MRSA: The new four letter word in veterinary dermatology (Proceedings)


Staphylococcus aureus is an opportunistic pathogen that is normally found on the skin, nasopharnyx, and perineum of humans and animals.


Staphylococcus aureus is an opportunistic pathogen that is normally found on the skin, nasopharnyx, and perineum of humans and animals. Staph. aureus is a gram positive, non-correlating cocci that is found singly, in pairs, short chains, or in irregular clusters. The virulence factors found in S. aureus allow it to adhere to environmental surfaces, damage or avoid the immune system and can cause toxic effects1 .

The overuse of antibiotics to treat simple infections in human medicine has lead to the development of methicillin resistant Staphylococcus aureus (MRSA). The first reported case of MRSA in human medicine dates back to 1961. Since that time, MRSA and methicillin resistant Staphylococcus intermedius (MRSI) have been diagnosed in multiple species of animals (ie dogs, cats, horses, cattle, rabbits, chickens and some other birds). MRSI tends to occur more commonly in animals than in people. Interestingly, MRSI studies have shown that 45% of the owners are colonized with the same strain of bacteria as their dogs.1 The suspicion is that these owners acquired the MRSI from their pet.

MRSA and MRSI can be resistant to different antibiotics depending on the strain present. Some are resistant to amikacin, clindamycin, trimethoprimsulfa, ciprofloxacin, and vancomycin, where as other strains are resistant to gentamicin and tetracycline as well. Some strains are only susceptible to vancomycin. Because of this, there are concerns about vancomycin resistance particularly with MRSA strains. In 1996, this fear came true and the first reported cases of vancomycin resistant MRSA were reported. 1 Another concerning finding is the existence of antedoctal reports of fluroquinolone resistant MRSA in veterinary medicine.


About 10% of the veterinary staff at any given clinic is thought to have methicillin resistant Staph present. Approximately 10% of healthy dogs and 15% of healthy cats are colonized with methicillin resistant Staphylococcus (MRS). Humans working in a health care profession have a higher risk of carrying MRSA than the general population.

Mortality rate

The mortality rate for humans and animals vary due to the underlying problem. The mortality rates are lower for superficial when compared to a higher mortality rate in serious invasive infections.2,3 Usually; the more severely affected animals are euthanized. The pet owners reasons for euthanizing these animals are financial constraints or non-responsive infections.1 The owner has an increased chance of getting an infection from their animal if the animal has a resistant infection. In order to completely treat an animal with a non-responsive infection, the owner may need to be treated as well, as long as they have a positive culture. The owner and/or pet should not be treated for a bacterial infection without a positive culture. This will help to reduce the chances of creating resistant bacteria.4

Zoonotic concerns

In the beginning it was not determined whether animals were the primary source of resistant infections, or if these animals were colonized after contact with human carriers. Now studies show that most canine and feline resistant infections are acquired from their owners.5,6 Healthy pets and humans may be colonized but will not get an infection, the concern is for the immunocompromised patients. If the owner is colonized and works in the health care profession, they could unknowingly be infecting immunosuppressed patients.7 Also, if an owner has an infection that is unresponsive to treatment; the animal can be a reservoir and be inoculating the owner again. Thus treatment of both the owner and the animal is necessary for the infection to be cleared.8

Necropsy on a MRS patient is a high risk procedure. There must not be any non-essential personnel around, and personal protective equipment (PPE) must be worn. This includes but is not limited to: latex gloves, mask, face shield, some sort of cut-proof gloves, and respiratory protection if a band saw or power equipment is used.3

Transmission of MRSA occurs in several different ways. Different modes of transmission for MRSA include: environment to human, human to environment, human to pet, pet to human, pet to environment, and environment to pet.9 Cross contamination can occur when a contaminated environment shared by humans and animals.6 Multiple incidences of MRSA being isolated from a dog and transmitted to the staff at a vet hospital have been documented. In turn, a second dog was inoculated by the staff members. These staff members were referred to their physicians for treatment, no other animal or staff members were infected.7 The majority of resistant infections in animals are high risk animals that acquired the MRSA from their owners .6


Transmission is usually by direct contact with the nasal passages (ie sneezing), throat, and skin; or by indirect contact from bedding, counters, dishes, equipment, floors, supplies, and walls.2,3 . MRSA can also be transmitted during surgery through endotracheal tubes, during the birthing process and bone fragments which have infected exudates on them. 1,10 In order to decrease the chance of an owner acquiring MRS from their pet, the owner should not share food with their pet. In addition, the pet owner should not allow their pet to lick their face or lick open wounds.5

Clinical Signs

Clinical signs associated with a MRS (methicillin resistant Staphylococcus sp.) infections are similar in humans and animals. Humans can have skin and soft tissue infections such as impetigo, furunculosis, abscesses, folliculitis, cellulitis, and wound infections. It can also present as pneumonia, endocarditis, septic arthritis, osteomyelitis, meningitis, and septicemia. Animals can also present with skin and wound infections such as abscesses, dermatitis, post operative infections of the surgical site, fistulas, infections at the intravenous catheter site, and surgical implant site. Internal infections can be pneumonia, rhinitis, bacteremia, septic arthritis, osteomyelitis, urinary tract infections, merits, and mastitis.1 For species other than companion animals, the presentations could be different. These include bovine mastitis, porcine exudative epidermitis, and equine draining tracts and nodules.12 In veterinary medicine, antedoctal reports exist that TECAs infected with MRSA can have very small insignificant looking skin lesions.

MRSA in veterinary medicine appears to be more common in patients immunosuppressed from cancer treatments, steroid use, and surgery, or if the patient has severe skin damage such as burns and extensive abrasions. Deeper, more purulent lesions can occur with MRSA than a traditional skin infection. One unique clinical presentation can occur when exotoxins are released in a MRSA infection (condition called Panton Valentine Loucicidin or PVL). These exotoxins cause tissue necrosis, leukocyte distraction, and severe inflammation. Other strains of MRSA produce exfoliative toxins that can cause toxic shock syndrome or can cause the superficial dead skin layers of the epidermis to separate from the living layers to cause scalded skin syndrome.

MRSI is rarely a component of normal human flora, but it can be colonized in people who have a lot of contact with animals6,11 . This bacteria usually causes pyodermas and otitis in animals, and tends to be a more superficial infection.12

Several clinical situations are at a greater risk of a MRSA or MRSI may be present. Warning flags should be thrown in the following situations: 1) known MRSA or MRSI household, 2) non-healing wounds, 3) infections that are non-responsive to antibiotics, 4) septic or invasive infections, 5) sores resembling spider bites, 6) recent or frequent antibiotic use, 7) recurrent urinary tract infection (UTI), 8) draining tract lesions.13-15 Typically infections appear between 4 and 10.13,14 days, but asymptomatic colonization is common and the disease may not occur until several months after colonization.1 If there is evidence the patient had been on fluoroquinolones, there may be enhanced resistance to methicillin in S. aureus isolates. Despite client communication about the seriousness of MRS infections, pet owners often deny or fail to recognize the seriousness of this type of infection. Many veterinarians are concerned that these infections may be under reported due to the lack of some clinics performing bacterial cultures on these high risk cases.

Clinic Prevention

The best way to avoid MRSI/MRSI infections at your clinic is to minimize the use of antibiotics to try to avoid creating resistance. If you need to prescribe an antibiotic for an infection then prescribe the appropriate dose and for the appropriate period of time. Many veterinary dermatologists will treat a superficial pyoderma for a minimum of 3 weeks or 1 week beyond when the skin appears normal and a deep pyoderma for 4 weeks or 2 weeks beyond when the skin appears normal. Do NOT use antibiotics for prevention, this will only cause resistance in the patient.4

The best defense for treating MRS is a good defense. Treat all patients with any of the MRS symptoms as a positive until proven otherwise by culture. The front desk staff of the clinic should know the symptoms as well as all the staff in the clinic. This allows for clients coming in with the symptoms to be identified prior to admittance .2

Special precautions should be taken with suspect or positive animals. These animals should be given the latest appointment of the day if possible and placed directly into an examination room or more ideally infectious control room upon arrival. Suspect or confirmed animals should either be carried or placed on a gurney, or have an alternative entrance to keep the front waiting area clean. If the animals must be walked into the clinic, the front lobby and any area where the animal walks should be mopped as soon as possible. The patient should remain in the room until all diagnostics and treatments are finished since one room is easier to clean than the whole clinic. Other protocols include: washing hands between patients, place a suspected animal into any exam room just disinfect well afterwards. For most of the clinics represented a single staff member was assigned to the case, and some would wear gloves, and some would not. If possible, have a scale in the room that you are using for your MRS patients. This way you don't have to worry as much about spreading the infection around the hospital. You should also keep all the equipment that you think you will need in the room (ie bandage supplies scissors, stethoscope, thermometer).3 All wounds should be covered as soon as possible to prevent any kind of leakage, and be sure to wear the appropriate protective equipment. Usually gloves and disposable gowns are all that is needed for bandaging. One person of your staff should be assigned to taking care of a particular MRSI or MRSI case. Staff members with skin barrier defects (ie eczema, psoriasis, or open wounds) should not be assigned to MRS cases since they are at increased risk of picking up these infections. The assigned MRS member for a particular case should not be allowed to handle sick or immunocompromised animals since these animals are more predisposed to picking up infection.

If the MRS animal must be hospitalized, they should be kept isolated from all other animals (ie isolation ward). The assigned staff should reduce the contact with other sick and compromised patients. Always wear your PPE anytime the patient is handled. The patient should have certain equipment assigned to them until they are cleared or are discharged. The equipment should include, but is not limited to pens, stethoscopes, thermometers, leashes, bowls, and anything else that touches the patient or is touched in the process of handling the patient. If the equipment can be completely disinfected, it can be reused on another patient, if not it must be disposed of. During cleaning of the patients cage, the bedding must be disposed of or laundered at 60°C (140°F), and the cage and surrounding area should be disinfected once a day. If possible, it is best if the patient is bathed every 2-3 days in an antibacterial shampoo. The owner can visit the patient, but they must wear proper PPE and wash their hands before leaving.13

For deceased or discharged patients, all lesions must be covered. Deceased patients must be sealed in an impervious bag and cremated. If a necropsy is warranted, advise the necropsy staff there is an infectious disease or a possible infectious disease, and have them incinerate the remains. Only discharge if the animal is clinically fit and culture the lesions prior to leaving. Have the owner sign a consent acknowledging that the animal is carrying a possible zoonotic disease and the owner is at risk of infection.13

The best prevention of an outbreak in your clinic is good hand hygiene and environmental disinfection. You should always wash your hands before and after each patient with a good liquid antimicrobial soap. Avoid bar soap since bacteria can start to grow on it. Anytime you are washing your hands you should scrub at least 15 seconds. Disinfect all stethoscopes, thermometers, leashes, muzzles and dishes at least once a week and keep all scrapes and cuts covered.15 When in contact with an infected surface or animal, wash your hands prior to putting on the non-sterile gloves. Put the gloves on immediately prior to touching the patient and take them off immediately after or prior to touching any other surface. Wash your hands again after taking the gloves off in case of microscopic holes in the latex gloves.4 Appropriate PPE will vary depending on whether you are bandaging or surgically removing infected tissue. Typical PPE used include disposable gowns and gloves, and foot baths. Face and eye protection may be warranted if lancing or draining an abscess.

Some personnel are more susceptible to colonization. Increased susceptibility comes from poor hand hygiene, close physical contact, contaminated laundry, burns, cuts, or abrasions .4 If a patient is suspected of having a resistant bacteria, be sure to culture it and send it to a reputable lab. Remember to cover existing wounds, especially if they are draining. Staff with cuts on their hands should avoid contact, and if the drainage cannot be contained, limit physical contact with the exudate and patient.14

MRSA can survive up to 12 months in bedding, clinic dust and clothing. Environmental screening of the clinic should be used to monitor cleanliness. These cultures should be done monthly to ensure your cleaning protocol is adequate. However, antedoctal reports suggest that a more typical routine culturing protocol (ie door handles, keyboards, water fountains, clinic pets) is 3 to 4 times a year .2 Many clinics do not culture their clinics at all. This lack of routine clinic culturing is concerning. Unfortunately, this routine culturing is difficult to track and enforce since this problem is an issue of consent and confidentiality. Staff screening should be done if an epidemic of MRS is present in your clinic. When staff screening is performed, the cultures should be done 5-7 days apart for 3 swabs and at different times of the day.2 These records should be held in strict confidence. Personnel who are exposed to infected patients can attend to normal duties pending results of the cultures. Personnel not complying with sampling policy should be removed from patient care until culture samples are obtained. Negative culture results should be delivered privately and no further action is warranted. A positive result should also be delivered privately and the staff member sent to their physician for treatment. The staff member should be removed from patient care until treatment is initiated and should submit documentation from a physician to return to patient care. Additional testing may need to be done to ensure the risk of transmission is minimized.16 It is unclear if surveillance of the clinic staff is effective in determining and managing the risk of MRSA.

Animal assisted therapy is becoming popular in the companion animal world. These animals visit places like children's hospitals, nursing homes and grief meetings. Animal assisted therapy animals and their owners/handlers should be routinely monitored for MRS colonization.6 . These animals should be groomed and bathed prior to their visit and they should not be taken to see patients known to have MRS.2

Horses have recently been shown to have MRS at an increasing rate. Many horses are carriers of MRSA in their nasal passages. This situation would allow MRSA to become a huge problem in the equine world. No proven, safe and acceptable options exist for eradication of this colonization. This makes it difficult to manage equine MRS cases .17

Cleaning recommendations after infectious patients

No set disinfection protocol for infectious disease. Recommended disinfectant solutions are ones that contain gluteraldehyde, formaldehyde, an iodine/alcohol mixture, or bleach.1 The recommended dilution for bleach is 1cup bleach to 9 cups water.14 These are not the only disinfectants available for infectious diseases; you can use any EPA product that has the MRSA/MRSI bacteria on its label.18

The rules for disinfection are simple. A concentration too weak is ineffective and too strong is a waste since it does not work any better. Water should be the only ingredient added to the cleaning solution unless the label directs you to do otherwise. A solution of disinfectant should be made up daily or sooner if the solution becomes contaminated with grossly visible material.19 Even though a surface appears to be clean, it does not mean that the surface is actually clean. About eighty two to ninety one percent of the visibly clean surfaces are actually only 30 to 45% clean. Contact time is the hardest part of the disinfection process when treating a MRS contaminated environment. The best way to deal with MRS cases is to terminally clean all work areas, treatment areas and lobby areas weekly.13 This includes cleaning the wall and surfaces that come in contact with the infected or colonized animal. Cages of suspect or positive MRS patients should be cleaned daily and any bedding changed daily. When disinfecting, always wear gloves, a disposable gown and face guard. This protective clothing will protect you from splash back while you are cleaning. Always remember to rinse the cage to remove all gross material, dirt and body excretions. The cleaning solution should be diluted and allowed to stand according to the manufacturer's recommendation period. You should remove any residue from the environment which may be left over after the recommended contact period. The disinfected surfaces should be allowed to air dry or a high temperature steamer should be used to treat the kennel area.13 Many clinics are too busy to allow a room to be closed down for a period of time.

In the case of horses, a positive horse should be housed in an isolation stall with the smallest amount of shavings possible. An alcohol based cleaner located right outside the stall. These animals should be handled with gloves, gowns and booties. Once discharged, the stall should be thoroughly cleaned and all surfaces scrubbed. The walls, doors, water buckets, feed troughs and anything that the horse sneezed on should be scrubbed and cleaned. These stalls should be maintained so no cracks are present in painted areas or concrete which would make the environment difficult to disinfect. Once disinfected, the stalls should be left overnight to air dry. This protocol should be repeated once a day for 2 days. Since rope twitches, hay nets and halters are difficult to thoroughly disinfect, these items should be thrown away. in the paint or concrete that are visible since these areas are very difficult to disinfect.10

Regardless of the species of animal with MRS, all draining wounds should be covered as soon as they are admitted. This can create a problem when the bandage needs to be changed or the lesion examined since it will increase the chance of contamination to the environment. Gloves should be worn when touching the bandage material. A designated pair of scissors should be used to change out a MRS bandage. If possible, you should cut the bandage off in one piece. Less bandage pieces will help to reduce the chances of contaminating a larger area. The bandage should be double bagged and disposed of in the routine garbage that is changed daily. The PPE worn can also be placed in the routine garbage unless it is heavily soiled, then it will also need to be double bagged.

Although not typically done, the MRS literature recommends that the entire clinic should be cleaned and disinfected once a week to ensure there will be no epidemic. The recommended places within the clinic to clean are table surfaces and sides, anesthetic machines, floors, walls, cages, door knobs, push plates, sinks, water fountains, soap dispensers, towel dispensers, telephones, keyboards, remotes, scissors, clippers, toys and furniture in the lobby .2,4,18 . Equipment and instruments should be disinfected and sterilized; if possible, using the manufacturers recommended dilution.

Surfaces where animals are housed, examined, or treated should be made of nonporous material that is easy to clean.3 Organic material can and does inactivate disinfectants, so always remove gross contamination prior to disinfection (Disinfection).

Comments and Conclusions

MRSA and MRSI are of concern both in human and veterinary medicine. It is important to remember that both of these organisms can be zoonotic in nature. Therefore, the clinician needs to be concerned about both humans and animals when dealing with these cases. PPC should be worn when handling MRS cases. Terminal cleaning should be performed. Contact time is extremely important when decontaminating the environment. The examination room will need to be shut down for a period of time so that appropriate cleaning protocols can be adhered to. With a little foresight, understanding of transmission, and prevention, the chances for an epidemic MRS situation can be greatly reduced.


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13. Methicillin Resistant Staphylococcus aureus (MRSA). British Small Animal Vet Assoc. 2006 Nov;

14. Borlaug, G. Davis, J. Fox, B. Community Associated Methicillin Resistant Staphylococcus aureus (CA MRSA). 2005 Oct.

15. Facts for Veterinarians, Methicillin Resistant Staphylococcus aureus. Ohio State University.

16. Sasaki, T. Kikuchi, K. Tanaka, Y. et al. Methicillin Resistant Staphylococcus pseudointermedius in a Veterinary Teaching Hospital. J Clin Microbiol 2007 Apr;45 :1118-25.

17. Weese, J.S. Archambault, M. Willey, B.M. et al. Methicillin Resistant Staphylococcus aureus in horses and horse personnel, 2000-2002. Emerg Infect Dis. 2005;11:430-5.

18. Combating Community-Acquired MRSA. Envirox.

19. Darling, K. Disinfection Protocols. Texas A&M University Infection Control Policies. 2007.

20. Disinfection best management practices. Minnesota Technical Assistance Program. 2008 Feb.

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