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# Use of Risk Model to Calculate Resistance Thresholds

SLIDE 1:

Use of Risk Model to Calculate Resistance Thresholds

Mary Bartholomew
Expert Mathematical Statistician
Center for Veterinary Medicine
January 24, 2001

SLIDE 2: Risk-based Approach

 Human Cases Caused by Resistant Bacteria(attribute) k-res<----- Quality of Meat with Resistant Bacteria(exposure)

SLIDE 3: A Measurable Human Health Impact, H(x)

H(x) = current measurable level of human health impact resulting from the use of an antimicrobial drug in food-producing animals causing resistance to an antimicrobial drug used in human medicine

SLIDE 4: For Measurable Values Q and H(x) it Follows That

k-res = H(x)/Q

SLIDE 5: Since T(x) is a Particular Value of H(x), There is a Particular Value of Q for Which the Same Relationship Holds,

k-res = H(x)/Qt(x)

SLIDE 6: How Q is estimated

Q =( total pounds of product consumed) x (proportion of sampled pounds containing bacteria) x (proportion of samples from which resistant bacteria are isolated (h))

SLIDE 7: Defining Q and Qt(x)

Proportion of samples from which resistant bacteria are isolated is

• called h in defining Q (as just shown)
• called t(x) in defining Qt(x)

SLIDE 8:

t(x) - the maximum allowable prevalence of resistant bacteria isolated from animal-derived food that does not cause an unacceptable impact on illness x in humans

SLIDE 9: During a period of stable

• Consumption
• Rates of carriage of bacteria on product
• Rates of human prescriptions

SLIDE 10:

H(x)/Q = T(x)/Qt(x)

and further,

H(x)/h = T(x)/t(x)

SLIDE 11:

This allows a solution for t(x)

t(x) = [T(x) *h]/H(x)

SLIDE 12: Maximum Impact Method

• Without current information, can use Max
• Assume all isolates from food commodity are resistant
• Assume all human cases attributable to the food commodity are caused by resistant bacteria
• Then h = 1, Hmax = all attributable cases

SLIDE 13:

A graph titled, "Human health impact of antimicrobial drug resistance attributed to use of an antimicrobial drug in food-producing animals.

X axis - Prevalence (impact) of resistance among food product exposure unit (h)

Y axis - Human Health Impact H(x)

SLIDE 14: Thresholds for Multiple Species

• Σai[ti(x) k-resi] T(x)
• uses weights, ai, to allocate t(x) for each species so that the total is less than T(x)
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