Genetic hitchhiking and resistance evolution to transgenic Bt toxins: insights from the African stalk borer Busseola fusca (Noctuidae).

Since transgenic crops expressing Bacillus thuringiensis (Bt) toxins were first released, resistance evolution leading to failure in control of pests populations has been observed in a number of species. Field resistance of the moth Busseola fusca was acknowledged 8 years after Bt maize was introduced in South Africa. Since then, field resistance of this corn borer has been observed at several locations, raising questions about the nature, distribution and dynamics of the resistance trait. Using genetic markers, our study identified four outlier loci clearly associated with resistance. In addition, genetic structure at neutral loci reflected extensive gene flow among populations. A realistically parameterised model suggests that resistance could travel in space at speed of several kilometres a year. Markers at outlier loci delineated a geographic region associated with resistance spread. This was an area of approximately 100 km radius, including the location where resistance was first reported. Controlled crosses corroborated these findings and showed significant differences of progeny survival on Bt plants depending on the origin of the resistant parent. Last, our study suggests diverse resistance mutations, which would explain the widespread occurrence of resistant larvae in Bt fields across the main area of maize production in South Africa.

Peak shift and epidemiology in a seasonal host-nematode system.

Insight into the dynamics of parasite-host relationships of higher vertebrates requires an understanding of two important features: the nature of transmission and the development of acquired immunity in the host. A dominant hypothesis proposes that acquired immunity develops with the cumulative exposure to infection, and consequently predicts a negative relationship between peak intensity of infection and host age at this peak. Although previous studies have found evidence to support this hypothesis through between-population comparisons, these results are confounded by spatial effects. In this study, we examined the dynamics of infection of the nematode Trichostrongylus retortaeformis within a natural population of rabbits sampled monthly for 26 years. The rabbit age structure was reconstructed using body mass as a proxy for age, and the host age-parasite intensity relationship was examined for each rabbit cohort born from February to August. The age-intensity curves exhibited a typical concave shape, and a significant negative relationship was found between peak intensity of infection and host age at this peak. Adult females showed a distinct periparturient rise in T. retortaeformis infection, with higher intensities in breeding adult females than adult males and non-breeding females. These findings are consistent with the hypothesis of an acquired immune response of the host to a parasite infection, supporting the principle that acquired immunity can be modelled using the cumulative exposure to infection. These findings also show that seasonality can be an important driver of host-parasite interactions.

Investigations of cattle herd breakdowns with bovine tuberculosis in four counties of England and Wales using VETNET data.

Cattle herd breakdown (HBR) with bovine tuberculosis (BTB) was investigated for farms in four counties of England and Wales outside southwest England from 1986 to early 2000. Data from the national database of TB testing history (VETNET) were used. Factors that influenced HBR included calendar time, herd size, number of cattle tested, the test type, the inter-test interval and spatial grouping of farms. Herd tests other than routine herd tests had an increased risk of HBR in all four counties. In all counties, the risk of HBR increased with calendar time and in Shropshire a test interval of 3 years was associated with an increased risk of HBR compared with a 1-year test interval. In Staffordshire and Sussex, a 4-year test interval was associated with a lower risk of HBR compared with a 1-year test interval. There was no evidence of spatial clustering of HBR in West Glamorgan (equal spatial risk in a 15-30 km radius) and weak evidence of spatial clustering in Shropshire (7-15 km) and Sussex (5-10 km). In Staffordshire, there was evidence of spatial (2-4 km) and time (3-4 years) clustering of HBR. The locally increased rate of testing following a confirmed HBR increased the detection of infected herds but did not prevent local spread in two of the four counties (Shropshire and Staffordshire) since the rate of HBR increased linearly from 1988 to 2000. The main conclusion is that there were both local and distant components of spread.

Spatial parasite transmission, drug resistance, and the spread of rare genes.

The transmission of many parasitic worms involves aggregated movement between hosts of "packets" of infectious larvae. We use a generic metapopulation model to show that this aggregation naturally promotes the preferential spread of rare recessive genes, compared with the expectations of traditional nonspatial models. A more biologically realistic model also demonstrates that this effect could explain the rapid observed spread of recessive or weakly dominant drug-resistant genotypes in nematode parasites of sheep. This promotion of a recessive trait arises from a novel mechanism of inbreeding arising from the metapopulation dynamics of transmission.

Dynamics of the 2001 UK foot and mouth epidemic: stochastic dispersal in a heterogeneous landscape.

Foot-and-mouth is one of the world's most economically important livestock diseases. We developed an individual farm-based stochastic model of the current UK epidemic. The fine grain of the epidemiological data reveals the infection dynamics at an unusually high spatiotemporal resolution. We show that the spatial distribution, size, and species composition of farms all influence the observed pattern and regional variability of outbreaks. The other key dynamical component is long-tailed stochastic dispersal of infection, combining frequent local movements with occasional long jumps. We assess the history and possible duration of the epidemic, the performance of control strategies, and general implications for disease dynamics in space and time.

Is it feasible to plan secondary care services for coronary heart disease rationally? A quantified modelling approach for a UK Health Authority.

BACKGROUND: Coronary heart disease (CHD) is the major cause of mortality in the UK. This paper explores the difficulties facing health authorities in applying a rational and needs based approach to the planning of hospital based services and describes a simple model used to bring available information to bear on this problem. METHOD: Published estimates of CHD incidence were identified and methodologies were critically appraised. Estimates were extrapolated to a district population. A three month cohort study of patients with suspected CHD was undertaken within a district general hospital and a model of these clinical pathways was used to examine the volumes of patients and services required to meet the estimated levels of need. RESULTS: From published studies, estimates of CHD incidence ranged from 83 to 3600 per 100 000. From the cohort study, of patients referred with possible CHD 62% received a definitive diagnosis of CHD, 56% underwent an exercise ECG, 16% received an angiogram, 4% received a CABG and 2% a PTCA. Using these figures together with the cohort study, estimated activity ranges from 247 to 6475 surgical interventions per million population compared with the National Service Framework for Coronary Heart Disease recommendations of 1500 procedures per million. CONCLUSIONS: Current research on CHD incidence gives a very wide variation in estimated need. This makes its value for service planning questionable and the model highlights a need for further high quality research. The model provides a link between epidemiological research and secondary care service planning and supports the implementation of recommendations within the National Service Framework for Coronary Heart Disease.

Drug-resistant parasites and aggregated infection--early-season dynamics.

We investigate the effect of spatial aggregation in the infection dynamics of nematode parasites in ruminants. We show that a high degree of spatial aggregation is likely to lead to a dramatically enhanced rate of invasion by drug-resistant strains.

Public health and primary care collaboration--a case study.

This paper describes the approach of one total purchasing project (TPP) to health needs assessment (HNA) and identifies issues that require consideration when undertaking HNA in primary care. It discusses the advantages of adopting a Health Authority (HA) HNA strategy and the development of a decision-making process for implementing this and other strategies to address possible areas of identified need. The prerequisites, advantages and potential difficulties of this approach are highlighted and related to the needs of primary care groups (PCGs) for addressing health needs and population health.