Staphylococcus (pseud)intermedius meningitis in a child
A paper in the International Journal of Infectious Diseases (Durdik et al 2010) describes a case of meningitis in an 11-month-old child caused by S. intermedius. (Presumably, the bacterium was actually S. pseudintermedius and they’re behind the times on identification of / nomenclature of this bacterium). This is the first report of this bacterium as a cause of meningitis in people, and obviously it’s a concern because of the potential severity of meningitis. Fortunately, the child made a full recovery with proper treatment.
S. pseudintermedius is a normal inhabitant of the skin and other body sites in dogs, and is found less commonly in cats. In this case, the child’s family owned a dog, but the dog lived outside and no direct contact was reported between the child and the dog. Indirect contact, such as someone bringing the bacterium in on their hands after touching the dog, would certainly be a possible route of transmission. Unfortunately, the authors of this study did not investigate the dog as the potential source and there was no attempt to isolate the same bacterium from the dog. The dog is certainly a likely source of infection here since S. pseudintermedius is not commonly found in people, and when it is, it is often associated with dog-contact.
There seems to have been an increase in reports of Staphylococcus intermedius/pseudintermedius infections in people lately. Reports are still very rare but there have been a couple in the past six months. That could be because there are more infections, but it could also be that people are just writing up the cases or that labs are getting better at identifying the organism. Overall, the number of apparent human infections caused by this dog-associated bacterium is very low. While it is clearly a bacterium that can infect people, the risks to people in contact with pets is also very low. “Low” doesn’t mean “no”, however, and the very low but not negligible risk of S. pseudintermedius infection is just one of many reasons to pay close attention to good hygiene practices around pets, and ensure that your physician knows if you have pets.
Staph pseudintermedius infection in a person
When I talk about methicillin-resistant Staphylococcus pseudintermedius (MRSP), I usually say that the human health risks are low because human infections are very rare. However, rare doesn't mean it can't happen, as demonstrated by a case report entitled "Beware of the Pet Dog: A Case of Staphylococcus intermedius Infection" published in the American Journal of Medical Sciences (Kempker et al 2009).
This paper reports about a post-operative sinus infection in a 28-year-old woman. Cultures were taken and the bacterium was initially misidentified as a coagulase-negative Staphylococcus. It was then misidentified as S. aureus, and finally determined to be S. intermedius. In reality, that's probably another misidentification because the bug almost certainly was truly S. pseudintermedius. (It's become clear over the past couple years that S. intermedius is basically non-existent in dogs and that what has been called S. intermedius in the past is truly S. pseudintermedius).
It's important to remember that human infection with S. pseudintermedius is a rare event. Whenever you see a single case reported, you know it's a pretty uncommon or novel event. Further, this was a post-operative infection, not a spontaneous infection occurring in a low-risk person. At the same time, we need to make sure we don't completely ignore the potential risks. While the risk of transmission of S. pseudintermedius (including MRSP) seems to be very low, we shouldn't ignore it completely. Isolation and other strict measures aren't indicated when dealing with a pet with S. pseudintermedius infection, but general attention to basic hygiene practices and avoiding contact with the infected site is still a good idea.
Methicilin-resistant Staphylococcus schleiferi in pets
.jpg)
When it comes to methicillin-resistant staphylococci in pets, MRSA (methicillin-resistant S. aureus) gets most of the attention. That's fair since it's emerging as an important health problem, and can be transmitted between pets and people. Now another staph, MRSP (methicillin-resistant S. pseudintermedius) is getting more attention, and it's actually a more common cause of infections in dogs and cats compared to MRSA. There are also some other methicillin-resistant staph that get much less attention. One is methicillin-resistant S. scheliferi (MRSS).
There are actually two different subspecies of this bacterium, S. schleiferi subsp. coagulans and S. schleiferi subsp. schleiferi. Staphylococcus schlieferi subsp. coagulans is the coagulase-positive subspecies. (Coagulase testing is one of the main ways staph species are classified.) Sta[hylocccus schleiferi subsp. schleiferi is coagulase-negative. In general, coagulase-negative staph are considered to be minor concerns and rare causes of disease other than in sick, compromised individuals in hospitals. However, it looks like S. schleiferi subsp. schleiferi is an exception to that rule, as it is able to cause disease in otherwise healthy dogs and cats.
Both S. schleiferi subtypes predominantly cause skin and ear infections. As with other staph, methicillin-resistance is a concern and is increasing. Methicillin-resistant S. schleiferi (MRSS) rates appear to be increasing, which is a concern because methicillin-resistant staph infections are harder to treat due to their resistance to many antibiotics.
One factor that limits our knowledge of the role of MRSS (and really, S. schleiferi in general) in disease is the fact that many, if not most, diagnostic laboratories don't try to differentiate it from S. pseudintermedius because the two species are very similar. (Sometimes, labs don't even try to differentiate any of the coagulase positive staph, including S. aureus).
While MRSA in pets is a public health concern, there is probably much less to fear from MRSS. Staphylococcus schleiferi infections in people are quite rare and there is currently no indication that pets are an important source of human infection. However, given our limited knowledge of this bacterium, it's wise to take some degree of precaution around animals with MRSS infections, particularly basic measures such as avoiding direct and indirect contact with infected sites, and good handwashing habits. These are the same general recommendations for pets with MRSP, and more details about this are available on the Worms & Germs Resources page.
Tea tree oil in dogs
Skin and soft tissue infections increasingly caused by highly drug-resistant bacteria, along with various concerns about antibiotic use, have led to a desire to find non-antibiotic approaches to treatment of these infections. Tea tree oil has some potent antibacterial properties when tested in the lab, and there are some studies indicating it might be effective for the treatment of certain infections. Some work that we've done in my lab shows promising activity of a few different essential oils against MRSP. Some of these oil may be similarly useful treatments for certain infections.
However, as I've stated before, we need to make sure that we adequately investigate safety of any new drug or therapy. All natural does not mean safer. If something kills bacteria, we need to make sure that it doesn't also harm an animal's cells and tissues.
Tea tree oil can cause damage to skin and soft tissue cells, but it's unclear whether this is really a problem during short courses of treatment. Nonetheless, in humans it has been recommended that tea tree oil not be used for treatment of burns because of concerns about tissue damage.(Faoagali et al, Burns 1997)
Another concern is toxicity from ingestion. This isn't usually a concern in adults, but there are a couple reports of children that became seriously ill (neurological abnormalities, progressive unresponsiveness... fortunately temporary) after ingestion of small volumes of tea tree oil. This leads me to have concerns about ingestion of the oil by dogs and cats if they lick areas where it has been applied, or eat bandages soaked in oil. They probably wouldn't ingest that much, but it's possible.
At this point, the jury is still out on the usefulness of tea tree oil. There are some potentially beneficial aspects and some safety issues that need to be clarified. In the interim, if you want to use tea tree oil:
- Recognize it's not a proven therapy. Don't use it in place of conventional treatment recommended by your vet.
- Keep it out of the reach of children and pets.
- Be judicious about the amount you use, and make sure pets don't lick it off.
- If the infected site seems to get worse after tea tree oil is used, stop applying it and see your veterinarian.
Should I isolate my dog with MRSP/MRSI?
Methicillin-resistant Staphylococcus pseudintermedius (MRSP, sometimes misidentified as methicillin-resistant S. intermedius (MRSI)) is an increasing problem in dogs and cats. This highly drug-resistant bacterium is a particularly problem in skin and ear infections, and the number of infected animals appears to be increasing significantly. A related bug, methicillin-resistant S. aureus (MRSA) has been a major problem in people for decades, and MRSA is now being found with increasing frequency in animals as well. Sometimes people get confused when they are dealing with methicillin-resistant staph, and it's important to realize some of the differences between these two related bugs:
- MRSA is a huge problem in people and can be transmitted between animals and people. The role of animals in human disease is unclear, but there is concern that people can develop infections due to contact with infected or colonized animals.
- MRSP is rarely identified as a cause of infection in people. Transmission of MRSP between animals and people has been reported. However, this is much less concerning than with MRSA because people are much less likely to carry, transmit or develop infection from MRSP than MRSA.
- The recommendations that have been made for management of animals with MRSA largely involve improving general household infection control practices. These guidelines are only based on expert opinion (i.e. there are no studies (yet) to back them up), but they are reasonable and practical.
- Even less information is available regarding MRSP, mostly because it's not considered a major human health issue. That being said, you don't want to get a multidrug-resistant bacterial infection, even if it's uncommon. Therefore measures to reduce the risk of transmission of MRSP from pets to people is should still be considered.
- Strict isolation of infected pets is probably excessive. General infection control practices (e.g. handwashing after contact with the animal, avoiding contact with the infected site, limiting contact overall) are probably adequate, especially in households with no high-risk people (e.g. people with weakened immune systems, infants, elderly individuals).
More information on both MRSP and MRSA can be found on the Worms&Germs Resources page.
How long do animals carry MRSP/MRSI?
I've had this question a lot lately. Methicillin-resistant Staphylococcus pseudintermedius (MRSP), which is sometimes misidentified as methicillin-resistant S. intermedius (MRSI), is an important and increasing cause of infections in dogs and cats. After an animal has had an MRSP/MRSI infection, a question people often ask is how long they will carry the bacterium?
MRSP can be carried in the nose, intestinal tract or on the skin of a small percentage of normal animals. The implications of this are not clear, but it is reasonable to assume that carriers are more likely to develop infections in certain situations (e.g. if they sustain a wound or need to have surgery), and can transmit it to other animals (and possibly people, but that's much less of a concern with MRSP than with MRSA).
Back to the question... To be perfectly honest, we really don't know. However, I think it's reasonable to assume that some animals could carry MRSP for a long period of time - certainly weeks or months, maybe even years. Staphylococcu pseudintermedius is a common bacterium in healthy dogs and cats, and has basically evolved to survive on these animals. The methicillin-resistant versions are likely no different in this respect, so it's reasonable to assume that some animals could be long-term carriers. This makes controlling MRSP more difficult. In contrast, MRSA appears to be only transiently carried by dogs and cats, probably because it is better adapted to living on humans than pets.
What you should do in the meantime if your pet has MRSP:
- Treat any infection as per your vet's instructions.
- Always complete the full treatment course.
- Wash your hands after contact with your pet, healthy or not.
More information about different types of staph bacteria can be found in the previous Worms&Germs post entitled Methicillin-Resistant Staph - What's In A Name?
My dog has an MRSI infection, should I be worried?
Infections caused by methicillin-resistant staphylococci are an increasing problem in veterinary medicine. Staphylococci are a group of bacteria that can cause various infections in many different animal species, including people. The one that gets the most attention is methicillin-resistant Staphylococcus aureus (MRSA). There is more information about MRSA in animals on the Worms & Germs Resources page and in our MRSA archives.
While MRSA gets most press, infections by methicillin-resistant Staphylococcus pseudintermedius (MRSP) are more common in dogs. (More information about the issues with naming staphylococci (particularly MRSI vs MRSP) can be found in the post "Methicillin-resistant Staph: What's in a name?"). These canine MRSP infections creat many frustrating and concerning situations. They can be very difficult to treat because they are resistant to a large number of antibiotics. However, with the proper testing we can usually identify an effective antibiotic to treat these infections, and the prognosis for dogs with MRSP infections is usually good, unless they have a very severe or invasive infection.
I get advice calls about management of MRSP on almost a daily basis. The first question is usually what treatment should be used. Almost inevitably, the second question is about human health concerns, because of the awareness of MRSA in people. MRSA can be transmitted from pets to people, although it probably more often goes from people to pets. MRSP can also be found in people, but it is very rarely identified as a cause of disease in humans. Someone in contact with an infected dog is probably more likely to carry MRSP in their nose, but they are unlikely to develop an MRSP infection. However "unlikely" doesn't make it impossible.
If someone has a dog with an MRSP infection, it's important to know that this is different than MRSA. The risks of human infection are likely much lower. Nonetheless, the last thing you want is an infection with a multidrug-resistant bacterium, so some basic measures should be employed to reduce the risks:.
- Avoid direct contact with the infected site(s). If you have to touch infected site (e.g. when applying ointment or changing a bandage), wear disposable gloves and always wash your hands well when you take the gloves off.
- Keep infected sites covered with a bandage if possible.
- Wash your hands or use an alcohol-based hand sanitizer regularly after contact with the dog, and always after contact with the infected site (even if gloves were worn) or the dog's nose or hind end. Remember that MRSP can be carried in the dog's nasal passages and intestinal tract.
- Avoid contact with the dog's stool.
- People with weakened immune systems, very young children and elderly individuals are probably at higher risk for infections, and should therefore avoid contact with infected dogs whenever possible.
- Don't let infected dogs sleep on the bed or on other areas where people have close and frequent contact (e.g. couch).
- While the risks are low, if you have any concerns, talk to your physician.
Chloramphenicol: the good, the bad, and the "beware"
As we see more and more infections caused by antibiotic-resistant bacteria, we have to re-think our approach towards antibiotic therapy. This often involves using new drugs, but sometimes it also involves considering the use of older drugs that we haven't used very much for a long time.
One such drug is chloramphenicol. Years ago, this antibiotic was widely used, and is still used in people and animals in some situations. In some respects, it is a very good antibiotic - it is often effect against many bacteria including those that are resistant to many other drugs, such as MRSA and MRSI/MRSP. Chloramphenicol can also be given orally, and it's relatively cheap. 
Unfortunately this drug can also be very toxic, both to the animals being treated with it and to people that come in contact with with it in the process. In some animals, chloramphenicol can cause suppression of the bone marrow, where red and white blood cells are produced. This is more of a concern with long term use, but if the bone marrow does become suppressed, stopping treatment with the chloramphenicol typically resolves the problem. Unfortunately, this bone marrow suppression is much more of a concern in people - the supression is very severe, and can occur with exposure to even a very low dose (or probably single dose) of chloramphenicol. This results in a condition known as aplastic anemia, which it typically fatal. Fortunately this reaction is very rare in people, but there is no way to predict who might develop this condition, and since it is usually fatal, we obviously need to be cautious about using this drug. In some countries, chloramphenicol use in banned in all animals. In many others, it cannot be used in food animals, but can be used in pets.
- Chloramphenicol should only be used as a drug of last resort. There are situations where it is useful and may be life-saving, but the human health risks cannot be overlooked.
- If chloramphenicol is being considered, it is critical that people who would need to handle the drug understand the risk and how to safely handle the drug to avoid exposure.
- Chloramphenicol tablets should not be crushed or otherwise processed at home because of the potential for breathing in the drug when it is in powder form.
Direct contact with pills or liquids should be avoided by use of gloves or other safe handling practices.
- If a liquid form is used and is squirted onto food, the food bowl should be handled as if it is contaminated.
- If pills are being used, the animal should be observed to ensure that the pill is ingested and not spit up and left on the floor.
- Contact with the mouth and face and animals that are being treated with chloramphenicol should be avoided in case drug residues are present.
If people are unable or unwilling to follow safe handling recommendations, they should not use this drug.
Treatment of MRSA and MRSP: more than meets the eye
Methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-resistant S. pseudintermedius (MRSP) are important causes of infection in pets, and are resistant to many different antibiotics. It's therefore very important that these bacteria are cultured and tested in a lab to determine what antibiotics may be effective. Even then, choosing an effective medication can be difficult due to the limited number of potentially effective drugs.
Treatment selection is further complicated by problems that can occur when methicillin-resistant staphylococci are tested against certain drugs in the lab - problems that are not readily apparent unless extra testing is done. One "problem" drug is the antibiotic clindamycin. Clindamycin can be an effective treatment for MRSA or MRSP infections, but some MRSA strains may be resistant to it despite appearing to be susceptible based on normal laboratory testing. This is because the bacteria can have "inducible resistance". Inducible resistance can be detected by a special test called a D-test (see picture), but this test is not widely performed in veterinary laboratories.
In recent study presented by Dr. Meredith Faires (University of Guelph) at a conference last week, 55% of MRSA from dogs and cats that were reported to be resistant to erythromycin and susceptible to clindamycin were actually inducibly resistant to clindamycin. But no MRSA that were susceptible to erythromycin were inducibly resistant to clindamycin. In contrast, inducible resistance to clindamycin was not identified in any MRSP. Therefore, in the absence of specific testing for inducible resistance (i.e. a D-test), it is wise to consider all erythromycin resistant MRSA to be clindamycin resistant as well, regardless of the results of traditional antibiotic susceptibilty testing.
Methicillin-resistant staph: what's in a name?
There is a lot of concern and confusion about methicillin-resistant staphylococcal infections in pets. "Staphylococcus" is genus of bacteria which contains numerous different species. All these species can come in 'methicillin-resistant' forms that are resistant to many antibiotics, but some of these species are of greater concern than others. The main species of concern are discussed below.
Methicillin-resistant S. aureus (MRSA): This species is the big concern. MRSA is an important cause of disease in people and an increasing problem in pets. It can cause serious infections and be transmitted between animals and people (in both directions).
Methicillin-resistant S. intermedius (MRSI): Actually, what most laboratories still call S. intermedius appears to be a related species called S. pseudintermedius. Regardless, MRSI (or MRSP) can cause a variety of infections in pets - usually skin and ear infections. While it can be spread from animals to people, this is likely rare and there is much less concern about MRSI compared to MRSA in terms of risk to humans from infected pets.
Methicillin-resistant S. schleiferi (MRSS): This species is closely related to (and often misidentified) as S. intermedius. It is most commonly found in skin and ear infections. There is no information about the risk of transmission to people from pets. It is probably possible but very unlikely.
Methicillin-resistant coagulase-negative staphylococci: This is a large group of staphylococcal species that are usually lumped together. They are commonly found in or on healthy dogs and cats as part of the body's normal bacterial population. They are less common causes of disease. There is currently minimal concern about transmission to humans.
MRSA is the main concern with regard to staph infections, and warrants particular attention when it causes infection in a pet. In these cases, important precautions to take include washing your hands frequently, avoiding contact with the infected part of the animal's body (plus the nose, where MRSA often hangs out), keeping the infected site(s) bandaged (if possible), keeping infected pets off the bed, and restricting contact with high risk people (e.g. young children, elderly, immunocompromised). For the other staph species, the risks to people are much lower, but it is still reasonable to implement these measures to further reduce the risks.
Information Sheets for Pet Owners
INFORMATION SHEETS specifically for KIDS, for VETERINARIANS, for PHYSICIANS and for PUBLIC HEALTH PERSONNEL are also available on the Worms & Germs RESOURCES page!
Click on the highlighted topics below for information sheets. Topics that are not highlighted are in development and coming soon. Sheets for other animal species and diseases are also under development and will be added when they are available.
| Animals | Diseases | Other |
| Dogs | Rabies | Litter Boxes |
| Cats | Giardia | Sandboxes |
| Turtles | Toxoplasma | Cat Bites |
| Hamsters | Leptospira | Raw Meat |
| Rabbits | Clostridium difficile | Petting Zoos |
| Pet Birds | Cryptosporidium | Needlestick Injuries |
| Reptiles | MRSA | |
| MRSP | ||
| Salmonella | ||
| Ringworm | ||
| Campylobacter | ||
| Bartonella henselae | ||
Please Remember:
- Your veterinarian and physician are your ultimate resource for information about the health of your pets or your family.
- Information provided here is accurate to the best of our knowledge, but infectious diseases can be unpredictable and these sheets are for general information purposes only.
- There can be great variation in disease risks in different geographic areas. The information provided was developed for Ontario, Canada, but most of the information is relevant for other regions as well.
