M. E. Arnold

Contact structures in the poultry industry in Great Britain: Exploring transmission routes for a potential avian influenza virus epidemic

BackgroundThe commercial poultry industry in United Kingdom (UK) is worth an estimated £3.4 billion at retail value, producing over 174 million birds for consumption per year. An epidemic of any poultry disease with high mortality or which is zoonotic, such as avian influenza virus (AIV), would result in the culling of significant numbers of birds, as seen in the Netherlands in 2003 and Italy in 2000. Such an epidemic would cost the UK government millions of pounds in compensation costs, with further economic losses through reduction of international and UK consumption of British poultry. In order to better inform policy advisers and makers on the potential for a large epidemic in GB, we investigate the role that interactions amongst premises within the British commercial poultry industry could play in promoting an AIV epidemic, given an introduction of the virus in a specific part of poultry industry in Great Britain (GB).ResultsPoultry premises using multiple slaughterhouses lead to a large number of premises being potentially connected, with the resultant potential for large and sometimes widespread epidemics. Catching companies can also potentially link a large proportion of the poultry population. Critical to this is the maximum distance traveled by catching companies between premises and whether or not between-species transmission could occur within individual premises. Premises closely linked by proximity may result in connections being formed between different species and or sectors within the industry.ConclusionEven quite well-contained epidemics have the potential for geographically widespread dissemination, potentially resulting in severe logistical problems for epidemic control, and with economic impact on a large part of the country. Premises sending birds to multiple slaughterhouses or housing multiple species may act as a bridge between otherwise separate sectors of the industry, resulting in the potential for large epidemics. Investment into further data collection and analyses on the importance of industry structure as a determinant for spread of AIV would enable us to use the results from this study to contribute to policy on disease control.

A modelling approach to estimate the sensitivity of pooled faecal samples for isolation of Salmonella in pigs

The objective of this study was to develop and parametrize a mathematical model of the sensitivity of pooled sampling of faeces to detect Salmonella infection in pigs. A mathematical model was developed to represent the effect of pooling on the probability of Salmonella isolation. Parameters for the model were estimated using data obtained by collecting 50 faecal samples from each of two pig farms. Each sample was tested for Salmonella at individual sample weights of 0.1, 0.5, 1, 10 and 25 g and pools of 5, 10 and 20 samples were created from the individual samples. The highest test sensitivity for individual samples was found at 10 g (90% sensitivity), with the 25 g test sensitivity equal to 83%. For samples of less than 10 g, sensitivity was found to reduce with sample weight. Incubation for 48 h was found to produce a more sensitive test than incubation for 24 h. Model results found increasing sensitivity with more samples in the pool, with the pools of 5, 10 and 20 being more sensitive than individual sampling, and the pools of 20 being the most sensitive of those considered.

Sensitivity of environmental sampling methods for detecting Salmonella Enteritidis in commercial laying flocks relative to the within-flock prevalence

The objective of this study was to estimate the sensitivity of three different sampling/testing methodologies for the detection of Salmonella Enteritidis in commercial egg-laying flocks relative to the within-flock prevalence. The following methods were compared on 21 farms: (1) The European Union (EU) baseline survey method (five faecal and two dust samples); (2) an in-house method that involved collecting 10 dust and 10 faecal samples into jars with buffered peptone water, and (3) a method involving single samples of pooled faeces and dust that has been adopted as a monitoring method for the National Control Programme across the EU (the NCP method). Testing of individual bird ovaries/oviduct and caeca was carried out on each flock, and the sensitivity of each sampling method was estimated relative to the within-flock prevalence using Bayesian methods. Results showed that the sensitivity of all the sampling methods increased as the within-flock prevalence increased, and that all were more efficient than individual bird sampling for detection of S. Enteritidis in commercial flocks. The in-house method was the most sensitive of the methods compared, with a 98% power to detect a 0.1% prevalence, and the NCP method the least sensitive, with a 93% power to detect a prevalence of 20%.

Dynamics of Salmonella transmission on a British pig grower-finisher farm: a stochastic model

Previous modelling studies have estimated that between 1% and 10% of human salmonella infections are attributable to pig meat consumption. In response to this food safety threat the British pig industry have initiated a salmonella monitoring programme. It is anticipated that this programme will contribute to achieving a UK Food Standards Agency target for reducing salmonella levels in pigs at slaughter by 50% within 5 years. In order to better inform the monitoring programme, we have developed a stochastic transmission model for salmonella in a specialist grower-finisher pig herd, where data from a Danish longitudinal study have been used to estimate some of the key model parameters. The model estimates that about 17% of slaughter-age pigs will be infected with salmonella, and that of these infected pigs about 4% will be excreting the organism. In addition, the model shows that the most effective control strategies will be those that reduce between-pen transmission.

Pathogenesis of experimental bovine spongiform encephalopathy (BSE): estimation of tissue infectivity according to incubation period.

This paper reports the results of tissue infectivity assays of bovine spongiform encephalopathy (BSE) agent in orally exposed cattle at stages during the incubation period. Estimations of the titre of infectivity in central nervous system (CNS), certain peripheral nerve ganglia and distal ileum tissue were made according to time post exposure from the relationship between incubation period and dose for RIII mice and C57bl mice using data from titrations of brain material from cases of BSE. The rate of increase of infectivity in the bovine CNS was then estimated, taking into account these tissue infectivity titres, the variability of the brain titre of clinical field cases of BSE, and the probability density of the expected number of months before clinical onset of each infected bovine. The doubling time for CNS was shown to equal 1.2 months. The titre in the thoracic dorsal root ganglia (DRG) was, on average, approximately 1 log units less than CNS, and cervical DRG approximately 0.5 log less than thoracic DRG. The pattern of increase of infectivity in the distal ileum is that of an initial increase up to 14–18 months post exposure, followed by a decrease, which is likely to be highly variable between animals. These results will be informative for future risk assessments of BSE, especially in relation to reviewing current control measures.

Modelling studies on bovine spongiform encephalopathy occurrence to assist in the review of the over 30 months rule in Great Britain

The objective of this study was to contribute to a risk assessment to review the over 30 months (OTM) scheme in cattle, whereby all cattle aged over 30 months slaughtered in the UK are removed from the human food chain. We use back–calculation methods to estimate the impact of changes to the OTM rule, by using passive and active surveillance data collected between 1 July 2001 and 30 June 2002. There are two types of change considered: increasing the age limit and allowing animals born after a certain date into the food chain. Results indicate that under the OTM rule less than 1 animal in the last 12 months of the incubation period would enter the food chain in 2003. The birth date changes considered and small changes to the upper age limit would increase this number by a relatively small amount.

Estimation of the Rate of Egg Contamination from Salmonella-Infected Chickens

Salmonella enterica serovar Enteritidis (S. Enteritidis) is one of the most prevalent causes for human gastroenteritis and is by far the predominant Salmonella serovar among human cases, followed by Salmonella Typhimurium. Contaminated eggs produced by infected laying hens are thought to be the main source of human infection with S. Enteritidis throughout the world. Although previous studies have looked at the proportion of infected eggs from infected flocks, there is still uncertainty over the rate at which infected birds produce contaminated eggs. The aim of this study was to estimate the rate at which infected birds produce contaminated egg shells and egg contents. Data were collected from two studies, consisting of 15 and 20 flocks, respectively. Faecal and environmental sampling and testing of ovaries/caeca from laying hens were carried out in parallel with (i) for the first study, testing 300 individual eggs, contents and shells together and (ii) for the second study, testing 4000 eggs in pools of six, with shells and contents tested separately. Bayesian methods were used to estimate the within‐flock prevalence of infection from the faecal and hen post‐mortem data, and this was related to the proportion of positive eggs. Results indicated a linear relationship between the rate of contamination of egg contents and the prevalence of infected chickens, but a nonlinear (quadratic) relationship between infection prevalence and the rate of egg shell contamination, with egg shell contamination occurring at a much higher rate than that of egg contents. There was also a significant difference in the rate of egg contamination between serovars, with S. Enteritidis causing a higher rate of contamination of egg contents and a lower rate of contamination of egg shells compared to non‐S. Enteritidis serovars. These results will be useful for risk assessments of human exposure to Salmonella‐contaminated eggs.

Bovine spongiform encephalopathy: the effect of oral exposure dose on attack rate and incubation period in cattle - an update

BackgroundTo provide information on dose–response and aid in modelling the exposure dynamics of the BSE epidemic in the United Kingdom groups of cattle were exposed orally to a range of different doses of brainstem homogenate of known infectious titre from clinical cases of classical bovine spongiform encephalopathy (BSE). Interim data from this study was published in 2007. This communication documents additional BSE cases, which occurred subsequently, examines possible influence of the bovine prion protein gene on disease incidence and revises estimates of effective oral exposure.FindingsFollowing interim published results, two further cattle, one dosed with 100 mg and culled at 127 months post exposure and the other dosed with 10 mg and culled at 110 months post exposure, developed BSE. Both had a similar pathological phenotype to previous cases. Based on attack rate and incubation period distribution according to dose, the dose estimate at which 50% of confirmed cases would be clinically affected was revised to 0.15 g of the brain homogenate used in the experiment, with a 95% confidence interval of 0.03–0.79 g. Neither the full open reading frame nor the promoter region of the prion protein gene of dosed cattle appeared to influence susceptibility to BSE, but this may be due to the sample size.ConclusionsOral exposure of cattle to a large range of doses of a BSE brainstem homogenate produced disease in all dose groups. The pathological presentation resembled natural disease. The attack rate and incubation period were dependent on the dose.

Allelic variants at codon 146 in the PRNP gene show significant differences in the risk for natural scrapie in Cypriot goats

Previous studies have shown the association between the polymorphisms serine (S) or aspartic acid (D) at codon 146 of the PRNP gene and resistance to scrapie. All goats aged >12 months (a total of 1075 animals) from four herds with the highest prevalence of scrapie in the country were culled and tested, of which 234 (21·7%) were positive by either the rapid test or immunohistochemistry (IHC) for any of the tissues tested. The odds of scrapie infection occurring in NN146 goats was 101 [95% credible interval (CrI) 19-2938] times higher than for non-NN146 or unknown genotypes. IHC applied to lymphoreticular tissue produced the highest sensitivity (94%, 95% CrI 90-97). The presence of putatively resistant non-NN146 alleles in the Cypriot goat population, severely affected by scrapie, provides a potential tool to reduce/eradicate scrapie provided that coordinated nationwide breeding programmes are implemented and maintained over time.

Estimating the impact on the food chain of changing bovine spongiform encephalopathy (BSE) control measures: the BSE control model.

The decline in the bovine spongiform encephalopathy (BSE) epidemic in Great Britain (GB) demands a review of control strategies to ensure that they remain proportionate. Amongst controls that are subject to review are those intended to minimise the risk of BSE exposure of consumers through food. Such risk mitigation steps are costly, and the relative impact of each in terms of human exposure to BSE infectivity is not known. This risk assessment, termed the BSE Control Model, aims to estimate by use of stochastic simulation the impact of testing of cattle at slaughter and the removal of Specified Risk Materials (SRM) on potential BSE infectivity consumed. This paper describes the use of the model to investigate the effect of different risk management methods that have been or could be implemented between 2005 and 2010. Our results suggest that the amount of infectivity consumed in 2005 with the Over Thirty Month (OTM) rule in place was a mean of 0.03 bovine oral ID(50) (BO ID(50)). This is an extremely low amount, particularly considering that it would be spread over, on average, 236 infected carcases that would be further sub-divided into portions for human consumption. The highest contributor to the total amount of infectivity consumed per year is spinal contamination at carcase splitting (35%). In 2006 the OTM scheme was discontinued and head meat was again permitted into the food chain. These changes resulted in an increase in the amount of infectivity consumed, rising to an estimated 28 BO ID(50) in 2006, and 19 BO ID(50) in 2007. In 2008 the age at removal of vertebral column was raised from 24 to 30 months, and an estimated 24 BO ID(50) of infectivity was consumed. At the beginning of 2009 the age at testing of cattle was raised to 48 months for healthy slaughter, emergency slaughter and fallen stock. Under these conditions, an estimated mean of 24 BO ID(50) will be consumed in 2009, decreasing to 20 BO ID(50) in 2010. Even though presented in terms of bovine rather than human oral ID(50), such estimates represent an extremely low exposure of the British population. Considerable uncertainty would surround any attempt to try to convert such exposure into estimates of new cases of vCJD, but the most recent estimates of the size of the species barrier between cattle and humans (4000, EFSA, 2006) suggest that there would be few, if any, new cases of vCJD arising from such exposure levels.