The impact of respiratory disease in the GB pig industry
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Research partner: University of Warwick
Industrial partner: BPEX
Duration: 2005 - 2009
Respiratory disease is the most significant cause of morbidity and mortality in pigs post-weaning. This project used data gathered as part of a DEFRA/BPEX-funded project on post weaning multisystemic wasting syndrome (PMWS) to investigate respiratory diseases in 116 pig herds. The specific aims of the project were to:
- Estimate the prevalence and incidence of common respiratory diseases in England and investigate their associations with one another and with post-weaning mortality
- Investigate antibody profiles for porcine reproductive and respiratory syndrome virus (PRRSV)-infected herds and to determine management factors associated with differences in profiles between herds
- Investigate how PRRSV could persist within infected herds using mathematical models and available data
- Determine the impact of PRRSV infection on disease and explore theoretically how this could be reduced by different control and elimination strategies
From the results, it was shown that PRRS was frequently associated with raised mortality and morbidity when present with other disease causing agents in a herd. Antibody profiles for 103 herds were investigated (50 pigs were sampled in each herd). In 35 herds no pigs had PRRSV antibody. These were considered PRRSV negative. These herds were more likely to have less than 250 sows and be located more than two miles from another pig farm. Sixty eight herds had pigs with antibody to PRRSV (twenty seven of which were vaccination). Higher antibody levels were present in antibody positive herds that did not isolate gilts for >6 days. In 39% of antibody positive herds that were not using vaccination, only breeding sows were antibody positive, suggesting that there was antibody on the farm but no virus because the growing pigs were antibody negative. This is indicative of fade out of virus, ie its disappearance from a herd in which it has been present. In our mathematical models fade out was also more likely in small herds with little/no reintroduction of infectious stock.
To investigate the mechanisms that could lead to fade out of PRRSV, within-herd transmission dynamics were investigated using a mathematical model. When virus was introduced into a herd via gilts there was a high frequency of fade out in breeding pigs before the virus reached young stock. Once the virus had reached young stock there was an increased probability of virus persisting in the herd. This was also more likely in large herds, herds with increased contact between age groups and herds that had frequent re-introduction of virus (for example through purchasing infectious pigs, or wind borne spread). The results highlight the importance of determining whether a herd is truly infected or not and appropriate sourcing and isolation of new stock. The patterns observed suggest that the processes of re-introduction, persistence and fade-out of PRRSV play critical roles in the spread of PRRSV throughout the pig industry.
Mathematical models were also used to investigate the range of clinical disease observed as a result of infection with PRRSV and the result of strategies for control and elimination. PRRSV was difficult to eliminate without targeting both rearing pigs and sows. Rapid vaccination of sows once there was an increase in stillbirths reduced the risk of spread of virus to rearing pigs but not in all simulations. In areas where the density of herds is low it might be possible to control PRRSV by elimination in individual herds. In larger herds in pig dense regions, elimination might be difficult and control of disease might give more stability. The long-term benefits of elimination will depend on the risk of re-introduction of virus into the herd.
More in depth results from this research can be found in the BMC Veterinary Research and Preventive Veterinary Medicine journals, or for full details of this project contact BPEX.
*Evans, C.M., Medley, G.F., Green, L.E., 2008. Porcine reproductive and respiratory syndrome virus (PRRSV) in GB pig herds: farm characteristics associated with heterogeneity in seroprevalence. BMC Veterinary Research 4, 48 (published paper)
Page last updated 08 June 2010