By Montse Torremorell, University of Minnesota College of Veterinary Medicine associate professor, and Marie Culhane, University of Minnesota College of Veterinary Medicine Department of Veterinary Population Medicine associate professor
Pneumonia, fever, coughing and sneezing caused by influenza A virus in pigs are common and can result in significant economic losses for producers. Flu never seems to go away in some herds and that is because there are groups of pigs, or subpopulations, that are able to maintain and spread the flu virus.
One of the most important subpopulations that have been identified as sources of virus on a farm is the piglets. Piglets may be infected, but may not show any signs of disease, and as a result, are silent spreaders of flu. Then, at weaning, a small, but significant, percentage of the piglets can be subclinically infected with flu, meaning they appear healthy but are shedding flu at the nursery or wean-to-finish site.
Piglets are born free of flu but in endemically infected farms, those that have flu problems continuously, they usually become infected during their second week of life. The occurrence of flu positive piglets is highest at weaning and thus piglets are not just a source of flu infections in the breeding herd but also a major source of infection to growing pigs.
Understanding how piglets become infected while still on the sow and preventing piglets from getting infected in the first place are necessary to decrease the risk of spreading flu to other farms. Controlling flu in farrowing can be very difficult because every day, new litters of pigs are born that are susceptible to flu and can really increase the virus levels in the farrowing rooms. Even when we do our best to protect the piglet by vaccinating the sow and making sure all piglets get their protective colostrum, the piglets still have flu and can spread it to their littermates and other pigs they contact. That is because the protection the sow gives to the piglet, known as the maternal immunity, is not perfect.
Maternal immunity is still important and helps to improve the performance of infected pigs, helping them get through their flu infections and become robust piglets. Sow vaccination is also important to reduce the prevalence of flu-infected pigs at weaning but sow vaccination alone is not enough to consistently wean flu-negative pigs or pigs with very low levels of flu.
At the University of Minnesota, we have been measuring the impact of piglets on the spread of flu for years. We found, in a study by Allerson of 52 swine breeding herds in the United States, 23 herds (44%) tested IAV RT-PCR positive at least once during a six-month study period. Groups of piglets from those herds also tested positive for flu at weaning about 25% (75 of 305) of the time.
Along those same lines, Chamba and partnering sow farms reported that out of the 34 farms studied for more than five years, all sow farms tested positive for flu at one time or another and the level of flu infection in the groups of weaned pigs ranged from 7% to 57%. More importantly, in this study, approximately 28% (427 of 1,523) of groups of pigs tested positive at weaning. With almost a third of the weaned pigs flu-positive when placed in the nursery, the impact of these flu-positive piglets can be tremendous for the other pigs they are placed with when growing.
Ultimately, the successful control of on-going flu infections in growing pigs will depend on the sow farm’s ability to wean a negative pig or a pig that does not pose a risk of spreading flu at weaning. Therefore, it is extremely important that we have a full understanding of what factors contribute to flu infections in breeding herds.
Unfortunately, flu viruses in pigs have changed significantly during the last few decades with the introduction of multiple viruses from humans and birds. This has resulted in an increase in viral genetic complexity that has made flu infections particularly difficult to control. More flu research is under way and is very much needed. On the horizon are new tools to control flu in piglets, such as vaccines designed to give a more complete immune response and given specifically to the growing pig and not the sow.
The research should help us determine the risk factors present on a sow farm that could increase the chance of flu infection for the piglets. Translating the research to on-farm use and applying new tools when they become available will be important to do control in swine breeding herds and protect the piglets.