A Quantitative Risk Assessment of Pathogens in Irrigation Water ...

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A Quantitative Risk Assessment of Pathogens in Irrigation Water ...

A Quantitative Risk Assessment of

Pathogens in Irrigation Water used for

Fresh Produce

Charles P. Gerba

University of Arizona

Dept. of Soil, Water and Environmental Science

and

Epidemiology and Biostatistics

Tucson, Arizona


Quantitative Microbial Risk Assessment

Identify

pathogen

of concern

Dose-response

data from

humans

Model

infection

probability

Validate

model from

outbreak data

Predict

probability

of disease

from exposure

Clinical data to

estimate probability

of disease and

mortality


Quantitative Microbial Risk Assessment

Concentration

of

pathogen

In irrigation

water

Survival

on crop

After

harvesting

Method

of irrigation

Contamination

of the

During

harvesting

(Soil)

Type of

crop

Survival on crop

in the field


Problems with Irrigation Waters

‣ 70% of irrigation in the

world occurs in

developing countries

‣ Most of the irrigation

waters in the developing

world are impacted by

human waste

‣ Currently no standards

for irrigation waters to be

used for produce

production from nonreclaimed

wastewater

source

‣ All produce in the

Western U.S. and Mexico

is irrigated


Microbial Water Quality Standards for

Irrigation Water

‣ Geldreich and Bordner (1971) suggested 1,000

fecal colifroms/100mL

‣ WHO reclaimed wastewater. 1000 E. coli/100

mL

‣ Recently Suggested Guidelines by producers in

the United States

• 126 E. coli /100 ml – geometric average

• No more than 235 E. coli / 100 ml in a single sample

• No greater or equal of 576 E. coli / 100 ml geometric

average of 5 samples


Occurrence of Pathogens in Irrigation Waters

used for Produce Production

‣ Central America (2002,2005) - ~50% of

samples positive for Giardia and/or

Cryptosporidium

‣ Nigeria (2001) – 2 to 14% positive for

Salmonella

‣ Brazil (2006) – 23.5% positive for

Salmonella

‣ Arizona (2005-2006) 20% positive for

norovirus , 21% positive for Salmonella


Large Scale Man-made Irrigation

Systems

Water travels

hundreds of miles

though these systems

– very complex

systems

‣ The ecology of

pathogens in these

systems has not been

given serious study


1

5

4

3

2

6

14

7

19

20

8

13

9

10

11

12

18

15

17

21 16


Birds

Storm water

Runoff

Livestock

Pets

Urban Centers

Irrigation return

flows


Bathers in Irrigation Source

Water


Summary of microbial indicators for irrigation water samples

collected in both Yuma and Maricopa counties

County

and

sampling

period

Statistical

parameter

Total

coliforms

MPN/100 mL

E. coli

MPN/100

mL

Enterococcus

MPN/100 mL

Clostridium

perfringens

CFU/100 mL

Yuma

Arithmetic average

9665

43

458

3.5

Geometric average

2056

6

82

2

June 2001

through March

2003

Median

Minimum

Maximum

2310

57

>241,920

6

1,300

8

2,419

1

2,419,200

15

2,419,200

159

241,920

2

300

p value >0.05 for each microbial indicator when comparing Yuma and Maricopa counties as demonstrated

by one-way ANOVA.


Some locations always positive for

E. coli (Yuma, AZ)

‣ Ten sites monitored for six months

‣ E. coli range in the water 3 to 461/100 mL

‣ E. coli range in sediments 10 to 9,208/100

grams

‣ E. coli survival (T 99 = time in days for a

99% decrease)

• T 99 water 4 days

• T 99 sediment 8 days


Microbial Water Quality Standards for

Irrigation Water

‣ Use risk assessment to develop water

quality guidelines for use of reclaimed

wastewater for food crops

‣ Develop quantitative exposure data

relative to

• Type of irrigation method

• Type of pathogen

• Type of food crop


The University of Arizona

Corn, cotton, etc.

FI vs. SDI

Lettuce, cantaloupe, etc.

Hypothesis: Under SDI, when the upper soil layers are kept dry, health

and environmental risks are minimized.

Wet

Surface?

12 in

6 in


Log (PFU or CFU / g)

C. P.

E. coli

PRD1

C. P.

E. coli

PRD1

C. P.

E. coli

PRD1

C. P.

E. coli

PRD1

C. P.

E. coli

PRD1

C. P.

E. coli

PRD1

The University of Arizona

4

Crop Contamination

3

2

1

0

-1

-2

-3

SDI FI SDI FI SDI FI

Cantaloupe Lettuce Bell pepper

INNER SMALL BOX REPRESENT 95 % CONFIDENCE

INTERVAL OF THE MEDIAN AND + INDICATES OUTLIERS.


The University of Arizona

Time (days) to Achieve 99.9% Reduction

Organisms

T 99.9

Cantaloupe Lettuce Bell Pepper

Dry Humid Dry Humid Dry Humid

E. coli O157:H7 3 >14 1 1 5 1

E. coli ATCC 25922 10 >14 3 1 1 1

S. sonnei 10 14 5 3 3 3

S. enterica 5 >14 1 14 1 1

C. perfringens >14 14 >14 >14 10 >14

PRD1 >14 10 >14 >14 >14 >14

HAV >14 >14 >14 10 >14 >14

FCV 14 3 3 3 3 5


Risk Assessment: Lettuce and FI

Steps in Risk Assessment

Assumptions

Annual risk of infection 1:10,000

Fresh produce consumed

% Pathogen transferred

to produce surface

Salmonella pre-harvest survival

Salmonella post-harvest survival

4416.5 g

0.00007% (average)

Decay rate (0.35): estimate

1 and 14 days die-off

No inactivation or growth

Concentration in irrigation water

150 cfu/100 mL (1 day)

4.2 x 10 9 cfu/100 mL (14 days)


Maximum Level of Salmonella allowable to

achieve 1:10,000 Risk of Infection per year

Crop

organisms/100 mL

Cantaloupe 0.1* 8,700**

Lettuce 91 2,900,000,000

________________________________

*assumes harvested one day after irrigation

**assumes harvested 14 days after irrigation

Assumes no growth of Salmonella on the produce


Maximum Level of Hepatitis A Virus

Allowable to Achieve 1:10,000 Risk of

Infection per Year

Crop

virus/100 liters

Cantaloupe 2

Lettuce 4.2

____________________________

assumes last irrigation events was 14 days

before harvest


Recommendations

‣ Need to develop risk based water quality

standards for irrigation waters

‣ Develop “Water Safety Plans” based on

irrigation water source, irrigation methods and

type of crop

‣ Consider development of different water quality

standards for “sensitive” crops

‣ Assess the impact of harvesting methods on

crop contamination by pathogens

‣ Assess potential for regrowth of Salmonella and

E. coli on crops growing in the fields

‣ Drip irrigation does not offer equal protection to

all crops from pathogens nor from all types of

pathogens. Needs to be taken into consideration

in standard development

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