Simulation Based Epidemiological Modeling Approach

We briefly introduce another approach that performs an epidemiological assessment by the use of a simulation model (Yamamoto et al., 2008). The back-calculation method usually uses mathematical equations to describe the model for calculating the number of infections in the past. In this approach, the simulation model is used to backward estimate the number of BSE-infected cattle that could have been a source of infection to humans instead of mathematical models.

First, the dynamics of BSE-infected animals in the cattle population are modeled by birth cohort, taking account of the coverage of the BSE surveillances currently in place. This individual-based simulation model generates the year of death and the final deposition of BSE-infected cattle, such as death on farm and detection at slaughter. The final dispositions of BSE-infected cows can be described and categorized as shown in Table 24.2. Parameters such as age at infection, incubation period, and age at slaughter/death are incorporated as probability distributions to generate stochastic effects. As a result, some infected cattle die before disease onset, while others live long enough to exhibit clinical symptoms. The probability of being tested is influenced by the year when an infected animal is slaughtered or dies, because the intensity of surveillance is different in different years. Infected cattle are assumed to be detected by surveillance if the animal is in the last stage of incubation period.

The output of the simulation model from a large number of iterations, with various numbers of initial infected animals, is compared to the observed BSE cases by each birth cohort. Then, the value with the highest likelihood is obtained as the maximum likelihood estimator for the number of infected animals in the birth cohort. The estimated number of infected cattle was then used to deduce the year and cause of slaughter/death of these BSE-infected cattle.

Table 24.2 Classification of the final disposition of BSE-infected cattle

Items

Classification

Died due to clinical onset of BSE ? Slaughtered or died on farm ? Tested by surveillance ? Detectable infectivity ?

Slaughter

Yes Yes

No Die No No

Year of death

Fig. 24.7 The estimated number of the BSE-infected cattle which are in the last stage of incubation period

Year of death

Fig. 24.7 The estimated number of the BSE-infected cattle which are in the last stage of incubation period

Figure 24.7 shows the estimated number of the BSE-infected cattle which are in the last stage of incubation period and are entered into the human food chain in Japan (Yamamoto et al., 2008). In the original study, a wide range of input parameters were tested in the sensitivity analysis.

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