Porcine Reproductive Respiratory Syndrome (PRRS) is one of the most important diseases in the domestic swine industry. As the name implies, PRRS causes reproductive and respiratory disease. PRRS actually is a new worldwide swine disease that was not even recognized before the late 1980s. The disease pattern in domestic swine herds caused by PRRS virus infection was actually called “mystery swine disease” for several years until the cause was discovered and proven to be a virus in 1991. It was named the Lelystad virus because the location of its discovery at the Central Veterinary Research Institute in Lelystad, Netherlands. Since that time, many strains of PRRS virus have been recognized. With the exception of mallard ducks, other animals and humans are not known to be hosts for the PRRS virus of swine.
It is still unknown where PRRS virus originated or how it was introduced into domestic swine. Free-roaming feral swine in the US and wild boar in other countries were thought to be likely sources. Obviously feral and wild swine were in existence a long time before PRRS emerged, but they could have been infected with PRRS virus from some unknown source and then made contact with domestic swine and infected them. In a joint Texas Agricultural Extension Service/Texas Parks & Wildlife 1996 – 1999 survey in the Texas Rolling Plains, only 1/135 samples tested positive by the PRRS ELISA antibody test. This PRRS positive animal was a twenty-month-old, 155 pound boar. 1 In a US survey of feral swine, all sera collected and tested between 1976 and 1993 were negative and only 2 positives were found in 1994.2 Only 2/1250 wild boar samples collected in Europe from 1991 through 1996 were PRRS positive.2 From these US and European surveys it was speculated that domestic swine probably transmitted PRRS virus to feral and wild swine, and not vice versa. 1, 2
Another wild species, the mallard duck, has been proven to be susceptible to PRRS virus infection and has extended virus shedding (39 days after exposure in some).3 It is interesting that PRRS virus is very fragile and is inactivated in an unprotected environment at room temperature, but can survive in room temperature well water and city water for 8 and 11 days, respectively. Mallard ducks can be infected through PRRS-contaminated water and shed virus into feces to infect other mallard ducks. PRRS virus isolated from mallard duck feces has been shown to be infectious for pigs when challenged intranasally; these infected pigs were then able to infect susceptible contact pigs. Scientists discount the idea that mallard ducks have spread PRRS virus to and between swine herds probably because duck to pig contact is very infrequent in modern swine production. However, many swine farms around the world with less intensive production practices have frequent duck to pig contact. Although it is unknown where PRRS virus originated, scientist do agree that mallard ducks could be a possible host for introduction of different strains of PRRS virus into swine in the future.3
It was not likely that PRRS virus infection in US domestic swine originated from PBPs since there was already a low PRRS prevalence in domestic swine in Iowa by 1985 (shown by retrospective studies)2, and PBPs did not become popular as US pets until the late 1980’s (the time of emergence of “mystery swine disease”). However, it would be interesting to know if there is currently any evidence of PRRS virus infection in the PBP population of the US.
Recently, 29 sera from normal PBPs of various ages, sex and weights from a Florida location were blood sampled and tested for PRRS antibody by the ELISA test. All 29 sera were negative. When infection to PRRS virus in swine does occur, antibody to the PRRS virus detectable by ELISA persists for about 4 to10 months. Since the average age for these 29 PBPs was 2.6 years (9 month – 6 years range), it is possible that PRRS virus infection in some animals could have occurred previously but antibody was no longer detectable at the time of sampling. Although this is a very small survey and has the aforementioned limitation, it may indicate that, like feral swine, PRRS virus infection is probably not important as a cause of disease in PBPs. Testing other populations of PBPs could substantiate this hypothesis.
1. Lawhorn B. Texas Rolling Plains Feral Swine Survey, Proceedings of the 1999 National Feral Swine Conference, Ft. Worth, Texas, 124-127.
2. PRRS Compendium. National Pork Producers Council, Des Moines, Iowa, 1998, 61, 78.
3. Benefield DA, Collins JE, Dee SA, et al. Porcine Reproductive and Respiratory Syndrome. In:Straw BE, D’Allaire S, Mengeling WL, et al, eds. Diseases of Swine, Eighth edition, Ames: Iowa State University Press, 1998; 201-232.