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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • Staphylococci are Gram-positive, opportunistic pathogens that can cause a variety of infections, including skin and soft tissue infections
  • Methicillin (meticillin) resistance is conferred predominantly by the mecA gene, which encodes an PBP2a that confers resistance to almost all beta-lactam antimicrobials (penicillin, cephalosporins, carbapenems). MRS are also often resistant to many other drug classes
  • MRSA is a potential zoonotic pathogen
  • Staphylococci (including MRS) are commensal organisms and are often found as mucosal colonizers and on the skin
  • There are two main classifications of staphylococci—coagulase positive and coagulase negative:
    • Coagulase-positive staphylococci are the most common causes of disease. S. aureus is the main coagulase-positive Staphylococcus in horses. S. pseudintermedius is a common canine pathogen that has sporadically been identified in horses
    • Coagulase-negative species are generally less pathogenic and are most often identified as contaminants or commensals
    • Any Staphylococcus can acquire methicillin resistance genes. Methicillin resistance does not make a Staphylococcus more virulent, but it complicates treatment when infections develop
  • MRS cause infections at various body sites. Dermatitis may occur alone or associated with infections at other sites

Genetics!!navigator!!

No genetic predisposition has been identified.

Incidence/Prevalence!!navigator!!

  • The incidence of staphylococcal dermatitis is unclear but is likely low
  • MRSA colonization can be found in the nares of 0–2% of healthy horses in most regions, with higher prevalences sporadically identified. MR-CoNS colonization is much more common, ranging from 30% to 60%, and high rates of skin carriage or colonization are likely
  • Staphylococcal skin diseases are virtually always secondary to an inciting cause that damages the skin barrier or host immune response. Infections more common in the spring and summer in most regions, likely because of factors such as heavy riding/training, higher temperature, higher humidity, increased time outdoors and in rainy conditions, increased biting insect populations and shedding

Geographic Distribution!!navigator!!

MRSA infections have been reported in many countries in North America, Europe, and Asia. It is likely that MRSA is distributed in the horse population worldwide. MR-CoNS are ubiquitous.

Signalment!!navigator!!

There are no breed, age, or sex predilections.

Signs!!navigator!!

Infections caused by MRS are not clinically discernable from those caused by susceptible strains. Clinical signs vary with the type of disease, and details are provided under specific topics.

Causes!!navigator!!

Causes of specific staphylococcal dermatologic diseases are covered elsewhere. There should be no difference for infections caused by MRS.

Risk Factors!!navigator!!

Risk factors for methicillin-resistant staphylococcal dermatitis have not been reported. A history of MRSA infection or colonization on the farm should increase suspicion. Prior antimicrobial therapy and hospitalization are associated with a higher risk of colonization by MRSA. Infections often occur in the absence of identifiable risk factors.

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

Differential diagnoses vary with the type of dermatologic disease and are covered elsewhere.

CBC/Biochemistry/Urinalysis!!navigator!!

These may be indicated to identify an underlying disease (e.g. hyperadrenocorticism).

Other Laboratory Tests!!navigator!!

  • Cytologic examination—an abundance of neutrophils and clusters of intra- and extracellular cocci is suggestive of staphylococcal infection, but cannot identify MRS
  • Identification of MRS involves isolation of the Staphylococcus spp. and identification of oxacillin or cefoxitin resistance, detection of mecA by PCR, and/or identification of PBP2a
  • Interpretation of cultures of superficial skin surfaces can be difficult because staphylococci, particularly coagulase-negative species, are common on normal skin. Sampling should be done to limit the likelihood of contamination (e.g. sampling of intact pustules vs. superficial swabs of affected skin surfaces). Isolation of MRS, particularly MR-CoNS, from skin does not necessarily imply relevance

Imaging!!navigator!!

N/A

Other Diagnostic Procedures!!navigator!!

None

Pathologic Findings!!navigator!!

N/A

Treatment ⬆ ⬇

Outline

TREATMENT

Aims!!navigator!!

Aims of treatment are to eliminate the clinical signs of infection and to limit the risk of transmission to other horses or humans. Addressing underlying risk factors is critical. Elimination of MRS from the body, at either the site of infection or colonization sites (e.g. nose, pharynx) would be desirable but is not the goal, because colonization with MRS after a successful clinical response is not uncommon.

Appropriate Health Care!!navigator!!

Most cases can be managed on the farm or as outpatients. The main reasons for hospitalization would be severe infection and an inability of the owner to properly treat the horse.

Nursing Care!!navigator!!

Nursing care is dependent on the specific staphylococcal disease. Infection control precautions should be instituted as described below.

Activity!!navigator!!

Medically, there is no reason to limit activity. The main reason to limit activity is for infection control. MRSA is transmissible to other horses and humans, and infected horses should be isolated. There is less concern with other MRS.

Diet!!navigator!!

There are no specific requirements.

Client Education!!navigator!!

  • MRSA is a zoonotic disease and clients should be counseled on the risk of transmission
  • Barrier precautions (gloves, protective outerwear) should be used when handling affected horses
  • Hand hygiene (handwashing, use of alcohol-based hand rub) should be performed when in contact with the horse or its environment. The affected horse should be isolated from other horses until clinical signs have resolved. Some risk is still present after clinical cure because of the potential for post-treatment colonization, and the infection control approach is a case-by-case basis, considering the risk status of other horses and people on the farm, whether MRSA likely originated on the farm, the ability to use isolation measures on the farm, and the intended use

Surgical Considerations!!navigator!!

N/A

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Dermatitis caused by MRS should be approached identically to disease caused by susceptible staphylococci, with the exception of the chosen antimicrobial
  • Topical therapy (e.g. chlorhexidine bathing) can be effective as the sole treatment of superficial disease. Shampooing affected areas 3 or 4 times per week is commonly used in dogs and would likely be similarly effective in horses. Topical antimicrobials include mupirocin and fusidic acid, although resistance to these drugs has emerged and there are public health concerns about use of these drugs in horses
  • With deeper involvement, poor response to topical therapy, or when proper topical therapy is not possible, systemic antimicrobials are indicated. Antimicrobials should be selected based on culture and susceptibility testing. MRS should be considered resistant to all beta-lactam antimicrobials, regardless of in vitro results. Fluoroquinolones should be avoided if possible because resistance can develop quickly and clinical response may be unpredictable. MRS may be susceptible to rifampin (rifampicin), but this drug should always be used with another antimicrobial to which the isolate is susceptible
  • For superficial infections, 2–4 weeks should be adequate, with shorter durations possible in some cases, particularly when topical therapy can also be used and when an underlying problem can be corrected (e.g. irritation from tack) or is not persistent (e.g. surgical wound)
  • For deep infections, it is important that treatment duration is adequate; 10–12 weeks of therapy (or more) may be required in some cases

Contraindications!!navigator!!

None

Precautions!!navigator!!

Horses should be prevented from licking sites where topical antimicrobials have been applied, as that might reduce efficacy or pose a risk for antimicrobial-associated diarrhea.

Possible Interactions!!navigator!!

N/A

Alternative Drugs!!navigator!!

There are ethical concerns about the use of drugs that are important in human medicine such as vancomycin and linezolid, and there is minimal pharmacokinetic and safety information available for horses. Use of these drugs should be discouraged.

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Clinical response should be monitored. If there is poor initial response, the diagnosis should be reconsidered
  • Culture of the affected site after clinical cure is not recommended because the presence of MRS after clinical cure is not uncommon
  • Post-treatment testing for MRSA colonization (e.g. nasal swab culture) can be indicated as part of the infection control response in some situations

Prevention/Avoidance!!navigator!!

Prudent antimicrobial use is important to decrease the prevalence of MRSA in the population and to reduce the chance of a horse acquiring MRSA. The same probably applies to many other MRS, although it is likely that MR-CoNS are part of the normal microbiota and there are no effective means of eliminating normal commensals. Application of general infection control practices may be useful for restricting the spread and impact of MRS infections.

Possible Complications!!navigator!!

Systemic manifestations secondary to dermatitis are rare.

Expected Course and Prognosis!!navigator!!

The prognosis is more dependent on the type of disease than the pathogen involved and is good as long as an appropriate antimicrobial can be identified and administered.

Miscellaneous ⬆ ⬇

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MISCELLANEOUS

Associated Conditions!!navigator!!

None

Age-Related Factors!!navigator!!

None

Zoonotic Potential!!navigator!!

MRSA is a zoonotic pathogen, and transmission from colonized and infected horses to humans has been documented. Infection control precautions should be implemented to limit contact with infected horses. These include the use of barrier precautions (gloves, dedicated outerwear) whenever the horse or its environment is contacted, restricted contact, and careful attention to hand hygiene. People at higher risk for developing an MRSA infection, such as immunocompromised individuals, should not have contact with infected or colonized horses. The risk with other MRS is unclear and is likely minimal; however, it is sensible to implement the same precautions with any multidrug-resistant infection.

Pregnancy/Fertility/Breeding!!navigator!!

There are no additional concerns.

Synonyms!!navigator!!

N/A

Abbreviations!!navigator!!

  • MR-CoNS = methicillin-resistant coagulase-negative staphylococci
  • MRS = methicillin-resistant staphylococci
  • MRSA = methicillin-resistant Staphylococcus aureus
  • PBP2a = penicillin binding protein 2a
  • PCR = polymerase chain reaction

Suggested Reading ⬆ ⬇

Bergström K, Bengtsson B, Nyman A, et al. Longitudinal study of horses for carriage of methicillin-resistant Staphylococcus aureus following wound infections. Vet Microbiol 2013;163:388–391.

Author(s) ⬆ ⬇

Author: J. Scott Weese

Consulting Editor: Gwendolen Lorch

Additional Further Reading ⬆

Click here for Additional Further Reading