Animal Waste, Water Quality and Human Health

18Animal Waste, Water Quality and Human Healthanimals to humans. Recent developments in genotyping and genome sequencinghave allowed the ecological distinction of many subtypes of pathogenic microbesthat may have different host preferences. Information generated by highresolutiontyping has provided evidence which either supports or casts doubt onassertions that particular pathogen subtypes are truly zoonotic. Certain protozoanspecies such as Cryptosporidium parvum and Giardia lamblia (duodenalis) werepreviously all considered to be zoonotic. It is now recognized, however, that alimited number of genotypes or assemblages within these species complexes areindeed transmitted from animals to man (Fayer et al. 2010, Feng & Xiao 2011,Thompson & Smith 2011). Similarly, among species of bacterial and viralpathogens of animal origin, genetic subtypes or lineages differ significantly in thefrequency with which they are associated with human disease and in the severityof this disease (Lan et al. 2009, Pavio et al. 2010, Sheppard et al. 2010, Teshaleet al. 2010, Zhang et al. 2010, Medina et al. 2011).Crossing of the species boundary and changes in the spectrum of hosts which canbecome infected appear to occur more readily with organisms such as influenzaviruses (Medina & García-Sastre 2011). In other pathogenic microbial species,such host specificity mutations may have occurred thousands of years ago(Sheppard et al. 2010). Truly zoonotic pathogens are capable of infecting bothhuman and animal hosts, but often the animal populations serve as reservoir andamplifying hosts. Co-evolution of the pathogen and the reservoir host maydampen the effects of infection on the animal and overt clinical disease mayeventually be no longer evident (Karmali et al. 2010). Humans, by contrast, areoften aberrant or “dead-end” hosts for these pathogens and may not play asignificant role in pathogen maintenance or perpetuation. However, thesepathogens can cause severe disease in humans. The risks of infection and theadverse health consequences are, as a rule, significantly greater for individualswhose immune status is somehow compromised than for healthy members ofthe community.This immuno-compromised group includes otherwise healthy individualswhose immune systems have not yet matured or who have been compromised byinfectious, physical or chemical agents, or inherited defects in the immunesystem; they include specific age and gender classes such as infants, the elderly,and pregnant women. A much greater number of opportunist agents are likely tobe associated with sporadic infections in immuno-compromised individuals thanoutbreaks at the community level. Such outbreaks of, for example, waterbornedisease do occur, however, as a consequence of intake of high concentrations ofparticular pathogens which overcome defence mechanisms of otherwise healthyindividuals, and infections with particularly virulent pathogenic species orsubtypes within a species.