Effective age models for imperfect maintenance

A large number of maintenance models are available in the literature. Most of these usually assume that the effect of maintenance interventions is as good as new. This amounts to assuming the maintenance action undergone by a system, be it preventive or corrective, is equivalent to its replacement. This hypothesis is, of course, questionable in many cases. Maintenance without replacement can lead to a significant level of rejuvenation of a system, either preventively or after repair. However, the restoration of the performances of the system is most of the time incomplete. The effect of such an imperfect maintenance has been described in different ways, which can be split in two main categories: reduction of the value of a degradation variable embodying the 'health' of the system, or modification of the lifetime distribution of the system. This paper focuses on the latter approach, and is structured in two parts. First, it reviews different approaches of imperfect maintenance, modelling the gain in residual lifetime either by a decrease of the failure rate value or by a reduction of the system effective age. Second, an innovative model based on the concepts of elasticity and inescapability of aging is described, in order to introduce more intuitive observations on the results of repeated maintenance actions on a system. This new approach is illustrated using a numerical example.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

First evidence of Johne's disease in farmed red deer (Cervus elaphus) in Belgium.

&lt;p&gt;In a deer farm, chronic diarrhoea was seen in a 4-year-old hind. This animal died in poor condition on the farm and Johne&#039;s disease was suspected. Ziehl-Neelsen staining of the faeces of this hind were positive for the presence of clumps of small acid-fast bacilli, but faecal cultures remained negative. Direct and indirect tests were performed on 24 hinds and stags (yearlings, 2- and 4-year-old animals). The indirect tests performed were serology (Mycobacterium paratuberculosis antibody ELISA, HerdChek, Idexx), comparative cervical skin test (CCT) and lymphoproliferation test (LT) using Mycobacterium bovis purified protein derivative (PPD) and Mycobacterium avium PPD as antigens. Three positive serological results, three positive CCT and eight positive LT were observed in hinds and stags older than 2 years. No positive serological results were observed in the yearling group, whereas some sensitisation was observed in the CCT as well as in the LT for the same group of animals. The degree of concordance between these indirect tests was poor. The three seropositive animals were slaughtered and subjected to post-mortem examination. Histopathology was performed on mesenteric lymph nodes and on the terminal ileum. Visual changes in some mesenteric lymph nodes were observed, no gross lesion was seen in the intestine. Although Ziehl-Neelsen staining yielded no positive results, a catarrhal focal necrotic enteritis associated with a granulomatous lymphadenitis compatible with Johne&#039;s disease was evidenced. The mycobacterial cultures on organ samples from slaughtered animals were positive after 2 months for M. avium subspecies paratuberculosis and negative for M. bovis and M. avium. This is the first description of Johne&#039;s disease in a deer farm in Belgium.&lt;/p&gt;</p

Comparison of approaches for incorporating depredation on fisheries catches into Ecopath

Ecosystem-based approaches are increasingly used in fisheries management to account for the direct trophic impacts of fish population harvesting. However, fisheries can also indirectly alter ecosystem structure and functioning, for instance via the provision of new feeding opportunities to marine predators. For instance, marine depredation, where predators feed on fishery catches on fishing gear, is a behaviour developed by many marine species globally. This behaviour can modify both the ecological role of predators and fisheries performance. Yet, these ecosystem-wide effects of depredation are rarely considered holistically. In this study, we explored different ways of incorporating depredation into an Ecopath trophic model. We assessed, through a subantarctic case study, how three alternative model structures can account for depredation effects on fishery catches, predator and non-commercial prey populations, as well as target fish stocks. While none adequately addresses all facets of depredation, the alternative models can to some extent capture how depredation can lead to increased fishing pressure on stocks. As structural specificities of Ecopath prevented us from representing other depredation effects such as provisioning effects for predator populations, we conclude this study with a set of guidance to effectively capture the complex effects of depredation in marine ecosystems and fisheries models