Influenza has gained renewed interest in swine in part due to the increased difficulty to control the infection at the farm level. Part of the problem is due to the co-circulation of genetically distinct strains of influenza virus. The vaccination protocols that are currently being used, either in pregnant sows prior to farrowing or as whole herd protocols,may decrease the prevalence of influenza at weaning but they are not able to completely stop the transmission of the virus. Current vaccination programs may improve the clinical outcome and decrease the economic impact of the disease.
In a recent study conducted at the University of Minnesota to study the maintenance and introduction of influenza virus in sow herds, replacement animals recently introduced, replacement animals that had been on-site for at least 30 days and piglets were tested monthly for a year. In this study, the newly introduced gilts and the piglets were the most important pig populations harboring the virus. About 8% of piglets and 19% of the newly introduced replacement animals tested positive. It is important to mention however, the significant differences observed between farms and the fact that there was not a common pattern observed among them.
Replacement animals can play an important role in the epidemiology of influenza virus. Replacement animals can be responsible for introducing new viruses into farms, but also they may act as amplifiers of circulating viruses already present in the farm if these animals are negative and become infected upon introduction. Nowadays is unrealistic to demand influenza negative breeding stock animals. However, ideally the introduction of immune animals that do not shed influenza virus is desirable. A way to achieve this is by isolating the replacement animals in isolation or quarantine units, monitor them and only introduce them when they are no longer shedding virus. The use of ropes to collect oral fluids may be the way to go to monitor groups of animals such as gilts often housed in pens. In addition, it is important to consider vaccine programs that include replacement animals prior to introduce them into the breeding herd.
Another important population is the piglets prior to weaning. Piglets ready to wean represent a significant challenge since they can be influenza positive despite the lack of clinical signs, mostly due to the presence of protective maternal antibodies. It is important to recognize that influenza virus does not cross the placenta, therefore piglets are born virus free.
Longitudinal studies have shown that the risk of infection in piglets increases with age and that is after 10-14 days of age (second week), and just prior to weaning, when the number of positive piglets is higher. In general, the prevalence of influenza virus in ready to wean piglets in endemically infected farms is low which often requires extensive diagnostics with large number of animals to know whether the virus is present or not. Unfortunately, maternal antibodies are not capable to fully block virus transmission, thus piglets ready to wean play a central role at maintaining the virus in the farms. An additional challenge of multi-site systems is that piglets may be weaned to farms located a significant distance away from site 1, thus piglets at weaning are considered an important source of virus dissemination between farms and between regions. About half of breeding herds may wean positive pigs to influenza virus in the U.S (results not published).
In addition, in the same study we demonstrated that is common to find multiple strains co-circulating in breeding herds. However, not all the strains seem to persist the same way at the population level and its appearance and disappearance may fluctuate. The influenza strains co-circulating may belong to the same subtype or different subtypes which difficulties even further the control of influenza virus on farm. Indeed, in this study we also found new strains that were the result of reassortants that took place at the farm, which emphasizes the need to eliminate the strains that are present at the farm before new viruses are introduced.
The findings of our studies emphasize the need to have programs to prevent and control influenza virus, in particular programs that minimize the introduction of new viruses via replacement animals, and also programs that minimize or eliminate the presence of influenza virus in ready to wean piglets with the goal that weaned pigs do not become a silent source of virus transmission between farms.