Biomedical subjects

G F Medley

Publications and source records attributed to G F Medley.

At least 19 recordsLinked to original sources

PubMed2006

Looking after the individual to reduce disease in the flock: a binomial mixed effects model investigating the impact of individual sheep management of footrot and interdigital dermatitis in a prospective longitudinal study on one farm.

A total of 160 ewes on one farm in England were studied for 18 months. The incidence of footrot and interdigital dermatitis in individually identified sheep and treatment and flock control measures were recorded. A binomial mixed effect model with the incidence of footrot or interdigital dermatitis as the outcome was used to investigate patterns of association between treatments, flock control measures and the incidence of footrot or interdigital dermatitis. In this one flock, the incidence of footrot and interdigital dermatitis was positively associated with the incidence of footrot and interdigital dermatitis and with trimming of feet and negatively associated with the use of parenteral antibiotics and topical antibiotic sprays in either the first and/or second 2-week period prior to the incidence of interest. These results provide two hypotheses: one that 'footrot and interdigital dermatitis are infectious diseases that can be controlled, in part, through the use of antibiotic therapy, which acts to reduce the infectious period of diseased sheep' and two, that 'routine trimming of diseased and healthy feet exacerbate disease, through environmental contamination and/or through increased susceptibility of sheep with recently trimmed feet'.

Animal Husbandry↗

PubMed2006

Screening strategies in surveillance and control of methicillin-resistant Staphylococcus aureus (MRSA).

With reports of hospital-acquired methicillin-resistant Staphylococcus aureus (MRSA) continuing to increase and therapeutic options decrease, infection control methods are of increasing importance. Here we investigate the relationship between surveillance and infection control. Surveillance plays two roles with respect to control: it allows detection of infected/colonized individuals necessary for their removal from the general population, and it allows quantification of control success. We develop a stochastic model of MRSA transmission dynamics exploring the effects of two screening strategies in an epidemic setting: random and on admission. We consider both hospital and community populations and include control and surveillance in a single framework. Random screening was more efficient at hospital surveillance and allowed nosocomial control, which also prevented epidemic behaviour in the community. Therefore, random screening was the more effective control strategy for both the hospital and community populations in this setting. Surveillance strategies have significant impact on both ascertainment of infection prevalence and its control.

Cross Infection↗

PubMed2006

Agricultural restructuring and gastrointestinal parasitism in domestic ruminants on the rangelands of Kazakhstan.

In order to evaluate the epidemiology of parasitism in farmed ruminants in the changed agricultural context in Kazakhstan, 505 cattle, sheep and goats were surveyed for gastrointestinal parasitism using coprological examination, and a further thirty sheep and four goats using post mortem extraction of helminths. The dominant helminth genera were Marshallagia, Nematodirus and Trichostrongylus. Haemonchus was also present. Faecal egg counts were low (mean abundance 0-115 eggs per gram in different groups), and there was no relationship between faecal egg density and body condition score. Eggs of Nematodirus spp. were more common in sheep less than 1 yr of age, whereas those of the Trichostrongylidae were generally more common in adult sheep. Based on faecal egg counts, sheep grazing on common land close to permanent human settlements were generally no more heavily infected than those grazing the remote open plains. This could be due to a non-linear relationship between livestock numbers and stocking density. We suggest that cessation of strategic nematode control in the region following post-Soviet agricultural reform was offset by a general reduction in stocking density, such that parasite burdens remained low. However, recovery in livestock numbers is likely to lead to increased levels of infection and production loss unless sustainable control strategies are put in place.

Age Factors↗

PubMed2006

The transmission and control of mastitis in dairy cows: a theoretical approach.

A multi-species model that incorporates the transmission of both major and minor mastitis pathogens as well as the interaction between them via coinfection of a quarter is fitted to data from seven dairy herds. The results suggest that major and minor pathogens can interact, on occasion, in a counter-intuitive way with implications for the control of clinical mastitis. The key finding is that delaying culling of cows with major pathogen infections for more than 100 days post infection could result in a higher prevalence of major pathogen infections, whereas early culling would reduce the levels. A theoretical exploration of current and proposed control strategies is carried out, informed by parameters estimated from the model and data. The results at each stage suggest of areas of further research such as: field-testing of the hypotheses presented; the exploration of a stochastic formulation of the model; analysis of the raw repeated measures data; application of control theory to determine the most effective combination of control strategies; inclusion of economic factors into the modelling framework.

Animals↗

PubMed2006

Stochastic dynamics of immunity in small populations: a general framework.

Assessment of immunological status is a powerful tool in the surveillance and control of infectious pathogens in livestock and human populations. The distribution of immunity levels in the population provides information on time and age dependent transmission. A stochastic model is developed for a livestock population which relates the dynamics of the distribution of immunity levels at the population level to those of pathogen transmission. A general model with K immunity level categories is first proposed, taking into account the increase of the immunity level due to an infection or a re-exposure, the decrease of the immunity level with time since infection or exposure, and the effect of immunity level on the susceptibility and the infectivity of individuals. Numerical results are presented in the particular cases with K=2 and K=3 immunity level categories. We demonstrate that for a given distribution of the immunity levels at the population level, the model can be used to identify quantities such as most likely periods of time since introduction of infection. We discuss this approach in relation to analysis of serological data.

Algorithms↗

PubMed2005

Dynamical behaviour of epidemiological models with sub-optimal immunity and nonlinear incidence.

In this paper we analyze the dynamics of two families of epidemiological models which correspond to transitions from the SIR (susceptible-infectious-resistant) to the SIS (susceptible-infectious-susceptible) frameworks. In these models we assume that the force of infection is a nonlinear function of density of infectious individuals, I. Conditions for the existence of backwards bifurcations, oscillations and Bogdanov-Takens points are given.

Communicable Diseases↗

PubMed2005

Prevalence and associations between bacterial isolates from dry mammary glands of dairy cows.

To assess the prevalence and patterns of bacterial isolates, cultures were made from the dry mammary glands of dairy cows in six commercial dairy herds in the UK. Milk samples were taken from all four quarters of 480 cows at drying off and at weekly intervals from 14 days before to seven days after calving. A major mastitis pathogen was isolated from at least one quarter of 220 (45.8 per cent) of the cows and from more than one quarter of 90 (18.8 per cent) of them. During the late dry to calving period, of the 957 quarters with three culture results, a major mastitis pathogen was cultured from 236 (24.7 per cent) quarters of 186 (38.8 [corrected] per cent) cows. The most commonly isolated major pathogen was Escherichia coli, followed by Streptococcus uberis and coagulase-positive staphylococci. There were significant differences between the patterns of isolates from different farms and in different calving months, suggesting that the rate of infection was partially dependent on external conditions. The isolation of E. coli, S. uberis or coagulase-positive staphylococci from a cow during the late dry/periparturient period was associated with an increased risk of that cow being culled in the next lactation. Bayesian general linear mixed models were used to assess the associations between the different bacterial species. The probability of isolating either E. coli or S. uberis was significantly greater when the other organism was cultured in a milk sample; this was also true of coagulase-positive staphylococci and S. uberis. When Corynebacterium species were isolated from a milk sample, the probability of isolating coagulase-positive staphylococci or S. uberis decreased significantly, and when coagulase-negative staphylococci were isolated the probability of isolating coagulase-positive staphylococci was reduced.

Animals↗

PubMed2005

The transmission dynamics of groups A and B human respiratory syncytial virus (hRSV) in England & Wales and Finland: seasonality and cross-protection.

Human respiratory syncytial virus (hRSV) transmission dynamics are inherently cyclical, and the observed genetic diversity (between groups A and B) also appears to have a repeating pattern. A key unknown is the extent to which genetic variants interact immunologically, and thus impact on epidemiology. We developed a novel mathematical model for hRSV transmission including seasonal forcing of incidence and temporary intra- and inter-group partial immunity. Simultaneous model fits to data from two locations (England & Wales, UK, and Turku, Finland) successfully reproduced the contrasting infection dynamics and group A/B dominance patterns. Parameter estimates are consistent with direct estimates. Differences in the magnitude and seasonal variation in contact rate between the two populations alone could account for the variation in dynamics between these populations. The A/B group dominance patterns are explained by reductions in susceptibility to and infectiousness of secondary homologous and heterologous infections. The consequences of the observed dynamic complexity are discussed.

Adolescent↗

PubMed2005

Multilocus sequence typing of Staphylococcus aureus isolated from high-somatic-cell-count cows and the environment of an organic dairy farm in the United Kingdom.

An outbreak of Staphylococcus aureus intramammary infections on an organic dairy farm was monitored for 10 months. Environmental and milk samples were collected from a total of 26 cows and a group of 21 purchased heifers about to be introduced into the milking herd. There was variation in the rate of isolation of S. aureus (9.5 to 43.8%) from individual mammary quarters, although no S. aureus isolates were detected in the milk samples collected from the heifers. One hundred ninety-one S. aureus isolates were detected from cow milk samples (n = 182), milking machine clusters (n = 4), farm personnel (n = 4), and the environment (n = 1). Multilocus sequence typing (MLST) had a typeability of 100% when it was applied to the 191 isolates. Among the 191 isolates there was limited strain diversity, with seven sequence types (STs) dominated by two strains with closely related STs that differed at a single locus. Within individual mammary quarters there were naturally occurring dual infections, although this was identified in only 0.4% of milk samples. Different strains were associated with variable persistence within quarters. MLST is clearly a very suitable tool for the differentiation and analysis of S. aureus populations detected on dairy cattle farms.

Animals↗

PubMed2005

Multilocus sequence typing of intercontinental bovine Staphylococcus aureus isolates.

A total of 258 bovine-associated Staphylococcus aureus isolates from the United States, Chile, and the United Kingdom, plus the reference isolate S. aureus Newbould 305 (NCIMB 702892), were analyzed by multilocus sequence typing (MLST). A collection of previously characterized United Kingdom isolates were also included in the analysis. The results demonstrated that MLST is suitable for the differentiation of bovine S. aureus isolates from various sites (milk, teat skin, milking machine unit liners, hands, and bedding) and countries. The theory of the host specificity of S. aureus is supported by the detection of a previously undescribed clonal complex that comprised 87.4% of the isolates studied, with representatives from all geographic locations investigated. This suggests that a single clonal group has achieved a widespread distribution and is responsible for the majority of infections. Some sequence types (STs; ST25, ST115, ST124, and ST126) demonstrated site specificity, as they were significantly (P < 0.05) associated with milk or teat skin.

Animals↗

PubMed2004

Isolation measures in the hospital management of methicillin resistant Staphylococcus aureus (MRSA): systematic review of the literature.

OBJECTIVE: To evaluate the evidence for the effectiveness of isolation measures in reducing the incidence of methicillin resistant Staphylococcus aureus (MRSA) colonisation and infection in hospital inpatients. DESIGN: Systematic review of published articles. DATA SOURCES: Medline, Embase, CINAHL, Cochrane Library, System for Information on Grey Literature in Europe (SIGLE), and citation lists (1966-2000). REVIEW METHODS: Articles reporting MRSA related outcomes and describing an isolation policy were selected. No quality restrictions were imposed on studies using isolation wards or nurse cohorting. Other studies were included if they were prospective or employed planned comparisons of retrospective data. RESULTS: 46 studies were accepted; 18 used isolation wards, nine used nurse cohorting, and 19 used other isolation policies. Most were interrupted time series, with few planned formal prospective studies. All but one reported multiple interventions. Consideration of potential confounders, measures to prevent bias, and appropriate statistical analysis were mostly lacking. No conclusions could be drawn in a third of studies. Most others provided evidence consistent with a reduction of MRSA acquisition. Six long interrupted time series provided the strongest evidence. Four of these provided evidence that intensive control measures including patient isolation were effective in controlling MRSA. In two others, isolation wards failed to prevent endemic MRSA. CONCLUSION: Major methodological weaknesses and inadequate reporting in published research mean that many plausible alternative explanations for reductions in MRSA acquisition associated with interventions cannot be excluded. No well designed studies exist that allow the role of isolation measures alone to be assessed. None the less, there is evidence that concerted efforts that include isolation can reduce MRSA even in endemic settings. Current isolation measures recommended in national guidelines should continue to be applied until further research establishes otherwise.

Cross Infection↗

PubMed2004

The use of Markov chain Monte Carlo for analysis of correlated binary data: patterns of somatic cells in milk and the risk of clinical mastitis in dairy cows.

Two analytical approaches were used to investigate the relationship between somatic cell concentrations in monthly quarter milk samples and subsequent, naturally occurring clinical mastitis in three dairy herds. Firstly, cows with clinical mastitis were selected and a conventional matched analysis was used to compare affected and unaffected quarters of the same cow. The second analysis included all cows, and in order to overcome potential bias associated with the correlation structure, a hierarchical Bayesian generalised linear mixed model was specified. A Markov chain Monte Carlo (MCMC) approach, that is Gibbs sampling, was used to estimate parameters. The results of both the matched analysis and the hierarchical modelling suggested that quarters with a somatic cell count (SCC) in the range 41,000-100,000 cells/ml had a lower risk of clinical mastitis during the next month than quarters <41,000 cell/ml. Quarters with an SCC >200,000 cells/ml were at the greatest risk of clinical mastitis in the next month. There was a reduced risk of clinical mastitis between 1 and 2 months later in quarters with an SCC of 81,000-150,000 cells/ml compared with quarters below this level. The hierarchical modelling analysis identified a further reduced risk of clinical mastitis between 2 and 3 months later in quarters with an SCC 61,000-150,000 cells/ml, compared to other quarters. We conclude that low concentrations of somatic cells in milk are associated with increased risk of clinical mastitis, and that high concentrations are indicative of pre-existing immunological mobilisation against infection. The variation in risk between quarters of affected cows suggests that local quarter immunological events, rather than solely whole cow factors, have an important influence on the risk of clinical mastitis. MCMC proved a useful tool for estimating parameters in a hierarchical Bernoulli model. Model construction and an approach to assessing goodness of model fit are described.

Animals↗

PubMed2004

Methicillin-resistant Staphylococcus aureus in hospitals and the community: stealth dynamics and control catastrophes.

Methicillin-resistant Staphylococcus aureus (MRSA) represents a serious threat to the health of hospitalized patients. Attempts to reduce the spread of MRSA have largely depended on hospital hygiene and patient isolation. These measures have met with mixed success: although some countries have almost eliminated MRSA or remained largely free of the organism, others have seen substantial increases despite rigorous control policies. We use a mathematical model to show how these increases can be explained by considering both hospital and community reservoirs of MRSA colonization. We show how the timing of the intervention, the level of resource provision, and chance combine to determine whether control measures succeed or fail. We find that even control measures able to repeatedly prevent sustained outbreaks in the short-term can result in long-term control failure resulting from gradual increases in the community reservoir. If resources do not scale with MRSA prevalence, isolation policies can fail "catastrophically."

Community-Acquired Infections↗

PubMed2004

Estimating the transmission parameters of pneumococcal carriage in households.

This paper analyses Streptococcus pneumoniae transmission dynamics in households using longitudinal data on pneumococcal (Pnc) carriage in the United Kingdom. Ten consecutive swabs were taken at 4-week intervals from all members of 121 households. The family status is derived from the observed Pnc carriage status of each family member. Transition matrices are built for each family size and composition containing the observed frequency of transitions between family statuses over a 28-day interval. A density-dependent transmission model is fitted to derive maximum-likelihood estimates of the duration of carriage and acquisition rates from the community and from infected individuals within the household. Parameter values are estimated for children (< 5 years) and adults (5+ years). The duration of carriage is longer in children < 5 years of age than in older family members (51 vs. 19 days). Children are 3-4 times more likely than adults to acquire Pnc infection from the community. Transmission rates within the household suggest that adults are more infectious but less susceptible than children. Transmission within the household is most important in large families. The proportion of household-acquired infection ranges from 29 to 46% in households of three persons to 38-50% in larger households. Evidence of density-dependent within-household transmission is found, although the strength of this relationship is not clear from the model estimates.

Adolescent↗

PubMed2004

Somatic cell count distributions during lactation predict clinical mastitis.

This research investigated somatic cell count (SCC) records during lactation, with the purpose of identifying distribution characteristics (mean and measures of variation) that were most closely associated with clinical mastitis. Three separate data sets were used, one containing quarter SCC (n = 1444) and two containing cow SCC (n = 933 and 11,825). Clinical mastitis was defined as a binary outcome, present or absent, for each lactation, and SCC were log (base 10) transformed. A generalized linear mixed model within a Bayesian framework was used for analysis. Parameters were estimated using Markov Chain Monte Carlo with Gibbs sampling. Results from the 3 data sets were similar. Increased maximum and standard deviation log SCC during lactation, rather than increased geometric mean, were the best overall indicators of clinical mastitis. Distributions of SCC were also investigated separately for different mastitis pathogens. Increased maximum log SCC was associated with clinical mastitis caused by all pathogen types. Increased standard deviation log SCC was associated with Staphylococcus aureus, and Streptococcus uberis clinical mastitis and increased coefficient of variation log SCC (standard deviation divided by mean) was associated with Escherichia coli clinical mastitis. Increased geometric mean lactation SCC was associated with an increased risk of Staph. aureus clinical mastitis but a reduced risk of E. coli clinical mastitis. Our results suggest that using measures of variation and maximum cow SCC would enhance the accuracy of predicting clinical mastitis, compared with geometric mean SCC, and therefore improve genetic programs that aim to select for clinical mastitis resistance. The results are also consistent with low SCC increasing susceptibility to some mastitis pathogens.

Animals↗

PubMed2003

Serological survey of anti-group A rotavirus IgM in UK adults.

Rotaviral associated disease of infants in the UK is seasonal and infection in adults not uncommon but the relationship between these has been little explored. Adult sera collected monthly for one year from routine hospital samples were screened for the presence of anti-group A rotavirus immunoglobulin M class antibodies as a marker of recent infection. Anti-rotavirus IgM was seen in all age groups throughout the year with little obvious seasonal variation in the distribution of antibody levels. IgM concentrations and the proportion seropositive above a threshold both increased with age with high concentrations consistently observed in the elderly. Results suggest either high infection rates of rotavirus in adults, irrespective of seasonal disease incidence in infants, IgM persistence or IgM cross-reactivity. These results support recent evidence of differences between infant and adult rotavirus epidemiology and highlight the need for more extensive surveys to investigate age and time related infection and transmission of rotavirus.

Adolescent↗

PubMed2003

Systematic review of isolation policies in the hospital management of methicillin-resistant Staphylococcus aureus: a review of the literature with epidemiological and economic modelling.

OBJECTIVE: To review the evidence for the effectiveness of different isolation policies and screening practices in reducing the incidence of methicillin-resistant Staphylococcus aureus (MRSA) colonisation and infection in hospital in-patients. To develop transmission models to study the effectiveness and cost-effectiveness of isolation policies in controlling MRSA. DATA SOURCES: MEDLINE, EMBASE, CINAHL, The Cochrane Library and SIGLE (1966-2000). Hand-searching key journals. No language restrictions. REVIEW METHODS: Key data were extracted from articles reporting MRSA-related outcomes and describing an isolation policy in a hospital with epidemic or endemic MRSA. No quality restrictions were imposed on studies using isolation wards (IW) or nurse cohorting (NC). Other studies were included if they were prospective or employed planned comparisons of retrospective data. Stochastic and deterministic models investigated long-term transmission dynamics, studying the effect of a fixed capacity IW, producing economic evaluations using local cost data. RESULTS: A total of 46 studies were accepted: 18 IWs, 9 NC, 19 other isolation policies. Most were interrupted time series, with few planned formal prospective studies. All but one reported multiple interventions. Consideration of potential confounders, measures to prevent bias, and appropriate statistical analysis were mostly lacking. No conclusions could be drawn in a third of studies. Most others provided evidence consistent with reduction of MRSA acquisition. Six long interrupted time series provided the strongest evidence. Four of these provided evidence that intensive control measures which included patient isolation were effective in controlling MRSA. In two others IW use failed to prevent endemic MRSA. There was no robust economic evaluation. Models showed that improving the detection rate or ensuring adequate isolation capacity reduced endemic levels, with substantial savings achievable. CONCLUSIONS: Major methodological weaknesses and inadequate reporting in published research mean that many plausible alternative explanations for reductions in MRSA acquisition associated with interventions cannot be excluded. No well-designed studies allow the role of isolation measures alone to be assessed. Nonetheless, there is evidence that concerted efforts that include isolation can reduce MRSA even when endemic. Little evidence was found to suggest that current isolation measures recommended in the UK are ineffective, and these should continue to be applied until further research establishes otherwise. The studies with the strongest evidence, together with the results of the modelling, provide testable hypotheses for future research. Guidelines to facilitate design of future research are produced.

Cost-Benefit Analysis↗

PubMed2003

Survival of Theileria parva in its nymphal tick vector, Rhipicephalus appendiculatus, under laboratory and quasi natural conditions.

Groups of nymphal Rhipicephalus appendiculatus Muguga, having a mean of 1 or 9 Theileria parva Muguga-infected salivary gland acini per tick, were kept under quasi-natural conditions at an altitude of 1950 m or 20 degrees C at a relative humidity of 85% in the laboratory and their survival and infection prevalence and abundance determined over time. Theileria parva infections for both categories of ticks survived in the nymphal ticks for 50 or 26 weeks post salivary gland infection under quasi-natural or laboratory conditions respectively. There was a distinct decline in infections in the more heavily infected nymphae under both conditions of exposure, reflecting an apparent density dependence in parasite survival. Nymphal ticks having an average infection level of 1 infected salivary gland acinus per tick, survived for up to 69 or 65 weeks post-repletion under quasi-natural or the laboratory conditions respectively. Nymphae having an average infection level of 9 infected salivary gland acini per tick survived for a similar duration under each of the 2 conditions. The infection level of 9 infected salivary gland acini per tick did not seem to significantly affect the survival of the tick vector compared to those having an average of 1 infected salivary gland acinus per tick.

Animals↗