MANCHESTER — Horse-riding advocates and a Manchester Water Works official released literature on Tuesday that they hope will bolster their case on Thursday, when the Manchester Board of Water Commissioners votes on whether to restrict horses from Water Works property.
Horse advocates noted that scientific studies have found that horse manure rarely contains Cryptosporidium and Giardia, two organisms responsible for most water-borne illnesses in the United States.
Manchester Water Works answered with material from the Massachusetts Department of Conservation and Recreation, which says horse manure can impair water quality and contain pathogens potentially harmful to humans.
Water Works Director David Paris acknowledged the two pathogens are seldom found in most horse droppings.
"However the risk is not zero," Paris wrote in an email, "and while horse feces is not 'hazardous' by the definition of a chemical, ignitable, reactive or corrosive, it does contain nutrients ... (nitrogen and phosphorous in particular) and can have crypto, giardia and coliform bacteria which are listed tap water microbiological contaminants."
Horse riders have planned a rally for 3:30 p.m. Thursday in front of Water Works headquarters on Lincoln Street.
The proposed regulations would:
• Allow horses on gravel fire roads as long as they wear some kind of diaper or waste-collection device.
• Allow horses on watershed property as part of a Water Commission-approved, organized riding event, as long as participants immediately remove horse droppings.
• Prohibit horses from water, the shoreline, beaches, boat launches or public parks.
The regulations would cover Water Works property within the watershed of Lake Massabesic, the drinking water supply for 160,000 residents of greater Manchester.
Horse riders showed the New Hampshire Union Leader the results of microbiological analysis of two samples of horse manure that horse riders had submitted to the University of New Hampshire Veterinary Diagnostic Laboratory. They showed no traces of pathogens. One was analyzed this year, one three years ago.
The also shared a paper, written in 1998 by the California organization EnviroHorse and revised in 2001. The review of scientific literature found few instances of pathogens in horses used in trail riding in California and Colorado.
The EnviroHorse author said lactating horses and foals are more likely to have the dangerous organisms. But adult recreational trail horses are not a "significant source of Cryptosporidium environmental contamination in watershed areas," it reads, quoting a 1996 study in Colorado.
Paris said the study did not rule out the pathogens altogether. He said state standards say no pathogens should be in drinking water. He also said the EnviroHorse paper was written for horse-riders in the western United States.
"Back country is not the same as watershed, where almost every trail goes close to water at some point, so the risk from even one infected horse could be significant," Paris said.