There was a time when Influenza Type A -swine (IAV-s) would enter a naïve pig herd, infect every pig at the same time, stop shedding in a short time frame as active antibodies increased in all (or almost all) pigs at the same time. Consequently the virus would be eliminated from the entire population of both breeding stock and growing pigs as there were no susceptible pigs to be had. As herd size has increased over time this scenario is much more uncommon. IAV-s is much more likely to become enzootic (endemic and persistent) in growing pigs. Previously exposed or vaccinated sows will provide passive protection via colostrum to their piglets. Colostrum quantity and quality, however, will vary from sow to sow and the colostrum intake by individual piglets can also be quite variable. The result is that the piglet’s passively acquired Maternal Derived Antibody (MDA) may be depleted at a variety of piglet ages (usually post weaning). A delay in infection for some pigs helps to ensure that there is always a source of viral shedding at the same time that there is a continuing source of IAV-s growing pigs. Putting more kindling on the fire!
One of the most widely used IAV-s control measures is the vaccination of sows. In the past , vaccination of sows with a commercial IAV-s vaccine might have provided adequate cross protection. As IAV-s have diverged genetically over time it has become more common for sow herds to be vaccinated with an autogenous vaccine specifically tailored to IAV-s types that are present in that geographic area (regional IAVs vaccine). It is well known that MDA cannot fully protect piglets from infection but it is able to reduce the clinical severity of disease. MDA may also temporarily block the development of an active immune response by the growing pig. Piglets, on occasion , may become susceptible to re-infection with the same IAV-s subtype within a short time frame. Enhancing the availability of MDA is an important tool but it is not a “silver bullet”. Other tools such as batch farrowing, internal biosecurity and strict all-in-all-out management in the farrowing rooms are other important tools in IAV-s control. As is the case with many infectious diseases, the clinical course of the disease is influenced by the dose and virulence of the virus. “The Solution to Pollution is Dilution!”
These German and French researchers studied disease transmission dynamics in weaned piglets in five enzootically IAV-s infected swine herds Germany. These herds had a history of recurrent respiratory problems associated with continuing influenza A virus infection. Participating farms ranged from 600 to1850 sows. Piglets were not vaccinated against IAV-s influenza A virus while IAV-s vaccination was performed in all sow herds, except for one, which discontinued vaccination during the study.
The researchers found the following:
- All nasal swabs collected from suckling piglets were PCR-negative for IAV-s
- All nasal swabs collected from sows associated with the piglets in the first week after farrowing were also negative. The researchers commented given that the sample size of 30 or 40 piglets per herd, it must be considered (even if unlikely) that a IAV-s infection prevalence below 10% might have existed but was not detected in nasal swabs.
- Nasal swabs PCR-positive for IAV-s (n = 216) were only found in weaned pigs housed in the nursery with first detections at 4 weeks of age and cohorts of pigs up to 10 weeks of age showing evidence of infection. (Chart 1)
- There was evidence of simultaneous circulation of two different IAV-s subtypes in one herd and co-infection with two subtypes in individual animals. ( Increasing the potential for recombination of genetic material )
- Prolonged shedding of at least 13 days was obtained in one piglet based on two consecutive IAV-s positive samplings. Shedding of IAV-s usually persists for about 7 days.
- Possible re-infection was suspected in twelve piglets based on three samplings, the second of which was IAV-s negative in contrast to the first and third sampling which were IAV-sages:
- In these IAV-s herds it was predominantly weaned piglets that were infected.
- There was no evidence of IAV-s transmission, in this endemically infected sow herd from sow to piglet
- Prolonged virus shedding by individual piglets as well as the co-circulation of different IAV-s subtypes in a group or even individuals emphasize the potential of IAV-s to increase viral genetic variationemphasize the potential of swIAV to increase genetic variation
- Given the demonstrable opportunity for viral genetic change it is important to continue herd surveillance such that emerging IAV-s can be incorporated into the latest autogenous vaccines.
Reference: Kathrin Schmies , Christin Hennig , Nicolas Rose , Christelle Fablet , Timm Harder , Elisabeth Grosse Beilage , Annika Graaf-Rau Dynamic of swine influenza virus infection in weaned piglets in five enzootically infected herds in Germany, a cohort study Porcine Health Manag . 2024 Oct 1;10(1):36. doi: 10.1186/s40813-024-00390-w.
