Staphylococcus bacteria are commonly found in equine microflora, including on the skin, in the nasal passages, and in the intestines. These bacteria typically do not cause infection or disease in horses.

However, under certain circumstances, the bacteria may proliferate where there is significant tissue damage or a weakened immune system, producing a symptomatic infection.

Staphylococcal infections (or staph infections) are most common in the skin, soft tissues, bones, and joints in horses. Symptoms in horses can include fever, swelling and loss of function in the infected area, and pain. Humans are also susceptible to staph infections and can acquire the bacteria from an affected or carrier horse.

Veterinarians diagnose staphylococcal infections by submitting samples for bacterial culture. Treatment of staphylococcal infections involves antibiotics, and most staph infections have a good prognosis for survival.

Some strains of Staphylococcus aureus, called methicillin-resistant S. aureus (MRSA), are resistant to the antibiotics commonly used against staphylococcal infections. These strains require stronger antibiotics to eliminate the infection, and may have poorer outcomes.

Causes of Staphylococcus Infection in Horse

Staphylococcus are a group of bacteria that are part of normal microflora (bacterial population) of horses. [1] Studies suggest that virtually all horses carry Staphylococcus bacteria either within their body or on their skin. [1] They most commonly reside in the nasal passages, intestines, skin, and conjunctiva (membranes around the eyes). [1]

Staphylococcus bacteria species associated with horses include: [1]

  • S. aureus
  • S. pseudintermedius
  • S. delphini
  • S. hyicus
  • S. epidermidis

Of these, Staphylococcus aureus is the most common cause of staphylococcal infection in horses. [1]

Risk Factors

When part of the horse’s normal microflora, Staphylococcus bacteria do not cause disease or infection. [1] In these scenarios, the protective (mucosal) layers lining the skin, intestines, and other tissue surfaces prevent the bacteria from entering deeper tissues. [1]

If bacteria do penetrate the protective layers, the horse’s immune system rapidly neutralizes them before an infection develops. [1]

However, damage or injury to tissues can allow a large amount of bacteria to enter the deeper tissues. [1] When this occurs, the immune system may have difficulty removing all the bacteria, resulting in an uncontrolled infection. [1]

Since pre-existing damage or injury is necessary for Staphylococcus to cause an infection, veterinarians consider them an opportunistic pathogen, a disease-causing agent that only causes infection under certain conditions. [1]

The main risk factors for Staphylococcus infection in horses are: [1]

  • Surgery
  • Traumatic injuries
  • Other types of infections causing tissue damage
  • Skin wounds

Horses that are immunocompromised (have a weak immune system) are also more likely to develop Staphylococcus infections. [1] These horses may develop infections without tissue damage or injury, as their weakened immune system may not remove the small number of bacteria that penetrate the body’s protective layers.

Methicillin-Resistant Staphylococcus

Methicillin-resistant Staphylococcus aureus (MRSA) refers to strains of S. aureus which are resistant to methicillin-type antibiotics. [1] These strains have an additional protein, PBP2a, which prevents these antibiotics from binding to the bacteria, blocking the antibiotic’s effects. [1]

MRSA is particularly problematic as methicillin-type antibiotics are the most common antibiotic group used for treating Staphylococcus infections. [1]

Without successful identification of methicillin resistance early on, veterinarians may continue treating an infection according to traditional antibiotic regimens, allowing the infection to worsen or spread before MRSA is identified. [1] This typically results in poorer outcomes, prolonged treatment, and increased costs. [1]

MRSA Risk Factors

Like other Staphylococcus bacteria, MRSA can be part of the normal microflora of horses in rare cases. [1] Many studies on the prevalence of MRSA in horse populations show no or low rates of MRSA in the equine microflora. [2][3] Humans can also be carriers of the bacteria in their microflora. [1]

Studies suggest that strains of MRSA spread easily between carrier humans and horses through direct contact, creating more carriers. [1]

Therefore, although MRSA is rare in horses overall, the spread of MRSA from carriers can result in the bacteria becoming endemic in a local area, such as on a specific farm or within an equine hospital. [1] Studies in affected areas show that up to 45% of horses in endemic areas can test positive for MRSA. [2]

A study examining horses carrying MRSA in their microflora identified several risk factors, including: [4]

  • Previous treatment with antibiotics
  • Living on a farm where other horses have tested positive for MRSA
  • Admission to the intensive care unit as a foal

Studies in human medicine commonly identify antibiotic treatment as a risk factor for developing MRSA. [4] This likely occurs because antibiotic treatment eliminates susceptible bacteria, freeing up nutrients and other resources for the resistant bacteria to proliferate. [1]

In horses, studies show that previous treatment with trimethoprim-sulfa (TMS), penicillin, ceftiofur, amikacin, or gentamicin result in a higher risk of becoming a MRSA carrier. [4][5]

Symptoms

The symptoms of staphylococcal infections in horses depend on which tissue is affected, and whether infection is localized or widespread (systemic).

The most common tissues affected by Staphylococcus in horses are: [1]

  • Skin, including wounds
  • Bones
  • Joints and tendon sheaths
  • Surgical sites
  • Soft tissues under the skin (cellulitis)

However, Staphylococcus can cause infection in almost any tissue it gains access to. [1] Other reported sites of infection include the uterus, kidneys, trachea, lungs, sinuses, and brain. [1][2]

Common symptoms of infection in horses include: [1]