Farm Characteristics Affect PRRSV in UK Pig Herds

Charlotte M. Evans and colleagues at the Department of Biological Sciences at the University of Warwick in the UK have published a paper on their studies investigating the farm characteristics associated with the heterogeneity in seroprevalence in Porcine Reproductive and Respiratory Syndrome virus (PRRSV) in British pig herds. The observed variation in disease levels in infected herds appears to be related to the virus gradually fading out of the herd, and then being re-introduced rather than the persistence of the virus itself.

31 December 2008

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The between- and within-herd variability of porcine reproductive and respiratory syndrome virus (PRRSV) antibodies were investigated in a cross-sectional study of 103 British pig herds conducted 2003-2004.

Fifty pigs from each farm were tested for anti-PRRSV antibodies using ELISA.

A binomial logistic model was used to investigate management risks for farms with and without pigs with PRRSV antibodies and multilevel statistical models were used to investigate variability in pigs' log ELISA IRPC (relative index x 100) in positive herds.

Results

Thirty-five herds (34.0%) were seronegative, 41 (39.8%) were seropositive and 27 (26.2%) were vaccinated.

Herds were more likely to be seronegative if they had less than 250 sows (OR 3.86 (95% CI 1.46, 10.19)) and if the nearest pig herd was more than two miles away (OR 3.42 (95% CI 1.29, 9.12)).

The mean log IRPC in seropositive herds was 3.02 (range, 0.83 - 5.58).

Proportion of pigs seropositive by age for 25 positive herds than had seropositive young stock, 16 positive herds that had seronegative youngstock and 27 vaccinated herds
^{Bars indicate 95% confidence intervals.

Lines are included for visual ease of presentaion only.}

Sixteen seropositive herds had only seropositive adult pigs. In these herds, pigs had -0.06 (95% CI -0.10, -0.01) lower log IRPC for every mile increase in distance to the nearest pig unit, and -0.56 (95% CI -1.02, -0.10) lower log IRPC when quarantine facilities were present.

For 25 herds with seropositive young stock and adults, lower log IRPC were associated with isolating purchased stock for more than 6 days (coefficient -0.46, 95% CI -0.81, -0.11), requesting more than 48 hours 'pig-free time' from humans (coefficient -0.44, 95% CI -0.79, -0.10) and purchasing gilts (coefficient -0.61, 95% CI -0.92, -0.29).

Conclusions

These patterns are consistent with PRRSV failing to persist indefinitely on some infected farms, with fade-out more likely in smaller herds with little or no reintroduction of infectious stock. Persistence of infection may be associated with large herds in pig-dense regions with repeated reintroduction.

Reference

Evans C.M, G.F Medley and L.E. Green, 2008. Porcine reproductive and respiratory syndrome virus (PRRSV) in GB pig herds: farm characteristics associated with heterogeneity in seroprevalence. BMC Veterinary Research 2008, 4:48doi:10.1186/1746-6148-4-48

December 2008