Risk Maps for the Spread of Highly Pathogenic Avian Influenza in Poultry
Citation: Boender GJ, Hagenaars TJ, Bouma A, Nodelijk G, Elbers ARW, et al. (
Risk Maps for the Spread of Highly Pathogenic Avian Influenza in Poultry
Gert Jan Boender 0 1 2
Thomas J. Hagenaars 0 1 2
Annemarie Bouma 0 1 2
Gonnie Nodelijk 0 1 2
Armin R. W. Elbers 0 1 2
Mart C. M. de Jong 0 1 2
Michiel van Boven 0 1 2
Abbreviations: AIC, Akaike's Information Criterion
0 Current address: Department of Virology, Central Institute for Animal Disease Control, Wageningen University and Research Centre , Lelystad , The Netherlands
1 Editor: Lauren A. Meyers, University of Texas Austin , United States of America
2 1 Animal Sciences Group, Wageningen University and Research Centre , Lelystad , The Netherlands , 2 Faculty of Veterinary Medicine, Department of Farm Animal Health, Utrecht University , Utrecht , The Netherlands , 3 Animal Health Service , Deventer , The Netherlands
Devastating epidemics of highly contagious animal diseases such as avian influenza, classical swine fever, and footand-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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Outbreaks of highly contagious animal infections such as
foot-and-mouth disease, classical swine fever, and highly
pathogenic avian influenza traditionally have been and
continue to be important loss factors in production animals
throughout the world. In recent years, several large epidemics
have occurred with serious socioeconomic consequences [1
3] and, in the case of highly pathogenic avian influenza
viruses of the H5 and H7 subtypes, also with possible public
health implications [47]. Improved understanding of the
factors facilitating the introduction and subsequent spread of
these viruses is crucial for effective control. An important
common characteristic of these past epidemics is that a large
fraction of farm infections occurred through local spread
between nearby farms [814].
To explain the observed patterns of infection of highly
pathogenic avian influenza virus between farms, and to be
able to evaluate the potential effectiveness of control
measures, we adopt a phenomenological modelling approach.
Similar approaches have been used in modelling studies of
the interfarm spread and the effectiveness of control
measures during the foot-and-mouth epidemic in the United
Kingdom in 2001 [8,9,1418]. The present analysis allows us to
produce geographic risk maps for the spread of highly
pathogenic avian influenza virus between poultry farms.
These risk maps are based on the calculation of a local
reproduction number, and are constructed so as to apply to a
given intervention strategy.
For estimation of the model parameters, we use an
extensive dataset that was collected during an outbreak of a
highly pathogenic H7N7 avian influenza virus in The
Netherlands in 2003. Shortly after the detection of virus circulation,
the Dutch authorities undertook an aggressive control
strategy that consisted of an animal movement ban and
enhanced biosecurity measures in the affected regions,
tracing and screening of suspected flocks, and culling of
infected and contiguous flocks. In all, 241 commercial flocks
became infected during a period of 9 wk, and more than 30
million birds died by infection and culling.
A striking characteristic of the 2003 epidemic was that
most of the infected farms were confined to two areas with a
high density of poultry farms. In fact, it was noted that
proximity to an infected herd was a significant risk factor for
acquiring infection [19]. This (...truncated)