Porcine reproductive and respiratory syndrome (PRRS) is a viral disease that causes a decrease in reproductive performance in breeding animals and respiratory disease in pigs of any age. PRRS is the most economically significant disease affecting U.S. swine production.
In the United States, the clinical disease was first discovered and described in 1987-88 in North Carolina, Iowa and Minnesota. PRRS spread rapidly, both in Europe and North America. By the end of 1992, the disease was reported in Canada, Great Britain and several European countries.
The disease was first described as a syndrome and confused with several other diseases. It was referred to as swine mystery disease (SMD) or swine infertility and respiratory syndrome (SIRS), before porcine reproductive and respiratory syndrome (PRRS) became the generally agreed-upon name.
Since the PRRS virus is made up of RNA genetic material (instead of DNA), there is a lot of variation in the virus makeup, due to mutations. Two distinct strains, one identified in Europe (Type I) and one identified in the United States (Type II), have been characterized. Both strains can be found globally.
During the past 20 years, we’ve greatly improved our understanding of the PRRS virus and how to control it; however, there is still much to learn. Swine industry consolidation in the past 15 years has led to entire production systems being designed around strategies for controlling or eliminating this disease.
This analysis is developed as part of the Swine Health Monitoring Program (SHMP) and shows both the change in PRRS incidence over time, as well as the seasonality of PRRS impact (virus incidence most commonly spikes in late fall/early winter).
An important feature of the PRRS virus is the ability of infected pigs to transmit the virus for up to 100 days. Infected pigs with few or no symptoms are “shedding carriers” and are probably the most common way that the PRRS virus is introduced into a population of pigs. The virus is highly infectious. The infectious dose may be as little as 10 virus particles.
Possible routes of PRRS virus transmission:
Pregnant sows infected with PRRS can deliver PRRS virus-infected piglets. PRRS can be transmitted from infected piglets or sows to other piglets. The cycle of virus shedding and infection can continue well into the nursery phase in situations in which the sow herd is actively infected.
Older infected pigs held back or crossfostered in farrowing rooms often are a source of virus for younger pigs. Likewise, older pigs and their secretions can be a source of infection to younger pigs on premises where pig flow is continuous/multi-aged or where biosecurity between groups is lacking.
Boars are known to shed PRRS virus in semen. The pattern of shedding via semen can be variable and intermittent among boars with studies reporting shedding periods lasting 21 to 35 days. Infection of sows via infected semen can occur through natural breeding or artificial insemination.
Impact in the United States
The economic impact of PRRS is substantial and global. The disease’s economic impact occurs through pig death losses, poor reproductive performance, poor growth rates and increased use of medications. Studies in the United States have estimated economic losses due to PRRS at $664 million per year - $1.8 million per day. Studies estimate that the costs due to PRRS may range from $27 to $156 per litter, and approximately $4.32 per marketed pig.
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