A Bayesian zero-truncated approach for analysing capture-recapture count data from classical scrapie surveillance in France

Capture-recapture (CR) methods are used to study populations that are monitored with imperfect observation processes. They have recently been applied to the monitoring of animal diseases to evaluate the number of infected units that remain undetected by the surveillance system. This paper proposes three bayesian models to estimate the total number of scrapie-infected holdings in France from CR count data obtained from the French classical scrapie surveillance programme. We fitted two zero-truncated Poisson (ZTP) models (with and without holding size as a covariate) and a zero-truncated negative binomial (ZTNB) model to the 2006 national surveillance count dataset. We detected a large amount of heterogeneity in the count data, making the use of the simple ZTP model inappropriate. However, including holding size as a covariate did not bring any significant improvement over the simple ZTP model. The ZTNB model proved to be the best model, giving an estimation of 535 (CI(95%) 401-796) infected and detectable sheep holdings in 2006, although only 141 were effectively detected, resulting in a holding-level prevalence of 4.4‰ (CI(95%) 3.2-6.3) and a sensitivity of holding-level surveillance of 26% (CI(95%) 18-35). The main limitation of the present study was the small amount of data collected during the surveillance programme. It was therefore not possible to build complex models that would allow depicting more accurately the epidemiological and detection processes that generate the surveillance data. We discuss the perspectives of capture-recapture count models in the context of animal disease surveillance.     

A deterministic and stochastic simulation model for intra-herd paratuberculosis transmission

In France, the considerable economic losses due to paratuberculosis introduction in cattle herds may justify the development of a herd-level certification procedure. For a cost/benefit analysis purpose, a simple deterministic and stochastic simulation model for intra-herd paratuberculosis transmission has been developed to evaluate the economic consequences of the purchase of a single infected heifer in a French average herd. The values of the epidemiological parameters were provided by a panel of French paratuberculosis experts. The results were in adequacy with field observations. A sensitivity analysis was conducted. The model was however difficult to validate rigorously, since few data on the intra-herd paratuberculosis true prevalence level are currently available.     

A SURVEY OF DYSTOCIA IN BEEF CATTLE IN SOUTHERN QUEENSLAND

Data relating to dystocia were obtained for 22% of beef herds in the Roma region of southern Queensland. In 1973 dystocia incidence in 23,129 maiden heifers was 12%; 4% of heifers and 7% of calves died. Thirty-five per cent of the 444 herds surveyed had more than a 10% incidence of dystocia, and 10% had more than a 30% incidence of dystocia. Only 6% of herds had no dystocia. Among the 73% of producers considering dystocia a problem in most years, 14% regarded it as a major problem. The incidence of dystocia in heifers was significantly higher in Poll Hereford than Hereford herds, which in turn were higher than other breeds of cattle. Poll Hereford and Hereford owners were the most concerned about dystocia but did not supervise calving more frequently. Dystocia and attitude were also examined in relation to degree of supervision at calving and size of the breeding herd.     

Simulated economic consequences of foot-and-mouth disease epidemics and their public control in France

The efficient management of foot-and-mouth disease (FMD) epidemics in France was examined through a simulation model which combines epidemiological and economic modules. From the reactions of the importing countries in terms of the products subject to import bans and the regionalization commitments, the economic module assesses the financial consequences of FMD outbreaks borne not only by the breeding sector but also by the other economic sectors on regional and national levels. Among the control options for FMD, the strategy of stamping out infected herds and dangerous in-contact herds most often contributes to reducing the economic consequences of FMD epidemics. Implementing a campaign of emergency vaccination is socially optimal if the additional export losses associated with the delay of slaughtering the vaccinated animals are offset by the gains of reducing the duration of the FMD epidemic. The importance of reducing as much as possible the total duration of the import bans is stressed by the estimated cost of an extra week of import bans. The optimal control strategy was unaffected by the introduction of stochastic parameters.     

Quantification of porcine circovirus type 2 (PCV-2) within- and between-pen transmission in pigs

PCV-2 within- and between-pen transmission was quantified by estimating the daily transmission rate beta and the basic reproduction ratio (R(0)) using a stochastic SEIR (Susceptible, Exposed, Infectious, Removed) model fitted on experimental data. Within-pen transmission was quantified by using four groups of eight SPF (specific pathogen-free) pigs (four infected and four susceptible pigs having direct contact). Between-pen transmission was studied in two groups of 16 SPF pigs (eight infected and eight susceptible pigs having indirect contact (10 cm distance)). Pigs were monitored twice a week (blood samples) and were tested for PCV-2 antibodies (ELISA test) and viral genome load in sera (real-time PCR). Transmission parameters beta(within) and beta(between) were estimated using a maximum likelihood method and the duration of infectiousness, to compute R(0), was estimated with a parametric survival model. Different assumptions were made to determine the end of infectiousness (seroconversion, seroconversion and decline in viral genome load, permanent infectiousness). R(0[within]) (8.9 (5.1-15.4)) was greater when the end of infectiousness was assumed to be related to both seroconversion and a decline of PCV-2 genome load in sera (average duration of infectiousness = 32 days) compared with only seroconversion as the indicator of recovery (R(0[within]) = 5.5 (3.3-9.0)). Whatever the assumption, between-pen R(0) (0.58 (0.23-1.47)) was always significantly lower than within-pen R(0). Only beta(within) was sensitive to the assumption on end of infectiousness and decreased with increasing duration of infectiousness. These results showed that PCV-2 transmission is influenced by contact structure that appears worth being taken into account in an epidemic model.     

A metapopulation model to simulate West Nile virus circulation in Western Africa, Southern Europe and the Mediterranean basin

In Europe, virological and epidemiological data collected in wild birds and horses suggest that a recurrent circulation of West Nile virus (WNV) could exist in some areas. Whether this circulation is permanent (due to overwintering mechanisms) or not remains unknown. The current conception of WNV epidemiology suggests that it is not: this conception combines an enzootic WNV circulation in tropical Africa with seasonal introductions of the virus in Europe by migratory birds. The objectives of this work were to (i) model this conception of WNV global circulation; and (ii) evaluate whether the model could reproduce data and patterns observed in Europe and Africa in vectors, horses, and birds. The model was calibrated using published seroprevalence data obtained from African (Senegal) and European (Spain) wild birds, and validated using independent, published data: seroprevalence rates in migratory and resident wild birds, minimal infection rates in vectors, as well as seroprevalence and incidence rates in horses. According to this model, overwintering mechanisms are not needed to reproduce the observed data. However, the existence of such mechanisms cannot be ruled out.     

Differential expression of the heat shock protein Hsp70 in natural populations of the tilapia, Sarotherodon melanotheron, acclimatised to a range of environmental salinities

Background  

The relationship between environmental variation and induction of heat shock proteins (Hsps) has been much documented under experimental conditions. However, very little is known about such induction in natural populations acclimatised to prevailing environmental conditions. Furthermore, while induction of stress proteins has been well documented in response to environmental contaminants and thermal stressors, little is known about whether factors, such as extreme salinity, are also potential inductors. The black-chinned tilapia Sarotherodon melanotheron is unusual for its ability to colonise estuarine environments in West Africa that are characterised by extremely high salinities. The relationships between mRNA levels of the 70 kDa heat shock protein (Hsp70) and Na⁺, K⁺-ATPase1α (Naka) in the gills, environmental salinity, and a life-history trait (condition factor) were investigated in wild populations of this species sampled from three locations in the Saloum estuary, at salinities ranging from 40 to 100 psu.