Risk maps for the spread of highly pathogenic avian influenza in poultry - PubMed (original) (raw)

Risk maps for the spread of highly pathogenic avian influenza in poultry

Gert Jan Boender et al. PLoS Comput Biol. 2007.

Abstract

Devastating epidemics of highly contagious animal diseases such as avian influenza, classical swine fever, and foot-and-mouth disease underline the need for improved understanding of the factors promoting the spread of these pathogens. Here the authors present a spatial analysis of the between-farm transmission of a highly pathogenic H7N7 avian influenza virus that caused a large epidemic in The Netherlands in 2003. The authors developed a method to estimate key parameters determining the spread of highly transmissible animal diseases between farms based on outbreak data. The method allows for the identification of high-risk areas for propagating spread in an epidemiologically underpinned manner. A central concept is the transmission kernel, which determines the probability of pathogen transmission from infected to uninfected farms as a function of interfarm distance. The authors show how an estimate of the transmission kernel naturally provides estimates of the critical farm density and local reproduction numbers, which allows one to evaluate the effectiveness of control strategies. For avian influenza, the analyses show that there are two poultry-dense areas in The Netherlands where epidemic spread is possible, and in which local control measures are unlikely to be able to halt an unfolding epidemic. In these regions an epidemic can only be brought to an end by the depletion of susceptible farms by infection or massive culling. The analyses provide an estimate of the spatial range over which highly pathogenic avian influenza viruses spread between farms, and emphasize that control measures aimed at controlling such outbreaks need to take into account the local density of farms.

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Conflict of interest statement

Competing interests. The authors have declared that no competing interests exist.

Figures

Figure 1

Figure 1. Map of The Netherlands Indicating the Physical Locations of All 5,360 Commercial Poultry Farms

Farms that were infected during the 2003 epidemic of avian influenza are represented by black dots, and farms that were not infected are represented by yellow dots.

Figure 2

Figure 2. Distance Distribution of Potential Transmission Events

(A) The frequency distribution of the distances of potential infection events. Notice that the majority of potential infections occur within a radius of 25 km around an infected farm. (B) The proportion of farms infected within different distance categories from a potential source farm, averaged over all possible source farms (i.e., over all farms confirmed positive during the 2003 epidemic).

Figure 3

Figure 3. The Transmission Kernel as a Function of Interfarm Distance for the Parameter Estimates of Table 1

The 95% confidence areas of the transmission kernel are represented by the shaded area.

Figure 4

Figure 4. High-Risk Areas for Epidemic Spread of Avian Influenza Virus Based on the Transmission Kernel of Figure 3

See Table 1 for parameter estimates. For each farm, an individual reproduction number Ri is calculated on the basis of Equation 5. Infected farms with Ri < 1 infect, on average, less than one susceptible farm and pose no risk for epidemic spread (yellow dots). Infected farms with _Ri_ > 1 are expected to infect more than one susceptible farm in the early stage of an epidemic and thus constitute a risk of epidemic spread (red dots). Pink dots represent farms with Ri < 1 for the maximum likelihood estimate of the transmission kernel, but with _Ri_ > 1 for the upper boundary of the 95% kernel confidence area (Figure 3). Note that most of the farms that were infected during the epidemic in The Netherlands in 2003 (Figure 1) are classified as high-risk farms.

Figure 5

Figure 5. High-Risk Areas for Epidemic Spread for Various Local Culling Strategies in the Central High-Risk Area of The Netherlands

(A) Results for the default scenario (no culling). (B) Results for a scenario with immediate culling of all farms within a range of 1 km around an infected farm. (C,D) Culling is carried out in a range of 3 km and 5 km around infected farms, respectively. Farms in yellow pose no risk of epidemic spread for the chosen control strategy, while farms in red constitute a risk of epidemic spread even with the control strategy in place.

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