Strep zoo infections in people
Streptococcus zooepidemicus is an important cause of infections in horses. This bacterium can also be found in healthy horses. When you consider the large number of horses that are infected and the larger number of healthy horses that are carriers, along with the close contact that people have with horses, it's pretty obvious that people are regularly exposed to "Strep zoo". This bacterium is not well-adapted to survive in people and cause infections, so human infections are quite uncommon, but they can occur. There are periodic reports of S. zooepidemicus infections in people, with varying degrees of association with horse-contact.
An upcoming edition of journal Epidemiology and Infection contains a report of S. zooepidemicus meningitis in a 51-year-old woman (Minces et al, 2010). This person had a mild upper respiratory tract infection, then developed signs of meningitis (including fever, unresponsiveness, respiratory distress). A spinal tap was performed and S. zooepidemicus was isolated. The woman fortunately responded to treatment and recovered.
Upon initial questioning of the patient's mother, no animal contact or ingestion of unpasteurized dairy products (another risk factor) was reported. However, it was later revealed that the woman's daughter had started horseback riding at a friend's farm approximately one month earlier. The type of contact that the woman had with horses (if any) at the farm was not reported, nor was there any investigation of S. zooepidemicus shedding by horses on the farm.
Based on the fact that this is typically an equine-associated bacterium and the history of contact (albeit potentially limited or indirect) with horses, horse contact was blamed for the infection. It's a reasonable conclusion but it's far from certain because of the nature of the contact, the lack of any proof of the same strain of S. zooepidemicus in horses on the farm, and previous reports of infections occurring in people with no contact with horses.
Exposure to S. zooepidemicus is an inherent risk of having contact with horses. It's nothing to lose sleep over and is probably relatively low on the list of potential health problems associated with horse contact. The risk is probably greatest in people with compromised immune systems and other general risk factors for disease such as advancing age and pregnancy. Good general hygiene measures, avoiding contact with sick horses and close attention to hand hygiene probably minimize these already low risks.
This Worms & Germs blog entry was originally posted on equIDblog on 25-May-10.
Bare feet and horse bugs
I assume that people wouldn't voluntarily and regularly walk around barefoot on dog feces (or feces of any type), yet it's perplexing that some people regularly clean out horse stalls in bare feet (I've seen it done!). While horse manure may not be as inherently gross as dog poop, it's still feces, and like all feces contains a huge population of various bacteria, some of which can be harmful. The risks of barefoot mucking may also extend to bare feet inside boots, although I don't think sock-averse people need to panic.
An article in the International Journal of Infectious Diseases (Friederichs et al) describes infectious arthritis of the shoulder of a horse owner that was caused by Streptococcus zooepidemicus, a bacterium commonly found in horses but rarely associated with disease in people. The person didn't have a wound in the shoulder area or any other obvious route for the bacterium to get to the shoulder joint. They searched for a source of the infection and all they found was a chronic lesion on the person's foot. This, combined with the patient's history of taking care of his horses in "bare feet in boots", led them to implicate the foot as the source of infection.
The idea, I guess, is that socks would be a barrier to help prevent contamination of the foot wound. That makes sense to a degree - the person could contaminate his foot with S. zooepidemicus from his hands (probably acquired from touching the horse's nose) while removing the boots, or manure could work its way into boots and directly contaminate the wound. Both are possible, but we have to be a little cautious in interpreting these conclusions. However, this is a bacterium that is associated with horses and the foot lesion is certainly a possible route of entry.
Overall, this should be considered an interesting report of a very rare problem, not something that indicates a major concern. However, there are a few good points to take away from this story:
- If you have a wound or chronic lesion of any sort, make sure you take measures to reduce the risk of bacterial contamination when working around horses. This might be as simple as making sure it's covered by clothing, or something more involved like using an impermeable bandage.
- Hands are probably the major source of infection transmission, and good hand hygiene is important after horse or stall contact, particularly if you have an underlying disease.
Another fatal strep outbreak at a shelter
A very poorly-written and confusing report suggests that another Streptococcus zooepidemicus outbreak is underway in dogs in a shelter in Ohio. Five of 175 dogs on the premises died suddenly of hemorrhagic pneumonia. The report variably mentioned a "virus that mutated from horses," that it's thought to be "not contagious" despite multiple dogs being affected, and that it's a "rare form of streptococcus" (a bacterium). Presumably, they are dealing with a group of dogs with Streptococcus zooepidemicus pneumonia (technically, Streptococcus equi var. zooepidemicus). This bacterium predominantly lives in horses but periodically causes infections in other species. Outbreaks in dogs are uncommon but have been reported in other shelters. I assume that cultures from the dead dogs identified the bacterium, otherwise other possible causes such as canine influenza would also have to be considered.
The statement about it not being contagious is bizzare. Obviously, it is contagious between dogs. It may have been referring to dog-to-human transmission, but while that's rare it has been reported.
The shelter is apparently treating all dogs with penicillin prophylactically (i.e. to prevent any more dogs from getting sick). There's no clear guidelines regarding management of S. zooepidemicus outbreaks in kennels. It's now known whether mass antibiotic treatment does anything helpful, but it has been used in other outbreaks. I think it's likely that these outbreaks stop on their own, rather than penicillin having a major impact, and that there's probably another underlying cause such as a viral infection to account for outbreaks of this rare disease. However, that's just speculation for now. Hopefully this outbreak will cease with whatever treatment and infection control measures they put in place (or on it's own). Hopefully a good review of routine infection control practices will be performed at the same time, as routine practices (or lack thereof) are often a major problem in shelters.
Group A strep and dogs
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.
Horse Strep in a person via a dog
Streptococcus equi subspecies zooepidemicus (usually just called Strep zooepidemicus) is a common cause of infection in horses. It is an "opportunist" that is often found in healthy horses, but which can cause disease in certain situations. While horses are the natural host of this bacterium, sporadic infections and outbreaks are occasionally reported in dogs at cats, particularly in shelters or other crowded situations. Severe (including fatal) pneumonia can occur, as was reported in a recent outbreak in a humane society in Ottawa. Rarely, S. zooepidemicus can also cause infections in people.
A report in the Journal of Medical Microbiology (Abbott et al) describes a serious S. zooepidemicus infection in a person, that was traced back to a dog. The dog lived on a farm that also had horses. It developed pneumonia and S. zooepidemicus was isolated from its respiratory tract. The dog was treated and recovered. However, the dog owner also became ill with fever, headache, a stiff neck and general malaise. Penicillin was prescribed, but the person's condition did not improve and he/she ended up in the hospital. Streptococcus zooepidemicus was also isolated from this person's nose and throat. When the dog and human strains were compared using molecular tests, they were related. An investigation of the farm was performed, and while all the horses present at the time were negative for S. zooepidemicus, the bacterium was isolated from a healthy dog.
This is a rare situation and one that shouldn't result in too much concern. It does highlight a couple points that are good to remember:
- Getting cultures is very important for obtaining a diagnosis.
- Animal contact and pet health should be considered whenever someone is sick with a potential infectious disease. Physicians need to know whether their patients have contact with animals. They need to be told if a sick animal is present so they can consider whether the pet and human illness might be related. Knowing to what someone may have been exposed might speed up diagnosis and appropriate treatment.
- Rare things are rare, but they happen. We shouldn't focus on rare events but we have to keep our minds open and recognize that strange things happen with infectious diseases.
"Horse strep" in people
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I received the following question the other day: "I have a friend who had chemo embolization on tumor on liver in late June. She is in hospital now, and an abscess was discovered on liver. Pathology results said "equine strep". Her brother visited immediately after procedure, and he works with horse full time."
Streptococcus is a group of bacteria that includes many different species. There are two main species in horses Streptococcus equi subsp. equi (aka S. equi, the cause of strangles) and Streptococcus equi subsp. zooepidemicus (aka S. zooepidemicus, a cause of various types of infections). As you can guess by the 'equi' name, their natural host is the horse. Strep infections are very common in people, but rarely involve these two species. Nonetheless, infections with either Streptococcus equi or S. zooepidemicus can be found in people, but S. zooepidemicus is most common. Usually, these infections develop in people who are already sick for another reason, have compromised immune systems, or in young children. Interestingly, not everyone that is infected reports direct or even indirect contact with horses.
Back to the question: it's hard to say what's going on here based on the the general term "equine strep", but presumably the person has an infection with S. equi or S. zooepidemicus. Whether horses are actually involved will be tough (or impossible) to determine. It's a tempting hypothesis that the patient's brother carried the bacterium from the farm to the hospital, but I'd be wary about making a definitive statement about the bacterium's origin solely based on that. There are ways to investigate this further, such as trying to isolate Streptococcus species from horses on the farm, typing them and comparing them to the strain that caused disease in the person, but this type of testing is very costly and almost never performed, as human infection with these species is so uncommon.
This should be a good reminder that people who are sick and in hospital are at higher risk for developing infections, and they can get infections from bacteria that rarely cause disease in healthy individuals. While there is no proof of a link to horses (at least in this case), good infection control practices should be used whenever anyone visits someone in the hospital. That would include not wearing barn clothes to the hospital and paying close attention to handwashing.
This Worms & Germs blog entry was originally posted on equIDblog on 26-Aug-09.
Group B Streptococcus: Don't blame the dog
I received the following question from a reader the other day: "I'm currently pregnant and was bitten by my grandmother's German Shepherd. The bite was on my ankle and broke the skin in several places. I went to the doctor and was prescribed antibiotics and the wound has seemed to heal fine. This is my second pregnancy and I have been diagnosed as group B strep positive, which I wasn't with my first child. I know that dogs can't spread strep throat to humans, but is it possible that I picked up group B strep from the bite?"
The short answer is that it's extremely unlikely there's an association.
Group B Streptococcus is predominantly a problem in people. Most people that carry this bacterium have no problems, although it can cause infections in some situations. It is of particular concern in pregnant women, because in 1-2% of exposed newborn babies the bacterium can cause serious infections such as bloodstream infections, meningitis and pneumonia. That is why pregnant women are often screened for Group B Streptococcus shortly before their due date, by taking a swab from the vagina and rectum. Approximately 10-30% of pregnant women carry Group B Streptococcus. Pregnant women that are carriers are usually given antibiotics shortly before delivery to reduce the risk of infection of the baby.
What about the role of pets? Group B Streptococcus is mainly found in people, and is quite common in healthy people. It is rare in pets, although it can cause various types of infections in animals too. Group B Streptococcus infections in dogs might actually represent human-to-dog transmission, although this hasn't been proven. In the case described above, a dog bite on a person's leg would not be a high risk for transmitting this bacterium to the intestinal tract or vagina. If a dog was carrying this bacterium in its mouth, it could cause a bite wound infection, but it is very unlikely that the bacterium would spread to other parts of the body in a healthy person. Other bacteria in the dog's mouth would be more likely to infect such a wound, even if Group B Streptococcus was present. If dogs were common carriers of this bacterium (which they are not), the main risk of transmission would be from regular contact, not bites.
So don't blame the dog... at least not for the Group B Streptococcus. The bite itself is another issue.
Pets and strep throat
Strep throat is caused by Group A Streptococcus, a bacterium that can be found in the throat and on the skin of some healthy people. Strep throat and impetigo are the most common diseases caused by Group A Streptococcus, although severe (‘invasive’) infections can occur, including ‘flesh-eating disease’. Group A Streptococcus is typically spread between people, both from people that are sick and healthy carriers.
Group A Streptococcus carriage by dogs and cats is extremely rare, and it is unlikely that they are involved in transmission to people. There were some older studies implicating dogs in transmission of Group A Strep, however there were weaknesses in the methods used by those studies which probably lead them to misidentify other types of Streptococcus that are often found in dogs as Group A Streptococcus. There is currently no convincing evidence that pets are a source of strep throat infection, although the possibility cannot be completed dismissed.
I have had questions about treatment of pets when recurrent strep throat infections were present in a household, which is not supported by any evidence and could lead to problems like antibiotic resistance and side-effects from antibiotic use such as diarrhea. It’s hard to say whether there is any indication to test dogs or cats when recurrent strep throat is present in a household. Collection of a throat swab by a veterinarian and culture of the swab is fairly easy to do. It’s not unreasonable to consider that but a few things must be remembered:- Even if Group A Streptococcus is found in a pet, it does not mean that the pet is spreading it. The pet might just be an ‘innocent bystander’ that was infected by a family member. It makes no sense to test the pet if the rest of the household is not being tested.
- Proper identification must be performed by the laboratory to differentiate Group A Strep from other strep. Just finding ‘Streptococcus’ is not useful.
- There are no guidelines for what to do in the unlikely event that a pet is identified as a carrier.
Overall, pets are not likely a major (or even minor) source of strep throat. If strep throat is circulating within a household, it's most likely being spread between people.
