Horse manure is recycled grass. No major disease has ever been linked to exposure to horses or horse manure, even among veterinarians or others who care for horses. However, poor hygiene while handling manure or contaminated water can create health problems for people with weak immune systems. Most problems come from ingesting contaminated water or food, not inhaling airborne particles.
Generally speaking viruses are specific to a particular species and do not transfer from one type to another. Giardia and Cryptosporidium are heath concerns that can affect humans. However, these organisms transfer through contaminated food and water, and are not airborne. Horses intestines don’t contain high levels of bacteria dangerous to humans,such as e coli or Salmonella. And horse manure is 70-80% water further diluting these pathogens.
However, recent research raises some questions about inhaling dust contaminated with a soil bacteria, rhodococcus equi,. A few cases of pneumonia have been attributed to inhaling manure by people with compromised immune systems such as AIDS or immuno-suppressive therapy patients.
It is also unknown how long a pathogen can remain active once dried out. Clostridium bacteria are known to have the ability to form spores that are resistant to drying out even over long periods of time. There are documented cases of soldiers or athletes getting tetanus from spores in fields where horses lived years earlier. However, this information was published in 1956 and no mention of these findings are mentioned in more recent research.
Inhaling manure is something that should be avoided for your horse as well. It is healthier to feed hay from the ground rather than from hay nets to avoid inhaling too much dust. Rhodococcus equi bacteria mentioned above grow rapidly in young foals under 12 weeks. A dusty environment can cause foal pneumonia and can be lethal to young horses. However, it does not present a danger to adult horses.
Horse manure has been used as a fertilizer for thousands of years and is found in many commercial garden products such as topsoil, soil conditioner for clay soil, mulch for garden plants, fertilizer. It is a helpful addition to a compost pile. and It is commonly used in vineyards.
While the actual risk of heath problems from inhaling horse manure is very slight, a simple solution to possible health concerns is to wear a mask when shoveling manure or fertilizing the garden. Manure should be removed from stalls daily to avoid a dusty environment and control flies and odors.