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]

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Diagnosis

Identifying a staphylococcal infection typically requires bacterial testing to confirm the type of bacteria causing disease. [1]

Samples suitable for bacterial testing include: [1]

  • Tissue biopsies
  • Swabs of the infected area
  • Aspirates of infected material using a needle and syringe
  • Joint fluid samples

To confirm diagnosis, the laboratory cultures (grows) the bacteria present in the sample, then identifies it using various test methods. [1] Some laboratories may use PCR testing, a test that identifies if the DNA sequence of a particular bacterial species is present in a sample. [1]

Antibiotic Susceptibility

When submitting for bacterial testing, most veterinarians also request a sensitivity panel as part of the testing process. [1] This test subjects the cultured bacteria to various types and concentrations of antibiotics, to determine which treatment protocol will be most effective in eliminating that strain of bacteria. [1]

Sensitivity panels are particularly important if the veterinarian suspects MRSA. To identify methicillin-resistance, the laboratory uses the methicillin antibiotic cefoxitin to determine whether this group of antibiotics can successfully kill the bacteria in the sample. [1]

If the bacteria do not respond to cefoxitin, the laboratory can advise the veterinarian to use other types of antibiotics to treat the horse. [1]

Treatment

The main treatment for Staphylococcus infections is systemic (whole body) antibiotics. [1] Providing antibiotics to the entire body ensures that the antibiotic reaches all potential areas of infection. [1] The choice of medication depends on the sensitivity panel, which informs the veterinarian of the type of antibiotic and dose necessary to kill the bacteria. [1]

For typical Staphylococcus infections, veterinarians commonly use methicillin-type antibiotics such as penicillin, ceftiofur or imipenem. [1] MRSA infections typically require the use of stronger antibiotics such as: [1]

  • Chloramphenicol
  • Doxycycline
  • Amikacin
  • Enrofloxacin

Veterinarians use these types of antibiotics very cautiously, as there is a risk of increasing antimicrobial resistance for many types of bacteria when these antibiotics are used. [1] Most veterinarians only use these antibiotics when the sensitivity report indicates they are effective against the infectious agent in question. [1]

Topical Treatment

For wounds, skin infections, or surgical site infections, veterinarians may prefer to use topical treatments to deliver a high dose of antimicrobial compound directly to the infected area. [1][2]

Topical treatments may include: [1]

  • Silver sulfadiazine
  • Hydrogen peroxide
  • Honey
  • Chlorhexidine
  • Mupirocin

Regional Therapy

For bone and joint infections, regional therapies can provide a high dose of antibiotic in the infected area, allowing for improved treatment outcomes. [1][2]

Regional therapies include: [1]

  • Regional limb perfusion, where the veterinarian injects antibiotic into a blood vessel near the infected area
  • Injecting the antibiotic into a joint space
  • Intraosseous infusion, where the veterinarian injects antibiotic directly into a bone
  • Injection of antimicrobial-containing gel into the area
  • Antimicrobial-containing implants

In most cases, veterinarians combine regional therapy with systemic antibiotics to ensure maximum efficacy. [1]

MRSA-Positive Horses

Due to the risk of MRSA spreading to other horses or people, treating MRSA carriers would be beneficial to stop spread of disease within the same operation. [6] However, there is currently minimal information on effective treatments for carrier animals. [6]

Studies show that colonization of MRSA in the normal microflora is transient. [6] Most MRSA-positive horses without symptomatic infection no longer test positive for MRSA after several weeks. [6]

Additionally, studies show that antibiotic treatment is not effective in removing MRSA from normal microflora. [1] Therefore, current recommendations for treating MRSA carriers are several weeks of isolation, until they no longer test positive for the bacteria. [1][6]

Prognosis

The prognosis for Staphylococcus infections depends on the location and severity of the infection and the response to antibiotic treatment. [1] Early treatment with an effective antibiotic produces the best prognosis. [1]

Although MRSA can be difficult to treat due to antibiotic resistance, it does not necessarily have a poor prognosis. [1] One study showed that MRSA infections have an 84% survival rate in horses. [7]

Prevention

Since Staphylococcus infections require tissue injury or a compromised immune system to develop, prevention primarily focuses on maintaining the horse’s health. [1]

Preventative strategies for horses include: [1]

  • Prompt veterinary care when illness develops
  • Proper wound care
  • Minimizing the use of antibiotics
  • Proper sterilization of surgical equipment, needles, and other invasive devices

Equine Hospitals

At equine hospitals, MRSA is a particular concern due to the risk of spread to other horses or people. [1] One study examining the rate of MRSA transmission within an equine hospital showed that 23 out of every 1000 horses admitted to the hospital became MRSA carriers. [5] Of these horses, 1.8 in 1000 horses developed a MRSA infection. [5]

Due to the transmission risk, some hospitals use MRSA screening programs to identify carrier horses before admission. [5] MRSA-positive horses are managed as infectious disease patient, including using quarantine and isolation practices such as: [1][2]

  • Personal protective equipment for all individuals contacting the horse
  • Disinfection of any items used in managing the horse
  • Disinfection of the stall or paddock after use
  • Separate disposal of waste and bedding from affected horses
  • Minimizing traffic in the isolation area

Thorough disinfection practices are crucial as one study identified MRSA on many veterinary hospital surfaces, including twitches, muzzles, stall walls, hay nets, feed bins, and water buckets. [6]

Horse Farms

Farms where MRSA is endemic may use similar biosecurity, isolation and quarantine practices. [1] Eliminating MRSA from a farm can be difficult due to the high rate of spread between carriers and other animals. [1]

Strategies for reducing the spread of MRSA include: [1][6]

  • Minimizing contact between resident horses and horses that frequently attend equine events
  • Isolation of new arrivals to the farm
  • Reducing direct contact between large groups of horses
  • Increased hand washing practices for all individuals handling horses
  • Frequent testing of all horses
  • Restricted movement of horses off the endemic farm

Ideally, MRSA-positive horses should be housed separately from MRSA-negative horses for several weeks. [1] This time period allows the carrier horses’ immune system to clear MRSA from their microflora, eliminating their carrier status. [1]

Transmission to Humans

Horses are a potential source for humans to acquire MRSA. [1] Studies show that horse owners, equine veterinarians, and other members of equine veterinary staff have high levels of MRSA-positivity in their microflora. [1] Additionally, there are several reports of equine-origin MRSA causing skin and soft tissue infections in humans. [1]

Currently there is minimal information about preventing zoonotic (animal to human) spread of MRSA. [1] Typical infection control practices such as frequent hand washing and the use of personal protective equipment, including gloves and protective coverings, when handling known MRSA-positive horses are likely effective. [1]

Summary

Staphylococcal infections in horses develop when Staphylococcus bacteria in the microflora gain access to compromised tissues and proliferate.

  • Staph infections are most common in the skin, soft tissues, bones, and joints
  • Humans can acquire Staphylococcus bacteria from affected horses, including MRSA
  • Symptoms of staph infections include fever, swelling, pain, and loss of function
  • Treatment involves antibiotics with proven efficacy against the strain of bacteria present
  • Most staph infections have a good prognosis

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References

  1. Sellon, D. C. and Long, M. T., Eds. Equine Infectious Diseases. Second edition. Saunders/Elsevier, St. Louis, Missouri. 2013.
  2. Maddox, T. W. et al. A Review of the Characteristics and Treatment of Methicillin-Resistant Staphylococcus Aureus (MRSA) in the Horse and a Case Series of MRSA Infection in Four Horses. Equine Veterinary Education. 2010.
  3. Boyle, A. G. et al. Prevalence of Methicillin-Resistant Staphylococcus Aureus from Equine Nasopharyngeal and Guttural Pouch Wash Samples. Journal of Veterinary Internal Medicine. 2017. View Summary
  4. Weese, J. S. and Lefebvre, S. L. Risk Factors for Methicillin-Resistant Staphylococcus Aureus Colonization in Horses Admitted to a Veterinary Teaching Hospital. The Canadian Veterinary Journal. 2007. View Summary
  5. Weese, J. S. et al. Methicillin-Resistant Staphylococcus Aureus in Horses at a Veterinary Teaching Hospital: Frequency, Characterization, and Association with Clinical Disease. Journal of Veterinary Internal Medicine. 2006. View Summary
  6. Weese, J. S. Methicillin-Resistant Staphylococcus Aureus in Horses and Horse Personnel. Veterinary Clinics of North America: Equine Practice. 2004. View Summary
  7. Anderson, M. E. C. et al. Retrospective Multicentre Study of Methicillin-Resistant Staphylococcus Aureus Infections in 115 Horses. Equine Veterinary Journal. 2009. View Summary