New research shows impact of pandemic swine influenza A virus on reproductive performance of sows

New research published in Porcine Health Management demonstrates that poor reproductive performance is associated with infection with the influenza A (H1N1)pdm09 virus and by vaccinating against the virus, an improvement in reproductive performance can be achieved in infected sow herds.
calendar icon 2 April 2020
clock icon 7 minute read

The study, conducted by researchers from the Clinic for Swine at the Centre for Clinical Veterinary Medicine, LMU Munich, Germany, investigates a phenomenon documented in previous research trials. Infection of sows with influenza A (H1N1)pdm09 virus has been shown to lower reproductive performance in the infected sows when compared to non-infected animals but, according to the review conducted in the current study, few studies have focused on the pathogenesis and the clinical signs of the infection. Additionally, it has been difficult to experimentally reproduce the clinical outcome of poor reproductive performance.

Influenza A virus (IAV) is caused by a number of closely related influenza A viruses that are noted for their ability to change their antigenic structure and create new strains. SI can be introduced by infected people, carrier pigs and probably on the wind although this has not been proved. Birds particularly water fowl, are reservoirs of infection. The virus is a potential pathogen of zoonotic disease and causes worldwide important economic losses.

There are three important periods when infection with the H1N1 virus causes infertility in sows. First, if sows are ill in the first 21 days post-service pregnancy their developing embryos may not get established and an increase in 21 day returns results. If pregnancy has been established 14-16 days after mating, and it then fails returns will be delayed. Second if infection occurs in the first five weeks of pregnancy, there could be total embryo mortality and absorption with sows becoming pseudo-pregnant and not in-pig. Litter size may also be affected at this stage due to absorption of embryos. Towards the end of the pregnancy period abortions or late mummified pigs at farrowing may also be experienced. The third major effect is on the boar, where high body temperatures affect semen and depress fertility for a 4 to 5 week period.

Subsequent to the emergence of the pandemic influenza A virus, a new vaccine against this sub-type was developed. As reported in the current study, the results of the clinical studies for efficacy and safety of this vaccine were verified under field conditions in 315 farms infected with pandemic influenza A virus. In these trials, the majority of the sows in the influenza-affected farms showed significantly reduced reproductive performance.

In light of this, the current study aimed to evaluate the effect of pandemic Influenza A on a set of reproductive parameters monitored in conventional sow herds, and how vaccination interacts with this reproductive performance.


A total of 315 sow herds infected with pandemic influenza A virus were included. Infection with the pandemic influenza A virus was verified by laboratory investigations. In 43 farms of the finally assessed 137 farms (see below), the infection with the aforementioned subtype was confirmed by detection of pandemic IAV-RNA by polymerase chain reaction (PCR). Due to the short period of virus shedding after infection, farms (94 farms) with the presence of antibodies against pandemic IAV detected by haemagglutination inhibition (HI) were also included in the study.

The 137 included farms comprised 104 piglet producing farms, 27 farrow-to-finish herds and 6 multiplier herds. In total, 111 farms were already vaccinating against other subtypes of IAV prior to the implementation of the pandemic IAV vaccine. The herd size ranged from 38 to 5600 sows, with a mean of 448 sows. In total, 60,153 sows were included.

The first step of the present study was to collect farm-specific data as well as information on the clinical presentation of the infection on the 315 farms via a standardised questionnaire. Based on the results of the questionnaire, data from only 137 farms were statistically analysed.

Primary vaccination was performed on all farms as a classical mass vaccination of all sows, consisting of two vaccinations at an interval of 3 weeks. The time period between the first occurrence of clinical signs related to influenza infection in the herd and the diagnosis and administrative approval differed between herds from 3 to 12 weeks.

To evaluate reproductive performance, the production parameters of the sow herds were recorded for 6 months before implementation of the vaccine and 6 months after completion of primary vaccination. Because the primary basic vaccination consists of two vaccinations at an interval of 3 weeks, this period of 3 weeks was not included in the data records.

worker performing an ultrasound on a sow

The following parameters were assessed based on routinely recorded production data that were obtained monthly whenever possible:

  • return to oestrus rate;
  • abortion rate;
  • stillbirth rate;
  • piglets born alive per litter;
  • pre-weaning mortality rate; and
  • number of piglets weaned per sow per year.

The farms were categorised by means of the return to oestrus rate before vaccination. The categories were chosen by the following benchmarks: return to oestrus rate < 10 percent (category 1), return to oestrus rate ≥ 10 percent and < 20 percent (category 2) and return to oestrus rate ≥ 20 percent (category 3), based on a literature review and industry reports characterising average German performance data.


  • Clinical signs prior to vaccination were monitored and a reduced reproductive performance was observed in 79.8 percent (n = 103/129) of the farms. Clinical signs included fever (62.8 percent), respiratory disease (cough) (61.2 percent), dyspnoea (17.1 percent), reduced feed intake (39.5 percent) and apathy in (14.7 percent).
  • In 74.8 percent of the farms a significant reduction in the return to oestrus rate was observed after implementation of vaccination. In those farms, the mean return to oestrus rate was significantly reduced by 5.1 percent.
  • After immunisation, the abortion rate decreased significantly by an average of 1.8 percent in 57 percent of the farms.
  • After immunisation, the number of piglets born alive increased significantly in 70.4 percent of the farms by an average of 0.6 piglets.
  • Analysis of the pre-weaning mortality rate after immunisation resulted in a significant average reduction of 2.29 percent in 49.6 percent of the farms.
  • Concurrently, an increase of an average of 1.98 piglets weaned per sow per year was observed in 77.1 percent of the farms after immunisation. However, in 18.1 percent of the farms, the number of piglets weaned per sow per year decreased by 1.08 piglets.
  • Analysis revealed no association between herd size and reproductive performance parameters. Additionally, no association between pre-vaccination of farms against other IAV subtypes and the return to oestrus rate, the abortion rate, the stillbirth rate, the number of piglets born alive/litter or the pre-weaning mortality rate was observed.


Reproductive performance of sows infected with the influenza A (H1N1)pdm09 virus can be improved by vaccination against this virus. The large number of herds and animals included in this study is indicative of the importance of influenza virus infection on the productivity of animals in conventional swine farms.

Read the full study, including all results, in Porcine Health Management.

Sophie Gumbert, Sebastian Froehlich, Anna Rieger, Julia Stadler, Mathias Ritzmann & Susanne Zoels
(2020) Reproductive performance of pandemic influenza A virus infected sow herds before and after implementation of a vaccine against the influenza A (H1N1)pdm09 virus. Porcine Health Management 6:4

Emily Houghton

Editor, The Pig Site

Emily Houghton is a Zoology graduate from Cardiff University and was the editor of The Pig Site from October 2017 to May 2020. Emily has worked in livestock husbandry, and has written, conducted and assisted with research projects regarding the synthesis of welfare and productivity of free-range food species.

© 2000 - 2024 - Global Ag Media. All Rights Reserved | No part of this site may be reproduced without permission.