Strangles Management and Prevention

New strategies for management and prevention.

It is not uncommon for strangles infections to recur on a farm, and until recent years there have been misconceptions about how this disease is maintained in a population of horses. These days scientists understand the phenomenon and can better explain it. We now know a farm that has experienced an outbreak of strangles, caused by Streptococcus equi spp bacteria, might have an ongoing problem, not because the bacteria remain in the environment, but because they persist within the horse, specifically within the guttural pouches or sinuses. Even though a horse might appear to have recovered, he remains an avenue to transmit disease to others by harboring the bacteria within his guttural pouches.

In the interest of eradicating strangles infection from endemic farms (those on which the disease is recurrent) and from the horse population at large, researchers have suggested new strategies to manage horses and facilities to reduce risk of an outbreak. They've also described ways to eliminate the bacteria from carrier horses.

Disease Opportunity

Josie Traub-Dargatz, DVM, Dipl. ACVIM, professor of equine medicine and epidemiology at Colorado State University's veterinary school, has been instrumental in developing prevention and recognition programs to protect the equine industry from contagious diseases. Aligning with this focus, she is part of the study group that has been making industry recommendations on equine strangles control.

Traub-Dargatz explains that a horse might be exposed to the causative S. equi bacteria either by the nasal or oral route. Nose-to-nose contact and shared water supplies are efficient means of transmission. She suggests paying close attention to the use of shared water sources, since S. equi is easily transmitted through contaminated water.

Rob Holland, DVM, PhD, a private practitioner and an equine field veterinarian for Pfizer Animal Health, is working with the American Association of Equine Practitioners' (AAEP) Infectious Disease Task Force to formulate the most-up-to-date strategies for equine immunization, and he has extensive experience with the dynamics of strangles transmission. Holland concurs, "This disease is often spread through oral contact, such as in water, on bits, or lip chains and twitches. Studies have shown that S. equi obtained through the oral route goes right to the tonsillar region quickly."

Traub-Dargatz adds, "Even a water hose that has been contaminated with the bacteria--because the hose was touched by people with the bacteria on their hands or the end of the hose was dropped into the bucket of an infected horse--can serve as a source of infection. When the hose is dunked into a bucket as it fills, it then inoculates the water with infectious bacteria."

She stresses that while these bacteria survive most efficiently in the horse, in the environment S. equi bacteria persist more effectively in water as compared to soil--the bacteria can survive for three to four weeks in water tanks contaminated by discharges. Although solar radiation and normal soil bacteria play a role in killing off S. equi, how effective they are in killing the bacteria is variable, depending on the surface and the weather conditions, as well as the dose of bacteria present.

1. A hose can carry bacteria from one bucket to another when you dunk the end in each bucket to fill.

2. S. equi can survive for three to four weeks in water in tanks contaminated by discharges.

3. Bacteria shielded in pus can live longer than if just contained in a drop of clear watery fluid from the horse's nose.

4. In wintertime, pus and contaminated discharges that freeze can persist to infect others when the ground thaws.

5. S. equi lives three days in the soil, seven days on a fence post, and 30-40 days in a water trough.

6. At events, if possible, it is best to leave your horse by your trailer rather than using shared stalls that might be contaminated.

7. Water buckets should not be shared at horse shows.

8. Some horses that continue to shed S. equi past 30 days might clear infections on their own, but it is possible for a horse to shed for weeks, months, or years.

9. Ask owners of visiting or new resident horses about the strangles status of these horses and consider screening for shedding, especially if there is a history of strangles.

10. Contaminated pastures should be empty for at least a month.

11. Water tanks should be cleaned/disinfected daily during an outbreak.

12. When possible, assign specific personnel to handle only the sick animals. Clean hands thoroughly after handling each horse.

13. Work with sick animals last if you are handling healthy and sick horses.

13. Supply disposable coveralls and foot covers or rubber boots that are to be worn only when in contact with sick horses, and arrange for specific changing areas for putting on protective clothing.--Nancy S. Loving, DVM

"It can persist for days in the environment, but not likely for months," she says. "If bacteria are shielded in pus, then there is potential for longer persistence than if it is just contained in a drop of clear watery fluid from the horse's nose."

In wintertime, pus and contaminated discharges that freeze can persist to infect others when the ground thaws.

Holland explains how outbreaks occur in large herds: "S. equi typically lives three days in the soil, seven days on a fence post, and 30 to 40 days in a water trough. So year after year, it stays on a farm, not in the soil, but rather in carrier horses within the guttural pouch. These horses field-vaccinate themselves, so you may only see one or two horses per year with a strangles infection, but the rest of the herd has high levels of immunity until high loads of bacteria overwhelm herd immunity, then large outbreaks cycle every three to four years."

At events Traub-Dargatz and Holland recommend keeping your horse under close supervision and in your immediate control, and to refrain from sharing tack and equipment. Traub-Dargatz says, "Weather permitting and when the duration of the event is short, it is best to leave your horse by your trailer rather than using shared stalls that may have been contaminated."

Holland says, "Common water buckets should not be shared at shows, since water and the oral route may be a more likely route of entry than the respiratory route."

They both urge good hygiene in all cases--cleaning horse trailers between every load of horses, particularly if they are not from the same pasture or stabling group, taking fastidious care taken if ill horses have been hauled in the trailer.

Traub-Dargatz advises, "Using your own horse trailer and your own buckets and equipment when going to equine events can reduce your horse's risk. Also, take care to keep the hose end out of buckets when filling them."

The Hidden Reservoir

Traub-Dargatz describes the presence of S. equi in the guttural pouch: "The guttural pouch is an air-filled expansion of the eustachian tube. Horses are the only domestic animals that have this structure. It functions to cool the air going to the horse's brain during extreme exercise, but it is also a site where S. equi can persist. The bacteria enter the guttural pouch by one of two routes--either from the throat via a natural opening into the pouch or because an abscessed lymph node in the throat region ruptures into the floor of the pouch, with pus then accumulating in the pouch."

This pus contains large numbers of bacteria, and as Traub-Dargatz explains, "Bacteria within the guttural pouch are shed out of the horse's nose periodically or intermittently, in spite of the horse appearing perfectly normal. There are rare instances when bacteria are harbored in a sinus in the horse's head instead of the guttural pouch, and this may be another place to investigate if a horse is identified as a shedder.

"One estimate suggests that up to 10% of horses develop a persistent infection and may shed past 30 days after recovery from illness," she adds.

Holland reports that, on average, farms with endemic (recurring year after year) infections of strangles will have 20% of the horses serving as carrier reservoirs (it ranges from 4-50%).

It is these shedding horses that pose a risk to others, particularly since they can only be identified with diagnostic testing. Traub-Dargatz says, "It is impossible to just look at the horse and determine if it is a shedder or not. Further examination, including collection of samples and testing of these samples, is required to determine the horse's infective status with any certainty."

Some horses that continue to shed past 30 days might clear infections on their own, but Traub-Dargatz notes that it is possible for a horse to shed for weeks, months, or even years. There are reports that the average period carriers shed is 4.5 months, with one horse shedding for as long as 42 months (3½ years!). Traub-Dargatz urges, "If we want to prevent spread of strangles to new groups of horses, it is important not to move recovered or exposed horses around the farm or to a new premises for at least a month. In addition, if we want to use all the tools available to us, these horses should be screened for shedding."

Traub-Dargatz describes her personal experience that impressed her on the value of identifying shedders: "I performed testing on a group of research ponies after an outbreak of strangles to determine their shedding status. Based on that experience, I will never think of strangles the same again. I was very surprised to see the persistence of disease and the bacteria in the pouches of several of the ponies, while others had completely resolved both the disease and the infection. All these ponies were housed together and had the same management, yet their status regarding resolution of the infection was quite variable. Based on a physical examination, I could not have told you which was shedding and which was not. It was only with further testing that we were able to determine their status.

"It made me a believer in the fact that some horses and ponies will persistently shed the bacteria and, thus, pose a risk to susceptible horses," she says. "Since then, I advise people who are having visiting or new resident horses come to their operation to ask about the status of horses regarding strangles and to consider screening for shedding if they are receiving new arrivals, especially if there is a history of strangles."

The American College of Veterinary Internal Medicine (ACVIM) has formulated useful strangles control guidelines ( to assist veterinarians and owners.

Testing Methods

Traub-Dargatz describes the best means of obtaining samples for testing. She suggests that if a horse shows signs of illness, then it is probably adequate to collect and culture purulent nasal discharge with a swab or collect pus from an abscess. For horses that have recovered from illness where there is no active abscess or discharge, the nasopharyngeal area can be lavaged (flushed) to collect fluid that has been in contact with many areas of the pharynx and both nasal passages. A lavage is like giving the nasopharynx a bath with sterile fluid, then collecting what runs out the nose for further testing.

She explains: "This optimizes the likelihood that we will find the bacteria, if present. In those horses that are not showing signs of active illness, we still do culture the material, but it is best if PCR (polymerase chain reaction) testing follows the nasopharyngeal lavage."

Due to its sensitivity, the PCR testing method allows detection of very small numbers of organisms. The PCR test detects DNA from both living and dead S. equi bacteria and could be more than three times more sensitive than bacterial culture. In persistent or chronic shedders the number of bacteria present is much lower than what is found in a horse with an active infection and showing clinical signs of disease.

"Caution must be taken in conducting a PCR nasal wash, since if one uses a twitch, dead DNA on the twitch can trip the test to give a positive result, even if the twitch has been immersed in antiseptic solutions," notes Holland. "A lip chain over the upper gum won't contaminate the sampling. The lip chain should sit for at least 10 minutes in a chlorhexidine solution before using it on another horse, or you can sedate the horse to get the sample."

Traub-Dargatz reports, "Nasopharyngeal swab samples can be tested by PCR, but in my experience, nasopharyngeal lavage is better-tolerated than the nasopharyngeal swab sampling. In addition, the lavage may achieve better results, since the collected fluid comes from both nostrils as well as the pharynx, whereas the swab only samples a single nasal passage and part of the pharynx."

Ideally, a horse is considered not to be a carrier if he has three negative PCR and culture results based on testing of nasopharyngeal swabs or lavages for S. equi over a two- to three-week period. Nasopharyngeal bacterial cultures detect 60% of carriers. Combining culture testing with PCR testing increases detection of carriers to 90%.

However, Traub-Dargatz notes that for recovered horses or those being screened for shedding, "Some veterinarians feel it is more expedient to examine the horse with an endoscope and look into the guttural pouches and do a lavage of that site. This allows a visual evaluation in addition to gathering samples for testing."

Samples taken from the guttural pouches can be cultured and tested with PCR for final confirmation.

As for blood testing, Holland says, "As yet, there is no blood test to confirm the presence of carriers, yet it would be an excellent tool to have a stall-side diagnostic blood test. Currently, an ELISA (enzyme-linked immunosorbent assay) blood test checks for M-protein antibodies, the main protein that the horse develops immunity to for S. equi (and assists the veterinarian in knowing whether or not to vaccinate the horse). The M-protein antibody test is good if one is worried about antibody load in an individual horse, but ... it only shows that a horse has come into contact with Strep equi or has been recently vaccinated. Because it only measures systemic levels of M-protein, this blood test doesn't let you know how much protection the horse has against infection, since mucosal immunity in the airways and throat is different from systemic immunity."


The American Association of Equine Practitioners (AAEP) has formulated practical recommendations and biosecurity measures for disease control. These specific protocols provide guidance for containment and cleanup. Work with your veterinarian to achieve best results.

Once a horse is identified with suspect or confirmed strangles, minimize horse movement and limit mingling of exposed and nonexposed horses.

"It is a good plan to separate a sick horse in an isolation area away from all other horses," recommends Josie Traub-Dargatz, DVM, Dipl. ACVIM, professor of equine medicine at Colorado State University’s veterinary school. "But it’s not a good idea to move exposed horses in with yet-unexposed horses just to get them away from a sick horse."

It is particularly important to protect pregnant mares and youngsters.

She says that a single fenceline between horses might protect horses from injuring one another, but it does little to protect against disease. Primary means of disease transmission, such as nose-to-nose contact and shared water sources, can still occur.

"An isolation area must be located a sufficient distance from highly trafficked areas," says Traub-Dargatz. Contaminated pastures should be kept free of horses for at least a month. Water tanks should be cleaned and disinfected daily.

Check rectal temperatures twice daily on those horses not yet sick, and immediately isolate those showing any indication of illness.

When possible, assign specific personnel to handle only the sick animals. Keep the number of these caretakers to a minimum to limit transfer of infective material around the farm.

These biosecurity measures should remain in effect at least three to four weeks following resolution of the last case of illness on a farm. Contact regulatory agencies and others whose horses might be affected by contact with the sick horses, for example, van companies, show barns, race barns, and sale barns. Apprise owners of incoming broodmares about the situation, and communicate about the steps being taken for control and eradication, while also updating owners whose horses are confined to the farm.

See more strategies at article #10526. --Nancy S. Loving, DVM

A Pound of Prevention

Traub-Dargatz stresses that "the devil is in the details" in constructing management strategies against an infectious disease such as strangles. She observes, "Because we cannot put horses in bubbles and have them do the many things we want them to do, we need to implement strategies that will reduce risk of exposure and infection. Options for management of exposure risk are not as easy as vaccination. How many strategies one adopts depends on an owner's aversion to risk. These strategies may take time, and may be inconvenient, and they will have an associated cost. Each owner must decide how much he or she wishes to take on."

The incubation period for strangles ranges from three to 14 days, in general. Holland notes that a horse that is developing a strangles infection usually develops a fever 1½ to two days prior to lymph node enlargement or shedding of large numbers of the organism. As soon as a fever or other signs of contagious disease occur, personnel should isolate the horse from others, in addition to taking precautions such as routing of equipment. This limits the spread of bacteria until a veterinarian examines the horse. If there is suspicion of strangles, then laboratory testing of nasal-pharyngeal swabs or lavage samples can yield specific information.

"In my opinion, antibiotics given before lymph nodes enlarge may help to head off the disease," says Holland. "Antibiotics may be indicated if there has been a positive identification of Strep on at least one animal on the farm--this increases the probability that the fever is associated with S. equi. If a lymph node becomes enlarged, I stop antibiotics and begin supportive care, unless it is life-threatening, in which case I continue antibiotics and supportive care."

Early detection of fever or illness allows the horse to be isolated before exposing others. Traub-Dargatz urges owners to keep ill horses as far from unexposed horses as possible, preferably in a separate pen or barn. "Based on field experience, it does not appear that S. equi is spread very far in the air," says Traub-Dargatz. "For example, it would be unlikely to spread from one barn aisleway to another if the walls are high or to the ceiling between the two aisleways. Experience at the racetrack has shown that spread from one side of the shed row to the other side did not occur as long as movement of horses and people is stopped between those two sites."

She advises designating isolation boundaries clearly with signs and specific directions on infection control protocols, in both English and Spanish when appropriate.

Farm managers should have an established plan that prevents contact between horses in isolation and others on the farm, not only by physical separation, but also by considering anything or anyone that might move between the groups of horses. Traub-Dargatz emphasizes that people (caretakers, veterinarians, farriers, trainers, and suppliers) and the things that move with people, such as equipment and vehicles, can be responsible for spreading infection between groups of horses on a farm and between farms. Facility design should route people, horses, vehicles, bedding, and contaminated materials as efficiently as possible, with limited crossover between groups of horses that are maintained in separate groups for infection control purposes.

Traub-Dargatz realizes that not all facilities and schedules allow isolation of individual animals. "Perhaps horses can be grouped by a category of contagious disease risk, as, for example, by age, foaling date, or mobility on or off the farm," she says. "Horses that are kept together will have equal exposure, but by keeping the number of individuals per group as small as possible, the impact of a strangles outbreak might be minimized."

Proactive Measures

To prevent a strangles outbreak on a farm, only those horses with a veterinary health inspection certificate--issued within the immediate time period prior to moving the horse--should be allowed onto the farm. Many states allow a 30-day window from when the exam is done and the health certificate is issued and when the horse actually travels--this is problematic, since it exceeds the incubation time of many strangles infections, so it may nullify the value of a "health" certificate.

"The closer the date the physical inspection occurs to the time the horse is to travel to a farm or an event, the better," says Traub-Dargatz. "It is wise to insist on a health certificate for entry of all visiting and new resident horses.

"The NAHMS (National Animal Health Monitoring System) Equine 2005 study results suggest that very few operations ask about previous strangles history or test newly introduced horses for shedding of strangles, yet screening is an extremely effective method to control spread from new arrivals," she says.

For results of this study, please refer to screening tests for strangles in this document:

Using these screening tools to limit exposure would be invaluable in protecting resident horses on a farm. Obtaining strangles history on an incoming horse requires simple communication, which might prevent an outbreak.

All arriving horses should be examined and, if feasible, isolated, and they should be monitored for signs of disease for a few weeks after arrival. One primary tactic is to refrain from allowing new horses to contact others on the farm until isolated for at least two to three weeks. Caretakers monitor these horses carefully by taking rectal temperatures twice daily and inspecting daily for signs of disease (such as fever, cough, and nasal discharge). All findings should be recorded in a log. Horses returning from events that allowed commingling with outside horses also pose a risk to resident horses, and proactive measures should be taken to ensure this risk is minimized.

"Strep (S. equi) is very contagious," states Traub-Dargatz. "It causes infection in the upper airway and in lymph nodes around the head and neck, but can become more serious and systemic, forming internal abscesses (bastard strangles)."

Good horsekeeping strategies rely on knowing each individual animal within a herd, and knowing how each horse appears when robust and healthy, so you can identify when the horse is out of sorts. The health of a farm depends on excellent handlers who readily recognize subtle signs of malaise, lethargy, and loss of appetite.

Vaccination Protocols

For many viral respiratory diseases immunization is one useful method for prevention and control. But for strangles, Holland stresses that vaccination (even when given per manufacturer's recommendations) cannot give complete assurance that a horse won't contract the disease.

"Strangles vaccines reduce severity of signs by about 60-70%, but none of the available vaccines will create sterile immunity," he advises.

He stresses that excellent management--in conjunction with screening and vaccines--significantly decreases risk.

Holland discusses vaccination rationale: "If a horse is going to travel to an area where risk is elevated, then vaccination may be warranted, following protocols of manufacturer's label instructions. The horse should be fully vaccinated at least one month prior to expected exposure. But, if it is believed that risk is not that great, then surveillance and management can be effective in limiting disease. However, keep in mind that it may be difficult to determine which horses in a herd are incubating disease, and which have not yet been infected."

In the face of an outbreak, researchers have demonstrated that vaccinating nonsick animals might reduce the rate of illness by half. However, this strategy could have limited usefulness in horses that have never been vaccinated against strangles. To ensure the maximum protective effect, the nonsick horses must receive the full protocol of two vaccines spaced two to three weeks apart. Sufficient immunity might not be present in time to protect against exposure during an outbreak. Horses on a prior strangles vaccine program can be "boostered" with one dose of vaccine, and this should stimulate some immunity to limit the severity of the infection.

However, Holland cautions that vaccination in the face of an active outbreak can be dangerous, since infected horses show such a wide range of signs, as noted earlier. "If you vaccinate an infected horse thinking the horse is negative for strangles infection, or if the horse is an asymp­tomatic carrier, problems can arise," he says. "If a horse has high antibody levels to S. equi, the vaccine may cause the horse to react back on itself, leading to purpura hemorrhagica (overzealous immune response to S. equi by the horse's body that results in pinpoint hemorrhages along the mucous membranes as well as areas of firm, pitting edema on the lower abdomen and lower limbs). For a horse with no outward signs, or one that is an asymptomatic carrier, you may end up vaccinating the horse with the problem. For this reason be careful and call your veterinarian to help you vaccinate."

Most reactions to strangles vaccines result from the horse responding to the antigen; typically it is a hard-core response to the M-protein of the strangles organism, not the adjuvant (carrier) in the vaccine. Once a horse has had an adverse reaction, Holland cautions that he is likely to do it again. "Any foreign protein or vaccine can cause a reaction, not just strangles vaccines," he says. "When this happens, advise your veterinarian so he or she can contact the pharmaceutical company to ensure there are no problems with a specific lot of vaccine."

Other vaccination concerns exist. It might be inappropriate to handle and immunize nonsick horses in a herd during an outbreak, for fear of spreading disease from those that are shedding bacteria to those not yet exposed. And, horses that have high antibody titers from a previous infection or vaccination need not be immunized again.

"If a horse has gotten over strangles in the past year, don't vaccinate," Holland advises. "If worried about which way to go, have an antibody test run for M-protein before vaccinating, or wait a year before vaccinating. If sensitized for M-protein through active infection, then vaccination may elicit problems with purpura hemorrhagica.

"In nonstrangles herds purpura occurs at rates of one in 100,000," he states. "In herds with active strangles, there is an increase in incidence of purpura following vaccination. We know the ability of an animal to test positive for Strep increases the chance of developing purpura since the vaccine may overwhelm the horse's immune system."

On endemic farms vaccination of mares is warranted. Inquisitive foals readily come into oral contact with many surfaces as they investigate their environments, especially if fed in a common bucket or creep feeder. This disease usually hits weanlings the most, since during the first two to three months, foals born to vaccinated mares have some protection from maternal antibodies obtained in colostrum. Holland notes that young horses often become infected from an asymptomatic carrier mare that has not been tested or screened.

Once recovered from strangles infection, 75% of horses develop a relatively lasting immunity of one to two years. Holland stresses, "There are rumors that a horse that has had strangles will never get it again, but that's not true--in about two to three years a horse is completely susceptible again. But for at least a year the horse has some degree of immunity, so after a case recovers it is best to wait at least a year to vaccinate."

Because protective immunity is expressed in the tonsil, an intranasal vaccine facilitates immune response. However, according to company data, 40% of horses vaccinated with the available intranasal vaccine (Pinnacle I.N. by Fort Dodge) that are challenged with the organism still develop clinical signs of disease. But note that 60% of unvaccinated horses that are challenged with the organism get the disease. Extent of illness is decreased with vaccination, and of those that do get sick, clinical signs are reduced by 65% as compared to unvaccinated horses.

Another important point Holland makes is that intranasal vaccination success is dependent on accuracy of administration. He says, "The intranasal applicators are lengthy for a reason. The ciliated hairs within the first six inches of the nose tend to beat material out of the nose, while beyond the first six inches, these ciliated hairs beat upward toward the back of the throat. This has significant ramifications for both repelling bacterial contact, and for efficacy in use of intranasal vaccines--efficacy of the vaccine is dependent on correct administration and skill of the veterinarian in placing the applicator far enough into the horse's nose."

Future Developments

Holland says currently researchers are studying the mucosal vaccine route to improve quality and length of immunity. He notes that in Europe there is an upper lip strangles vaccine that might have future potential in the United States. Of special significance is the work in progress to develop a diagnostic blood test that will allow veterinarians to detect guttural pouch carriers. Disease experts remain hopeful that this test will be available for stall-side use for immediate results. Currently all that is available on blood sampling is to test for M-protein titers, but this still confounds efforts at control, since in an outbreak all horses probably have high blood titers, not just the carriers.

Take-Home Message

It's crucial to prevent the introduction and spread of contagious diseases such as strangles. The likelihood of exposure increases for highly mobile populations of horses, so it is especially important to remain vigilant in handling prevention and control measures in these groups. But, not every case can be prevented. Consider all means of contagious disease spread and routes of exposure on a farm, and apply control programs as they are feasible. Strangles is a three-pronged disease: there are horses with fever prior to developing lymph node enlargement, there are cases of full-blown infection with clinical signs, and there are carriers. Recognize any horse that has been exposed to strangles as a potential carrier of S. equi bacteria within the guttural pouches and sinuses. Implement screening and diagnostic testing protocols to identify potential shedders that pose a risk to others within a herd, and screen incoming horses before admitting them to a herd.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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