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Swine & U column:  Swine influenza research continues

By Diane DeWitte, UMN Extension swine educator
Originally printed in The LAND - as December 22, 2023 Swine & U column

Winter has arrived and we have seasonal disease concerns, both in the pig barn and with the caretakers. For the past three years we have been immersed in learning all we can about COVID-19, but the usual winter worries on the pig farm always include Swine Influenza Virus (SIV). Swine Influenza Virus can be simply referred to as the flu, but there’s nothing simple about it in the herd.

Prior to COVID-19, the 2009 H1N1 (“swine flu”) pandemic was the worst human flu attack since 1918, and because its origins were a reassortment of human, bird and swine flu viruses, much research has occurred in the ensuing years to better understand flu in pigs and people.

Influenza in Pigs

Swine influenza virus is present in most swine herds across the country, and it appears to be a disease that many farms constantly live with. It’s a respiratory disease which affects all stages of production. Suckling piglets generally have immunity to the disease while with the sow. Coughing, pneumonia and fever are the most common symptoms. The high temperatures of SIV can cause abortions in pregnant sows; growing pigs will experience severe respiratory distress for 7-10 days, then return to normal over the next 7-10 days. In the Midwest, 90% of swine herds containing growing pigs have tested positive for SIV.

While commercial vaccines exist for swine influenza, many herds prefer to use an autogenous vaccine created specifically for them. These custom-made vaccines are formulated by using herd-specific antigens from the influenza-infected population.

U of M Studies on Influenza

The past decade has found the University of Minnesota on the forefront in conducting valuable applied research to try to understand influenza in swine, its control, and how it can be eradicated.

June 2023 studies led by researchers at the University of Minnesota College of Veterinary Medicine (CVM) reveal that farmworkers vaccinating and weaning 3-week-old piglets can indirectly and significantly contribute to the spread of the influenza A virus (IAV) on swine farms. This information can help pig producers target biosecurity measures more strategically, curbing swine flu transmission and enhancing swine production outcomes.

Flu limits the growth rate of young pigs, causing great financial losses to farmers and food security issues worldwide. IAV can also infect humans, leading to serious illnesses that have the potential to cause a pandemic. This two-way transmission of IAV between pigs and humans helps the virus evolve and evade vaccines. So, research clarifying how IAV spreads can help protect the pig industry and avoid pandemics.

Researchers, pig producers, and farmworkers alike know that handling infected pigs can carry and spread IAV. Farmworkers already take stringent precautions when handling infected pigs, but pinpointing which farm management activities most foster IAV’s spread can help them better understand when pigs are most infectious, and better focus their mitigation efforts.

In this study, the scientists collected and tested samples from farmworkers’ hands and clothes before and after the workers performed piglet processing, which takes place in the first few days of the piglets’ lives. The researchers also collected and tested samples from farmworkers again after vaccinating and weaning the piglets, which happens when the pigs reach about 3 weeks of age.

Only about 16% of the samples collected immediately after piglet processing tested positive for IAV, while 96% of the samples collected after vaccination and 94% of the samples collected after weaning tested positive. The hands of the farmworkers handling the infected 3-week-old piglets had a 91% contamination rate, while their clothes had a 97% contamination rate, and the scientists found live virus on both surface types.

“Results from this study should help producers and veterinarians to design more effective measures to control diseases in pigs and prevent flu virus transmission between pigs and people,” says Dr. Montse Torremorell, interim chair and professor in the Department of Veterinary Population Medicine at the CVM, who led the study.

Swine Farm Studies

Dr. Fabian Chamba Pardo recently published findings of his UMN study of the factors which affect influenza infection status of piglets at weaning time. From 2011 to 2017, Pardo collected samples at weaning on 83 swine farms in Iowa, Minnesota and South Dakota. These monthly samples were part of a routine surveillance program and yielded these results: Piglets who were raised by sows vaccinated against influenza were less likely to have the disease. Gilts who entered the sow herd and were influenza-positive were associated with positive piglets at weaning. Further details of Dr. Pardo’s study can be found at

A robust five-year study of 34 breed-to-wean farms by the team at the UMN Veterinary Population Department undertook the three-pronged challenge of estimating the prevalence and seasonality of SIV, investigating the correlation between the prevalence of SIV and weather, and studying the genetic diversity of the SIV on the farms over time.

The team found that the prevalence of influenza in herds over five years ranged from 7% to 57% with a median presence of 28%. Herd-level influenza occurrences followed a cyclical pattern with levels increasing during the fall, peaking in December and May, and subsiding in the summer. Researchers were able to correlate the prevalence of herd-level influenza with lower outdoor temperatures and low absolute humidity. The team’s research also showed that over time, there were genetically diverse influenza viruses co-circulating within the herd. (Frontiers in Veterinary Science, October 2017).

Building on Previous Influenza Research

A 2020 study by the University of Minnesota Veterinary College’s Dr. Jorge Garrido-Mantilla et al. evaluated if piglets put together with a nurse sow were more likely to be influenza A (SIA) positive and conversely, if a nurse sow could become infected when adopting a litter of positive piglets.

A common practice on pig farms is to use nurse sows to adopt piglets who fall behind and might otherwise die. Transmission of influenza virus from nurse sows to adopted pigs has been reported experimentally, however, until now, studies in actual production farms have not been conducted.

The study included a total of 184 sows in three breeding herds in Minnesota and Iowa. All three herds were positive for the swine influenza virus. The researchers collected oral swabs and udder wipes from the sows before the adopted pigs were placed with them, and after weaning. Oral swabs were collected from six piglets in each litter three times during the nursing period and at weaning.

Overall, this study showed that more sows were found positive at weaning compared to the beginning of the study when using oral swabs. However, no difference was found between the nurse sows and control sows. When looking at udder wipe samples, a higher proportion tested positive in the nurse group compared to the control group at the onset of the study. This difference was not found at the time of weaning. The udder wipe samples also tested positive by virus isolation, showing that nurse sows could serve as a mechanical means of transmission in addition to direct transmission from their own oro-nasal secretions. This study indicates that nurse sows can contribute to the transmission and perpetuation of influenza infections in pigs prior to weaning, particularly during the first week after adoption.

Protect People and Pigs

Although influenza is not a federally reportable or regulated swine disease, the USDA Animal and Plant Health Inspection Service (APHIS) in conjunction with the American Association of Swine Veterinarians (AASV) and National Pork Board have collaborated on the establishment of a national swine influenza virus surveillance program. Veterinarians can submit nasal fluid, oral swabs or lung tissue to a local lab to be evaluated. In Minnesota, the UMN Veterinary Diagnostic Lab on the St. Paul campus is the surveillance site. Information gathered there can help determine the presence of or changes in influenza virus on pig farms.

Because flu viruses can be transmitted between pigs and people, guidelines are in place for pig handlers and farm team members. As always, biosecurity is important to prevent the spread of influenza from pigs to people and from workers to pigs. Wearing personal protective equipment like gloves and masks that cover nose and mouth can reduce the transfer of flu virus. Workers should not eat, drink or put anything in their mouth in pig areas. Also important is hand-washing often with soap and running water before and after working with pigs. If soap and water is not available, an alcohol-based hand rub is recommended. Pig barn employees with flu-like illness should stay home for at least 24 hours after the fever ends.

The National Pork Board and the US Center for Disease Control recommend that people who work with pigs get a seasonal flu vaccination. Vaccinations are the most valuable tool for preventing flu transmission. Annual vaccination will prevent the spread of the flu between people and from people to pigs.

Flu season is here; Make sure that you, your family, and your pigs are protected against the disease.

Diane DeWitte, Extension Educator-Swine, University of Minnesota Extension

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