Microbial pathogens excreted by livestock and potentially transmitted to humans through water

Introduction Protecting surface water quality in the face of a burgeoning human population has become a major challenge for western states such as California. One of the many concerns regarding water quality is to minimize the concentration of pathogens in water and thereby minimize the risk of waterborne disease to humans or to animals. The focus of this article is to review the list of pathogens which can be shed in the excrement of livestock and transmitted to humans through water. The term, waterborne zoonotic disease, is commonly used by public health researchers to refer to pathogens which are transmitted via water from animals to man. Four primary steps need to occur for waterborne transmission of pathogens from livestock to humans. Eliminate any one of these steps and transmission of the specific pathogen from livestock to humans through water can be significantly reduced or even stopped completely. First, the pathogen must be excreted by livestock. Second, the pathogen must reach a water supply either by the animal defecating in water, by overland flow (runoff from a grazed pasture during rainfall, snowmelt, etc.), by subsurface flow, or by some combination of these three pathways. Third, the pathogen must retain the cellular functions necessary for initiating a new infection in humans during the time it is in the environment. Lastly, given that the pathogen is shed by livestock, reaches a water source, and remains infective until ingested by a human, the concentration of infective pathogens must be sufficiently high in order to initiate an infection. The minimum number of pathogens needed to initiate infection varies from pathogen to pathogen. For example, the protozoal parasites, Cryptosporidium parvum and Giardia lamblia, have a very low infectious dose (DuPont et al. 1995; Rendtorff, 1954). In contrast, for bacteria such as Campylobacter jejuni or Salmonella typhimurium, hundreds or thousands of those bacteria need to be ingested in order to initiate an infection (Robinson, 1981; Black et al. 1988).