Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Production of deterministic and stochastic models of map dynamics and risk

We aimed to assess the potential role of wildlife hosts in the within herd dynamics of Map and develop a framework for the assessment of the risk posed by wildlife hosts in the within-farm dynamics of Map-infection in dairy herds, beef suckler herds and sheep breeding flocks.

Two approaches have been followed
- A quantitative model of within farm transmission of Map in cattle farms and
- A qualitative assessment of the risk of a replacement animal becoming infected from infected wildlife in a dairy, beef suckler or sheep breeding farm.

For the first modelling approach, the models allowed both point estimates of parameters and distributions to be used as input permitting both deterministic and stochastic modelling. A high level of detail in the specification of livestock population dynamics and animal life-histories was achieved by linking a considerable number of management input parameters in addition to those used to model Map transmission. By initialising the models with different combinations of these parameters the within-herd dynamics of Map in different production systems (such as the dominant production systems in the project partner countries), under different control strategies and with different level of exposure to wildlife infection can be simulated. In this first modelling approach two separate models were used - the difference being the way transmission was modelled - a dairy-herd model and a beef-cattle model. In the dairy-herd model transmission of Map was modelled using the S-L-I-R model, where the animals of the herd were divided into: susceptible, latently infected, infectious (shedding the bacterium) and resistant (escaped from the disease). Estimates of the transmission parameter (beta) were obtained using empirical data from a previous field study in The Netherlands. This transmission parameter was estimated for farms with different risk profiles, the risk profile being an index of the potential for within-farm transmission based on calving management and hygiene.

The number of new infections per unit time due to Map contamination from wildlife was assumed to be proportional to
- The contribution of wildlife to environmental contamination relative to that of an infected cow,
- The transmission probability, due to environmental contamination, from an infected cow to a susceptible calf and
- The number of susceptible calves.

The contribution of wildlife to environmental contamination relative to that of an infected cow was estimated from
- Faecal contamination by wildlife (g per hectare),
- Prevalence of Map in wildlife and
- Infectivity of faeces (cfu per g of faeces).

In the beef-cattle model (due to the lack of suitable epidemiological data) the within farm transmission was modelled as one minus the joint probability of escaping the different contamination sources; these sources being not only clinically and subclinically infected animals but also wildlife hosts. Transmission probabilities from clinically and subclinically infected cows were obtained from recent literature and the number of new infections per unit time due to Map contamination from wildlife was modelled in the same way as described before for the dairy herd model.

For the second modelling approach we applied a standard risk-assessment methodology to obtain a framework to assess the risk that wildlife pose to dairy herds, beef suckler herds and sheep breeding farms in a more qualitative fashion than the previous approach.

The hazard was Map and the risk questions that we considered dealt with
- The risk of a replacement heifer infected from wildlife being introduced in the production group on a dairy herd,
- The risk of a replacement heifer infected from wildlife being introduced in the production group on a beef suckler herd and
- The risk of a replacement ewe infected from wildlife being introduced in the production group on a sheep breeding flock.

For each of the questions we formulated the risk pathways starting with a physical pathway, which included a postpartum period followed by time spent outdoors (on pasture) and/or indoors. The sources of the agent could be infected cows/ewes and wildlife hosts and from these sources, via different routes and through a series of dependencies a susceptible animal may become infected and may be introduced in the production group. By decomposing each route in a series of individual events and assessing the likelihood of its occurrence we formally assessed the risk.

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Reported by

School of Veterinary Medicine, University of Thessaly
224 Trikalon
43100 Karditsa
Greece
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