Veterinary Services 

Centers for Epidemiology and Animal Health January 2009 

_________________________________________________________________________________________________________________________

PRRS Seroprevalence on 

U.S. Swine Operations 

Background 

First reported in the United States in the late 1980s, 

porcine reproductive and respiratory syndrome (PRRS) 

severely impacts the economic well being of the Nation’s 

swine industry.1 PRRS causes both late-term 

reproductive failure and postweaning respiratory disease 

in swine. In addition, the PRRS virus has a high mutation 

rate which has made control of the disease difficult.2

Although PRRS virus is highly contagious, 

transmission generally requires direct pig-to-pig contact. 

However, virulant strains can aerosolize over short 

distances.2

 PRRS virus also is transmitted indirectly via 

insects and fomites,3 and is found in and spread to other 

pigs by saliva, nasal secretions, urine, feces, needles, 

and semen.2 3 Infected animals may shed the PRRS 

virus up to 157 days post infection,3 and is found in the 

blood for up to 23 days post infection. 

Reproductive losses among affected sows include 

premature farrowing, increased abortions, and increased 

stillbirths/mummies. Affected sows may stop producing 

milk, resulting in increased preweaning mortality. Liveborn piglets infected in utero are viremic at birth and are 

susceptible to secondary bacterial diseases seen in the 

nursery. Grower/finisher pigs infected with PRRS may 

experience respiratory disease.2

 Highly virulent strains of 

virus may cause death losses of 10 percent and higher 

in sows and growing pigs.4

The control of PRRS remains an important topic in 

animal heath. Control of this disease includes 

vaccination and proper herd management.2

 Diligent 

biosecurity including air filtration, careful management of 

breeding stock immunizations before entry, herd closure, 

and all-in/all-out pig flow can aid in controlling PRRS on 

a swine operation.3

PRRS prevalence on U.S. swine operations 

In 2006, the USDA’s National Animal Health 

Monitoring System (NAHMS) conducted a study on 

swine health and management practices from a random 

sample of swine production sites in 17 States*

 divided 

 

*

Regions/States 

North: Michigan, Minnesota, Pennsylvania, and Wisconsin 

West Central: Colorado, Kansas, Missouri, Nebraska, and South 

Dakota 

East Central: Illinois, Indiana, Iowa, and Ohio 

South: Arkansas, North Carolina, Oklahoma, and Texas

into 4 regions. These States represented about 

94 percent of the U.S. pig inventory and 94 percent of 

U.S. pork producers with 100 or more pigs. Overall, 

2,230 swine production sites participated in the first 

interview from July 17 to September 15, 2006. 

Producers participating in the NAHMS Swine 2006 

study had the opportunity to submit up to 35 blood 

samples from grower/finisher pigs to be tested for 

antibodies to PRRS virus. From September 5, 2006, 

through March 15, 2007, 6,234 samples were collected 

from 185 swine sites and tested using the IDEXX 

enzyme linked immunosorbant assay (ELISA) test. 

Because the ELISA test does not differentiate titers 

due to field virus exposure from PRRS vaccination, 

estimates reported here are from unvaccinated 

grower/finisher pigs only. Of the 6,234 samples tested 

for PRRS antibodies, 5,793 (92.9 percent) were from 

173 sites that did not vaccinate grower/finisher pigs for 

PRRS virus. These 173 sites were used in all 

subsequent calculations. 

Overall, 49.8 percent of unvaccinated 

grower/finisher pigs were positive for PRRS virus 

antibodies and 71.1 percent of sites that did not 

vaccinate for PRRS had at least one positive sample. 

The percentage of sites with at least one sample 

positive and the percentage of samples positive in 

unvaccinated herds is shown by herd size in figure 1. 

0

20

40

60

80

100

Percent samples

Percent sites

Herd Size (Total inventory June 1, 2006)

Percent

Figure 1. Percentage of Sites and Samples Positive for PRRS Virus

Antibodies in Unvaccinated Herds, by Herd Size

Small

(fewer

than 2,000)

Medium

(2,000-4,999)

Large

(5,000 or more)

All

sites/samples

71.1

49.8

65.1

40.8

76.3

54.9

77.4

62.1
Figure 2 shows that nearly one-half of sites 

(46.2 percent) had between 0.0 and 29.9 percent of 

samples positive for PRRS virus antibodies, while 

36.4 percent of sites had 90 to 100 percent of their 

samples positive. 

0

5

10

15

20

25

30

35

40

Percent

Figure 2. Percentage of Sites by Within-herd Prevalence for PRRS

Virus Antibodies in Unvaccinated Herds

28.9

17.4

4.6

12.7

36.4

0.0 0.1-29.9 30.0-59.9 60.0-89.9 90.0-100.0

Percent Positive Samples

PRRS prevalence by region 

Figure 3 shows the percentage of unvaccinated sites 

that were positive for PRRS virus antibodies (had at 

least one positive sample) and the percentage of 

positive samples. The East Central and West Central 

regions had more than 70 percent of sites positive, while 

less than one-half of the samples were positive. 

0

20

40

60

80

100

Percent samples

Percent sites

Percent

Figure 3. Percentage of Sites and Samples Positive for PRRS Virus

Antibodies in Unvaccinated Herds, by Region

Region

North West Central East Central South

56.8 59.3

70.5

41.7

77.1

30.2

77.3

69.6

References 

1 Neumann, J. Eric, J.B. Kliebenstein, and C.B.Johnson, 

et al. 2005. Assessment of the economic impact of 

porcine reproductive and respiratory syndrome on swine 

production in the United States. J Am Vet Med Assoc. 

227(3):385-392. 

2 Straw, B.E., J.J. Zimmerman, S. D’Allaire, and D.J. 

Taylor, eds. 2006. In: Diseases of Swine 9th ed. Iowa 

State University Press, Ames, IA. p. 201-244. 

3 Cho, J.G., and S.A. Dee, 2006. Porcine reproductive 

and respiratory syndrome virus. Theriogenology 66:655-

662. 

4 Zhou, Y.J., X.F. Hao, Z.J. Tian, et al., 2008. Highly 

virulent porcine reproductive and respiratory syndrome 

virus emerged in China. Transbound Emerg Dis 55:152-

164. 

For more information, contact: 

USDA:APHIS:VS:CEAH 

NRRC Building B, M.S. 2E7 

2150 Centre Avenue 

Fort Collins, CO 80526-8117 

970.494.7000 

E-mail: NAHMS@aphis.usda.gov 

http://nahms.aphis.usda.gov

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