Veterinary Services 

Centers for Epidemiology and Animal Health January 2009 

_________________________________________________________________________________________________________________________

Swine Influenza Virus 

(H1 and H3) Seroprevalence on 

U.S. Swine Operations

Background 

Since 1998, two main swine influenza virus 

hemagglutination types have circulated in the 

United States—H1 and H3.1

 Viruses of these 

hemagglutination types can cause respiratory 

disease within a swine herd and pose a public 

health threat. Swine can act as a reservoir for 

influenza viruses that affect humans.2

Swine influenza is often a seasonal disease, 

usually affecting swine herds between October and 

March. Acute respiratory signs include a barking 

cough, nasal discharge, and sneezing.2

Occasionally, reproductive problems occur, 

including abortions and infertility. While this disease 

has a high morbidity, its mortality rate is low.2

While current vaccine strategies provide some 

protection, herds are still susceptible to swine 

influenza because of the multiple subtypes and 

variants of the virus.3

 Due to interspecies 

transmission and the infectious nature of swine 

influenza, standard biosecurity measures need to 

be implemented to help control this disease.2

Swine influenza prevalence on U.S. swine 

sites 

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 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 

 

*

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

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 H1 and H3 swine influenza 

viruses. From September 5, 2006, through March 

15, 2007, a total of 6,235 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 

between titers due to field virus exposure and titers 

from swine influenza vaccination, estimates 

reported here are from unvaccinated 

grower/finisher pigs only. Of the 6,235 samples 

tested for swine influenza antibodies, 5,307 were 

from 158 sites that did not vaccinate grower/finisher 

pigs for H1 or H3 virus. These 158 sites were used 

in all subsequent calculations. 

Overall, 25.5 percent of samples tested were 

positive for H1, 26.1 percent were positive for H3, 

and 38.6 percent were positive for either. In 

addition, 7.4 percent were suspect for H3. 

 A site was considered positive if at least one 

sample tested positive. Overall, 58.2 percent of 

sites that did not vaccinate for either swine 

influenza hemagglutination type was positive for 

H1, 57.6 percent was positive for H3, and 

71.5 percent was positive for either.

 The percentage of sites with at least one 

positive sample and the percentage of all samples 

positive in unvaccinated herds is shown for each 

hemagglutination type in figure 1. 
0

20

40

60

80

100

Samples

Sites

Swine Influenza Hemagglutination Type

Percent

Figure 1. Percentage of Sites and Percentage of Samples Positive 

for H1 and H3 Swine Influenza Virus Antibodies in 

Unvaccinated Herds

58.2

25.5

57.6

26.1

71.5

38.6

H1 H3 Either

 Published literature suggests that influenza 

outbreaks occur more often in the fall and winter.2

Table 1 shows the percentage of unvaccinated 

animals that tested positive for H1, H3, and either 

strain, by time of year the samples were collected. 

An association was observed between the time 

of year and the presence of antibodies to H1, H3, 

or either strain. In winter, 30.8 percent of 

unvaccinated animals were H1 positive compared 

with 17.5 percent in fall. 

Table 1. Percentage of Sites and Percentage of 

Samples Positive for H1, H3, and Either 

Serotype, by Season 

 Winter1 Fall2 Total 

Hemagglutinatio

n Type 

Pct. 

Sites 

Pct. 

Samp. 

Pct. 

Sites 

Pct. 

Samp. 

Pct. 

Sites 

Pct. 

Samp.

H1 65.3 30.8 47.6 17.5 58.2 25.5 

H3 62.1 27.7 50.8 23.7 57.6 26.1 

Either 76.8 44.1 63.5 30.3 71.5 38.6 

1

January, February, and March. 

2

September, October, November, and December. 

For sites that did not vaccinate against flu, the 

percentage of positive sites is shown in table 2. 

Over 8 of 10 sites in the East Central region had at 

least 1 positive sample. 

Table 2. Percentage Sites Positive for H1, H3, 

and Either Strain, by Region 

 Percent Sites 

 Region 

Hemagglutination 

Type North 

West 

Central 

East 

Central South 

H1 48.7 54.6 66.7 57.1 

H3 48.7 50.0 71.4 42.9 

Either 62.2 65.9 81.0 71.4 

References

1 Webby, R.J., K. Rossow, G. Erickson, Y. Sims, and R. 

Webster. 2004. Multiple lineages of antigenically and 

genetically diverse influenza–A virus co-circulate in the United 

States swine population. Proceedings of the First European 

Influenza Conference. Virus Res 103(1-2):67-73. 

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

D.J., eds. 2006. In: Diseases of Swine, 9th ed., Iowa State 

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

3 Zhou, N.N., D.A. Senne, J.S. Landgraf, et al. 2007. 

Emergence of H3N2 reassortment influenza A viruses in North 

American pigs. Vet Microbiol 74(1-2):47-58.

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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