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Influenza Season Requires Precaution in Handling Pigs

By Diane DeWitte, Swine Extension Educator
Originally printed in The Land - December 14/December 21, 2018

Winter has arrived and with it some particular disease concerns, both in the pig barn and with the caretakers. In swine, the "Big Four" diseases are PRRS (Porcine Reproductive and Respiratory Syndrome), Mycoplasma pneumonia, Porcine Circovirus (PCV) and 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.

Many people remember the 2009 human influenza pandemic which was originally referred to as “swine flu”. Indeed, it was a H1N1 virus which affected swine or humans, but connecting it to swine was a public relations disaster for the pork-producing community. At that time, in late August 2009, H1N1 was considered to be responsible for an 11% drop in global pork trade, with the underlying reason attributed to the name “swine flu”.

Because the 2009 H1N1 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.

Human Influenza Season
Flu viruses are most common in humans during the fall and winter, although, like in pigs, the virus can be detected any time during the year. Influenza generally makes an appearance in October, then peaks between December and February. The US Center for Disease Control (CDC) indicates that flu is contagious beginning 1 day before symptoms develop and up to 5-7 days after a person becomes sick. Symptoms begin 1-4 days after the virus enters the body. The virus can easily be spread without a person knowing that he/she is sick.

In humans, flu is most easily spread from droplets in the air made by people with flu. Droplets spread as far as six feet when a sick person coughs, sneezes or talks, and, to a lesser extent, flu virus is spread by a person touching a surface that has the virus on it, then touching their nose or mouth.

Influenza is caused by ever-changing RNA viruses. The change in the virus isn’t mutation, it’s called reassortment; when more than one similar virus is affecting the same cell, the viruses can exchange genetic material. This new combination virus has properties of the original viruses and is called a variant. The H1N1 influenza pandemic in 2009 happened when people were infected by a variant of a swine, avian and human virus.

Typically the seasonal human flu is caused by the H3N2 virus. In 2011, a variant of H3N2 was discovered in humans which contained a gene from the 2009 H1N1 virus. That H3N2 variant has been most commonly found in people who experienced prolonged exposure to pigs at fairs.

The Latest U of M Studies on Influenza 
The past six years have 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. Researchers at the University of Minnesota’s College of Veterinary Medicine have recently published influenza information which details the prevalence of certain variants of the virus during outbreaks, and how seasonality and weather can have an effect on influenza outbreaks.

Dr. Fabian Chamba Pardo recently published findings of his UM 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 UM 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).

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 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 UM 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 has arrived; 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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