Understanding Strangles and the Importance of Vaccination

Strangles is one of the most common and formidable bacterial infections affecting horses globally. Caused by *Streptococcus equi*, the disease is highly contagious and can lead to significant health and economic burdens if not properly managed. This article explores the disease, vaccination strategies, and essential biosecurity measures to prevent its spread.

What is Strangles?

Strangles primarily affects the lymph nodes around the throat and jaw, resulting in severe swelling, difficulty breathing, nasal discharge, and sometimes abscess formation. While it mainly targets younger horses, strangles can affect any age group, making comprehensive vaccination and management strategies critical.

Why We Vaccinate

Vaccination against strangles serves several crucial purposes:
  • Reduces Disease Severity: Vaccinated horses that become infected typically experience milder symptoms.
  • Limits Outbreaks: Vaccination helps control the spread of the disease within communities, reducing the overall incidence.
  • Promotes Herd Immunity: Protecting individual horses contributes to the broader health of the population.

How We Vaccinate

Two primary vaccine types are available in North America, each catering to different management needs and risks:
Intranasal Vaccine:
  • Applied via a spray into the nostrils, this vaccine provokes a strong local immune response.
  • It’s particularly effective in environments where exposure to strangles may high, such as boarding barns or training facilities.
Injectable Vaccine:
  • Administered intramuscularly, this vaccine induces a systemic immune response.
  • Recommended for horses that may find intranasal vaccination stressful or those with a history of adverse reactions to intranasal vaccines.

Specific Vaccine Schedule

Foals: Begin vaccination from 6 months of age, considering the maternal antibody status. Boosters should be given at 7 and 8 months.
Adult Horses: Annual boosters are recommended, with more frequent vaccinations advised for those in high-risk environments or where strangles is known to be endemic.

Risks of Vaccination

While vaccination is a critical tool in preventing strangles, there are potential risks associated with it that must be considered:
  • Adverse Reactions: Some horses may exhibit reactions to vaccines, such as fever, lethargy, or swelling at the injection site. The intranasal vaccine may occasionally cause mild and temporary respiratory symptoms.
  • Abscess Formation: Particularly with the injectable vaccine, there is a small risk of abscess formation at the injection site.
  • Immune Reaction: A rare negative side effect to the vaccine is the development of purpura hemorrhagica, a severe immune-mediated reaction. The risk of this occurring is higher when horses have previously been exposed to Strangles and then are vaccinated. 

Testing for Exposure

Before initiating a vaccination program, especially in adult horses or those with unknown vaccination history, it is advisable to test for prior exposure to the Streptococcus equi bacteria. In cases of suspected illness, testing is highly recommended. There are two primary types of testing commonly used:

  • Serological Testing: Measures antibody levels in the blood to determine previous exposure. High levels may suggest recent infection or an active infection, where vaccination should be delayed. This is the test we recommend prior to vaccinating a horse with unknown exposure history.
  • PCR Testing: Can detect the DNA of the *Streptococcus equi* directly from swabs taken from the nasopharynx or on guttural pouch sampling, providing a clear indication of the presence of the bacteria. This is the test that is typically recommended as a first step to determining which horses have been exposed in an outbreak situation. 

Testing helps tailor the vaccination strategy to each horse’s needs, reducing the risk of adverse reactions and ensuring the effectiveness of the vaccine. 

Biosecurity Measures to Prevent Strangles

Implementing stringent biosecurity measures is vital to prevent the introduction and spread of strangles:
  • Isolation Protocols: New arrivals should be isolated for a minimum of 2-3 weeks to monitor for signs of strangles.
  • Disinfection Practices: Regular cleaning of stalls, water troughs, and communal areas using effective disinfectants such as phenolics (ex: Virkon), which are known to kill bacteria, including Streptococcus equi.
  • Equipment Management: Use individual bridles, halters, and grooming tools. Disinfect these items regularly and do not share them between horses.
  • Environmental Controls: Manage traffic in and out of stables and limit horse-to-horse contact, especially in communal grazing areas.
  • Staff Education: Train all staff in recognizing symptoms of strangles and in proper hygiene practices to prevent transmission.


Vaccination against strangles, coupled with robust biosecurity practices, is essential for maintaining the health and welfare of horses. By understanding the disease, its transmission, and prevention strategies, horse owners and caretakers can significantly mitigate the risk of strangles outbreaks.

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