Strangles in Horses

Updated July 18, 2024 | By: Dr. Lydia Gray

What is Strangles in Horses?

Strangles is one of the common terms for the upper respiratory infection in horses caused by the bacteria Streptococcus equi subsp equi (or Strep equi for short). It is also sometimes referred to as equine distemper.

A highly contagious disease that occurs worldwide, strangles is on the USDA’s National List of Reportable Animal Diseases. This means that veterinarians (and others) are required by law to notify state and federal officials if the disease is diagnosed or even suspected.

It can occur in horses of any age, although young horses seem to show more severe clinical signs and recover more slowly. The prognosis is good in that strangles is rarely fatal, but complications occur in approximately 20% of cases. These complications include the spread of infection from the head and neck to other parts of the body (metastatic or “bastard strangles”) and autoimmune conditions that affect the muscles, heart, and kidney as well as potentially lead to “purpura hemorrhagica” a syndrome that includes edema and capillary bleeding.

How Strangles Spreads from Horse to Horse

The most obvious method for the strangles infection to spread is from horse to horse, also known as direct contact. That is, through nose-to-nose or head-to-head touching, a healthy horse comes into direct contact with contaminated nasal discharge or lymph node abscess material (pus) of a sick horse that is actively shedding the bacteria.

Another, not-so-obvious method for strangles to spread is through indirect contact. Streptococcus equi has been shown to survive less than 24 hours on surfaces exposed to direct sunlight. However, that is enough time for inanimate objects (such as water troughs, feed buckets, grooming tools, tack, stall cleaning equipment, and even clothes and shoes) to serve as fomites, transferring the bacteria from one horse to another.

While humans and other animals such as dogs, cats, and birds can transmit strangles from one horse to another, it is unlikely for another species to develop the disease itself.

Experts now believe that an even greater source of strangles transmission is from outwardly healthy horses that are:

1. in the early stages of the disease and not showing signs yet,
2. in the early stages of recovery but still harboring the bacteria, or
3. persistent, long-term carriers of the disease who remain asymptomatic themselves but can shed the bacteria to other horses for months or even years.

It is estimated that 10 to 40% of horses who have had strangles silently carry the disease and are able to transmit it to other horses without showing signs of the disease themselves.

Signs and Symptoms of Strangles in Horses

The incubation period for strangles is 3 to 14 days, with the first signs of illness being a fever for 24 to 48 hours before showing any other of the classic signs, such as:

• Thick white or yellow nasal discharge
• Abscesses in the lymph nodes between and behind the jawbones
• Inflammation of the throat leading to a reluctance to eat or drink
• Difficulty swallowing
• Wheezing or a harsh, high-pitched sound when breathing
• Coughing that is soft and produces mucus
• Holding the head (and neck) in an abnormal position

Shedding of bacteria usually begins 2 to 3 days after the onset of fever, and may continue for 2 to 3 weeks after recovery from clinical signs. However, horses should be considered infective for up to 6 weeks after recovery.

In some cases, horses may shed bacteria intermittently for months or even years if bacteria persist in their sinuses or in an area called the guttural pouch. These are air-filled spaces within the head on the right and left sides that are an expansion of the Eustachian tubes and connect the middle ear to the pharynx.

The only definitive way to determine if an individual horse is still shedding is to perform a diagnostic test for the S. equi bacteria.

Diagnosing Equine Strangles

A collection of experts in the field of infectious diseases of the horse published recommendations for diagnosing strangles in the American Association of Equine Practitioners (AAEP) Streptococcus Equi Infectious Disease Guidelines. Hopefully, technological advances in the diagnostic testing of S. equi will make it easier to identify this bacteria sooner and with more certainty.

Veterinarians may collect samples from an abscessed lymph node, a nasal swab, a nasopharyngeal swab, a nasopharyngeal wash, or a guttural pouch wash, also known as a lavage. Each method has its pros and cons and also must be performed at the correct stage of the disease for accurate results.

Once a sample is obtained, it can be cultured or undergo PCR testing, or both. PCR stands for Polymerase Chain Reaction and is a DNA amplification technique which makes it a very sensitive technique for detecting low levels of the bacteria.

In order to identify long-term carriers of the bacteria that appear outwardly healthy, these experts recommend that a sample be obtained from the guttural pouch using endoscopy, as then the area can be visually examined at the same time for inflammation and even pus.

Treatment for Strangles in Horses

Since strangles is a bacterial disease, it might seem obvious to reach for antibiotics as the first line of treatment. However, the use of antibiotics in this particular infection is controversial, because they may be unnecessary in mild to moderate cases with no complications and potentially even lead to problems.

In most situations, simple supportive care while the disease runs its course may be the best treatment, which includes:

• Rest in a comfortable, clean, well-ventilated, and dry environment that is neither too hot nor too cold.
• Soft, moist, tasty food of good quality.
• Food and water that is easily accessible because the head, throat, and neck of horses with strangles may be painful and they may be reluctant to stretch all the way down to the ground.

Severely affected horses or those with complications may be candidates for antibiotics and other treatment at the discretion of your veterinarian.

Preventing the Spread of Disease

A strangles outbreak on a farm can be costly, time-consuming, and inconvenient, not to mention the toll it can take on a horse’s health even when there are no complications. It’s best to take steps to protect horses from being exposed to an S. equi infection. These steps, or best practices for biosecurity on horse farms can be divided into three general areas:

1. Before an outbreak
2. During an outbreak
3. After an outbreak

Before an Outbreak of Strangles

The sayings “prevention is the best medicine” and “an ounce of prevention is worth a pound of cure” could not be more true than in the case of strangles. One of the best resources for preventing a strangles outbreak is the farm’s veterinarian. This person has been trained not only to recognize, report, and treat infectious diseases like strangles, but also understands the management practices and general good husbandry that can limit the exposure and prevent the spread of a contagious diseases.

A program developed with the farm’s veterinarian will likely include these four components:

A program developed with the farm’s veterinarian will likely include these four components:

1. Vaccination: The strangles vaccine is described as a risk-based vaccine in the AAEP Vaccination Guidelines. That means it is included in a vaccination program only after the risks of administering it are compared to the benefits. There are two strangles vaccines licensed for use in the United States – one is given intramuscularly and one is given intranasally – and each has its own list of pros and cons. The farm’s veterinarian is the best person to assess the particular situation and advise whether vaccination is appropriate or not.
2. Management practices: Farms that get into the habit of practicing good hygiene on a regular basis not only reduce the potential for disease transmission, but they are also more prepared in the event of an outbreak. Good hygiene on a horse farm includes:
   1. Washing hands or using hand sanitizer between horses
   2. Not allowing visitors to go from stall to stall petting or feeding horses
   3. Cleaning and disinfecting feed and water buckets regularly
   4. Using dedicated equipment for each horse and not sharing tools
   5. Controlling rodents so they do not spread disease
   6. Managing manure responsibly
3. Quarantine: Because the incubation period for the contagious diseases of concern is 14 days or less, experts recommend that all new horses be quarantined for at least two weeks, preferably three. During this time when they are kept separate from the other horses, they should have their temperature taken twice daily and be closely observed for any signs of illness. Some farms may even want to consider screening for active S. equi infection or carrier status during this period.
4. Develop an outbreak action plan: Should these best practices fail and horses become sick, knowing beforehand what steps to take next may help contain the spread of disease. For example:
   1. Already have on hand the equipment and products necessary during an outbreak such as boots, coveralls, gloves, and disinfectant.
   2. Know where sick horses will be isolated.
   3. Be able to access the medical records of all horses on the farm.
   4. Designate a point person to communicate back-and-forth with the farm veterinarian.
   5. Most importantly, educate everyone that steps foot on the farm (caretakers, trainers, owners, riders, etc.) on what to do and what not to do in the face of an infection.

During a Strangles Outbreak

If a horse on a facility develops a fever and/or signs of strangles (or any other disease), the first step is to isolate that horse then contact the farm’s veterinarian. With their help, the rest of the horses on the farm can be separated into three groups for observation and medical care:

1. Healthy, not exposed (green): No clinical signs of illness and have not had direct or indirect contact with sick horses. For example, horses in a completely different barn with different equipment, caretakers, turnout, and exercise areas might fall into this group. Temperatures should be taken twice daily.
2. Healthy, exposed (yellow): While these horses also have no clinical signs of illness, they may have come into contact with sick horses, and therefore could be incubating the disease. This includes horses in the same barn as the sick one, or that have shared tools, equipment, and tack. Temperatures should be taken twice daily. The attending veterinarian may also recommend diagnostic screening.
3. Sick (red): These are the horses that have a fever or are showing signs of strangles such as coughing, nasal discharge, or swollen lymph nodes. They should be isolated from the others and receive supportive care.

Ideally, the horses in the healthy, not exposed (green) group are kept in a clean area with separate caretakers and equipment while the horses in the sick (red) group are in an isolation area, also with separate caretakers and equipment.

However, if this is not possible, then the order of horse-handling is healthy to exposed to sick, or green to yellow to red in order to prevent the disease from spreading. The use of colored tape to designate not only the health status of parts of the farm such as barns and stalls but also items such as feed tubs, water buckets, muck buckets, manure forks, tack, and grooming tools can be helpful preventing the spread of disease.

Feed carts, tractors and manure spreaders, and wheelbarrows are shared equipment that may usually travel all over the farm but should be restricted during an outbreak.

Additional management tips during an outbreak include:

• Limit or halt the movement of horses on an off the farm
• Restrict farm visitors
• Dispose of manure, bedding, and leftover feed from the sick group responsibly

After an Outbreak

The attending veterinarian will determine when an outbreak of strangles is official over and things can return to normal on a farm. Generally, quarantine should last a minimum of three weeks past the resolution of the last case/clinical signs and all cases being declared S. equi negative.

The gold standard for when a horse is considered S. equi negative is one guttural pouch wash submitted for PCR testing that does not find the bacteria. If samples from another site are submitted, it may take as many as three negative samples to be confident that the healed horse is not a carrier.

Once the horses have been tested and found not to silently harbor the strangles bacteria, it is important to make sure the farm itself is properly cleaned and disinfected. Again, the barn’s veterinarian is the best resource for products and procedures, as certain surfaces (like wood) can make disinfecting challenging, and certain practices (like high-pressure washing) can actually make things worse.

Fortunately, the bacteria that causes strangles is susceptible to a wide range of disinfectants, such as hypochlorites (household bleach), phenolics (Lysol), and quaternary ammonium compounds (Microban). However, it is important to clean surfaces of dirt and filth first as this material can cause disinfectants to become deactivated.

Follow manufacturer directions carefully about rinsing, amount to be used, and air drying for the most effective cleaning and disinfection of stalls, aisle floors, trailers, tools, and equipment.

Video on How to Manage a Horse or Barn with Strangles

In this Ask the Vet video, Dr. Gray explains what strangles in horses is and what to do if your horse or multiple horses at your barn are diagnosed with this disease.