Not all pathogens are harmful to humans

In response to the coverage of fecal indicator bacteria concentrations and cattle grazing ("Fouled waters: Sierra lakes, streams show contamination from livestock," May 8, Page A-1): We think that this is a time for balance, solution-based science and collaboration.

Our shared challenge is to continue to identify and enact grazing practices which reduce pollution risks, enhance watershed health, and sustain agricultural enterprises. All of our local communities are reeling from budget blows and decreasing revenues. Our rural communities depend on livestock grazing and associated businesses for a stable economy. Sustainable food production and natural resources are crucial to our state, country and world. We all depend upon healthy watersheds.

Improper livestock management can increase fecal pollution indicators such as fecal coliforms and generic E. coli in streams. To a lesser degree, improper management increases the risk of pathogens such as Cryptodporidium parvum and E. coli O157:H7 entering streams.

Proper livestock management can significantly reduce this risk, but first a few points about that risk. It has often been assumed that many pathogens shed by cattle are infectious to humans. Recent research finds this is not always the case. Several species of the parasite Cryptosporidium shed by cattle are barely infectious for humans, or not at all, and often just a few adult range cattle are carriers. Many types of Giardia carried by livestock are not infectious for humans. Background levels of indicators and pathogens in streams can be very low, but also quite high. Many wildlife species carry surprisingly high levels of these microbes, and make daily contributions in our watersheds.

There are many science-based opportunities to address livestock-associated risks. For example, we can use careful placement of livestock attractants such as drinking water to move as much as 60 percent of cattle fecal waste away from streams. Even a few days a month spent herding cattle away from streams will increase stream health.

During summer months, almost all the C. parvum found in cattle fecal pats exposed to sunshine dies within one day due to lethal hot temperatures. Over 90 percent of the E. coli, C. parvum, Giardia and Salmonella found in cattle fecal pats are not transported more than a foot during rainfall-runoff events. A single additional yard of rangeland soil and vegetation can often filter 30 to 90 percent of these microbes flowing in surface runoff. Wetlands can filter up to 90 percent of E. coli in runoff from grazed pastures.

These are just some of the tools and opportunities available. We also know many of the management risk factors, and are working to find their solutions.

Over the past 15 years we, along with many colleagues, have conducted research and outreach on this topic in collaboration with ranchers, agencies, conservation organizations and anyone interested in the sustainable management of rangeland watersheds. These efforts continue today, and will not stop anytime soon. We welcome requests for more information, clarification of the real human health risks associated with range livestock production, and cordial communication and collaboration with anyone interested in working toward practical solutions.

Tate is a professor in the College of Agricultural and Environmental Sciences at UC Davis. Atwill is director of the Western Institute for Food Safety and Security, School of Veterinary Medicine, UC Davis.