Animal Waste, Water Quality and Human Health

Zoonotic waterborne pathogens in livestock 1234.3.3 Reducing exposure from high risk animalsThe law of the “vital few” and the “trivial many” applies to hosts as well as tozoonoses. For many diseases a minority of animals harbour a majority ofpathogens; this may be the result of density-dependent and/or densityindependentfactors, and is especially important for parasitic infections.“Supershedding” and “superspreading” are more recently identified phenomena:super-shedders are individuals which shed exceptional numbers of pathogens(often temporarily) while super-spreaders are individuals with exceptionalopportunities to infect other hosts (the phrase was first used in sexual diseasetransmission studies). As noted in Chapter 3, super-shedders are thought to bethe most important source of the zoonotic pathogen E. coli O157:H7 andpossibly other waterborne zoonoses (Chase-Topping et al. 2008) and arethought to be responsible for the perpetuation of herd infections.Carriers are asymptomatically infected animals that shed constantly,intermittently, or when stressed and for periods varying from days to throughouttheir lives: the carrier state is important in salmonellosis and leptospirosis.Carriers are difficult to identify without laboratory testing procedures such as theserological tests that are used to identify pigs asymptomatically carryingSalmonella, as part of the European SALINPORK programme (Wegener et al.2003).Understanding epidemiological phenomena that result in high-risk animalsoffers the possibility of new intervention modalities based on identification andremoval, treatment, or isolation for the period of shedding (where temporary).The carrier state presents a particular challenge in that control procedures suchas vaccination and antimicrobials may control the level of pathogens excretedbut not eliminate the carrier state and, indeed, simply mask hidden infections.However, for some pathogens (e.g. leptospirosis) antibiotics are effective ineliminating the carrier state.4.3.4 Reducing exposure from wildlifeAlthough briefly considered in the section on biosecurity, wildlife transmissiondeserves a place to itself.Many waterborne zoonoses have wildlife reservoir hosts and in many casesthese have key roles in disease epidemiology. And, while in wild animalpopulations, parasite and host may live in an ecological balance with little or noobvious effect on the natural host reservoir population, health effects are likelyto be observed when human hosts are accidentally exposed to these zoonoticagents. Eradication programmes have been, and continue to be, successfully