Biomedical subjects

M G Roberts

Publications and source records attributed to M G Roberts.

At least 19 recordsLinked to original sources

PubMed2006

The type-reproduction number T in models for infectious disease control.

A ubiquitous quantity in epidemic modelling is the basic reproduction number R(0). This became so popular in the 1990s that 'All you need know is R(0)!' became a familiar catch-phrase. The value of R(0) defines, among other things, the control effort needed to eliminate the infection from a homogeneous host population, but can be misleading when applied to a heterogeneous population for the same purpose. We have defined the type-reproduction number T for an infectious disease, and shown that this not only has the required threshold behaviour, but also correctly determines the critical control effort for heterogeneous populations. The two quantities coincide for homogeneous populations. In this paper we further develop the new threshold quantity as an indicator of control effort required in a system where multiple types of individuals are recognised when control targets a specific type.

Algorithms↗

PubMed2005

Vertebral shape: automatic measurement with dynamically sequenced active appearance models.

The shape and appearance of vertebrae on lateral dual x-ray absorptiometry (DXA) scans were statistically modelled. The spine was modelled by a sequence of overlapping triplets of vertebrae, using Active Appearance Models (AAMs). To automate vertebral morphometry, the sequence of trained models was matched to previously unseen scans. The dataset includes a significant number of pathologies. A new dynamic ordering algorithm was assessed for the model fitting sequence, using the best quality of fit achieved by multiple sub-model candidates. The accuracy of the search was improved by dynamically imposing the best quality candidate first. The results confirm the feasibility of substantially automating vertebral morphometry measurements even with fractures or noisy images.

Algorithms↗

PubMed2005

An integral equation model for the control of a smallpox outbreak.

An integral equation model of a smallpox epidemic is proposed. The model structures the incidence of infection among the household, the workplace, the wider community and a health-care facility; and incorporates a finite incubation period and plausible infectivity functions. Linearisation of the model is appropriate for small epidemics, and enables analytic expressions to be derived for the basic reproduction number and the size of the epidemic. The effects of control interventions (vaccination, isolation, quarantine and public education) are explored for a smallpox epidemic following an imported case. It is found that the rapid identification and isolation of cases, the quarantine of affected households and a public education campaign to reduce contact would be capable of bringing an epidemic under control. This could be used in conjunction with the vaccination of healthcare workers and contacts. Our results suggest that prior mass vaccination would be an inefficient method of containing an outbreak.

Algorithms↗

PubMed2004

Modelling strategies for minimizing the impact of an imported exotic infection.

The global epidemic of severe acute respiratory syndrome (SARS) in 2003 demonstrated the need to determine control strategies for exotic infections. The prior determination of such strategies, and the use of mathematical models to assist this, is hampered by the obvious lack of data. We propose an integral equation model of Kermack-McKendrick type that may be used to compare strategies based on the isolation of infectious individuals. The model structures the incidence of infection according to the location of an infected individual at exposure, and requires knowledge of the infectivity kernel and the initial rate of exponential increase of cases. The model's use in the design of strategies to minimize the risk of SARS in a previously unexposed community is demonstrated.

Communicable Disease Control↗

PubMed2003

A new method for estimating the effort required to control an infectious disease.

We propose a new threshold quantity for the analysis of the epidemiology of infectious diseases. The quantity is similar in concept to the familiar basic reproduction ratio, R0, but it singles out particular host types instead of providing a criterion that is uniform for all host types. Using this methodology we are able to identify the long-term effects of disease-control strategies for particular subgroups of the population, to estimate the level of control necessary when targeting control effort at a subset of host types, and to identify host types that constitute a reservoir of infection. These insights cannot be obtained by using R0 alone.

Animals↗

PubMed2002

The metapopulation dynamics of an infectious disease: tuberculosis in possums.

An SEI metapopulation model is developed for the spread of an infectious agent by migration. The model portrays two age classes on a number of patches connected by migration routes which are used as host animals mature. A feature of this model is that the basic reproduction ratio may be computed directly, using a scheme that separates topography, demography, and epidemiology. We also provide formulas for individual patch basic reproduction numbers and discuss their connection with the basic reproduction ratio for the system. The model is applied to the problem of spatial spread of bovine tuberculosis in a possum population. The temporal dynamics of infection are investigated for some generic networks of migration links, and the basic reproduction ratio is computed-its value is not greatly different from that for a homogeneous model. Three scenarios are considered for the control of bovine tuberculosis in possums where the spatial aspect is shown to be crucial for the design of disease management operations.

Animal Migration↗

PubMed2001

Does pet helminth prophylaxis increase the rate of selection for drug resistance?

There is a growing tendency to control helminths in pets by the prophylactic use of broad-spectrum drug combinations (Allwormers), some of which are of low efficacy. If similar treatment regimes were applied to livestock, parasite strains resistant to chemotherapy would be expected to evolve. The rate of selection for resistance depends significantly on epidemiological parameters and strategic recommendations based on experience with farm animals might not be applicable to pets without critical examinations. Also, the routine use of Allwormers reduces the level of interaction between veterinarians and pet owners and the valuable contribution by veterinarians towards educating pet owners about animal and public health issues in addition to parasite treatment.

Animals↗

PubMed2001

Modelling physical activity: a multi-state life-table approach.

OBJECTIVE: To develop a consistent set of epidemiological estimates (incidence, prevalence, remission, mortality) for physical activity in New Zealand; project these estimates in the light of demographic trends; and predict the effectiveness of different health promotion strategies. METHOD: Multi-state life tables were constructed using physical inactivity prevalence data from the 1996/97 New Zealand Health Survey, and estimates of the relative risk of mortality, and of remission rates, from the literature. Statistics New Zealand population projections were used to forecast these multi-state life tables to 2021. Two physical activity health promotion strategies -uptake (remission enhancement) and maintenance (incidence or relapse reduction)--were simulated by changing the relevant epidemiological variables. RESULTS: The current fatal burden of physical inactivity in New Zealand is estimated to be 2,600 deaths per year (9% of all deaths). By 2021, the prevalence of physical inactivity will rise 4% as a result of demographic trends. Relapse reduction (enabling active people to remain active) is about 50% more effective than uptake enhancement (enabling inactive people to become active) as a physical activity health promotion strategy, but the two approaches are additive. Maximum realistic changes in relapse prevention and uptake enhancement could reduce the prevalence of physical inactivity by about 30%. CONCLUSIONS AND IMPLICATIONS: Multi-state life table methods can be used to model health risks (such as behaviours), as well as (chronic) diseases. The model has provided valuable insights for policy makers into the burden of physical inactivity in New Zealand, the impact of demographic trends, and the relative effectiveness of different health promotion strategies.

Adolescent↗

PubMed2001

The use of multistate life-table models for improving population health.

We demonstrate how incidence, prevalence, remission, mortality (IPRM) models may be constructed on population life-tables, how the incidence of a condition may be calculated, and how the consequences of demographic changes and public health interventions may be predicted. We illustrate the methodology by applying it to the epidemiology of diabetes, physical inactivity and obesity in New Zealand.

Diabetes Mellitus↗

PubMed2000

Nematode parasites of sheep: a survey of epidemiological parameters and their application in a simple model.

We review the literature on parameter values relevant to the epidemiology of strongyle nematode infections of domestic sheep. Information is subdivided by parasite genus, country of origin and climate type. While field observations have been made in a large number of countries, the bulk of studies under controlled conditions have been conducted in Australia, New Zealand and the UK. For these countries, experiments and parameters are interpreted in terms of a previously published model of nematode dynamics, and are used to calculate the basic reproduction number. Average values range from less than 6 for Haemonchus contortus in New Zealand and a winter rainfall region of Australia, to more than 16 for Ostertagia circumcincta in New Zealand and the UK. Additional considerations of the effects of climate and the annual replacement of host stock show that for conditions favourable for parasite transmission this is a robust indicator of parasite epidemiology. When climate variation and annual replacement are added to the model, it is shown to reasonably describe the qualitative behaviour of an experimental data set, indicating it to be a useful tool for further investigation of some of the underlying assumptions of sheep-nematode dynamics.

Animals↗

PubMed2000

Predicting and preventing measles epidemics in New Zealand: application of a mathematical model.

A mathematical model of the dynamics of measles in New Zealand was developed in 1996. The model successfully predicted an epidemic in 1997 and was instrumental in the decision to carry out an intensive MMR (measles-mumps rubella) immunization campaign in that year. While the epidemic began some months earlier than anticipated, it was rapidly brought under control, and its impact on the population was much reduced. In order to prevent the occurrence of further epidemics in New Zealand, an extended version of the model has since been developed and applied to the critical question of the optimal timing of MMR immunization.

Adolescent↗

PubMed1999

The immunoepidemiology of nematode parasites of farmed animals: a mathematical approach.

The population dynamics of farmed animals are controlled by humans, and often involve high host densities, which encourage higher parasite burdens than would be usual in wild animals. As a result, the immunity to reinfection acquired by the host is an important determinant of parasite population dynamics. For example, lambs are highly susceptible to gastrointestinal nematodes as they begin to graze, but develop an immunity that accounts for the observed within-year variation in parasite load and pasture contamination. In the longer term, control measures are compromised by the development of parasite strains resistant to chemotherapy, focusing attention on the development of 'natural' measures, including the selection for resistant hosts and the development of antiparasite vaccines. Mick Roberts here considers the immunoepidemiology of parasites of farmed animals on three levels: the interaction between the parasite and the host's immune system determining the individual's level of protection; the development of acquired immunity determining the within-year parasite population dynamics; and the long-term effects of control measures on the between-year parasite population dynamics.

Animal Husbandry↗

PubMed1999

A comparison of wildlife control and cattle vaccination as methods for the control of bovine tuberculosis.

The Australian brushtail possum is the major source of infection for new cases of bovine tuberculosis in cattle in New Zealand. Using hypothetical values for the cost of putative cattle and possum Tb vaccines, the relative efforts required to eradicate Tb in cattle using possum culling, possum vaccination or cattle vaccination are compared. For realistic assumed costs for 1080 poison bait, possum culling is found to be a cost-effective strategy compared to cattle vaccination if the required control area is below 13 ha per cattle herd, while possum vaccination is cost-effective for control areas of less than 3 ha per herd. Examination of other considerations such as the possible roles of possum migration and heterogeneities in possum population density suggest that each control strategy may be superior under different field conditions. Finally, the roles of the possum in New Zealand, and the Eurasian badger in Great Britain and Ireland in the transmission of bovine tuberculosis to cattle are compared.

Animals↗

PubMed1999

A Kermack-McKendrick model applied to an infectious disease in a natural population.

The dynamics of a fatal infectious disease in a population regulated by density-dependent constraints are represented as a system of nonlinear integral equations. Survival probabilities and disease transmission coefficients may vary with the time elapsed since infection, and horizontal and vertical modes of transmission are allowed for. Criteria for the existence and stability of steady states are derived, and an example based on the dynamics of tuberculosis is presented. Finally, the relative merits of this approach, and the familiar compartmental models based on differential equations are discussed.

Communicable Diseases↗

PubMed1998

A simple parasite model with complicated dynamics.

During their first year of life sheep acquire parasites through grazing, and simultaneously build up an immunity to infection. At the beginning of each year non-immune lambs are introduced onto contaminated pasture. We represent this process by differential equations describing the within-year dynamics, and defining a difference equation that describes the between-year dynamics. An example with two system parameters is analysed in detail. It is shown that regions exist in parameter space where periodic (between-year) or aperiodic solutions occur. Parasite control schemes could change the system dynamics from a stable equilibrium to complicated long-term fluctuations.

Animals↗

PubMed1998

The dynamics of an infectious disease in a population with birth pulses.

In most models of population dynamics increases in population due to births are assumed to be time-independent, but many species of wild animal give birth only during a single period of the year. We propose a model for the dynamics of a fatal infectious disease in a wild animal population for which births occur in a single pulse once per time period. Periodic solutions are found and criteria for their stability determined. A simple example applied to tuberculosis in the possum is used to illustrate the effect of the birth pulse on critical population parameters.

Animals↗

PubMed1997

A model of bovine tuberculosis control in domesticated cattle herds.

A typical strategy for disease control in domesticated animals involves regular field tests and quarantine of infected herds. This prevents disease spread beyond the herd, while slaughter of diseased animals removes the infection from within the herd. A model of bovine tuberculosis (Tb) control in cattle is examined, which includes 'test and slaughter' combined with herd isolation and vaccination. Herd status is represented by an integral equation expressing the duration of herd isolation. The current Tb situation in New Zealand is used as an example, and vaccination strategy discussed. Extrapolation of existing management strategies indicate that a vaccine of efficacy greater than 96% would be required, reaching 95% of target Tb levels within six years. These results suggest that a complementary strategy of vaccination and vector control may be more promising than vaccination alone.

Animals↗

PubMed1997

An algorithm for the detection of surface-active alpha helices with the potential to anchor proteins at the membrane interface.

MOTIVATION: Surface-active peptides are amphiphilic in nature and have been shown to have the potential to interact at the membrane interface, possibly by lying parallel to the membrane surface. Present methodology for the identification of these helices uses a fixed window size, is based on a two-dimensional sum of hydrophobicity vectors and gives no measure of the statistical significance for any region identified as amphiphilic. Identification of weakly surface-active structures is difficult and here we have attempted to remedy this by introducing an algorithm which considers three-dimensional geometries and variable window size. RESULTS: A new measure of membrane-interactive potential is proposed, called the depth-weighted inserted hydrophobicity (DWIH), which is based on the sequestration of hydrophobic residues within a hydrophobic compartment, such as that produced by a membrane bilayer. A statistical significance for this measure has been derived using Monte Carlo techniques. The algorithm is applied to a set of proteins which are thought to anchor to the membrane via C-terminal amphiphilic alpha helices. The DWIH measure appears to allow the identification of this category of membrane-interactive helices which lie near the boundary of the hydrophobic moment plot and which have previously been hard to classify.

Algorithms↗