Dog and cat bite MRSA link

“Doctors treating dog and cat bites should be aware of the risks of MRSA infection,” BBC News reported. It said that US researchers have warned that MRSA caught in the community is becoming more common and, as a result, there are more cases of infection in domestic animals. They suggest that pets are being infected by their owners and then acting as “reservoirs” for the infection. However, the news website also reported a UK expert, Professor Mark Enright, as saying, “This is probably a marginal problem. It may be of more significance in the US where community-acquired MRSA is more of an issue."

This article provides an overview of the potential for infections from animal bites, which is well known, and highlights the potential for transmission of community-acquired MRSA from humans to pets and vice versa. The authors of the review suggest that most household pets are unlikely to be colonised with (carrying) MRSA. Therefore, pet owners should not be unduly worried about the possibility of getting MRSA from their pet. Rather, they should be aware that they can potentially catch infections from animal bites and should seek appropriate medical help if they are bitten.

Where did the story come from?

The review was written by Dr Richard Oehler and colleagues from the University of Florida College of Medicine. No sources of funding or conflicts of interest were reported. The study was published in the peer-reviewed medical journal The Lancet Infectious Diseases .

What kind of scientific study was this?

This was a narrative review that discussed the possible effects of dog and cat bites. The researchers searched online databases of scientific and medical literature for English language studies relevant to this topic. No specific criteria for selecting studies for inclusion were reported. The researchers discuss the findings of the studies that they identified, including how common bite-related injuries are, the types of organisms that cause bite-related infections, the consequences of these infections and how they are treated.

What were the results of the study?

The researchers discuss the history of pet ownership and say that about 63% of homes in the US and about 43% of homes in the UK have a pet. They say that this closeness with animals brings with it the risk of infections from at least 30 infectious agents. Although certain human infections from dog and cat bites are well known, such as bite wound infections and cat scratch disease, newer infections, such as community-acquired methicillin-resistant Staphylococcus aureus (MRSA), are becoming more common. The authors report that about 1% of emergency hospital visits in the USA and Europe are for dog and cat bites. Dog bites account for about 60% of animal bites and cat bites for about 10-20%.

They report that cat bites are more common in women and the elderly and the risk of dog bites is highest in boys aged five to nine years old. Cat bites are reported to be less damaging and life threatening than dog bites, but carry a higher risk of infection. People who go to hospital more than eight hours after being bitten often have infected wounds.
The researchers say that infections from cat or dog bites can be caused by a mixture of bacteria from the person’s skin or from bacteria in the animal’s mouth. Severe infections occur in about one-fifth of all dog and cat bites. Hands are the most common site of bite-related infection and of long-term problems caused by bites, with about 30–40% of hand bites becoming infected. Bites to the head and neck are particularly dangerous in children and infants, and dog bites can lead to skull fractures, severe bleeding and disfigurement of the face.

How should animal bites be managed?

The authors report that the treatment of dog and cat bites should include treatment of tissue damage from the bite and management of the risk of infection.

They discuss appropriate management of animal bites, including the following:

• Before wounds are treated, deep wound swabbing and testing should be carried out to determine the type of bacteria involved. Often, treatment is carried out based on experience of common infections rather than an identified pathogen.
• Careful assessment of wounds is essential for determining how to manage the wound, as deep wounds are more likely to become infected after closure.
• Prompt and thorough washing out of the wound with tap water or salt solution helps to remove any foreign particles and bacteria, and can reduce transmission of the rabies virus.
• Careful removal (debridement) of dead tissue and looking for and removing any embedded teeth or tooth fragments is essential.
• An X-ray of the site of the bite should be taken to rule out the presence of any foreign material or any bone involvement or bone fracture. Further testing with CT or MRI scans should be used where necessary.
• Involving orthopaedic specialists is important for hand bites, as are elevation and immobilisation of the hand and physiotherapy.
• For most head and neck bites, a plastic surgeon should be consulted. Consultation with a neurosurgeon may be necessary in children with possible head injuries.
• With any serious animal bite, a consultation with local public health authorities should be seriously considered, particularly if the animal was a stray, the attack was unprovoked or the animal cannot be caught.
• Rabies prophylaxis (preventative treatment) should be considered on the basis of infection rates in the local area and exposure risk, and a tetanus vaccine or booster should be considered.
• Prophylactic (precautionary) antibiotics are recommended unless the bite is superficial and easily cleaned. The authors discuss the most appropriate antibiotics for targeting the bacteria that are likely to be found in an animal’s mouth.
• Once an infection has taken hold, it is likely to require admission to hospital for surgical wound cleaning and drainage.

What damage can infection do?

If an infection spreads, it can lead to severe problems such as septic shock, meningitis and inflammation of the heart valves (endocarditis), particularly if the infection is caused by the bacteria Capnocytophaga canimorsus or Pasteurella multocida. They discuss in detail the epidemiology, clinical effects and management of infection with these two types of bacteria.

How is MRSA transmitted between animals and humans?

The researchers say that MRSA is a relatively new problem and the bug is being shared between domestic animals and their handlers. They discuss the epidemiology, clinical effects and management of bite-related MRSA infection in detail.

They say that MRSA infection arising in the community has become increasingly common over the past ten years. The strains that cause these community infections (most commonly the USA300 strain) are different from the hospital-acquired strains in that they are easily passed between individuals in the household, often cause skin and soft tissue infections, and are generally susceptible to most antibiotics, other than the commonly prescribed beta-lactams.

The authors say that, as these community-acquired MRSA strains have become more common, there has been growing evidence of the presence of MRSA infection in domestic animals such as dogs, cats and horses. These infections are thought to be acquired by the pets from their owners and could result in the infection passing in a cyclical fashion between pets and the humans they come into contact with.

They say that S. aureus is not the most common strain of staphylococcal bacteria in cats and dogs and accounts for less than 10% of strains. They then report various studies on S. aureus in animals.

The researchers say that the first published case of domestic animal MRSA transmission was in 1988 and occurred in a UK geriatric rehabilitation unit, where a ward cat was found to be colonised with MRSA and 38% of nursing staff were also colonised. The outbreak was controlled once appropriate infection-control measures were put in place and the cat was removed from the ward. They also report another UK case where a patient in an intensive care unit developed MRSA that was traced to a male nurse working in the unit and his wife, who was also a nurse but worked in a different ward. Despite attempts to stop the infection in these individuals, there was a further outbreak six months later. At this point, the nurses’ dog was found to have an eye infection caused by MRSA. Treating both the nurses and the dog successfully removed the MRSA colonisation.

The authors report other cases of MRSA transmission between humans and pets. They say that the management of these pet-acquired infections is similar to that for community-acquired MRSA and discuss appropriate antibiotic treatment. They say that most household pets are not likely to be colonised with MRSA and, therefore, contact with a pet that does not show symptoms of MRSA infection is not a risk factor for infection for sensitive patients or those whose immune system is compromised. They say that “much more remains to be learned about MRSA and pet-associated human infections.”

What interpretations did the researchers draw from these results?

The researchers concluded that “pet owners are often unaware of the potential for transmission of life-threatening pathogens from their canine and feline companions.” They also say that “clinicians must continue to promote loving pet ownership, take an adequate pet history, and be aware that associated diseases are preventable via recognition, education, and simple precautions.”

What does the NHS Knowledge Service make of this study?

This article highlights the potential for infections from animal bites, which is well known, and the transmission of MRSA from humans to pets and vice versa. There are a few points to note:

• The authors of this review are based in the US and their recommendations about management of animal bites are likely to reflect US practice rather than UK or European practice.
• Although the authors carried out searches of the literature, their review would not be classed as a systematic review as it did not have set criteria for including or excluding studies. Therefore, certain studies may have been missed and treatment recommendations may not reflect the entire body of relevant evidence.
• This study does not report on exactly how common MRSA infection is in pets in the US or UK, but it does state that most household pets are unlikely to be infected.

This article will be of interest to doctors and vets. Pet owners should not be unduly worried by this article. Rather, they should be aware that they can potentially catch infections from animal bites and seek appropriate medical help if they are bitten.