Quantification of transmission of foot-and-mouth disease virus caused by an environment contaminated with secretions and excretions from infected calves.
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This was done using a generalized linear model based on a 2 route (2R, i.e. direct contact and environment) SIR model that included information on FMDV survival in the environment.The study shows that roughly 44% of transmission occurs via the environment, as indicated by the reproduction ratio R0(2R)environment that equalled 2.0, whereas the sum of R0(2R)contact and R0(2R)environment equalled 4.6.We conclude that a contaminated environment contributes considerably to the transmission of FMDV therefore that hygiene measures can play a crucial role in FMD control.
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Affiliation: Central Veterinary Institute (CVI), part of Wageningen UR, P.O. Box 65, 8200 AB, Lelystad, The Netherlands. carla.bravoderueda@wur.nl.
ABSTRACT
Foot-and-mouth disease virus (FMDV) infected animals can contaminate the environment with their secretions and excretions. To quantify the contribution of a contaminated environment to the transmission of FMDV, this study used calves that were not vaccinated and calves that were vaccinated 1 week prior to inoculation with the virus in direct and indirect contact experiments. In direct contact experiments, contact calves were exposed to inoculated calves in the same room. In indirect contact experiments, contact calves were housed in rooms that previously had held inoculated calves for three days (either from 0 to 3 or from 3 to 6 days post inoculation). Secretions and excretions from all calves were tested for the presence of FMDV by virus isolation; the results were used to quantify FMDV transmission. This was done using a generalized linear model based on a 2 route (2R, i.e. direct contact and environment) SIR model that included information on FMDV survival in the environment. The study shows that roughly 44% of transmission occurs via the environment, as indicated by the reproduction ratio R0(2R)environment that equalled 2.0, whereas the sum of R0(2R)contact and R0(2R)environment equalled 4.6. Because vaccination 1 week prior to inoculation of the calves conferred protective immunity against FMDV infection, no transmission rate parameters could be estimated from the experiments with vaccinated calves. We conclude that a contaminated environment contributes considerably to the transmission of FMDV therefore that hygiene measures can play a crucial role in FMD control. No MeSH data available. Related in: MedlinePlus |
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Mentions: The transmission rate parameter β is defined as the average number of new infections caused by one typical infectious individual per day in a totally “susceptible” (not infected) population [16,28] (Additional file 2: equations 1 and 2, with references [16,28,29]). For the analysis, as described previously [28], it was assumed that the calves were infectious (I) when one of their samples (OPF swabs, urine or faeces) tested positive in the virus isolation assay at the start of the time interval. Contact animals were considered cases (C) when one of their samples (OPF swabs, urine or faeces) tested positive, for the first time, in the virus isolation assay at the end of the time interval. The number of new cases (C) during that time interval is binomially distributed with probability p (which is a function of the transmission rate parameter β, the number of infected animals (It) and the total number of animals (N)) and with binomial total St, the number of susceptible animals. Thus, the probability of a single susceptible animal becoming infected during a period Δt is, , where is the transmission rate parameter β. To quantify β, the data from the direct contact experiment were analysed using a generalized linear model (GLM) [30]. The GLM is based on the binomial distribution and the above-mentioned expression for p, using a complementary log-log link function, S as binomial total, a binomial error function and with as offset [16,28]. This model will be hereinafter referred to as the 1 route-SIR (1R-SIR) model. To quantify the contribution of the environment to the transmission of FMDV, as an extra route to the 1R-SIR model (Figure 2), we included the environment (E). In the new 2 route-SIR model (2R-SIR) we additionally assumed that the amount of FMDV present in the environment on a specific day (Et) depends on the secretion and excretion of FMDV by infectious individuals (either I or C) on the previous days, as well as on the remaining FMDV in the environment (E(t-1)), both weighted (discounted) by the FMDV survival rate (σ). Et is represented by the following equation: Et = σI(t − 1) + σC(t − 1)→ t + σE(t − 1) with starting condition E0 = 0 (Additional file 2: equation 3). We performed a sensitivity analysis in which we multiplied the new cases (C) in the equation above either by 0 or by 0.5, instead of 1 as it is in the above equation for Et, to check whether this affected the outcome. Additionally, we performed a sensitivity analysis in which we considered a latent period (counting the inoculated calves as infected but not yet infectious, (1, 2 and 3 days before virus shedding was detected), to check whether the use of an SEIR (susceptible-exposed-infected-recovered) instead of an SIR model would lead to different results for the estimated β and R values (i.e. if β is underestimated) and whether this affected the estimation of the environmental component.Figure 2 |
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Affiliation: Central Veterinary Institute (CVI), part of Wageningen UR, P.O. Box 65, 8200 AB, Lelystad, The Netherlands. carla.bravoderueda@wur.nl.
No MeSH data available.