Equid Blog : Horse Health & Diseases : University of Guelph : Ontario Veterinary College : Equine Infectious Diseases

Methicillin-resistant Staphylococcus aureus (MRSA) is a huge problem in people and is an emerging pathogen in horses. Most earlier reports of MRSA in horses involved one strain, called CMRSA-5 in Canada, USA500 in the US, and sequence type 8 (ST8) as a more general term. This human-origin strain seems to be adapted for survival in horses, and in North America, this strain has accounted for most MRSA infections in horses and MRSA infections in people linked to horse contact.

Another MRSA strain, ST398, has recently emerged as a big problem associated with livestock (particularly pigs). This strain is very common in pigs internationally, and is a major cause of infections in people in some European countries. There are also a few reports of ST398 in horses. Most are from Europe, although we have found this strain in one horse in North America. At last week's ASM Conference on Antimicrobial Resistance in Zoonotic and Foodborne Pathogens in Toronto, Dr. Engeline van Duijkeren from the Netherlands presented a case of human ST398 infection linked to a horse.

In the reported case, a 16-year-old girl had a lesion on her foot that was initial diagnosed as a spider bite infection (a common misdiagnosis of early MRSA skin infections). It didn't respond to initial treatment and MRSA was isolated on culture. The girl didn't have any history of contact with pigs or cattle, but had close contact with a foal. That foal had previously been in an equine hospital because of a wound infection, but the wound was not cultured. The same MRSA strain that caused the infection in the girl, however, was found in the foal's nose (the prime site for MRSA carriage by healthy horses). Fortunately, the girl's infection responded to treatment once treatment was adjusted for MRSA.

In some ways, this case is not too surprising, since we know ST398 MRSA can cause disease in people, and since it is found in horses, transmission from horses to people was likely inevitable. However, it's the first report of human infection with this strain associated with horse contact. MRSA exposure is a potential risk for anyone working with horses, since we know that this organism can be found in a small percentage of healthy horses. We don't have great information about how to prevent horse-human transmission, but simple things like only using antibiotics when needed and attention to hygiene (especially hand washing) when working with horses are presumably important factors.

More information about MRSA in horses can be found on the equIDblog Resources page.

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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.

(click image for source)

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Unfortunately, but not surprisingly, Hendra virus has resurfaced in Australia. Horses are the only known species that are naturally infected by this deadly virus, which is carried by flying foxes (bats). Death rates are high, and the infection can be transmitted to people, with fatal consequences. Four out the seven people that have ever been diagnosed with Hendra virus died.

In the latest equine case, nine people were exposed to the infected horse (that was euthanized on Monday), including a nine-year-old girl. All of these individuals are being monitored. No treatments are being given because there is no established way to treat or prevent the disease. Ribivirin, an antiviral drug, has been tried in some people that have been exposed to Hendra virus, but the drug is quite toxic and it's far from convincing that it works. So, for the moment, they are taking a wait-and-see approach. Fortunately, it seems like very close contact with secretions from infected horses is required for transmission, so hopefully the true risk of exposure for these individuals is low. It's going to be a tough couple of weeks for them, however, while they wait to see if any of them get sick.

This is a very tough situation to control and a major concern for people in Queensland.

• There is no clear way to prevent exposure of horses to this virus, since it has a wildlife reservoir.
• There is no vaccine available, although fundraising is underway to support vaccine research.
• There are no effective treatments.
• Measures to reduce the risk of transmission from horses are based on common sense, but have not been specifically investigated.
• Hendra virus is not always an obvious diagnosis when a horse first begins to show signs of illness, so people can be exposed before realizing they may be dealing with a deadly disease.

More information about Hendra virus is available in our archives.

Image: Australian Stock Horse (click for source)

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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.

This equIDblog entry was originally posted on our sister site, Worms & Germs Blog, on 20-Jan-10.

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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.

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