Methicillin-resistant Staphylococcus aureus infection

• Methicillin-resistant Staphylococcus aureus (MRSA) Methicillin-resistant Staphylococcus aureus are strains of the gram positive bacterium, Staphylococcus aureus Staphylococcus aureus, that are resistant to the beta-lactam antibiotic methicillin as well as all other beta-lactam antibiotics, including the penicillins, cephalosporins and carbapenems.
• Methicillin (also known as meticillin and more recently replaced by oxacillin), is a narrow-spectrum antibiotic of the beta-lactamases. It has been widely used to treat human infections caused by gram-positive bacteria, since the 1950s.
• Its use was primarily against Staphylococcus aureus bacteria that were resistant to most penicillins.
• The earliest recorded MRSA was identified in staphylococci isolated from humans in the UK in 1961.
• MRSA was first reported to be a major problem in human hospitals in the USA in the 1970s, and in the 1990s it became recognized as an important cause of hospital-associated infection Hospital-associated infections in human hospitals all over the world. These hospital acquired strains of MRSA (HA-MRSA) cause the majority of human infections.
• Recently, MRSA has become an important cause of infection in people in the general population (community-associated MRSA, CA-MRSA). Although these are less prevalent than HA-MRSA they are thought to be more virulent.
• In the UK, the most common strains of MRSA isolated from humans are epidemic MRSA (EMRSA) strains 15 and 16. These are both associated with HA-MRSA infections.
• The first isolates of MRSA from domestic animals were detected in milk from cows with mastitis in the early 1970s.
• MRSA is becoming an increasingly recognized problem in companion animal medicine. Over the past few years it has become apparent that MRSA infection in the UK is much more widespread in companion animals than previously thought.
• MRSA tends to occur as sporadic cases or outbreaks in veterinary hospitals and appears to be relatively uncommon in dogs in the community.
• Methicillin resistance is also found in other species of staphylococci isolated from animals, in particular pigs, poultry, cattle and sheep.
• Cause: methicillin-resistant Staphylococcus aureus (MRSA) Methicillin-resistant Staphylococcus aureus.
• Signs: colonization of nasal passages may occur with or without clinical signs. When clinical infections are apparent they can range from mild skin infections to severe or fatal bacteremia.
• Diagnosis: samples from suspected clinical cases, as well as samples from asymptomatic individuals (usually from nasal passages) can be submitted for bacteriological culture and species identification.
• Treatment: subsequent sensitivity testing can be used to determine the course of further antimicrobial therapy.
• Prognosis: good: the majority of MRSA infections can be treated successfully.

Presenting signs

• Not all animals that are infected with MRSA develop clinical signs.
• The most commonly reported clinical signs are post-operative and wound infections, with less reported incidences including urinary tract infections, pneumonia, and skin and ear infections.
• Clinical signs can vary from mild to severe and in some cases fatal bacteremia may result.

Acute presentation

• Acute bacteremia is possible, especially in compromised patients with hospital-acquired infections Hospital-associated infections.
• CA-MRSA tends to produce more virulent clinical signs than HA-MRSA.

Geographic incidence

• Worldwide.
• The prevalence of human S. aureus isolates that are methicillin-resistant varies between countries, being lower in northern Europe than in southern and western regions (range 1 to >40%), where successful control policies have been implemented in hospitals. In the USA some reported prevalence figures exceed >57%, particularly in patients within intensive care facilities.
• In areas such as the Netherlands, certain strains of human MRSA have been found to be closely related to those found in pigs and cattle.
• Canine MRSA infections have been reported from multiple locations it is likely disseminated widely internationally. Prevalence figures of MRSA in dogs range from 0.6% in healthy dogs to 23% in hospitalized dogs in the UK.

Age predisposition

• No epidemiological studies have been reported in rabbits.
• In humans the prevalence of nasal carriage of S. aureus is higher in children than in adults.

Breed/Species predisposition

• Little data is available for rabbits.

Public health considerations

• The natural habitat of S. aureus is the human nares (nostrils), but it can also be found in sites such as the groin, perineal area, armpit and throat. Nasal carriers also often carry S. aureus on the skin, and it may be transmitted via the hands.
• Although prevalence figures vary, it is thought that approximately 20% of the human population are permanently colonized, and up to 60% transiently colonized with S. aureus.
• Community-associated MRSA (CA-MRSA) infections tend to be associated with different strains of the bacterium and appear to be resistant to more classes of antimicrobials than the more frequently isolated hospital-associated MRSA (HA-MRSA).
• The prevalence of MRSA carriage in community surveys of healthy people is around 1-3%.
• Certain risk groups have higher carriage rates, for example in hospital staff, care workers and hospital outpatients MRSA prevalence can be up to 22%. In the UK and Ireland MRSA prevalence has been reported as 7-8%.
• MRSA ST398 (livestock-associated MRSA, LA-MRSA) was isolated from an industrial rabbit holding in Europe. Farm workers and a worker's relative were found to be carrying MRSA, and surface and air samples were also positive. In this report, clinical signs of disease were not present. Spa typing suggested porcine origin of the MRSA isolates detected.
• Dog owners and small animal practitioners also have higher carriage rates, with up to 12% MRSA prevalence reported in some studies.
• The proportion of human S. aureus isolates that are resistant to methicillin has increased greatly in recent years, eg the proportion of MRSA in cases of S. aureus bacteremia in the UK has risen from 12% in 1990-1992 to 40% in 2000. However, the rate of resistance varies between geographical areas.
• There are potential zoonotic implications for MRSA infections in companion animals, given the close social interaction between pets and humans.
• Healthy animal carriers have been associated with transfer of MRSA to humans. This has been documented in the general community, and is becoming increasingly documented in health-care settings and veterinary clinics.
• Typing studies have shown that MRSA isolated from dogs are identical to hospital-acquired MRSA in humans
• The majority of canine infections appear to be caused by strains common in the general human population, the most predominant being EMRSA strain 15 and to a lesser extent EMRSA strain 16.
• The first documented case of animal-to-human transmission was in 1994 when a husband and wife, who were both nurses, were re-infected with MRSA following initial clearance. Screening of the family dog confirmed the presence of the same MRSA strain. It is therefore likely that one or both were infected at work and originally transmitted the MRSA to the dog.
• Other domestic animals such as pigs have also been suggested as a source of human MRSA infection, since identical strains have been isolated from humans and pigs.

Cost considerations

• The cost of treating human cases of MRSA is increasing. Estimates suggest that MRSA infections cost the NHS approximately one billion pounds per year.
• It is likely that costs for treating companion animal MRSA infections are also increasing, due to increased hospitalization and antimicrobial therapies.