In addition, time that purchased stock spent in isolation was associated with lower IRPC values in pigs in computer virus positive herds (Model 3). Sixteen seropositive herds experienced only seropositive adult pigs. In these herds, pigs experienced -0.06 (95% CI -0.10, -0.01) lesser log IRPC for every mile increase in distance to the nearest pig unit, and -0.56 (95% CI -1.02, -0.10) lesser log IRPC when quarantine facilities were present. For 25 herds with seropositive young stock and adults, lower log IRPC were associated LY2608204 with isolating purchased stock for 6 days (coefficient -0.46, 95% CI -0.81, -0.11), requesting 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). Conclusion These patterns are consistent with PRRSV failing to persist indefinitely on some LY2608204 infected LY2608204 farms, with fadeout more likely in smaller herds with little/no reintroduction of infectious stock. Persistence of contamination may be associated with large herds in pig-dense regions with repeated reintroduction. Background Porcine Reproductive and Respiratory Syndrome (PRRS) caused by PRRS computer virus (PRRSV), was first reported in North America in 1987 and in the United Kingdom in 1991 [1]. Current estimates are that 79% of breeder to finisher models in the UK are affected with PRRSV or are using vaccination (National Animal Disease Information Support, UK, 2007). The disease causes significant economic losses to the pig industry, costing approximately $560 million per year in the United States alone [2]. The clinical indicators of PRRSV are reproductive loss in sows including return to oestrus, abortion, premature farrowing, mummified foetuses and stillbirths [3,4]. PRRSV causes high pre-weaning mortality in piglets infected em in utero /em [5] and immunosuppression and consequent increase in LY2608204 susceptibility to other infectious diseases, particularly respiratory diseases in pigs infected post-weaning [6]. The clinical disease caused by PRRSV is usually highly variable between farms. For example, whilst some seropositive herds have fairly consistent rates of respiratory disease [7,8], others have periodic outbreaks of reproductive disease in breeding sows [9] suggesting that the computer virus does not behave consistently between farms. There has also been a report of natural fadeout of PRRSV on a farm [10] and some reports of active removal of PRRSV from individual herds [11,9,13]. The role of fadeout and persistence in determining viral transmission dynamics has been recognised for some time, especially in the context of measles and other child years infections [14,15]. Periodic outbreaks of measles (and therefore episodes of fadeout) have been observed in small communities [14], with low rate of supply of susceptible individuals (births) and low rates of virus introduction [15]. Persistence of a virus in a host population is usually critically determined by the availability (proportion) of susceptibles in the population, which is determined by, em inter alia /em , transmissibility of the virus, infectious period and presence of alternate hosts or environment contamination [16-18]. Thus, for PRRSV, the observed variable clinical indicators and natural fadeout might occur because of variability in computer virus transmission within and between farms, different strains of computer virus, and/or because of transmission dynamic heterogeneity that results when most of the herd becomes immune. Anti-PRRSV antibodies (detectable by ELISA) arise approximately 9 C 13 days after contamination [19] and decay over time [19,12], persisting for up to 28 months [12]. Most pigs obvious computer virus within 3C4 months of exposure [20], so most PRRSV antibody positive pigs are computer virus negative and consequently seropositivity is an indication of past contamination or vaccination. Whereas seropositivity of adult pigs might have been acquired many months previously in a herd in which the virus has become absent, seropositivity of young stock born on a farm indicates virus Rabbit Polyclonal to OR5M1/5M10 presence on that farm. In this paper we present the farm and pig characteristics associated with herd seropositivity and pig heterogeneity in seroprevalence to PRRSV on 103 GB pig herds using ELISA antibodies as.