The interaction between the spread of disease both within and between-farm is vital to improving the understanding of the implications that industry contacts may have for the spread of poultry diseases within Great Britain. So say scientists working in Glasgow, who have found that farm risk profiles, combining both farm connectivity and transmission potential, could have useful implications for targeting disease control.

"For example biosecurity for farms where within-flock spread is more important than between-farm spread, and the targeting of 'high-risk' farm groups where between-farm spread is more important," explained Sema Nickbakhsh, who led a study to assess the interaction between the speed of within-farm spread and the pattern of between-farm spread, in order to determine the implications of considering the combined effect of transmission potential and connectivity for disease control.

Social network data can be useful for determining optimal control strategies through the identification of 'high-risk' groups, such as highly connected farms that exacerbate disease spread in a national livestock epidemic.

Modelling work, based on between-farm associations informed by the Poultry Network Database (PND), has so far highlighted some conditions under which a large highly pathogenic avian influenza (HPAI) outbreak in Great Britain is possible.

But until now, no study has incorporated the temporal patterns of movements on and off farms. In order to investigate the spread of disease between farms, both the within-farm spread of disease and this pattern must be considered, as transmission risk is a combination of the potential (via infected birds) and opportunity (via off-movements) to infect other farms.

The team of researchers constructed a model to look at within-farm disease spread, which incorporated a range of parameter values representing different poultry pathogens of varying virulence, transmissibility and bird susceptibility.

A dataset of temporally explicit movements of slaughterhouse vehicles to farms, consisting of individual records from a large catching company in England, was used to inform daily farm movements.

By matching the daily movement patterns per farm to each day of a simulated epidemic, an overall 'transmission potential' was calculated for each combination of model parameter values.

The relative importance of within- and between-farm disease spread was then investigated by comparing the sensitivity of within-farm transmissibility (as measured by the basic reproduction number and the 'transmission potential' across each model parameter value combination, by averaging these values across all farms.

"Preliminary results show that the within-farm spread of disease can affect farm-level disease transmission potential, but is dependent upon farm-level movement patterns," said Ms Nickbakhsh.

"For example, different farm movement frequencies and times between consecutive movements were found across farms with different between-farm association frequencies. These were related to farm-level demographic characteristics such as the number of birds and production type in review.

"For farms where the transmission potential was found to be more sensitive to other model parameter values than within-flock, the suggestion is that the temporal pattern of between-farm spread is important for determining onward transmission risk," she added.

Preliminary results also highlight the importance of considering both the 'static' farm connectivity (representing the potential epidemic final size) and temporally explicit movements (representing the potential for between-farm transmission) in combination as these are important implications for disease control.

"For example, farms with a moderate potential for disease transmission and relatively few associations may represent a lower risk group, compared to farms with a low potential for disease transmission but with a relatively high number of associations," Ms Nickbakhsh said.

Reference

Nickbakhsh A., Matthews L., Reid S.W.J. and Kao R.R. 2012. Investigating within-farm and between-farm disease transmission interactions: Implications for the control of British poultry diseases. Advances in Animal Biosciences.

November 2012