ABSTRACT
OBJECTIVE To measure incidence and estimate temporal and spatial dynamics of porcine reproductive and respiratory syndrome virus (PRRSV) infection in US sow herds.
ANIMALS 371 sow herds in the United States from 14 production companies.
PROCEDURES The exponentially weighted moving average was used to monitor incident PRRSV infections for onset of an epidemic. The spatial scan statistic was used to identify areas at significantly high risk of PRRS epidemics. A χ2 test was used to estimate whether there were significant differences in the quarterly and annual PRRS incidence among time periods, and a bivariable logistic regression model was used to estimate whether PRRSV infection during a given year increased the odds of that herd being infected in the following year.
RESULTS During the 4-year period of this study, 29% (91/319; 2009 to 2010), 33% (106/325; 2010 to 2011), 38% (135/355; 2011 to 2012), and 32% (117/371; 2012 to 2013) of the herds reported new infections. Weekly incidence was low during spring and summer and high during fall and winter. The exponentially weighted moving average signaled the onset of a PRRSV epidemic during the middle 2 weeks of October each year. Disease incidence was spatially clustered. Infection in the previous year increased the odds of infection in 2010 to 2011 and 2011 to 2012.
CONCLUSIONS AND CLINICAL RELEVANCE Results indicated a striking repeatability in annual PRRSV temporal and spatial patterns across 4 years of data among herds from 14 production companies, which suggested that efforts to control PRRSV at a regional level should continue to be supported.
Porcine reproductive and respiratory syndrome virus is a positive-stranded, enveloped RNA virus that is 50 to 65 nm in diameter with a smooth surface and cuboidal nucleocapsid core that has a diameter of 25 to 35 nm1,2Similar to other members of the Arteriviridae family, PRRSV is species specific (infects only swine) and highly variable. It is transmitted through all bodily secretions, including urine, feces, colostrum, milk, saliva, semen, and nasal secretions.3,4 The infectious dose appears to be low and variable and is dependent on the virus strain, route of entry, concurrent health conditions, and age of the pig.5–7
Clinical manifestation of PRRSV infection was first documented in the United States in North Carolina during the 1980s.8–10 Since then, PRRSV has rapidly spread throughout the US swine population and is now considered the most economically important disease of modern swine production, with annual production losses estimated at $560 million in 2005 and increasing to $664 million in 2011.11,12 Direct production losses associated with PRRSV infection are far-reaching. In sows, PRRSV infection causes abortions, stillborn piglets, mummified fetuses, irregular returns to estrus, and death.13,14 In suckling and growing pigs, PRRSV causes preweaning death, lethargy, respiratory tract disease, anorexia, and a decrease in daily weight gain.9,13,14 In boars of reproductive age, PRRSV causes acute respiratory tract disease, anorexia, lethargy, lack of libido, and a reduction in semen quality.9,15–17
Several studies18–21 have been conducted to determine the relationships between the probability of a herd developing a new PRRSV infection and various risk factors such as herd size, biosecurity score, season in which a herd was established as uninfected with PRRSV, and PRRSV status of neighboring farms. The Production Animal Disease Risk Assessment Program is a commonly used survey in the swine industry that allows herds to compute internal (practices done within farm) and external (conditions outside the farm) biosecurity scores, which can then be bench-marked against herds in a national database.19 One study18 identified a correlation between a new PRRSV infection and the season during which a herd was established as uninfected with PRRSV (PRRSV negative), with herds established as PRRSV negative in the winter having a greater risk of developing a new PRRSV infection than herds established as PRRSV negative in the summer. Additionally, results of that study18 indicate that herds with low overall external biosecurity risk scores as determined by the Production Animal Disease Risk Assessment Program survey have a significantly higher probability of remaining PRRSV negative longer than do herds with high overall external biosecurity risk scores. Other risk factors for PRRSV infection include the size of the sow herd20,21 as well as the distance to, size of, and duration of PRRSV infection on neighboring swine herds.21
Certain critical aspects of PRRSV epidemiology have yet to be elucidated. For example, to our knowledge, the nature and extent of temporal and spatial clustering of PRRSV-infected herds in the United States have not been evaluated. The goal of the project reported here was to provide metrics that describe the apparent repeatability of the temporal and spatial patterns of PRRSV epidemics on US sow herds.
Tousignant, S. Perez, A, Lowe, J, Yeske, P, Morrison, R. Temporal and spatial dynamics of porcine reproductive and respiratory syndrome virus infection in the United States. AJVR. Jan 2015. 76:1 pp 70-76.