APHIS Factsheet

Veterinary Services August 2016

Questions & 

Answers: Influenza A 

Virus in Swine (IAV-S)

 Type A influenza viruses can infect swine and 

cause a respiratory disease called Influenza A Virus 

in Swine (IAV-S). Influenza is present at low levels 

in pigs throughout the world and regularly causes 

disease outbreaks. It typically causes only minor 

clinical signs of illness. IAV-S is not a disease that 

needs to be reported to the World Organization for 

Animal Health (OIE), nor is it a regulated disease in 

the United States.

 There are many different subtypes of influenza 

A viruses. These subtypes differ and are classified 

based on a combination of two groups of proteins on 

the surface of the influenza A virus: hemagglutinin 

or “H” proteins, of which there are 17 (H1–H17), and 

neuraminidase or “N” proteins, of which there are 

10 (N1–N10). Many different combinations of “H” 

and “N” proteins are possible. Each combination 

is considered a different subtype and each subtype 

can be further divided into many distinct strains.

 Like human influenza viruses, there are multiple 

subtypes and strains of influenza in swine. There 

are currently three types of influenza A viruses 

commonly found in swine in the United States: 

H1N1, H1N2, and H3N2. Most influenza A viruses 

that affect swine are different from the ones that 

affect people. However, pigs may occasionally 

become infected by influenza viruses found in 

people or birds. This can potentially mix the viruses 

and create new and very different viruses that 

might be able to spread more easily to people from 

swine. It is important to remember that people 

cannot contract influenza from eating properly 

prepared pork.

IAV-S Surveillance

 

The U.S. Department of Agriculture (USDA) offers 

a voluntary influenza A virus in swine surveillance 

program. The cooperative program with State and 

industry swine health partners began in 2009 and 

was helpful when dealing with the pandemic H1N1 

outbreak later that year.

Q. Why is it important to monitor IAV-S?

A. Over the years, various influenza A viruses 

have emerged in swine. Because these viruses can 

change over time and even affect other species—

including people—it is important to monitor 

influenza in all species. This allows scientists to 

study any changes that appear, including those 

that could cause a vaccine to become ineffective. 

There’s also a benefit to human health, as public 

health officials can be alerted early if there are any 

changes that could potentially cause the virus to 

affect people.

Q. How does USDA monitor IAV-S?

A. USDA monitors influenza viruses in the U.S. 

swine population by looking for changes in how 

these viruses are made up over time. This 

knowledge helps animal and human health experts 

to update testing methods, anticipate new vaccine 

requirements, and develop disease response plans.

Q. How does the surveillance program work?

A. The program tests samples gathered in three 

ways: 

1. Samples from sick swine that are routinely 

sent to the National Animal Health Laboratory 

Network by veterinarians (accounts for 99 

percent of the surveillance program);

2. Samples from swine associated with public 

health investigations; and 

3. Samples from swine that have influenza-like 

illness at events where many animals gather, 

such as auctions, markets, fairs, or other swine 

exhibition events. 

 USDA’s National Veterinary Services Laboratories 

tests samples associated with human cases, samples 

that require whole genome sequencing, and samples 

for which the results are inconclusive or unusual. 

USDA maintains the test results and a collection of 

IAV-S samples, but genetic sequence information 

is deposited into GenBank®, a publicly available 

database managed by the U.S. Department of 

Health and Human Services’ National Institutes of 

Health.

Q. How often do you find IAV-S through your 

surveillance program?

A. Because IAV-S is common, but usually causes 

minor clinical signs of disease, our surveillance 
program does find many cases. From October 1, 

2010, through December 31, 2015, the program 

tested nearly 27,000 herds and identified nearly 

9,900 cases that were positive for swine influenza, 

with about one-third of those characterized via 

genetic analysis.

Q. How do you test for IAV-S?

A. Our surveillance program typically analyzes nasal 

swab samples and oral fluid collections from live 

pigs, as well as lung tissue samples from dead pigs, 

to look for the virus. The laboratories look at the 

virus’s genetic material to determine what subtype 

of IAV-S is present.

Recognizing and Preventing IAV-S

Q. What are the signs of IAV-S?

A. Influenza viruses are common in swine, usually 

causing a high rate of illness but few deaths. In 

many instances, swine show no signs of illness. Ill 

animals typically begin showing signs 1–3 days after 

being infected with the virus, including:

•	 Coughing or barking

•	 Sneezing

•	 High fever

•	 Difficulty breathing

•	 Runny nose

•	 Not eating

•	 Not wanting to get up and move around

If you see any of these signs in your swine, contact 

your veterinarian and isolate the sick animal(s) from 

the rest of the herd, if possible.

Q. How does IAV-S spread?

A. IAV-S is believed to spread like human influenza 

viruses do—mostly through close contact, coughing, 

and sneezing. It is also possible that contaminated 

objects (such as farming equipment) can move the 

viruses around and indirectly spread the disease. 

Q. How can I prevent or control IAV-S?

A. The best way to help keep your swine from 

getting sick, or reduce the severity of illness if they 

do, is to use the following management strategies:

•	 Vaccinate your herd for influenza A.

•	 Practice good biosecurity and hygiene.

•	 Use proper ventilation systems in all swine 

facilities.

•	 Ask that your staff get the seasonal flu vaccine 

and avoid contact with your animals if they 

have flu-like symptoms to lessen the chances of 

your swine being infected by human influenza A 

viruses.

Q. What biosecurity practices should I use?

A. Use these biosecurity measures to help keep your 

animals healthy:

•	 Permit only essential workers and vehicles onto 

your farm.

•	 Avoid visiting other livestock farms.

•	 Clean and disinfect shoes, clothes, hands, crates, 

vehicles, and tires—all of which can carry the 

virus from one location to another.

•	 Purchase animals from reputable sources and 

document their origin.

•	 Have new additions to your herd checked by a 

veterinarian.

•	 Do not share equipment or vehicles with other 

farms.

•	 Isolate and monitor new animals or animals 

returning from exhibitions.

•	 Feed and work with healthy animals first, and 

then care for sick animals to minimize the spread 

of disease among swine housed together.

•	 Use separate equipment, clothes, and shoes when 

working with healthy versus sick animals.

•	 Do not work with or handle swine if you are 

experiencing flu-like symptoms.

Q. What should I do if I am going to a fair or event 

with a lot of swine around?

A. Swine caretakers can contact their State 

animal health officials for guidance. General 

recommendations to lessen chances that exhibitors 

and fair visitors will become ill from swine at 

exhibitions can be found at: https://secure.in.gov/

isdh/files/NASAHO-NASPHV-InfluenzaTransmissionAtS

wineExhibitions2013.pdf 

 Members of the general public can contact their 

State public health office or visit the Centers for 

Disease Control and Prevention (CDC) Web site on 

swine influenza: www.cdc.gov/flu/swineflu

For More Information

To learn more about IAV-S, visit the Web sites listed 

below.

•	 USDA Animal and Plant Health Inspection Service: 

www.aphis.usda.gov/animal_health/animal_dis_

spec/swine 

•	 CDC: www.cdc.gov/flu/swineflu

•	 National Association of Public 

Health Veterinarians: nasphv.org/

documentsCompendiaZoonoticInfluenza.html

•	 National Pork Board: www.pork.org/

Resources/1389/influenza.aspx

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United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture