Animal Waste, Water Quality and Human Health

Comparative risk analysis 371For many diseases evaluation of ID 50 has come from clinical trials using healthyadult volunteers. It therefore ignores any elevated health risks that may be faced bychildren, by immuno-compromised people, and by the elderly (USEPA 2000b,Nwachuku & Gerba 2004, Wade 2008). The pattern for children may be ofconsiderable importance for developing countries where it is known thatchildren can exhibit campylobacteriosis rates many times higher than those foradults (Blaser 1997, Rao et al. 2001, Teunis et al. 2005). In developed countriessuch as New Zealand the reported illness rates for all five of the selecteddiseases exhibit higher rates among children (see www.nzpho.org.nz, Lakeet al. 2011) and similar findings have been made for Scotland (Strachan et al.2009) and in USA (Denno et al. 2009). Accordingly, differential rates betweenchildren and adults need careful attention. A good example of differentialsensitivity of identifiable sub-populations has been demonstrated for listeriosis:the susceptibility to infection from this food-borne pathogen ranges over1000-fold between the healthy, young adult, population and those who areimmuno-compromised due to underlying illness (e.g. AIDS) or medicaltreatment such as organ transplant recipients (Marchetti 1996).10.2.3 Assessing severityThe dose-response relationship also does not consider the severity of the illness,For example, Salmonella infections are usually self-limiting and of relativelyshort duration, while infections from enterohaemorrhagic E. coli often lead tolife threatening illness which is clearly more severe. Similarly, reliance only onclinical trial data frequently ignores any sequelae that may arise. For example,Guillain-Barré Syndrome may affect about 0.03% of people who havecontracted campylobacteriosis (McCarthy & Giesecke 2001). Infections withSalmonella or Campylobacter have been found to increase the short term risk ofdeath and long term mortality (Helms et al. 2003).One way of comparing disease severity is to use the metric of the “disabilityadjusted life years” (DALY) concept, originally developed by Murray andLopez (1996) and adopted by the World Health Organization to inform globalhealth planning (AIHW 2000, Kemmeren et al. 2006). The DALY is a measureof the years of “healthy” life lost due to illness or injury, that is, time lived instates of ‘less-than-full’ health. DALYs are calculated as the sum of years of lifelost due to premature death (YLL) and the equivalent years of “healthy” life lostdue to poor health or disability (YLD). The YLD considers the extent of thedisability that is endured, that is, YLD is weighted according to the severity ofthe disability. The origin and application of the DALY concept, particularly in