Impact of nurse sows on influenza A virus transmission

A study by the Torremorell group was recently published in the Preventive Veterinary Medicine Journal. In this publication, Dr. Jorge Garrido-Mantilla et al. evaluated if piglets put together with a nurse sow were more likely to be influenza A positive and conversely, if a nurse sow could become infected when adopting a litter of positive piglets.


This study was completed in three commercial farrow-to-wean farms in the Midwest. Ninety sows were enrolled across the various herd for both groups (nurse and control). Oral swabs and udder wipes were collected from sows before the piglet adoption and after weaning. At the same time, airborne particles were collected from the environment. Oral swabs were collected from six piglets in each litter at 2, 4, and 14 days post-enrollment and at weaning. All samples were tested by PCR and a subset were chosen for virus isolation.


Overall, this study showed that more sows were found positive at weaning compared to the time of enrollment when using oral swabs. However, no difference was found between groups. When looking at udder wipe samples a higher proportion tested positive in the nurse group compared to the control group at the time of enrollment. This difference was not found at the time of weaning. The udder wipe samples also tested positive by virus isolation, showing that nurse sows could serve as a mechanical mean of transmission in addition to direct transmission from their own oro-nasal secretions. The results from the piglets’ oral swabs are shown in the table below.

Sampling time Control   Nurse sow litters  
No.Pos/No.Total % Positive No.Pos/No.Total % Positive
Enrollment 11/94 a 11.7 11/90 a 12.2
2 DPE 14/94 a 14.9 26/86 b 30.2
4 DPE 20/87 a 23.0 31/84 a 37.0
14 DL 36/77 a 46.8 38/64 a 59.4
Weaning 55/86 a 64.0 44/78 a 56.4
Table 1. Number and percentage of influenza A virus (IAV) positive litters by cohort group (control and nurse sow litters) for all farms together.
a,b Differences in superscripts within a row indicate statistical differences at p<0.05.
** DPE: Days post enrollment.
*** DL: Days of lactation.

For more detailed results, read the entire publication on the journal website, available in open access.


Piglets prior to weaning play a central role in maintaining influenza infections in breeding herds and the use of nurse sows is a common practice to adopt piglets that fall behind and that otherwise would die. Transmission of influenza A virus (IAV) from nurse sows to adopted pigs has been reported experimentally, however, the importance of this route of transmission under field conditions has not yet been elucidated. A cohort study to assess the IAV status in nurse and control sows and their respective litters was carried out in three influenza positive breed-to-wean farms. A total of 94 control and 90 nurse sows were sampled by collecting udder skin wipes and oral swabs at enrollment (∼ 5–7 days after farrowing) and at weaning. Six piglets per litter were sampled randomly at enrollment, 2 days post-enrollment (DPE), 4 DPE, at day 14 of lactation (14DL) and at weaning. At enrollment, 76 % (69/91) of udder wipes and 3 % (3/89) of oral swabs from nurse sows were positive by rRT-PCR compared with 23 % (21/92) of udder wipes and 0 % (0/85) of oral swabs from control sows. Of the 94 control litters sampled, 11.7 %, 14.9 %, 22.9 %, 46.8 % and 63.9 % tested rRT-PCR IAV positive at enrollment, 2DPE, 4DPE, 14 DL and weaning, respectively. Corresponding prevalence for nurse sow litters were 12.2 %, 30.2 %, 37.0 %, 59.4 % and 56.4 %. The odds of IAV positivity were significantly higher (p < 0.05) for litters from nurse sows 2 DPE (odd ratio (OR) = 6.13, 95 % CI = 1.8–21.2), 4 DPE (OR = 5.5, 95 % CI = 1.7–17.8) and 14 DL (OR = 3.7, 95 % CI = 1.1–12.3). However, there were no differences in the proportion of positive samples at weaning. Moreover, approximately 18 % of the control sows and 11 % of nurse sows that tested IAV negative in oral swabs at enrollment, tested IAV positive at weaning. This study indicates that nurse sows can contribute to the transmission and perpetuation of IAV infections in pigs prior to weaning, particularly during the first week after adoption.