Equine strangles infection (caused by Streptococcus equi sp. equi bacteria) on a horse farm can cause big headaches for barn managers, horse owners and trainers. An infected property is typically quarantined to prevent spread to outside horses. This means that many horses won’t be able to participate in clinics and events while others suffer through the sickness.
The incubation period for strangles ranges from three to 14 days. A horse usually develops fever a day or two prior to lymph node enlargement or shedding of large numbers of the organism. S. equi is highly contagious, causing infection in the upper airway and in lymph nodes around the head and neck. There is a potential for it to become more serious and metastasize to form internal abscesses, called bastard strangles.
A horse can be exposed to S. equi bacteria by nose-to-nose (nasal) contact and/or shared water supplies (oral). Oral contact—in water or on bits, lip chains or twitches—is a common avenue for spread. Even a water hose contaminated with bacteria inoculates water with infective material (hoses become contaminated when handled with bacterial-contaminated hands or from making contact with a bucket from an infected horse). S. equi obtained through the oral route goes directly to the tonsillar region.
S. equi bacteria survive most efficiently in the horse, but bacteria can also survive in the environment. The bacteria persist better in water compared to soil, lasting up to 30-40 days in water in contaminated tanks. Outbreaks occur because of environmental persistence with survival for 34-72 days.
For example, S. equi bacteria can last for three days in soil, seven days on a fence post, and for considerable periods on webbed halters, even when cleaned and sanitized. The cooler the temperature, the longer the survival, such as up to 34 days in a wet bucket in winter. However, summer ambient temperatures and direct solar radiation can reduce survival on wood, metal or rubber surfaces to one to three days, and typically no more than nine days. Bacteria shielded in pus persist for longer than if contained in a drop of clear watery nasal discharge. In wintertime, pus and discharges that freeze can remain infective to others when the ground thaws.
The Hidden Reservoir
It is not uncommon for strangles infections to recur on a farm. It helps to know how this disease survives in a population of horses. Not only do the bacteria remain in the environment, but even a seemingly recovered horse can be a carrier for infection, harboring S. equi within the guttural pouches.
Bacteria enter the guttural pouch 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, a site where S. equi can persist.
Shedding of large numbers of bacteria from a horse’s nose occurs periodically or intermittently, even if the horse appears perfectly normal. In rare cases, bacteria live in a sinus in the horse’s head. Up to 10% of horses develop persistent infection and can shed for 30 days after recovery from illness. On average, farms with endemic, recurring strangles infections have a carrier reservoir in 20% of the horses, ranging from 4-50%.
The average period of carrier shedding is 9.2 months, with reports of up to 42 months. 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, veterinarians should screen these horses for shedding. You can only identify horses shedding strangles bacteria with certainty through diagnostic testing by collection of samples from either a nasopharyngeal swab or lavage.
The American College of Veterinary Internal Medicine (ACVIM) has formulated useful protocols to help veterinarians and their clients control strangles.
Testing Methods for Equine Strangles
Veterinarians can culture purulent nasal discharge or abscess drainage using a nasopharyngeal swab. For “recovered” horses with no active abscess or nasal discharge, lavage of the nasopharyngeal area collects fluid that has contacted many areas of the pharynx and both nasal passages, thereby yielding more reliable results.
PCR is also an effective testing method for strangles. A qPCR test detects DNA from both living and dead S. equi bacteria. It is up to three times more sensitive than bacterial culture. Take caution to avoid a false positive result that comes from dead DNA on a twitch; it is best to use a lip chain (soaked in chlorhexidine antiseptic) over the upper gum.
A horse is not considered a carrier once it has received three negative qPCR and culture results for S. equi over a two- to three-week period. Nasopharyngeal bacterial cultures detect 60% of carrier horses, but combining cultures with qPCR testing increases detection of carriers to 90%. Another expedient screening method visually inspects guttural pouches with an endoscope along with lavage of that site to submit samples for qPCR testing.
No blood test is available to confirm the presence of carriers. An ELISA blood test checks for systemic levels of M-protein antibodies, but it is not useful to determine a horse’s degree of protection against the disease. It only demonstrates that a horse has contacted S. equi or has been recently immunized.
Strangles Prevention Strategies
Management strategies are critical for protecting horses from exposure to strangles and preventing an outbreak. If a horse is showing any signs of strangles—fever, nasal discharge, lymph node enlargement—immediately isolate it from others. Diagnostic testing helps to identify the specific infectious disease. Antibiotics given before lymph nodes enlarge might mitigate the extent of the disease. However, veterinarians should not prescribe “preventive” antibiotics to potentially exposed horses.
Isolate Exposed Horses
Successful implementation of biosecurity practices relies on early detection of fever or illness. Separate ill horses as far as possible from unexposed horses in a separate pen or barn. S. equi does not appear to spread very far in the air.
Farm managers should establish a plan that prevents contact between isolated and unexposed horses on the farm. Physical separation alone is not sufficient; the plan should also account for people and equipment moving between horses. Caretakers, veterinarians, farriers and trainers, as well as equipment such as wheelbarrows, pitchforks, hose nozzles and vehicles, can be sources of infection between groups of horses on a farm and between farms. Designate isolation boundaries clearly with signs and specific directions on infection control protocols. Facility design should route people, horses, vehicles, bedding and contaminated materials as efficiently as possible with limited crossover between separated groups of horses.
Group Horses by Disease Risk
Not all facilities and schedules can isolate individual animals practically. Instead, facility owners should group horses by category of contagious disease risk, such as age, foaling date and mobility on or off the farm. Keeping the number of horses per group as small as possible helps mitigate the impact of a strangles outbreak.
Know Your Herd
Good horse-keeping strategies rely on knowing each individual animal within a herd. Caretakers should know how each horse appears when robust and healthy. Then, it is easier to identify a horse when it first becomes sick. The health of a farm depends on competent handlers who readily recognize subtle signs of malaise, lethargy and loss of appetite.
Remain Diligent at Horse Shows
At equine events, keep horses under close supervision. Refrain from sharing tack, equipment, feed and water buckets to protect against oral exposure, which is more likely than the respiratory route. For a short event in good weather, leave a horse tied to the trailer rather than using shared stalls that could be contaminated. Good hygiene is important, including thorough cleaning of horse trailers between use.
For many viral respiratory diseases, immunization is useful for control. But for strangles, a recommended vaccination protocol cannot give complete assurance of disease protection. Strangles vaccines reduce severity of signs by 60-70%. To control this disease, management is essential, especially when used in conjunction with screening and vaccines.
If a horse is going to travel to an area with an elevated risk of exposure, then vaccination might be warranted. The protocol would involve two injections spaced three weeks apart. Full immunization should be accomplished at least three to four weeks prior to expected exposure. The vaccine is boosted annually. Vaccination against S. equi is associated with a lower frequency of S. equi qPCR-positive cases.
Vaccination During an Active Outbreak
In the face of an active outbreak of strangles, vaccination of seemingly healthy horses is not without risk, especially if a horse is an asymptomatic carrier or is incubating disease. High antibody levels of S. equi can elicit a reaction to the vaccine, leading to purpura hemorrhagica. Only horses that have not yet been exposed to strangles should be immunized with the live intranasal vaccine during an outbreak. Vaccinating non-sick animals can halve the rate of illness. However, this strategy works best if naïve horses receive the full two-injection protocol in ample time prior to exposure. This might not be possible in an outbreak. Horses with a previous strangles vaccine history can be “boosted” with a single dose to stimulate immunity that limits severity of the infection.
If a horse has recovered from strangles in the past year, it is likely sensitized to M-protein. So, owners do not need to vaccinate. Once recovered from strangles infection, 75% of horses develop relatively lasting immunity for about one to two years. After a case recovers, it is best to wait at least a year to vaccinate.
Additional concerns about vaccinating against strangles exist. Contamination of remote injection sites with the attenuated, modified live intranasal strangles vaccine strain can create local abscesses. Strangles should be the last vaccine veterinarians administer on a visit, or better yet, the only vaccine (or injection) given that day.
Protecting Mares and Foals
On farms endemic for strangles, owners should immunize the mares. Foals readily come into oral contact with environmental surfaces as they investigate their environments, especially if fed from a common feeder. Weanlings are most at risk since during the first two to three months, foals born to vaccinated mares have some protection from maternal antibodies obtained in colostrum. Young horses often become infected from an asymptomatic carrier mare that has not been screened.
Because protective immunity is expressed in the tonsil, an intranasal vaccine facilitates response in mucosal tissues and tonsils. However, despite “protection” derived from the intranasal vaccine (Pinnacle I.N. by Zoetis), 40% of horses challenged with S. equi still develop clinical signs of disease compared to 60% of unvaccinated horses challenged with the organism. The extent of illness is decreased in vaccinated horses. Of those that get sick, clinical signs are reduced by 65% compared to unvaccinated horses. Just as with the intramuscular strangles vaccines, no horse can receive complete protection with the intra-nasal strangles vaccine when challenged.
Intranasal vaccination efficacy relies on accuracy and skill of administration. Veterinarians administer the vaccine with a nose applicator that reaches beyond six inches, where ciliated hairs beat upward toward the back of the throat. In comparison, material deposited within the first six inches of the nostrils is directed out of the nose by cilia. This technique has significant ramifications for repelling disease exposure.
Prevention against the introduction and spread of contagious diseases like strangles is important for a highly mobile population of horses. However, not every case can be prevented. Veterinarians should consider all means of contagious disease spread and routes of exposure on a farm. Then, they can recommend strategic biosecurity programs to owners accordingly.
To improve biosecurity measures as a veterinarian provider, change clothing and shoes between visits to different horses and different stables. It is also important to follow diligent hand-washing and equipment sanitization protocols and to implement other biosecurity measures.
Strangles is a three-pronged disease: a) horses develop fever prior to lymph node enlargement; b) full-blown infection is associated with clinical signs; and c) a potential carrier state persists within the guttural pouches. Veterinarians should implement screening and diagnostic testing protocols to identify potential shedders that pose a risk to others within a herd. Facility managers should screen incoming horses before admitting them to a herd. They should then isolate and monitor newcomers for two to three weeks.