# **A.3. Unrestricted Irrigation: Norovirus Infection Risks**

Recently it has become possible to use QMRA-Monte Carlo techniques to estimate norovirus infection risks (Mara and Sleigh 2010a,c). The dose-response data needed to do this were only published in 2008 (Teunis _et al_ . 2008), so it was not possible for norovirus to have been considered in the 2006 WHO Guidelines.

Norovirus (NV, formerly called Norwalk or Norwalk-like virus) is the major viral pathogen causing diarrheal disease in adults − in contrast rotavirus mainly affects children under 5, and commonly under 2, years of age, although NV does cause diarrhea in children (Patel _et al_ ., 2008). It is therefore a better reference viral pathogen than rotavirus for wastewater-use studies as young children are less exposed than adults, either as field-workers (although they may play in wastewater-irrigated fields while their mothers work in them) or as consumers (children under 2, especially, eat little wastewater-irrigated foods). That norovirus is a more suitable reference viral pathogen than rotavirus is illustrated by the fact that in the USA during 1998−2007 there were 1,773 confirmed foodborne norovirus outbreaks, but only 4 confirmed foodborne rotavirus outbreaks (CDC 2009).

Using a DALY loss per case of 9 × 10<sup>−4</sup> per case of NV disease (Kemmeren _et al_ ., 2006) and an NV disease **/** infection ratio of 0.8 (Moe 2009), the tolerable NV disease and infection risks corresponding to a tolerable DALY loss of 10<sup>**−4**</sup> pppy are:

|=<br>disease<br>NV<br>of<br>case<br>per<br>loss<br>DALY<br>pppy<br>loss<br>DALY<br>Tolerable<br>=<br>risk<br>disease<br>NV<br>Tolerable|10<br>×<br>9<br>10<br>4<br>-<br>4<br>-|pppy<br>11<br>.0<br>=|
|---|---|---|
|ratio<br>ection<br>isease/inf<br>d<br>NV<br>pppy<br>risk<br>disease<br>NV<br>Tolerable<br>=<br>risk<br>infection<br>NV<br>Tolerable|=<br>8.0<br>11<br>.0<br>=|pppy<br>14<br>.0<br>|

The NV dose-response dataset of Teunis _et al._ (2008) was used in place of the beta-Poisson equation in the QMRA-MC computer program developed to determine median NV infection risks pppy. The resulting estimates of median risk obtained, using the same assumptions as for the index pathogens in Table A.4, are given in Table A.5, which shows that a reduction of 4 log units results in an NV infection risk of 0.25 pppy, which is only marginally higher than the tolerable NV infection risk of 0.14 pppy determined above. Table A.5 also includes, for comparison, rotavirus infection risks – these are broadly similar to the norovirus infection risks.

# **_Lettuce consumption in urban Ghana: norovirus infection risks_**

Exposure to wastewater pathogens present in wastewater-irrigated foods varies with differences in consumption patterns which need to be accounted for in the risk calculations. For example, Seidu _et al_ . (2008) reported that people in urban Ghana commonly consume ~10−12 g of lettuce in ready-to-eat street-vended food on each of four days per week.<sup>35</sup> NV infection risks for a DALY loss of 10<sup>−4</sup> pppy and for this Ghanaian consumption of lettuce were determined by 10,000 Karavarsamis-Hamilton Monte Carlo simulations for various wastewater qualities (Table A.6). A 3-log unit NV reduction achieves an NV infection risk of 0.3 pppy, which is a little higher than the tolerable NV infection risk of 0.13 pppy. However, a 4-log unit reduction is feasible as it could be simply achieved by treatment (1 log unit) and produce disinfection (3 log unit) ( _cf_ . Table 4.2).

35 This refers to a specific situation in one developing country and may or may not be representative of what happens elsewhere; however, it is much less than the 100 g of lettuce consumed on alternate days used by Shuval et al. (1997) to reflect the situation in Israel (this value of 100 g per 2 days was also used in the 2006 WHO Guidelines).

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