The facts about...

PVL-positive Staphylococcus aureus/MSSA/CA-MRSA

What is Staphylococcus aureus?

Staphylococcus aureus is a common bacterium that probably has been around for millions of years. At any time about 25% of humans carry S. aureus inside the nostrils which is a favourite place for S. aureus to live, and intermittently on their skin, without any sign of infection. The great majority who carry S. aureus do not suffer an infection with this organism (these people are known as carriers). However, S. aureus is able to cause a variety of infections. Especially if the normal barrier of the intact skin has been breached, the organism can become invasive and multiply in the body. Infections may vary from minor skin infections such as boil or furuncle (common) to life-threatening infections such as sepsis and pneumonia (uncommon).

What are MSSA, MRSA, and HA-MRSA?

The emergence and spread of antibiotic resistant bacteria are a concern worldwide. Before the era of antibiotics, most S. aureus strains were sensitive to a powerful group of oral antibiotics that includes flucloxacillin, co-amoxiclav, and penicillin. Meticillin is an antibiotic not used in practice nowadays, but if a strain is resistant to meticillin this means it is resistant to all antibiotics in this group, and such S. aureus strains are widely known as MRSA (Meticillin Resistant S. aureus). MRSA are still sensitive to other groups of antibiotics. Since the 1980s, MRSA strains have emerged and spread in healthcare facilities in most countries, and in the UK have caused a significant burden of hospital-acquired and healthcare-associated infection since the 1990s. Such MRSA strains are now called Healthcare-Associated MRSA (HA-MRSA), to distinguish them from community-associated MRSA – see below. There is a separate fact sheet on this website on HA-MRSA. HA-MRSA do not spread easily in the community, and most (but not all) infections occur in hospital, especially in old or frail patients. In the UK, most S. aureus strains that are found in the community are still sensitive to flucloxacillin and these are now called MSSA (Meticillin Sensitive S. aureus) in order to distinguish them from MRSA.

What is Community-Associated MRSA (CA-MRSA)?

Unlike Healthcare-Associated MRSA (HA-MRSA), Community-Associated MRSA (CA-MRSA) have emerged outside hospitals in the community, hence the fact that they are resistant to flucloxacillin is usually not suspected. In contrast to HA-MRSA, CA-MRSA may spread easily in the community from person to person, especially where crowding and close contact occurs. Epidemic strains have emerged in the South-West Pacific, North America, Europe, and elsewhere. Infections occur in all age groups, but are most commonly seen in young adults and children. In North America a strain of CA-MRSA has emerged since 2001 that is called USA300. USA300 has become the most common cause of skin and soft tissue infection seen in A&E units in US cities. Most strains of epidemic CA-MRSA, including the USA300, are positive for PVL (a staphylococcal toxin – see below). While HA-MRSA do not spread easily in the community, CA-MRSA such as USA300 can spread in hospitals and are increasingly reported as a cause of healthcare-acquired infection in hospitals in the USA. CA-MRSA is currently thought to be rare in the UK, but is likely to be under reported for several reasons. Doctors such as GPs do not usually send samples for culture from patients with a minor boil or skin abscess. Furthermore, when samples are sent and MRSA is cultured, PVL CA-MRSA may not be recognised since these strains cannot be easily distinguished from HA-MRSA in hospital laboratories.

What is PVL?

PVL is an abbreviation of Panton-Valentine Leukocidin. PVL is a toxin that is produced by S. aureus strains if they carry a gene containing the genetic code for this toxin. PVL is a potent toxin that can cause cell death in skin and lung tissue. PVL can occur in both MSSA and MRSA. In the 1950s, PVL positive MSSA strains were common and notorious worldwide, but they became rare in the following decades. The HA-MRSA strains frequently found in NHS hospitals, as well as most MSSA strains found in the community in the UK, currently do not possess the PVL gene. In recent years the PVL gene, which sometimes moves between S. aureus strains, is increasingly found in S. aureus strains worldwide, especially in North America and Asia. Certain CA-MRSA strains appear to have acquired epidemic properties by acquiring the PVL gene.

Is PVL positive S. aureus dangerous?

S. aureus strains that are positive for PVL, and especially epidemic strains of PVL CA-MRSA, have an increased ability to cause spontaneous infection and recurrent disease as compared to toxin negative S. aureus strains. Most of these infections are superficial skin infection such as a boil or skin abscess. These tend to heal spontaneously or heal after incision and drainage. Thus, most infections caused by PVL S. aureus are not dangerous. However, a correct diagnosis is important since (i) recurrence of infection and spread between members of a household or close contacts is a common feature of epidemic PVL CA-MRSA, (ii) the recurrent boils are unpleasant, and (iii) the identification of MRSA would help to avoid the use of ineffective antibiotics. Rarely, PVL positive S. aureus causes severe and life-threatening infection such as pneumonia in previously healthy adults and children. Once such a severe infection has developed, the fatality rate is high. Fatalities in previously healthy adults due to PVL positive S. aureus infection including PVL CA-MRSA have occurred occasionally in recent years in the UK. Some fatalities in children have been reported in the USA.

When should PVL CA-MRSA or PVL MSSA be suspected?

PVL S. aureus should be suspected if someone suffers from spontaneous recurrent boils or eyelid infection (2 or more episodes in 6 months), especially if more than one member of a household or a group with close contacts is affected. A small, non-recurrent boil is not a sign of PVL infection and not a reason for concern. PVL S. aureus can spread between people in any area where crowding, close contact and/or skin abrasion occurs, such as sport activities, child day care centres, military recruit camps, prisons, and in healthcare facilities as described in the USA. PVL S. aureus should be suspected in any of these areas when several persons develop boils or soft tissue infection within a period of days or weeks. The skin infection that is typically caused by PVL S. aureus starts out as a small red bump that can quickly turn into a large pustule with central breakdown, or a deep, painful abscess with surrounding redness. The infected skin site may be confused with a spider or insect bite because the centre is often black and the boil is so painful. The skin tends to heal spontaneously leaving a scar, but recurrent infection often develops after days or weeks in a different location on leg, arm, trunk, face or neck. Mammals such as cats and dogs sharing the household may occasionally be affected too. People with recurrent infection due to PVL CA-MRSA often have a history of multiple visits to their GP, walk-in centre or A&E, and multiple courses with an ineffective antibiotic, before the bacterium is identified.

How can PVL CA-MRSA or PVL MSSA be identified?

If PVL S. aureus infection is suspected, a doctor should send a pus specimen or a swab to the local hospital laboratory, asking for the laboratory to look for PVL S. aureus. If no symptoms or only a healing skin site is present in a household with a history of recurrent boils, a selective MRSA screen by culture of swabs from nose and throat, in addition to swabs from any wound or roughened skin area, may reveal the presence of MRSA. If MRSA or MSSA has been found, further testing in a reference laboratory can confirm the presence of PVL. In the rare cases of severe infection caused by S. aureus, the bacterium may be cultured from blood, lungs or other tissues, and then investigated to see if it produces PVL toxin.

How is PVL CA-MRSA or PVL MSSA infection treated?

Treatment depends on the type and severity of infection. Minor boils may heal spontaneously, while a skin abscess can usually be treated successfully by incision and drainage alone. If cellulitis (redness and tenderness of the surrounding skin) is present, treatment with an active oral antibiotic is helpful. The rare cases of severe invasive infection require prompt recognition of severity, admission to a hospital with an intensive care unit, collection of appropriate samples for culture, empiric treatment with intravenous antibiotics active against MRSA, and sometimes surgery.

How is spread of epidemic PVL CA-MRSA prevented?

Spread of PVL positive CA-MRSA is prevented by prompt recognition of the presence of PVL CA-MRSA, followed by appropriate treatment and implementation of measures to prevent further spread and recurrence. PVL CA-MRSA may be found in several or all household members, indicating that transmission of PVL CA-MRSA has occurred within the household. Transmission mainly occurs by direct contact, but may also occur via indoor surfaces that are commonly touched by the inhabitants. MRSA can survive for weeks on indoor surfaces that are not cleaned. Further occurrences of PVL CA-MRSA infection in a household may be prevented by decolonisation treatment of all household members with washing of skin and hair with a special antiseptic lotion, and an antibiotic cream put inside the nostrils, and by thorough cleaning of the home environment. If an animal is involved, a veterinary surgeon should be consulted on treatment of infection and decolonisation of the animal. Personal hygiene such as frequent hand washing by all household members is important in the prevention of transmission, but may be difficult to stick to in households with infants.