Animal Waste, Water Quality and Human Health

136Animal Waste, Water Quality and Human Healthcan quickly be transferred both within and between bacterial species. This hascaused concerns about the long-term effectiveness of these measures and, moreimportantly, about the transfer of these genetic elements into a broader range ofhuman pathogens. While there are attempts to separate classes of antibiotics usedin animals for growth promotion from those used in human therapeutics, thetrend is to greater restriction on the use of antibiotics for growth promotion infarm animals. Between 1997 and 1999, the European Union implemented banson five different antimicrobials: avoparcin, spiramycin, tylosin, bacitracin, andvirginiamycin, which are used for growth promotion in animals (Frei et al. 2001,Emborg et al. 2003, Evans & Wegener 2003). It was initially feared that theseregulatory developments might increase levels of enteric pathogens in animalexcreta. However, Evans & Wegener (2003) have reported decreases inSalmonella levels in poultry and pigs and equivalent levels of Campylobacter inpoultry in Denmark three years after the ban on the use of these antimicrobialsin feeds.Crypto. parvum is naturally resistant to most anti-coccidial drugs (Coombs &Muller 2002). Joachim et al. (2003) reported that halofuginone decreases butdoes not eliminate faecal shedding of Cryptosporidium oocysts by calves andsuggested that the drug, used together with good sanitation and disinfectionprocedures, may limit infection by this parasite in calves. Likewise, prophylacticfeeding of decoquinate in experimentally challenged dairy calves did not reduceoocyst shedding nor clinical severity (Moore et al. 2003). Few, if anychemotherapeutic drugs have been adopted for treatment of this parasite inlivestock populations due to cost or lack of efficacy (Thompson et al. 2008).Chlorate compounds have selective toxicity for enteric pathogens such as E. coliand Salmonella. In these bacteria the intracellular molybdenum-containing enzymenitrate reductase changes chlorate to the toxic chlorite form. Interestingly, thischemical has little or no effect on most other members of the gastrointestinalmicrobial flora because they either lack this enzyme or possess a second enzymethat changes chlorite to the non-toxic chloride form. Sodium chlorate, given bymouth to cattle, sheep and pigs has been shown to reduce Salmonella serovarTyphimurium and E. coli O157:H7 in the intestine (Loneragan & Brashears2005). Similarly, chlorate administered to broilers prior to slaughter has beenshown to reduce Salmonella contamination in the crop and the caecum (Byrdet al. 2003, Byrd et al. 2008). Unfortunately, resistance to chlorate appears todevelop quickly in Salmonella and is likely to develop in E. coli O157:H7(Oliver et al. 2009).Short-chain fatty acids such as acetate, butyrate and formate have also beenshown to reduce Salmonella populations in the crops and caeca of broilerchickens (Immerseel et al. 2005). However, the use of combinations of agents