Is Horse Manure on Trails a Risk to Human Health?

by Marlissa Campbell, Contributing Writer

One of the concerns people sometimes have about sharing trails and other recreational spaces with equestrians is encountering the inevitable piles of equine fecal waste. Equestrians, for their (I should say "our") part are too often inclined to brush off such concerns as silly. If you have a horse, you spend a lot of time shoveling horse poop and it's never done you any harm, right? In fact, most of us kind of like the tangy smell of manure -- it means a healthy horse is nearby! But what are the actual facts about harmful micro-organisms making their home in that "brown gold?"

According to Rutgers University Equine Science Center, horse manure should not pose a significant health risk to people coming across it on the trail. Specific pathogens of potential concern include Salmonella, certain strains of E. coli (particularly the O157:H7 strain), Cryptosporidium, Giardia, and Campylobacter.

• Salmonella -- Salmonella can be present in horses, but has actually been found in so few horses (1 in 500) as to be considered insignificant.
• E coli -- No incidents of human disease caused by equine E. coli have been reported. E coli of the O157:H7 strain have been found in horse intestines, but only in insignificant amounts.
• Cryptosporidium parvum and Giardia -- Cryptosporidium has been reported at a frequency of 1 in 300 horses; Giardia at a frequency of 2 in 300 horses.
• Campylobacter -- No links have been shown between Campylobacter in the equine gut and human health.
• Leptospirosis -- Leptospira is a spirochete that can infect horses, but is not transferred in feces. It can be shed in other bodily fluids, such as urine, but horses have not been associated with the disease in humans.

Researchers from the University of California, Davis Medical Center collected samples of horse and mule manure along 186 miles of trails in major national parks and wilderness areas in California. Overall, the samples contained large numbers of normal equine gut bacteria, which do not cause human disease. Only 12 out of the 81 pack animal samples contained bacteria with the potential to cause diarrheal disease in humans. Particular attention was paid to any indication of E. coli O157 or Salmonella. Neither of these organisms were found in any of the pack animal manure samples. One pack animal sample was found to contain the disease-causing protozoa, Giardia. The authors concluded that while potentially pathogenic organisms were found in some samples of horse manure collected along trails, the prevalence was low.

These results could be compared to findings for samples of dog feces collected from the streets of St. Paul, Minnesota. Of those dog fecal samples that contained growing E. coli, more than half contained strains of E. coli identical to bacteria isolated from human patients suffering from serious infections including cystitis, pyelonephritis, bacteremia, or meningitis. The authors of this study concluded that dog feces, and hence domestic dogs, serve as a reservoir for strains of E. coli that are pathogenic (disease causing) in humans.

Presumably these differences between horses and dogs result from their contrasting diets and digestive systems, as well as the greater intimacy with which humans live with our canine pets. In other words, dogs have more opportunities to catch bad bugs from us, and then pass them back.

Unlike dog walkers, horse riders are generally not required to stop and clear up after their horses on the trails. This reflects the comparatively low health risk of horse manure. The potential for a dismounted rider to lose control of their animal while attempting to clear up is deemed to be the greater hazard to other trail users. Nonetheless, equestrians do have a responsibility to clean up where they reasonably can -- around their trailers at trail heads, and when their horses are securely tied at a camp or picnic site.


Marlissa writes about books (including horse books!), reading, and writing on her blog, "You Are What You Read."