 3. Only broodmares and longtime residents were tested on Farm 1.
"The economic losses and negative publicity sustained by farms with strangles outbreaks can be significant," says Smith. "This protocol for detection and treatment of silent carriers was cost-effective on all three farms, given the morbidity rates experienced in years past. The discovery and treatment of silent carriers allowed each of the farms to institute more thorough prevention and control programs with a clean slate," Smith notes.
"It also provided an opportunity for the veterinarian to review and update vaccination programs, quarantine and isolation procedures and other herd health practices on each farm."
Managing an outbreak
In order to limit the spread of strangles, it is important to recognize the disease early on and to properly manage and segregate infected from uninfected populations.
"Since each farm setting is different, the veterinarian is the key person to assess and understand the principles of managing an outbreak," says Sweeney.
A major concern to veterinarians is to have a plan in place to manage it. That begins with a thorough history of the premises.
"Immediately isolating the dirty horse or horses, quarantining those that are sick from those that are unaffected and have not been exposed is of critical importance," Sweeney says.
Though some may contend that the whole farm has strangles, that is not accurate, Sweeney explains. Farms are often of such large size and are set up so that exposed infected animals can be identified and kept separate.
"The object is not to have to shut operations down, but to let those with strangles get over the disease, be tested so they are not carriers and be able to mix them back into the general population," Sweeney says.
She recommends the following steps to help control a strangles outbreak:
Pastures where infected animals have grazed should be rested for at least four weeks.
On the premises, organic material is of most concern. "Use a phenolic compound to clean all water sources, stall areas, fencing, brushes, halters, etc.," Holland recommends.
Holland recommends several steps to clean a farm after clearing S. equi infection. These post-infection procedures include emptying, scrubbing and disinfecting waterers; fully cleaning and disinfecting stall walls, floors, feed buckets, water buckets and all other equipment in contact with infected horses, including wheelbarrows, pitchforks, muck tubs, and twitches, lead ropes and shanks.
Holland also suggests that field fencing be sprayed with a dilute disinfectant and rinsed thoroughly.
Ed Kane is a Seattle author, researcher and consultant in animal nutrition, physiology and veterinary medicine, with a background in horses, pets and livestock.