How to reduce the risk, recognize the symptoms, and take steps to manage an outbreak.

Equine Guelph

Strangles is a highly contagious infection caused by the bacterium, Streptococcus equi (S. equi). It is not an airborne virus. Rather, the bacteria spreads through contact, which could be direct nose-to-nose between horses, or via contaminated surfaces or equipment such as shared halters, lead shanks, cross-ties, feed tubs, stall walls, fencing, clothing, hands, the hair coat from other barn pets, grooming tools, water buckets, and communal troughs.

Although rarely fatal, strangles is a respiratory disease that can make your horse miserable for a few weeks, and recovering horses can shed the bacteria for four weeks or longer. Three to fourteen days is the typical length of time from infection to the manifestation of clinical sign. Swift control measures will help minimize transmission, reduce associated medical expenses, and decrease the duration of facility “lockdown.”

Signs of Strangles

Severe inflammation of the mucosa of the head and throat, often with extensive swelling and rupture of the lymph nodes below and behind the jaw, which can produce large amounts of thick, creamy pus, are classic signs of strangles.

Related: How Infectious Diseases Spread Among Horses

Still, not all horses will develop enlarged and abscessed lymph nodes. Some will present with only slight nasal discharge, while others may be lethargic, lose their appetite, and have difficulty breathing and swallowing due to lymph node enlargement around the throat area. In very rare circumstances, “bastard strangles” may develop with abscesses in the abdomen, chest or brain; others may develop immune-mediated diseases of skin or muscle.

A purulent nasal discharge, which may initially be clear, is a typical symptom of strangles. Once strangles is confirmed, the horse should be isolated for at least four weeks AFTER the resolution of clinical signs.

Fever is an initial sign of strangles, and the beginning of a brief window of one or two days before the horse starts shedding the bacteria from the nose, contaminating the environment and infecting other horses. Swift isolation and control measures are important.

Prevention

Good biosecurity practices should be employed both on and off your property. On the farm, these include establishing a two- to three-week quarantine period for new horses arriving, and screening horses for S. equi before or during quarantine after discussing with your veterinarian. Horses that travel should be grouped together in the same barn and paddocks, and equipment (leads, halters, tack, grooming) should not be shared between groups. Use a visitors’ log as part of your access management plan and implement a clean clothes and clean footwear policy for people accessing your facility. Use foot baths to mitigate risk. 

To curb the spread of strangles, good hygiene is essential, including hand washing with soap immediately after handling horses and their gear. Photo: Shutterstock/Pic by Femke

Off the property, employ trusted haulers that clean and disinfect their trailers between loads. On arrival at your destination, disinfect your stall before use focusing on high contact areas such as the bars on stall doors and windows. On the grounds, refrain from grazing your horse in communal areas and sharing water troughs. Avoid handling horses that are not in your group and do not let horses have nose-to-nose contact with other horses. Take temperatures and monitor horse health while away and after return.

Related: Sources of Infectious Disease in Horses

Strangles On Your Farm? Here’s What To Do

Immediately isolate horses with signs of strangles. Infected horses should remain in isolation until your veterinarian determines they are no longer infected. Testing should not occur until 30 days after the abscesses have healed, or other clinical signs have resolved.

Stop all horse movement on and off the premises and limit human traffic.

Set up red, yellow, and green zones and coordinate taping of halters, stalls, and equipment with matching colour tape or markers to avoid cross-contamination: red (isolated/infected), yellow (exposed/monitor), green (not exposed). Limit traffic and keep all but essential personnel out of red zones. Put up signs and clearly communicate the infection control measures you are implementing with your staff, service providers, customers, and community. 

The bacteria can spread via contaminated equipment such as shared feed tubs, halters, grooming equipment, stall walls, fencing, and even on the hair coat of other barn pets. Photo: Rita Kochmarjova

Monitor diligently. Take temperatures of all horses in the yellow zone twice a day and monitor for signs of infection. Fevers can be cyclical; some horses may develop fevers in the morning, while others may present with a fever in the afternoon.

Related: Getting Ahead of Strangles

Cleaning and disinfect. Stalls, trailers, and other areas that have contained horses with strangles should be cleaned of organic debris (manure, discharges, etc.) before being disinfected, as organic debris can deactivate disinfectants, particularly bleach. Prevail™ or Virkon® are the most common products used. It is important to make sure the disinfectant remains on the surface for the appropriate contact time as described on the label.

Practice good hygiene, paying particular attention to hand hygiene. Have staff dedicated to the different zones/horses (one for green, one for yellow, and one for red) but if that is not possible, green horses should be handled first, then yellow, then red, with staff changing clothes and boots or using foot baths before switching zones. 

If foot baths are used, boots should be cleaned first with a scrub brush and detergent. Boots should be in contact with the disinfectant for the appropriate contact time listed on the product label. Boot baths should be changed as soon as they appear dirty and at least daily. Common disinfectants include Prevail™, Virkon®, and diluted bleach (1:10 with water). Bleach is particularly deactivated by organic debris so boots etc. must be clean prior to contact. An alternative to a foot bath is to use dedicated rubber boots, which should remain in the zone in which they are used. 

If using foot baths as an infection control measure, boots must first be cleaned with a scrub brush and detergent, as organic debris can deactivate bleach. Photo: Shutterstock/Little Miss

Related: Blood Tests for Life

Use dedicated equipment (pitch forks, shavings forks, wheelbarrows, etc.) for each zone when possible. Coloured electrical tape on the handles can be used to indicate the appropriate zone. If this is not possible, clean and disinfect (using the appropriate contact time) between zones (going from clean to dirty or green to red). S. equi can survive in cold water for weeks and is easily transmitted in water, so don’t used shared water sources. Avoid submerging the end of a water hose in water buckets as you fill them to avoid cross contamination.

Curtail environmental survival. S. equi can survive well in certain conditions such as wet and cold conditions. One study found the bacteria can linger in the webbing of polyester halters unless exposed to washing at a scorching 60 degrees C! Rest paddocks/pastures that have housed infected horses for up to four weeks (depending on if it’s a grazing paddock or exercise paddock) to allow sunlight and dry weather to destroy the bacteria.

In a recent study, polyester halters underwent testing to evaluate cleaning protocols after contamination with S. equi. The study confirmed that unless exposed to washing at 60 degrees C (140 degrees F), bacteria can linger in the webbing of polyester halters. On the other hand, even without rigorous cleaning, leather appears to poorly support survival of S. equi. Photo: Alamy/Layne Naylor

Consult your veterinarian before administering any medication. Your vet is an excellent resource to help you with infection control measures and provide advice on testing so you can get your life back to normal as quickly as possible. 

Related: Sickness Prevention in Horses

How and When to Test for Strangles, and How to Detect Carriers

A veterinarian examining a horse that is suspected of having strangles will usually test the horse in one of the following ways. If there is pus draining from the nose or abscessed lymph nodes, they may take a direct swab for S. equi polymerase chain reaction (PCR) (DNA) testing and/or bacterial culture. If the lymph node is large but hasn’t ruptured, the veterinarian may use a needle to aspirate the pus from the lymph node for testing. If there is no nasal discharge or the abscesses haven’t started draining, they may take a sample from the back of the throat using a very long swab or inserting tubing and infusing fluid to wash the area and catch it as it comes out the nose. Some horses will have to be sedated for these latter procedures. The sample from the back of the throat is generally sent for PCR testing with or without bacterial culture.

As the bacteria can survive in cold water for weeks and is easily transmitted in water, shared water can be a source of strangles. Even submerging the end of a water hose in water buckets can spread contamination. Photo: Shutterstock/Katrin-ps 

Once strangles is confirmed, the horse should be isolated for at least four weeks AFTER the resolution of clinical signs. If the horse is tested too soon after clinical signs have resolved, it is more likely to be positive and increase the costs associated with repeated testing. After this time, it is recommended to test the horse to determine that it has cleared the infection.

The 2018 American College of Veterinary Internal Medicine (ACVIM) Consensus Statement on S. equi infection provides the following science-informed recommendations for determining that a horse is negative for S. equi infection: three negative PCR-tested nasopharyngeal swab/lavages one week apart, or a single negative PCR-tested, pooled guttural pouch lavage. Some veterinarians are using a combined guttural pouch/nasopharyngeal lavage, which is also appropriate.

Related: Diseases with Horse-to-Human Transmission

Carrier horses have had strangles at one point in time and never cleared the infection from their guttural pouches. These horses show no clinical signs of infection and are thought to be significant contributors to the spread of strangles. They generally shed low numbers of bacteria from the nose and may do this intermittently. It is likely that some factors, such as environmental stress, cause these horses to shed more bacteria at certain times leading to an infected horse appearing in the herd. The dynamics of S. equi shedding in carrier horses is still unclear, and it is possible for these carrier horses to live in herds for months or years without a strangles outbreak occurring. The only way to detect these carrier horses is through testing. PCR testing of guttural pouch lavages is the most sensitive way to detect carrier horses. PCR testing of nasopharyngeal swabs/lavages can also be used but may miss horses as it is not as sensitive a technique. Carrier horses should have their guttural pouches evaluated by endoscopy to determine if there are chondroids present (pebbles of pus) and/or pus which would need to be removed. Most often the guttural pouches need to be treated directly with antibiotics to clear the infection.

Endoscopy is used to check guttural pouches of a suspected carrier horse for chondroids. Photo courtesy of Equine Guelph

Testing horses for S. equi should only occur after a comprehensive discussion with the veterinarian about what a positive test will mean for the horse and the facility, and considering the following:

• The availability of isolation areas for positive horses;
• Cost of testing and the possibility for repeated tests and treatment;
• The risk assessment — Is there a significant risk of exposure to S. equi to require testing?
• The risk generally for S. equi infection for the horses on the premises; for example, if they compete; whether they are vaccinated; if you test all horses on the property and clean up any S. equi positives, are you prepared to then test every new horse coming on the property; and your policy about competition horses that come and go.

Should We Vaccinate for Strangles?

It is not advisable to vaccinate horses for strangles during an outbreak due to the risk of purpura hemorrhagica (an immune-mediated inflammation of the blood vessels in the skin) and confounding signs secondary to vaccination. An exception might be to do “ring vaccination” on large facilities where some barns are very far away from the affected barn with no possibility of contact. Your veterinarian will help you determine if this is an appropriate decision.

Generally, vaccinating for strangles will reduce the severity of clinical signs if your horse becomes infected with S. equi. It may also limit shedding of the bacterium. There are two forms of vaccine available; an injectable (killed vaccine) and intranasal (modified live vaccine).

Very rarely, horses which have been exposed to S. equi previously and vaccinated with either vaccine may develop purpura hemorrhagica. Performing a streptococcus equi M protein (SeM) antibody titre before vaccination can identify horses at risk of developing this complication. As well, some horses vaccinated with the intranasal vaccine may, in rare circumstances, show mild lymph node swelling after vaccination. The intranasal strangles vaccine should not be administered at the same time as other injections as there is a potential for contamination of those injection sites and subsequent abscessation. After intranasal vaccination, horses may test positive for S. equi by PCR testing for up to 60 days post vaccination.

Related: What Are You Vaccinating Your Horse Against?

Related: Biosecurity on the Horse Farm

Resources:

• Ontario Animal Health Network, Equine Strangles Resources
• Resources and an interactive Biosecurity Risk Calculator
• More on access management in Section 7 of the National Biosecurity Standard for the Equine Sector

Published with the kind permission of Equine Guelph.

Main Photo: Dreamstime/Zuzana Tillerova