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An opportunist pathogen: MRSA (methicillin-resistant Staphylococcus aureus) (Proceedings)

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Aug 01, 2009

Methicillin is a narrow spectrum beta-lactam antibiotic of the penicillin class. It was first introduced in 1959 for the treatment of penicillin-resistant staphylococcus infections. Staphylococcus aureus, literally the "golden cluster seed" or "the seed gold" and also known as golden staph) is the most common cause of staph infections. It is a spherical bacterium, frequently found in the nose and skin of a person. About 20% of the population are long-term carriers of S. aureus.[1] S. aureus can cause a range of illnesses from minor skin infections, such as pimples, impetigo (may also be caused by Streptococcus pyogenes), boils, cellulitis folliculitis, furuncles, carbuncles, scalded skin syndrome and abscesses, to life-threatening diseases such as pneumonia, meningitis, osteomyelitis, endocarditis, Toxic shock syndrome (TSS), and septicemia. Its incidence is from skin, soft tissue, respiratory, bone, joint, endovascular to wound infections. It is still one of the four most common causes of nosocomial infections, often causing postsurgical wound infections. Abbreviated to S. aureus or Staph aureus in medical literature, S. aureus should not be confused with the similarly named (and also medically relevant) species of the genus Streptococcus. S. aureus was discovered in Aberdeen, Scotland in 1880 by the surgeon Sir Alexander Ogston in pus from surgical abscesses. Each year some 500,000 patients in American hospitals contract a staphylococcal infection.

In 1962, only a little over two years later, the first case of methicillin-resistant staphylococcus aureus (MRSA) was reported. The incidence of MRSA has since escalated in the human population; hospital acquired strains of methicillin-resistant Staphylococcus aureus (HA-MRSA) have become the most prevalent pathogen implicated in nosocomial infections in people worldwide. [Nosocomial infection, are infections which are a result of treatment in a hospital or a healthcare service unit, but secondary to the patient's original condition].

Since the 1970's there have been numerous cases of infections due to MRSA in domesticated animal species. Species that have diagnosed with MRSA include: dogs, cats, horses, cattle, guinea pigs and rabbits. There have been individual reports also in a parrot, bat and turtle. Since then there have been an increasing number of reports of MRSA isolated from companion animals. Prior to 2000, MRSA was rarely isolated. Part of the reason for a perceived increase in MRSA cases in veterinary medicine may be due to increased awareness of this pathogen and increased likelihood that testing (bacterial culture and susceptibility) is performed. However it is also likely that MRSA infections are diseases of emerging importance in companion animal species, particularly dogs, cats and horses.

Patient population/epidemiology/risk

As opposed to humans where the nares and perineum have been identified as the most common sites for these organisms to originate from, a single representative carriage site has not been identified in the dog or cat. This makes identification of the exact prevalence of MRSA in the healthy pet population difficult to determine. In general, the frequency of isolated cases of lower than that of human counterparts most likely because the studies are designed for people and not animals. It is agreed upon though that there is cause for concern for the apparent increase in frequency of MRSA infections in small animals. As with humans, most of these documented infections are associated with post-operative infections and open wounds; however any opportunistic infection has potential to be caused by MRSA.

Pyoderma, otitis and urinary tract infections have also been commonly reported to be the initiating factor. Wound and surgical site infections also appear to be implicated more commonly, especially where implant or other fixation devices are present. Suture material and orthopedic devices act as foreign bodies with a large surface area, lending to an increase potential for bacterial adherence and colonization. Devitalization of tissue also increases the frequency of colonization by this opportunistic pathogen. Additional major risk factors to those listed include: patients with impaired immune systems or chronic illness being treated with steroids, long hospital stays, repeated use of broad-spectrum antibiotics, long surgical procedures as well as owners who work in a human health care settings.

Accessing the biophysical vulnerability of each patient will help prevent poor practices as well as identify those who are particularly at risk.