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. 2008 Jan 7;275(1630):107-15.
doi: 10.1098/rspb.2007.1154.

Modelling studies to estimate the prevalence of foot-and-mouth disease carriers after reactive vaccination

M E Arnold  1 , D J Paton, E Ryan, S J Cox, J W Wilesmith
Affiliations

Modelling studies to estimate the prevalence of foot-and-mouth disease carriers after reactive vaccination

M E Arnold et al. Proc Biol Sci. .

Abstract

Foot-and-mouth disease (FMD) is a highly contagious and economically significant viral disease of cloven-hoofed animals. Vaccination can be used to help restrict the spread of the infection, but evidence must be provided to show that the infection has been eradicated in order to regain the FMD-free status. While serological tests have been developed, which can identify animals that have been infected regardless of vaccination status, it is vital to know the probable prevalence of herds with FMD carriers and the within-herd prevalence of those carriers in order to design efficient post-epidemic surveillance strategies that establish freedom from disease. Here, we present the results of a study to model the expected prevalence of carriers after application of emergency vaccination and the impact of this on the sensitivity of test systems for their detection. Results showed that the expected prevalence of carrier-containing herds after reactive vaccination is likely to be very low, approximately 0.2%, and there will only be a small number of carriers, most likely one, in the positive herds. Therefore, sensitivity for carrier detection can be optimized by adopting an individual-based testing regime in which all animals in all vaccinated herds are tested and positive animals rather than herds are culled.

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Figures

Figure 1
Figure 1
Model assumptions regarding (a) the effect of distance on the probability of transmission between farms, (b) the effect on the number of cattle on the susceptibility and infectiousness of herds, (c) the reduction in the probability of infection of each vaccinated bovine with time after vaccination and (d) the reduction in the probability of a vaccinated bovine becoming clinical (as would be identified on a farm) with time after vaccination.
Figure 2
Figure 2
Outline of the steps to calculate the probability that an infected vaccinated farm has carriers at post-epidemic surveillance (infs, infecteds).
Figure 3
Figure 3
Histogram of the estimated number of initial infections from the 2001 UK FMD epidemic.
Figure 4
Figure 4
Comparison of the effect of vaccinating cattle farms within 10k of each IP versus no vaccination on (a) the final epidemic size and (b) the mean number of IPs each day after 100 simulation model replicates.
Figure 5
Figure 5
The probability of an infected vaccinated farm becoming a carrier as the number of days between vaccination and infection increases for (a) the number of initial infections from the 2001 epidemic and (b) assuming the number of initial infections is Poisson distributed with a mean=5% of the herd, assuming one clinical to be an IP.
Figure 6
Figure 6
The results of 200 runs of the simulation model of (a) the total number of vaccinated herds with at least one carrier, grouped by herd size and (b) the within-herd prevalence of carriers in vaccinated herds.
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References

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