What To Know About Strangles

Published: August 13, 2020 06:54 pm EDT

Dr. Alison Moore, Lead Veterinarian for Animal Health and Welfare at the Ontario Ministry of Agriculture and Food, has penned the following Q & A feature pertaining to equine strangles.

Michael Copeland

The following are general questions and answers related to strangles in the racing industry. Always consult your veterinarian if you have or suspect an infectious disease and follow their advice.

What is strangles?

Strangles is an upper respiratory disease caused by a bacterium called Streptococcus equi. It is NOT a virus. This is important because it is not spread through aerosol like respiratory viruses. It is spread through nose-to-nose contact between horses and from contact with contaminated surfaces (e.g stall walls, fencing, fur of barn cats and dogs)/equipment (e.g. twitches, lead shanks, buckets)/clothing and hands. It takes between 3 to 14 days from the day the horse is infected to show clinical signs.

Strangles is frequently diagnosed in Ontario and throughout the year. It is RARELY diagnosed in the racing industry. When it has been diagnosed in the racing industry over the years, it has usually been introduced by an infected horse from another jurisdiction, from auction (companion ponies/miniature horses bought at auction can be carriers) or returning from breaking/training.

What are the signs of strangles?

Classical signs include the development of abscesses under and/or behind the jaw that break open and drain pus. Some abscesses can be quite large and cause difficulty breathing or swallowing.

Not all horses develop abscess however, some only have nasal discharge.
Very rarely, horses can develop abscesses in other areas such as the abdomen and brain and may develop immune-mediated diseases affecting the skin or muscle. Although very rare, these consequences can be fatal.

A fever occurs before any clinical signs of infection are seen but it is often missed by the time the first case is noted. If a case is diagnosed, it is important, therefore, to take temperatures twice daily on any horses exposed to the infected horse. If a fever is detected, the horse should be isolated as there is a brief window (1-2 days) before the horse starts shedding the bacteria from the nose and contaminating the environment and infecting other horses.

Why do we have to take temperatures twice daily?

Fevers develop within cycles. Some horses may develop fevers in the morning, some in the afternoon and some have fevers consistent throughout the day. To maximize detecting a fever twice-a-day temperature taking is recommended. Local veterinarians have found the microchip temperature readings to be unreliable as they differ from the rectal temperature by 1 to 2 degrees.

How do you know if a horse has been exposed?

Any horse that has had nose-to nose contact with the infected horse, has used the same trailer (without cleaning and disinfecting), shared the same paddock, shared the same equipment and/or had the same staff/service suppliers in contact with them in the previous week would be considered exposed. This timeline may vary depending on the clinical signs of the horse and should be discussed with your veterinarian.

How is strangles diagnosed?

There are a few ways to diagnose strangles. The quickest and most sensitive way is by testing for the bacterial DNA through a test called a polymerase chain reaction or PCR test. You veterinarian will ideally do a nasopharyngeal wash which involves putting 30-60cc of warm saline up both nostrils to hit the back of the throat, collecting the fluid that comes back out and sending it off to the lab. Turn around time is 24-48 hours unless it’s over a weekend. Bacterial culture of the pus may be the preferred method in some situations or done in addition to PCR. This will take longer to get results though (up to a week). Any horse suspected of strangles should be isolated until proven otherwise.

My horse tested positive for strangles but has not shown any clinical signs. How can that be?

Some horses are tested as part of an outbreak to find horses shedding the bacterium. Sometimes a horse will be positive on PCR testing but is clinically normal. There are a few explanations for this.

  1. The horse is early in the disease process and may go on to develop clinical signs. The fever may have been missed.

  2. The horse was exposed to a low dose of the bacterium and is presently trying to clear it from its system

  3. The horse had a snotty nose that wasn’t identified as strangles and, although the signs have gone, the horse is still shedding the bacteria.

  4. The horse is a carrier. These are horses that at one time had strangles but didn’t completely clear the bacterium from their system. They can shed the bacterium on and off for months to years in rare cases. If you have had the horse for a while or it has been consistently racing the risk of the horse being is a carrier is low however it is worth discussing the possibility with your veterinarian.

How is strangles treated?

Typically, veterinarians don’t treat strangles once the abscesses are formed. Treatment with antibiotics at this time can delay maturation of the abscess and prolong the disease. If the abscesses are impairing breathing or swallowing, however, antibiotics are used as an emergency treatment. In some outbreak situations, and only in cases of exceptional biosecurity practices on the farm, veterinarians may treat horses that develop a fever with antibiotics prior to abscess development to stop the spread. Horses do not develop a strong immunity to the bacteria though and, if placed back into a contaminated environment, can become re-infected with the bacterium.

What biosecurity practices should be used?

  1. Isolation – any horse diagnosed with strangles should be isolated from other horses, ideally in a separate and empty barn. If no empty barn is available, the horse should be isolated away from high traffic areas. Horses should NOT leave the isolation barn to jog. They are in there because they are shedding the bacterium and pose a risk to contaminating the outside environment. Horses can be hand walked in a dedicated area not used by other horses or people if necessary. Discuss appropriate places with your veterinarian first.

    Any horse that has been exposed to the infected horse that develops a fever should also be isolated.

    If dealing with an outbreak, colour coding the horses has proven useful. Place red tape on the halters/stalls of those with clinical signs (in isolation), yellow tape for exposed horses and green tape for those not-exposed or showing signs. When mucking, feeding, jogging it is helpful to have staff dedicated to the different colours (one group works with the green horses, another with the yellow and another with the red). If you don’t have staff, then manage the green horses first then yellow then red. Essentially you are going from “clean” horses to “dirty”. This will help minimize spread of the infection in the barn.

  2. Clothing, equipment and supplies – for horses in isolation, dedicated equipment (wheelbarrows, shovels, forks, lead shanks, halters) should all stay in the isolation area and NOT leave. Put red electrical tape to mark them if necessary. People mucking, feeding and treating horses in isolation should wear dedicated foot wear (or use foot baths), gloves and coveralls (old training/driving suits work) which should not leave the isolation area. Change right before going in and change before coming out of the area. Staff handling these horses should not handle other (healthy) horses or should do them at the end of the day prior to going home. They should make sure they have a clean set of clothes and footwear and washed their hands prior to handling other horses. Don’t forget to clean the coveralls when the isolation period is over.

    Hay and grain for these horses should be located in the isolation area to minimize contact with the rest of the stable.

  3. Cleaning and disinfecting – All contaminated surfaces and equipment (stalls that infected horses have been in and any equipment they have touched e.g. trailers, bits, bridles if recently jogging) should be cleaned first with soap and water to remove debris and then disinfected. Typical disinfectants include Virkon and Prevail. Bleach is broken down by organic debris (manure, pus etc.) so is not ideal in this situation unless areas are thoroughly cleaned first. If you are using foot baths instead of a change of footwear in isolation, the baths must be changed when dirty or, at a minimum, daily.

  4. Paddocks – any paddocks used by an infected horse should be closed for 28 days to allow sunlight and dry weather to destroy the bacteria.

  5. Signage – people should know that an area contains a horse with an infectious disease and they should not enter so “do not enter” or similar signs should be posted. Service providers (hay and feed deliverers, veterinarians, farriers etc.) should be made aware of restricted areas.

When is it safe to allow a horse to leave isolation?
Horses should not leave isolation until they have tested negative for the bacterium. Testing should not start until around 30 days after the abscesses have healed. Healing of abscesses is not an indication that the bacterium is gone. Since the bacterium sets up shop in the guttural pouches (very rarely it hides in the sinuses), horses should either have three negative PCR tests by nasopharyngeal wash one week apart OR have a guttural pouch sample taken via endoscope (both pouches done) with a nasopharyngeal wash (to account for contamination of the sinus). The sample can be pooled and submitted as a single sample. These steps to have a negative test are necessary as 10% of horses infected with strangles can become carriers and carriers keep the infection going in the industry.

What if my horse stays positive on testing?

If your horse stays positive on testing, then the guttural pouches should be visualized via endoscopy. Any chondroliths (pebbles of pus) should be removed and the pouch treated with penicillin gel and testing repeated. Sometimes your veterinarian will also do a bacterial culture on samples from horses who repeatedly test positive as the PCR detects only DNA not live bacteria – meaning dead bacteria can also cause a positive test. This would be done after the pouches have been assessed.

Should we vaccinate for strangles?

It is recommended that horses who have been exposed to a horse with strangles should not be vaccinated as there is a risk of developing a serious immune mediated condition. If you have a horse on the same property but far away from the infected horse, you should discuss this pros and cons of vaccination with your veterinarian.

Other resources:



The views presented in Trot Blogs are those of the author and do not necessarily represent those of Standardbred Canada.

Have something to say about this? Log in or create an account to post a comment.