Improved use of abattoir information to aid the management of liver fluke in cattle
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Date
08/07/2017Item status
Restricted AccessEmbargo end date
08/07/2018Author
Mazeri, Stella
Metadata
Abstract
Fasciolosis, caused by the trematode parasite Fasciola hepatica, is a multi-host parasitic
disease affecting many countries worldwide. It is a well-recognized clinically and
economically important disease of food producing animals such as cattle and sheep. In
the UK, the incidence and distribution of fasciolosis has been increasing in the last decade
while the timing of acute disease is becoming more variable and the season suitable
for parasite development outside the mammalian host has been extended. Meanwhile
control is proving increasingly difficult due to changing weather conditions, increased
animal movements and developing anthelmintic resistance.
Forecasting models have been around for a long time to aid health planning related to
fasciolosis control, but studies identifying management related risk factors are limited.
Moreover, the lack of information on the accuracy of meat inspection and available
liver fluke diagnostic tests hinders effective monitoring of disease prevalence and treatment.
So far, the evaluation of tests available for the diagnosis of the infection in cattle
has mainly been carried out using gold standard approaches or under experimental settings,
the limitations of which are well known. In cattle, the infection mainly manifests
as a sub-clinical disease, resulting in indirect production losses, which are difficult to
estimate. The lack of obvious clinical signs results in these losses commonly being
attributed to other causes such as poor weather conditions or bad quality forage. This
further undermines establishment of appropriate control strategies, as it is difficult to
convince farmers to treat without demonstrating clear economic losses of sub-clinical
disease.
This project explores the value of slaughterhouse data in understanding the changing
epidemiology of fasciolosis, identifying sustainable control measures and estimating
the effect of infection on production parameters using data collected at one of the
largest cattle and sheep abattoirs in Scotland. Data used in this study include; a) abattoir
data routinely collected during 2013 and 2014, b) data collected during 3 periods
of abattoir based sampling, c) data collected through administration of a management
questionnaire and d) climatic and environmental data from various online
sources.
A Bayesian extension of the Hui Walter no gold standard model was used to estimate
the diagnostic sensitivity and specificity of five diagnostic tests for fasciolosis in cattle,
which were applied on 619 samples collected from the abattoir during three sampling
periods; summer 2013, winter 2014 and autumn 2014. The results provided novel
information on the performance of these tests in a naturally infected cattle population
at different times of the year. Meat inspection was estimated to have a sensitivity of
0.68 (95% BCI 0.61-0.75) and a specificity of 0.88 (95% BCI 0.85-0.91). Accurate
estimates of sensitivity and specificity will allow for routine abattoir liver inspection to
be used as a tool for monitoring the epidemiology of F. hepatica as well as evaluating
herd health planning.
Linear regression modelling was used to estimate the delay in reaching slaughter weight
in beef cattle infected with F. hepatica, accounting for other important factors such as
weight, age, sex, breed and farm as a random effect. The model estimated that cattle
classified as having fluke based on routine liver inspection had on average 10 (95% CI
9-12) days greater slaughter age, assuming an average carcass weight of 345 kg. Furthermore,
estimates from a second model indicated that the increase in age at slaughter
was more severe for higher fibrosis scores. More precisely, the increase in slaughter
age was 34 (95% CI 11-57) days for fibrosis score of 1, 93 (95% CI 57-128) days
for fibrosis score 2 and 78 (95% CI 30-125) days for fibrosis score 3. Similarly, in a
third model comparing different burden categories with animals with no fluke burden,
there was a 31 (95% CI 7-56) days increase in slaughter age for animals with 1 to 10
parasites and 77 (95% CI 32-124) days increase in animals with more than 10 parasites
found in their livers.
Lastly, a multi-variable mixed effects logistic regression model was built to estimate
the association between climate, environmental, management and animal specific
factors and the risk of an animal being infected by F. hepatica. Multiple imputation
methodology was employed to deal with missing data arising from skipped questions
in the questionnaire. Results of the regression model confirmed the importance of temperature,
rainfall and cattle movements in increasing the risk for fasciolosis, while it
indicated that the presence of deer can increase the risk of infection and that male cattle
have a reduced risk of infection.
Overall, this project has used slaughterhouse data to fill important knowledge gaps regarding
F. hepatica infection in cattle. It has provided valuable information on the accuracy
of routine abattoir meat inspection, as well as other diagnostic tests. It has also
provided estimates of the effect of infection on the time cattle take to reach slaughter
weight at different levels of infection and identified relevant risk factors related to the
infection. In conclusion, knowledge of the effect of infection on slaughter age, as well
as regional risk factors for F. hepatica infection, along with an improved use of abattoir
inspection results in the evaluation of treatment strategies, can provide farmers and
veterinarians with better incentives and tools to improve their herd health strategies and
in the longer term help reduce the incidence of liver fluke in cattle.