During 2007 the UK experienced outbreaks of three notifiable exotic livestock diseases;
Foot and Mouth Disease (FMD), Highly Pathogenic Avian Influenza (HPAI)
and bluetongue. Large epidemics of any of these diseases would have a serious impact
on animal welfare, farming, food production and the economy. In light of this,
understanding holdings which are most likely to acquire and spread infection and
being able to identify areas at higher risk of an epidemic is valuable when preparing
for and managing an epidemic. This thesis uses a spatial epidemiological framework
and the detailed disease and demographic data from the 2001 Great Britain (GB)
FMD epidemic to develop static models of the risk of FMD susceptibility and transmission.
These models are used to develop maps of FMD risk. These methods are
then applied to the outbreak of FMD in 2007.
The inputs for this analysis comprised a set of data relating to the farms diagnosed
with FMD and farms culled as part of the disease control measures. The cleaning of
these data is described and data which were estimated relating to dates of infection
and putative sources of infection are evaluated. The distribution of farm holdings
and animals is taken from the June 2000 GB agricultural census, off-fields of farms in
the agricultural census are recorded in other datasets and these have been identified
and linked to census holdings.
A model of holding level susceptibility is developed using both farm level variables
and measures of animal numbers in the locality of the holding as well as the distance to
the nearest farm infected before the ban on animal movements (seeds). The overall fit
of the model was very good with an area under the Receiver Operator Characteristic
(ROC) curve of 0.91. A further model was developed to describe the risk of FMD
transmission. However, due to incompleteness of transmission data, this was a model
of the risk of finding a subsequent Infected Premises (IP) within 3km of an IP. Risk
factors were a combination of holding level variables and locality measures as well
as data relevant to the infection, such as infectious period and the species initially
infected. The area under the ROC curve for this model was 0.71, which is regarded
as an acceptable fit. Geographical barriers to FMD transmission were investigated
using a case-control methodology, linear barriers comprising rivers and railways had
a significant protective effect with respect to disease transmission (odds ratio = 0.54,
95% CIs = 0.30,0.96, p=0.038).
Modelled values for the transmission and susceptibility models were transformed
to a raster surface in ESRI ArcMap for both the disease as it was seeded in the 2001
epidemic and a non-specific background risk surface independent of the distribution
of seeds. A risk map generated for the outbreak of FMD in Surrey in August 2007
suggested that there was little risk of a large outbreak in Surrey. Potential disease
introductions through livestock movements from Surrey into Scotland were identified
and these suggested that if the disease were introduced into Scotland there was great
danger of substantial local spread.
These methods described in this thesis have been used to map risk of FMD
and subsequently applied to inform the risk presented by a different outbreak of
FMD. The study underlines the value of detailed data both disease and demographic,
for epidemic management. Similar methods could and should be applied to other
infectious diseases threats of livestock such as HPAI and bluetongue