Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

Influenza A Virus in Swine Surveillance

Fiscal Year 2020 Quarterly Report

Surveillance Summary for Second Quarter Fiscal Year 2020: January 1 

to March 31, 2020

Report Summary1

 This report covers the second quarter (Q2) of fiscal year 2020 from January 1 to March 31, 2020.

 Where relevant, the report also includes previous years’ data for historical perspective. 

 The report provides data from both national and regional levels.

 In fiscal year 2020 Q2, there were 1,709 samples submitted for influenza A virus (IAV)

surveillance in swine from 1,620 accessions. 

 H1N1was the predominant subtype reported in USDA data in the first half of FY2020.

 Over the past 7 quarters, H1N2 was the main subtype in Regions 1, 3 and 5 while Region 2 and 

Region 4 most frequently isolated H1N1 and H3N2, respectively. For regions recorded as 

“unknown,” H1N2 was the most frequent subtype.

 Limited accessions from a region can skew data and lead to misinterpretation. Therefore, less 

inference can be applied to results from Regions 3, 4, and 5.

 All IAV-S submissions are voluntary and based on clinical case submissions to veterinary 

diagnostic labs. These data are not a statistically representative sampling of the U.S. swine 

population.

 Due to the voluntary nature of this surveillance, the information in this report cannot be used to 

determine regional and/or national incidence, prevalence, or other epidemiological measures, 

but it may help identify IAV-S trends. 

Introduction

This report, based on data received into the database as of July 15, 2020, provides a brief update on the 

status of national surveillance for IAV in swine for producers, swine practitioners, diagnosticians and the 

public. Summaries in this report may differ from those provided in past reports due to the regular 

addition of data from participating laboratories. Reporting months are based on the month the sample 

was collected. The IAV-S surveillance program is voluntary and, as a result, the accessions and samples 

submitted represent a subset of the swine population. Submitted samples should only be collected from 

 

1

In November 2016, VS modernized the process that prepares and stages laboratory results data for reporting. Consequently, 

VS recognizes there is a small difference in previously reported summary numbers for IAV-S surveillance. The results in this 

report reflect updated and corrected numbers achieved with the modernized data process.

Animal and 

Plant Health 

Inspection 

Service

Veterinary 

Services

July 2020
Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

animals displaying influenza-like illness. When the submitter does not report relevant information, data 

are recorded as “unknown.” Due to its voluntary nature, this surveillance system does not entirely 

represent the total U.S. domestic swine population. Therefore, the data cannot be used to determine 

IAV-S prevalence or other epidemiologic measures in the swine population. However, the data may help 

identify trends of influenza in swine. 

A laboratory accession generally represents a set of samples collected at a single premises on a single 

day and received at the laboratory. While a nasal swab or lung tissue sample represents a single animal 

within the herd, a single oral fluid sample may represent one to two pens of animals in a herd. A positive 

sample status is based on the screening real-time reverse transcriptase polymerase chain reaction (rRTPCR) on one or more samples within the accession. The subtype result is based on rRT-PCR-based 

subtyping assays. Virus isolation (VI) and sequencing in the NAHLN labs are only attempted on rRT-PCR 

positives meeting criteria with sequences deposited into GenBank, the public sequence database. On a 

monthly basis, USDA NVSL also performs whole genomic sequencing (WGS) on a selected subset of virus 

isolates received into the repository through the surveillance program and deposits those sequences

into Genbank. On a quarterly basis, a phylogenetic analysis is performed by ARS influenza researchers; 

phylogenetic analyses are based on all successful USDA surveillance sequencing results deposited into 

GenBank, the public sequence database.

Program Updates

Information on IAV-S and the IAV-S surveillance program, as well as previous IAV-S quarterly reports, are 

found at:

https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/animal-disease-information/swine-diseaseinformation/influenza-a-virus

The focus of IAV-S surveillance remains on acquiring and analyzing contemporary viruses from sick swine 

for ongoing genetic studies. The National Animal Health Laboratory Network (NAHLN) has several 

submission options to ensure that unusual viruses identified by methods other than standardized

NAHLN testing processes can be submitted into the program. An updated version of the IAV-S NAHLN 

testing guidelines and instruction sheet can be found at:

https://www.aphis.usda.gov/animal_health/animal_dis_spec/swine/downloads/appendix_c_testing

_guidelines.pdf

and 

https://www-author.aphis.usda.gov/animal_health/animal_dis_spec/swine/downloads/iav-salgorithm-instructions.pdf

IAV-S Surveillance Objectives

USDA’s National Surveillance Plan for Swine Influenza Virus in Pigs (July 2010) describes the current 

surveillance system for IAV in swine in detail. The surveillance objectives are to: 

1. Monitor genetic evolution of endemic IAV in swine to better understand endemic and emerging 

influenza virus ecology; 
Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

2. Make influenza isolates from swine available for research and establish a data management system to 

facilitate genetic analysis of these isolates and related information; and 

3. Select proper isolates for the development of relevant diagnostic reagents, updated diagnostic assays, 

and vaccine seed stock products. 

Objective 1. Monitoring Genetic Evolution of Endemic IAV in Swine to Better 

Understand Endemic and Emerging Influenza Virus Ecology

Objective 1 is met through the submission of diagnostic laboratory samples to the surveillance system, 

collection of the viruses that are isolated from the samples, and analysis of the hemagglutinin (HA) and 

neuraminidase (NA) sequences that are generated at the NAHLN laboratories. Each month, selected

viruses undergo whole genomic sequencing by the National Veterinary Services Laboratories (NVSL). 

Phylogenic analysis of the genetic sequences submitted through the surveillance program is provided 

through an interagency agreement with the USDA’s Agricultural Research Service (ARS) National Animal 

Disease Center (NADC). 

National Surveillance Data Summary

From fiscal year 2010 through fiscal year 2016, the total number of accessions and samples submitted 

increased. Changes initiated in fiscal year 2016 resulted in decreased laboratory accessions and samples,

but a higher percentage of accessions resulting in a virus isolate that can be sequenced and analyzed. In

fiscal year 2020, 1,709 samples have been tested from 1,620 accessions (Figure 1). Figure 2 shows the 

overall increasing trends in total accessions, rRT-PCR and VI positive accessions, and subtyped 

accessions.

Figure 1. Number of IAV-S laboratory accessions and samples tested in swine, fiscal year 2011 through fiscal 

year 2020 Q2.
Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

Figure 2. Accessions submitted, subtyped accessions, rRT-PCR positive accessions, and virus isolation positive 

accessions over time with trend lines for influenza A virus in swine, fiscal year 2011 to fiscal year 2020 Q2.

Figure 3 shows the number of subtype detections in fiscal year 2020 Q2. The total number of samples

subtyped was 306, including H1N1 (n=111), H1N2 (n=82), H3N2 (N=103), H3N1 (n=2), and mixed (N=8).

Figure 3. Number of influenza A subtype detections in swine fiscal year 2020 Q2.

Figure 4 breaks down accessions by rRT-PCR subtype from fiscal year 2011 to fiscal year 2020 Q2. H1N1 

was the predominant subtype detected in 2012, 2013, 2014, 2018, and 2020. H1N2 was detected most 

often in 2011, 2015, 2016, 2017, and 2019.
Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

Figure 4. Number of subtypes, fiscal year 2011 through fiscal year 2020 Q2.

Figure 5 displays the number of times VI was conducted in blue, the number of times VI was conducted 

and was positive in purple, and the number of viral isolates submitted to GenBank in green. Since the 

implementation of the June 2016 modifications to the program, almost all VIs attempted now yield a 

virus and the sequences are submitted to Genbank for analysis. Due to an unresolvable data processing 

coding error, samples sequenced and sent to Genbank during FY2019 Q1 appear greatly reduced in 

Figure 5. As with other fiscal years since FY2016, the samples sent during that timeframe closely follow

the number of attempted virus isolations as well as the number of positive virus isolation samples. 

Figure 5. Number of influenza A virus isolations attempted, number of virus isolations that were positive, and 

the number of viruses submitted to GenBank from fiscal year 2011 to fiscal year 2020 Q2.

Laboratory accessions were evaluated by age-class for the second quarter. The most common subtype 

isolated among the suckling age class was H3N2 and among the nursery age class it was H1N2. Among 

the grow-finish age class, H1N1 was the predominant isolated subtype. The sow/boar class had limited 

testing, with one isolate of H1N1 and one isolate of H3N2. Among isolates for which the age class was 

unknown or not recorded, H3N2 was the predominant subtype (Table 1). Nasal specimens (swabs or 
Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

wipes) were the most successful at providing a virus isolate which enables sequence submission to 

GenBank (Table 2).

Table 1. Number of positive accessions tested for influenza A virus in swine by age class and subtype fiscal year

2020, Q2.

Age class Number of accessions with 

subtype reported

Number 

of H1N1

Number 

of H1N2

Number 

of H3N1

Number 

of H3N2

Number 

of Mixed

Suckling 71 24 19 1 28 0

Nursery 126 37 42 1 40 7

Grow/finish 58 33 10 0 15 0

Sow/Boar 2 1 0 0 1 0

Unknown 47 16 11 0 18 1

Table 2. Number of positive accessions* tested for influenza A virus in swine by specimen type** and subtype fiscal 

year 2020 Q2.

Specimen 

Type

Number of 

accessions 

with 

subtype 

reported

Percent of 

subtyped 

accessions with 

positive virus 

isolation

Number 

of H1N1

Number 

of H1N2

Number 

of H3N1

Number 

of H3N2

Number 

of Mixed

Number of 

samples 

sequenced 

and sent to 

GenBank

Lung 253 90% 100 65 2 82 6 230

Nasal 39 92% 8 17 0 13 0 35

Oral Fluid 11 100% 3 0 0 6 2 9

Other 1 100% 0 0 0 1 0 2

*Accessions may include samples with multiple specimen types. In these cases, individual accessions are counted 

in more than one specimen type category. 

** Other includes specimen types recorded as swab, mixed tissue, or unknown.
Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

Regional surveillance data

In this section, we present data in five different regions (Figure 6). These regions are based on former 

USDA administrative districts only and do not represent specific industry distribution. Submissions are 

voluntary, as is any identifying information accompanying the submission (except the State of animal 

origin), and therefore no sampling strategies can be applied to the regions.

Figure 6. A map of the regions for national IAV-S surveillance.

Summary of Regional Data from ARS

Table 3. Summary of predominant subtypes in each region in a 2 year window 

(fiscal year 2018 Q3 through fiscal year 2020 Q2).

Region Total number HA/NA subtype

1 272 Gamma H1/Classical N1

Delta2 H1/1998-N2

2010.1 H1/2002-N2

2 1,367 Gamma H1/Classical N1

2010.1 H3/2002-N2

Delta2 H1/1998-N2

3 155 Alpha H1/2002-N2

Gamma H1/Classical N1

2010.1 H3/2002-N2

4 169 IV-A H3/2002-N2

Gamma H1/Classical N1

2010.1 H3/2002-N2

5 49 Low participation

2010.1 H3/2002 N2

Alpha H1/2002-N2

Delta1a H1/1998 N2

Most Predominant HA/NA phylotypes overall: H1N1 (Gamma H1/Classical N1), H3N2

(2010.1 H3/2002-N2) and H1N2 (Delta2 H1/1998-N2) 

Region 5 Region 3 Region 1
Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

Figure 7 shows the distribution of rRT-PCR subtyped accessions among the five regions for Q4 fiscal year

2018 through Q2 fiscal year 2020. Regions 1, 3 and 5 demonstrate H1N2 as the predominant subtype 

while Region 2 and Region 4 isolated H1N1 and H3N2 most frequently, respectively. For regions 

recorded as “unknown,” H1N2 was the most frequent subtypes.

Figure 7. Percentage of influenza A subtyped accessions for swine by region for fiscal year 2018 Q2 to fiscal year 

2020 Q2.

Regional phylogenetic analysis

Phylogenetic analysis of sequences from the IAV-S surveillance system

Phylogenetic analysis of gene sequences of IAV in swine is conducted to further examine the genetic 

changes that occur in HA, NA, and Matrix (M) genes of this rapidly changing virus. Through collaboration 

with ARS, a dataset2,3 of 379 isolates with published sequences in GenBank was characterized by 

phylogenetic analysis for the Q2 fiscal year 2020 report. This analysis provides information on the 

genetic diversity and evolution patterns of influenza in swine and allows for inferences about population 

and/or vaccine immunity.

 

2 Participating NAHLN labs included M gene sequencing in their testing until July 2016 because the 2009 H1N1 M gene was the 

predominant circulating gene.

3

The ARS dataset is comprised of IAV-S surveillance isolate sequences that were posted in Genbank. This represents only a 

subset of the complete IAV-S surveillance dataset that includes PCR diagnostic test-based results as well as sequencing results. 

Therefore, ARS dataset results, such as subtype percentages, differ from the complete IAV-S dataset results provided in other 

sections of this report.
Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

The following series of bar charts parse the data into an approximately 2-year window by quarters and 

subtypes for each region, followed by charts further describing the phylogenetic clades of H1 and H3 

subtypes. Regional charts depicting the various combinations of HA and NA are in Appendix 1. 

Figure 8. Temporal distribution of Influenza A virus subtype by region for Q3 fiscal year 2018 to Q2 fiscal year 

2020

Figure 8 demonstrates the four subtypes H1N1, H1N2, H3N2 and mixed subtypes across the five regions. 

Regions 1 and 2 reported the most submissions, with a mixture of mostly H1N1, H1N2, and H3N2. 

Limited accessions from a region can skew data and lead to misinterpretation and therefore, less 

inference can be applied to results from Regions 3, 4, and 5.

National phylogenetic HA gene information 

HA genes from H1 subtype viruses are classified as alpha, beta, gamma, delta-1, delta-2, or pandemic 

H1N1 2009 (H1N1pdm09) phylogenetic clades based on a previously published nomenclature system. 

Similarly, H3 subtype viruses are classified as Cluster IV, Cluster IV-A, Cluster IV-B, Cluster IV-C, Cluster 

IV-D, Cluster IV-E, Cluster IV-F, or human-like. In fiscal year 2020, Q2, the H1 subtypes gamma viruses 

are the predominant H1, with a total of 122 detected, with delta2 and alpha also frequently being 

detected. Gamma2-beta-like continue to be detected at low levels and is paired with MN99 N1 (Figure 

9).
Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

Figure 9. Temporal distribution of H1 phylogentic clades by region for fiscal year 2018 Q3 to fiscal year 2020 Q2.

In Q2 fiscal year 2020, there were 67 detections of H3s and human-like_2010.1 H3 was the predominant 

H3 (Figure 10). 2010.2 H3 was detected in Arkansas, Iowa, Illinois, Indiana, Ohio, and Oklahoma and is

paired with human seasonal N2. Cluster I H3 continues to be detected at low levels. TX98 N2 detected at 

low levels and predominantly paired with H3 Cluster I. Human-seasonal N2 is paired with a humanseasonal H3

Figure 10. Temporal distribution of H3 phylogentic clades by region for fiscal year 2018 Q3 to fiscal year 2020 Q2.
Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

National phylogenetic NA gene information 

Whole genome patterns with HA/NA pairs were reported and the dominant patterns are in descending 

order below with most frequent pattern by HA/NA pair: 

 TTTTPT: H3.2010.1/N2-02, H1.gamma/cN1, H1.delta1a/N2-02,

 TTTPPT: H1.delta2/N2-98, H1.gamma/cN1, H3.IVA/N2-02.

 TTPPPT: H1.gamma/cN1, H1.alpha/N2-02, H1.delta1a/N2-02. 

Leading vaccine constellation is TVVTPT, with other patterns circulating. Both the N1 and N2 subtypes 

are found in circulating swine viruses. Classical N1 continued to be the dominant cluster at 81.1 percent; 

and the 2002-lineage N2 represents 64 percent of N2 collections. 

Objective 2. Make Influenza Isolates from Swine Available for Research and 

Establish a Data Management System to Facilitate Genetic Analysis of these 

Isolates and Related Information

A primary goal of IAV swine surveillance is to share selected virus isolates obtained through the 

surveillance system with public health, animal health, and academic researchers to facilitate genetic 

analysis and research on viruses of interest. The NVSL Diagnostic Virology Laboratory maintains a 

repository of the viruses submitted into the surveillance system and provides these viruses upon 

request. 

In the second quarter of fiscal year 2020, the NVSL Diagnostic Virology Laboratory provided 30 isolates 

to two government, and one academic institutions. NVSL received 323 isolates into the repository in the 

second quarter. Table 4 reports the total number of virus isolates received into the repository each year 

FY2014 to FY2020. Table 5 reports the total number of isolates available in the repository by subtype for 

sharing. 

Table 4. Virus isolates received in repository. 

Virus isolates in the repository

2020 600

2019 1,055

2018 994

2017 844

2016 1,046

2015 883

2014 765

TOTAL TO DATE 6,187

 

 

Objective 3. Select Proper Isolates for Development of Relevant Diagnostic 

Reagents, Updating Diagnostic Assays, and Vaccine Seed Stock Products

USDA makes IAV-S isolates available in the public domain for further research. ARS-NADC conducts 

research on isolates obtained from the repository and sequences generated from the surveillance 

system. Genetic sequencing from the USDA program that is reported to GenBank is available for private 
Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

corporations, government entities, academia, and other scientific community partners for research and 

vaccine strain selection and efficacy testing. NVSL and ARS staff are consulted as subject matter experts 

when necessary. 

Conclusion

The IAV voluntary surveillance system in swine continues to provide insight into the genetic makeup of 

circulating influenza A virus in limited populations of commercial pigs. Genetic information and virus 

isolates are made publicly available for further research and possible vaccine strain selection and 

efficacy testing. Influenza A virus in swine remains a dynamic virus with high levels of genetic variability 

in the hemagglutinin and neuraminidase genes. 

Table 5. Total number of subtyped 

isolates available through repository.

Subtyped isolates available through 

repository

H3N2 2,120

H3N1 17

H1N1 2,767

H1N2 2,603

Mixed 302

TOTAL 7,809
Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

Appendix 1. Regional Charts of HA and NA Combinations by Percentage

The following charts present the percentages of combinations of HA and NA on the national and regional scales based on ARS-NADC phylogenetic 

analyses. The results are reported from April 2018 through March 2020. These “heat maps” represent the percentage of combinations by using a 

color gradient where a deeper gradient color represents a greater percentage occurrence for a particular HA-NA combination. HA clusters are 

listed on the left vertical axis of the chart and NA clusters are listed on the bottom horizontal axis. Line up the HA cluster with the corresponding 

NA cluster to determine the occurrence of that particular combination. 
Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

Region 1. Total HA & NA combinations –272 HA Clade
Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

Region 2. Total HA & NA combinations –1,370 HA Clade
Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

Region 3. Total HA & NA combinations - 155

HA Clade
Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

Region 4. Total HA & NA combinations – 169

HA Clade
Influenza A Virus in Swine Surveillance Quarterly Report for fiscal year 2020, Quarter 2

Region 5. Total HA & NA combinations – 49

HA Clade