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Chapter 16 : Quantifying Transmission

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Abstract:

Transmissibility is the defining attribute of infectious diseases, and it has profound consequences for their epidemiology. In contrast to noncommunicable diseases, the risk of an individual contracting an infectious disease increases with the number of infected and infectious individuals present in the population. This is positive feedback, and it makes the epidemiology of infectious diseases considerably more complex and often hard to understand intuitively. For example, reducing the per capita transmission rate is expected to decrease the size of an epidemic, but it is difficult to estimate by how much; because of the positive feedback, there is not a proportional (or linear) relationship between epidemic size and transmission rate. Less obvious still is the expectation that decreasing transmission rate may increase rather than decrease the duration of an outbreak ( ).

Citation: Woolhouse M. 2019. Quantifying Transmission, p 281-289. In Baquero F, Bouza E, Gutiérrez-Fuentes J, Coque T (ed), Microbial Transmission. ASM Press, Washington, DC. doi: 10.1128/microbiolspec.MTBP-0005-2016
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Figures

Image of Figure 1
Figure 1

Time line of exposure, infectiousness, and clinical signs. Following exposure to infection, there is a period when the host is infected but not yet infectious—the latent period (yellow)—and a period when the host is infected but not yet showing clinical signs—the incubation period (yellow and orange). In this example, there is a (brief) period when the host is infectious but asymptomatic (orange), as occurs for infections such as influenza or FMD.

Citation: Woolhouse M. 2019. Quantifying Transmission, p 281-289. In Baquero F, Bouza E, Gutiérrez-Fuentes J, Coque T (ed), Microbial Transmission. ASM Press, Washington, DC. doi: 10.1128/microbiolspec.MTBP-0005-2016
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Image of Figure 2
Figure 2

Infectiousness of FMDV in experimental infections of cattle. Distribution of the incubation period minus the latent period, predicted using a Bayesian analysis, where a negative value indicates when clinical signs appear before an animal is infectious. The plot shows that when infectiousness is measured directly by the exposure of in-contact susceptible cattle, the analysis indicates that only a small fraction of infectiousness is found to occur before the appearance of clinical signs (black line). In contrast, if infectiousness is measured indirectly using virus isolation from blood (red) or nasal fluid (green), the bulk of infectiousness is estimated to occur well before clinical signs appear. Data from reference ; figure kindly supplied by Simon Gubbins.

Citation: Woolhouse M. 2019. Quantifying Transmission, p 281-289. In Baquero F, Bouza E, Gutiérrez-Fuentes J, Coque T (ed), Microbial Transmission. ASM Press, Washington, DC. doi: 10.1128/microbiolspec.MTBP-0005-2016
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Figure 3

Transmission heterogeneities. (A) Heterogeneous infectiousness. Frequency distribution of bacterial counts for O157 in cattle fecal samples (horizontal axis, log scale). Raw data (histogram) are compared with a fitted mixture distribution log normal model that identified two distributions (red lines) and their sum (green). Arrowheads indicate the mean for each distribution. Reproduced with permission from reference . (B) Heterogeneous contact rates. The graph shows the cumulative, fractional contribution to the value of of individuals arranged in order (highest to lowest along the horizontal axis) of their observed contact rates. For a range of infectious diseases (multiple examples covering HIV/AIDS, malaria, schistosomiasis, and leishmaniasis), 20% of the population contribute 80% of the basic reproduction number, , as estimated given observed heterogeneities in contact behavior (sexual contacts, vector biting rates, water contact, as appropriate). Data from reference .

Citation: Woolhouse M. 2019. Quantifying Transmission, p 281-289. In Baquero F, Bouza E, Gutiérrez-Fuentes J, Coque T (ed), Microbial Transmission. ASM Press, Washington, DC. doi: 10.1128/microbiolspec.MTBP-0005-2016
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References

/content/book/10.1128/9781555819743.chap16
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