Dr. Stephen Page, a regular supplier of good blog material, sent me a couple papers from the Quarterly Journal of Medicine the other day. One was an interesting report of "Staphylococcus intermedius" infection in a person, entitled "A canine bug in a human heart" (Koci et al, Q J Med 2015;108:337-338).
It’s almost guaranteed that this wasn’t S. intermedius but rather S. pseudintermedius (some medical microbiology labs are apparently still a couple decades behind in identifying this bug). Regardless, it’s an interesting case of a 58-year-old man with a pacemaker that developed fever, chills and a headache. He reported that a neighbour’s dog had licked his hand a few weeks earlier. The pacemaker incision site was unremarkable but "Staphylococcus intermedius" was isolated from two different blood samples. That's a significant concern in a patient like this because of the potential for infection of the heart valves and/or the pacemaker leads. Infections like that can be very serious and hard to eliminate (especially since we know that S. pseudintermedius tends to produce biofilm, which helps it hang around sites like pacemaker leads and avoid antibiotics). Fortunately, after a couple rounds of antibiotics and removal of the pacemaker system, he recovered.
It’s interesting (and encouraging) that the dog exposure was reported. Whether he offered the information or they asked about dog contact isn’t clear, but this is the type of information that’s often missed.
Putting this report into perspective is important. This, and various other reports of S. pseudintermedius infections, show that this dog-associated bacterium can cause disease in people.
- Single cases continue to appear in the medical literature. That means it’s really rare (since a single occurrence is enough to prompt a publication).
- The relative risk from exposure is limited. Most dogs carry this bacterium and huge numbers of people are exposed every day. So, the incidence of disease with respect to exposure is incredibly low
While "low" is good, it’s not much consolation if you’re the one with the rare but life-threatening infection. So, some basic preventative practices are indicated. Avoiding contact of dog saliva with open wounds would be one. Good general hygiene practices (especially handwashing), avoiding contact with feces and similar common sense measures are probably the key. Making sure physicians know about animal contact and think about potential zoonotic infections is also important, particularly for people who are at increased risk of infection and disease.
Some people like to send me links to internet sites to see if they can get a rise out of me. There are a few usual suspects (both senders of information and places I get sent to) but a new one for me was tlcpetfood.com
For some reason, this site has a series of FAQ's completely unrelated to pet food. Some are rather bizarre, such as "My dog keeps getting pneumonia, and we just found out her internal organs are on the wrong side. Help? "
Many of the answers are fine. That's because they're plagiarized... verbatim text taken from reputable sites (mainly AAHA's Healthy Pet site) without attribution. Besides the whole violation of intellectual property aspect, it's at least good that the advice is sound.
Some of the other answers they provide (likely the ones that aren't plagiarized) are considerably less sound.
The one that got sent to me was "Is it okay for my dog to lick my son's face?"
This is actually a common question and a reasonable one. There's no perfect answer to it, but there are definitely some imperfect answers, such as this one:
(It starts out okay...)
Yes, it probably is.
- I'd agree with that statement.
(Then goes downhill quickly...)
The only disease that dogs and humans can pass back and forth through saliva is beta strep throat, which is relatively rare.
- This is a myth that just won't go away. There's no evidence that pets are relevant sources of strep throat. Furthermore, there are many other pathogens that can be transmitted from dogs' saliva to people. Disease isn't common but it does occur and it can be fatal in some situations.
And if your son has a weakened immune system, you may want to be careful about exposing him to the normal bacteria that's present in the saliva of healthy dogs.
- Good advice. (However, if their statement that strep is the only thing that can come from dogs was actually true, this one wouldn't make any sense.)
My response to this common question is that I don't particularly like being licked by my dog. It's a personal thing and not a germaphobic response. It's unlikely to harm me as an adult with a (hopefully) functional immune system. I don't hover around my kid to make sure they don't get licked, but I don't encourage it either.
Licks to young kids (especially around the face), licks that have contact with skin lesions or mucous membranes (e.g. mouth, nose) or licks to people with compromised immune systems (including people that do not have a functioning spleen) are higher risk. Strep throat isn't a concern, but many other things are. There's a cost-benefit. If it's an important part of someone's bond with his/her animal, that's fine. Individuals just need to understand the risks, and be aware of when the risks are higher. Part of that is getting good advice, which can be a challenge on the internet.
BMJ Case Reports has a recent paper entitled “Cirrhosis, cellulitis and cats: a ‘purrfect’ combination for life-threatening spontaneous bacterial peritonitis from Pasteurella multocida” (Hey et al 2012). (I don’t think we'd be able to use a title like that in a veterinary journal, but they often get away with titles playing on the animal side in medical journals.)
The case report describes a man who had liver cirrhosis from hepatitis C infection and alcoholism. He went into hospital with a chronic infection of his leg. He’d been seen various times for this problem and various bacteria had been isolated (leading to treatment with various antibiotics). This time, he had severe swelling of his leg that went from his calf to his groin, along with significant accumulation of fluid in his abdomen. Shortly after admission, he deteriorated and became septic (he developed an overwhelming bloodstream infection) and went into liver failure. Blood samples and samples of fluid from his abdomen were tested and the bacterium Pasteurella multocida was isolated. Fortunately, he responded to aggressive treatment.
Upon subsequent investigation, they found out that the patient had a cat, and that cat "had a disturbing habit of regularly licking the serous exudate [leaking fluid] from the patient’s chronic left leg leg cellulitis." (This probably occurs much more commonly than most people think.)
Pasteurella multocida is a bacterium that’s commonly linked to cats, and it’s a frequent inhabitant of the mouth of healthy cats. The cat wasn’t tested but it’s a reasonable assumption that this infection came from the pet. The patient was given “the appropriate advice with regard to the safe cohabitation with domestic animals” but unfortunately they don’t say what that advice was.
This is just one more in a series of reports that show the importance of asking people about pet ownership and pet contact. Asking that question initially can potentially help identify zoonotic disease risks, as opposed to asking the question after the zoonotic pathogen is eventually found when they’re trying to round out the story.
The authors list some ‘learning points’. I’ve copied two important ones below.
- Domestic animals can, under the appropriate clinical circumstances, pose a risk to an immunocompromised host, particularly if adequate hygienic practices are not adhered to.
- Questioning regarding exposure to domestic animals should form part of the clinical history when assessing immunocompromised hosts, allowing early institution of empiric antimicrobial therapy in the appropriate clinical scenario.
They’re both good points, although I’d remove "immunocompromised." While the risk is lower in people with normal immune systems, some risk still remains and pet contact should be queried all the time.
People sometimes get freaked out by the concept that they have approximately 10-times as many bacterial cells on them as all their own body cells combined.
- Yes, our cells are a minority in our own bodies, and amongst the trillions of bacteria we carry are many that could kill us given the opportunity.
Yet, we have somehow managed to survive, both individually and as a species. So, keeping things in perspective is important and, in reality, we need much of that bacterial population to keep us healthy.
Just like every person is carrying many bacteria on any given day that can cause illness or infection, every animal is carrying many different microorganisms that can infect a person. While infections from pets do happen, they are uncommon - we're not seeing dog owners dropping like flies on the street, which is a testament to our immune system and other body defenses and barriers (e.g. intact skin).
So, when studies come out describing various bugs at various sites in various animals, you have to put them into context. It's not that the studies are bad (my lab does a lot of work trying to define the complex bacterial populations of sites like the intestinal tract, oral cavity, respiratory tract and skin), it's that we need to think about what the results really mean and avoid sensational headlines in the press.
A recent paper in the Archives of Oral Biology (Yamasaki et al. 2012) is an example of this. The study looked at mouth bacteria in dogs and their owners. They used molecular testing developed for human oral samples and focused on bugs that have been implicated in dental disease - not the range of bugs that are more often associated with zoonotic infection. Not surprisingly, they found lots of different bacteria in the mouths of the dogs, including some bacterial species that were present in both dogs and their owners. No methods were used to type the bacteria to see whether the strains found in dogs and people were the same or whether dogs and people just normally have those bugs present, independently, in their mouths. However, we know that transmission of certain bacterial between people and their pets is a relatively common event, and it wouldn't be surprising if the same applies for oral bacteria, through direct contact (e.g. kissing/licking) or through indirect contact (e.g. a person touching a dog's face then his/her own face).
So, this was an interesting study... nothing earth-shattering but the first side-by-side comparison of oral microflora in people and dogs, and it provided some useful information for future research. The authors rightly discuss the limitations and things that need to be done to further investigate this, such as looking at strains and evaluating the bacterial population in relation to types of contact that people and dogs have (e.g. do certain activities increase the likelihood that people and pets share oral bacteria). They conclude by writing "In summary, we found that the distribution of periodontopathic bacterial species in dogs and their owners is diverse, though several species including P. gulae may be transmitted during close daily contact. Therefore, our findings could be significant in understanding the relationship between the oral health of humans and their companion animals"
Yet, headlines like "New study warns against kissing your dog" don't really reflect the true content... not uncommon but unfortunate.
I get asked about dogs licking a lot. My general line is that I don't particularly like to be licked but for the average healthy person, I don't get worked up about it. My kid were playing with puppies yesterday and were getting licked. I didn't fire up the power washer to hose them down after. If I had an infant, an immunocompromised child or some other high risk person it would be a different story. Licking around the ears is something I like to see avoided because of some links between this activity and certain ear infections in kids, but overall it's a relatively low risk situation that some people enjoy.
While rabies is classically transmitted from animals to people by bites, any situation that allows saliva from an infected animal to get past the body's protective skin barrier can result in infection. Graeme Anderson, a 29-year-old South African canoeist, recently died after contracting rabies from a sick dog for which he was caring. There was no history of a bite, but the dog had licked damaged skin on the man's hands, allowing the virus to enter the body.
Any contact with animals showing signs consistent with rabies needs to be investigated. Licks over damaged skin (or mucous membranes like the mouth) are classified by the World Health Organization as having the same level ("severe") risk of rabies exposure as bites, and post-exposure prophylaxis is indicated. Bites are the main source of rabies transmission, but not the only source, the fact of which situations like this remind us.
When it comes to public health concerns about staphylococcal bacteria from pets, most of the attention gets paid to methicillin-resistant strains like MRSA. That's not surprising considering how important MRSA is in human medicine. However, staph that aren't methicillin-resistant can also be a problem, since they can cause the same types of infections that resistant types can (they are just easier to treat). Another issue that often gets overlooked is staphylococcal food poisoning.
Staphylococcal food poisoning is one of the most common foodborne illnesses and results from growth of certain strains of staphylococci in poorly handled or stored foods. If staph get into food and the food is kept at improper temperatures, the bacteria can grow. If the strain of staph that's in the food is one that can produce enterotoxins, these toxins can accumulate in the food at high enough levels to cause food poisoning when eaten. In most cases, people are probably the origin of enterotoxin-producing staph that contaminate food, but pets are another possible source.
A recent study in Vector-borne and Zoonotic Diseases (Abdel-moein et al 2011) looked at enterotoxigenic staph in 70 dogs and 47 cats. Swabs were collected from the mouth, nose and wounds. Nasal swabs were also collected from 26 people. The researchers isolated enterotoxigenic Staphylococcus aureus (strains of S. aureus that possessed genes for enterotoxin production) from 10% of dogs and 2.1% of cats, as well as 7.7% of people. Most of the positive samples from pets were oral samples.
This study shows that dogs and cats can be potential sources of strains of S. aureus that cause food poisoning. Since the staph are often in the animals' mouths (and therefore presumably shed in saliva), animals can potentially contaminate food with these enterotoxigenic staph fairly easily, but it's unknown how often this occurs.
Prevention measures are pretty basic but should be considered, including:
- Keeping pets off kitchen counters.
- Discarding foods that pets have licked.
- Washing hands after pet contact, before handling food.
- Properly storing food, so that even if it gets contaminated with staph, the bacteria don't get the opportunity to grow and produce toxins.
- Photo from http://www.wagreflex.com/2009/06/taking-cat-ownership-to-the-next-level.html
Using logic akin to "Chocolate cake? It has flour, eggs and milk. It's virtually health food. You should eat it every day.", AllPetNews.com has an article on its site entitled "Dog saliva has healing properties." The article focuses on potential beneficial compounds in saliva, with specific reference to a University of Florida (Gainesville) discovery of nerve growth factor (NGF) in saliva (although I can't find any reference to nerve growth factor in dog saliva on PubMed).
The article states "Wounds that were treated with NGF actually healed twice as fast as untreated wounds, indicating that if a dog does lick a humans wound, it could in fact lead to a faster recovery."
The problem here is taking some controlled research data and spinning it out of control. Putting synthesized or concentrated nerve growth factor on a wound in a controlled manner is different from putting saliva on a wound, which is also different from having a dog lick it. You have to consider the whole picture when evaluating potential treatments. For example, rubbing alcohol kills bacteria nicely but that doesn't mean it's a good idea to put it on a wound to prevent infection. It hurts and it damages tissue. While it may kill superficial bacteria, the tissue damage can ultimately increase the risk of infection.
While there are certainly some compounds in dog (and human) saliva that stimulate healing, those potential benefits need to be weighed against the potential adverse effects, particularly infection. The oral cavity of the dog contains billions of bacteria from hundreds of different bacterial species. Many of these are able to cause infection given the right circumstances. A wound helps create the right circumstance by breaking down the body's normal protective barriers.
The article does mention some dangers, although only with licking of deep wounds (a wound doesn't have to be deep to become infected), and concludes with:
"Although the healing properties outweigh the negative impacts there will always be a number of people that find it in their best interest to treat a wound by cleaning it with soap and water...."
- At this point, there's no evidence that the beneficial properties of dog saliva outweigh the negative impacts. In reality, people would be better off if they carefully cleaned wounds with soap and water. Licking may not cause a problem in many (or even most) situations, but it can lead to serious infection, and the lack of clear evidence of benefit alongside documented risks indicate that this is an inappropriate activity.
I was asked this the other day, in regards to a post about pets and recurrent strep infections in people: "You listed a few things to remember and one of them was how the pet might be an "innocent bystander infected by a family member." Is there any indication that a dog might get sick from licking a person infected with Group A Strep?"
Streptococcal infections in dogs are very rare. When they occur, they are typically caused by Streptococcus canis, a Group G strep. Group B strep infections have also been reported. I'm not aware of any reports of Group A (Streptococcus pyogenes) infections in dogs, despite the fact that exposure is probably very common.
Group A strep is a predominantly, if not exclusively, human pathogen. It can be found in healthy individuals (e.g. in the throats of 10-15% of healthy kids) and is the main cause of strep throat. Group A strep also causes invasive infections such as cellulitis, various soft tissue infections, and in rare circumstances, necrotizing fasciitis (flesh-eating disease). Considering how commonly healthy people carry this bacterium and how common strep throat is, you have to assume that dogs are frequently exposed to this bacterium from household contacts.
Licking a healthy human carrier would certainly create an opportunity for a dog to be exposed. Licking wounds of patients with strep infections would probably be worse. Since exposure is probably common and we don't really identify problems in dogs with this bacterium, the risk of infection in dogs is presumably very low. However, basic measures should always be used to reduce the risk of exposure to infectious agents. Even though we don't recognize Group A strep as a problem in dogs, you don't want your dog to be the first case. Dogs shouldn't be allowed to lick infected wounds for both the health of the dog and the person. Strict avoidance of people with strep throat doesn't make sense, but licking should perhaps be avoided since the sick person could also be at higher risk for a secondary infection from the multitude of bacteria present in the dog's mouth.
Bottom line... the risk of Group A strep infection in dogs is minimal, but basic hygiene practices can make the risks even lower.