Current incidence, prevalence and state of control of PRRS virus in North America


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R.B. Morrison, D. Goede, S. Tousignant, A. Perez2015 International PRRS Symposium p.21

Porcine reproductive and respiratory syndrome virus (PRRSV) has been reported to be the most costly pathogen causing reproductive disease and decreased growth performance.  The virus continues to evolve and considerable variability exists among isolates in virulence, apparent protection from vaccination and our time required to eliminate the virus from sow herds. Lacking any data on prevalence or incidence in the industry, we started a Swine Health Monitoring Program in 2012 (Tousignant et al).

A convenience sample of volunteer herds were invited to participate and today, 538 sow herds are enrolled accounting for approximately 2 million sows, or about a third of the total sows in United States. Veterinarians provide weekly diagnostic data on PRRS status for these herds including new infections and progress made in controlling the virus. The data are analyzed to calculate incidence and prevalence. Location is also shared thereby facilitating temperospatial analysis.

These data indicate a decrease in incidence in 2013 and 2014 to approximately 25% compared to approximately 35% in previous years. This decrease may reflect increased knowledge and implementation of effective biosecurity measures, however, it remains to be seen if this decrease is sustainable. Substantial progress has been made in how we control PRRSV in sow herds. After a herd is infected, veterinarians commonly employ a program referred to as Load / Close / Expose (LCE).  The expectation is that protective immunity is built at the population level resulting in the eventual elimination of PRRSv from the breeding herd.

Linhares et al (2014) followed a cohort of 61 infected sow farms and reported the median time to eliminate virus from piglets as 26.6 weeks (25th to75th percentile, 21.6–33.0 weeks). The average production loss was 2.2 pigs / sow and it took an average of 16.5 weeks for herds to recover back to baseline. Herds that received modified live virus as the exposure program recovered production sooner and had fewer piglets lost but took longer to eliminate field virus than herds that received field virus as the exposure program. Recognizing that a herd’s risk of infection is partially related to PRRSV status of neighboring herds, has led to voluntary efforts to control the virus within regions of the country (Corzo et al). Health status is shared among participants as well as successes and failures with regards to control efforts.

There is an overall sense of progress and this has led American Association of Swine Veterinarians to propose a national PRRS control program. The intention is to build on the existing regional control programs in an attempt to reduce regional incidence of the virus, while encouraging PRRSV elimination programs to decrease the prevalence of infected herds. The program will be producer led, producer funded and voluntary, at least at its initiation.