CSF outbreaks can cause enormous losses in naïve pig populations and how best to minimise the economic damage and number of culled animals caused by CSF is therefore an important research area, according to S. Dürr and colleagues at the University of Berne in Switzerland.

In their paper in the journal, Preventative Veterinary Medicine, they explain that the baseline CSF control strategy in the European Union and Switzerland comprises culling all animals in infected herds, movement restrictions for animals, materials and people within a given distance to the infected herd and epidemiological tracing of transmission contacts. Additional disease control measures such as pre-emptive culling or vaccination have been recommended, based on the results from several simulation models; however, these models were developed for areas with high animal densities.

The objective of the Berne study was to explore whether pre-emptive culling and emergency vaccination should also be recommended in low- to moderate-density areas such as Switzerland. Additionally, the researchers studied the influence of initial outbreak conditions on outbreak severity to improve the efficiency of disease prevention and surveillance.

A spatial, stochastic, individual-animal-based simulation model using all registered Swiss pig premises in 2009 (n=9,770) was implemented to quantify these relationships. The model simulates within-herd and between-herd transmission (direct and indirect contacts and local area spread).

A total of 112 distinct scenarios were simulated by varying the following four parameters:

1. control measures
2. index herd type (breeding, fattening, weaning or mixed herd)
3. detection delay for secondary cases during an outbreak, and
4. contact tracing probability.

To assess the impact of scenarios on outbreak severity, daily transmission rates were compared between scenarios.

Compared with the baseline strategy (stamping out and movement restrictions) vaccination and pre-emptive culling neither reduced outbreak size nor duration.

Outbreaks starting in a herd with weaning piglets or fattening pigs caused higher losses regarding to the number of culled premises and were longer lasting than those starting in the two other index herd types. Similarly, larger transmission rates were estimated for these index herd type outbreaks.

A longer detection delay resulted in more culled premises and longer duration and better transmission tracing increased the number of short outbreaks.

Based on the simulation results, Dürr and colleagues concluded that baseline control strategies seem sufficient to control CSF in low- to medium animal-dense areas. Early detection of outbreaks is crucial and risk-based surveillance should be focused on weaning piglet and fattening pig premises.

Reference

Dürr S., Zu Dohna H., Di Labio E., Carpenter T.E. and Doherr M.G. 2012. Evaluation of control and surveillance strategies for classical swine fever using a simulation model. Prev Vet Med. 2012 Aug 1. [Epub ahead of print]

Further Reading You can view the full report (fee payable) by clicking here. Find out more information on swine fevers by clicking here.

October 2012