Maternal antibodies do not prevent infection and propagation of swine influenza virus in experimental conditions

Recurrent influenza infections in swine herds are characterized by swine influenza A virus (swIAV) infections occurring at a fixed age in successive batches, when a significant part of the piglets still have swIAV maternally derived antibodies (MDAs). Although passive immunity is known to provide partial protection against infection, its impact on transmission is not fully understood. The present study aimed at estimating the protective impact of MDAs derived from sow vaccination on swIAV transmission parameters.

A transmission experiment involving 72 specific pathogen free (SPF) piglets with or without MDAs was carried out. MDA-positive piglets were derived from vaccinated (Gripovac®3) SPF sows. In each group (MDA-positive/MDA-negative), 2 seeder-pigs per room, inoculated intratracheally with a H1N1 virus at 35 days of age, were put in contact with 4 direct- and 5 indirect-contact piglets (3 replicates per group, 2 pens per room). Individual virus shedding (RT-PCR) and MDA waning (ELISA test) were monitored from nasal swabs (daily basis) and blood samples, respectively. The duration of passive immunity persistence as well as the duration of shedding period were estimated using parametric survival analysis. Differential swIAV transmission rates depending on piglets’ initial serological statuses and contact structure (direct contact with penmates or airborne route) were estimated using maximum likelihood method, allowing the estimation of specific reproduction numbers.

Time to maternal antibody waning was 71.3 [52.8 – 92.1] days on average. The duration of shedding period was 6.1 days [5.9 - 6.4] for both groups. The airborne-related transmission rate was 0.69 [0.33 - 1.18] newly infected piglets per day leading to a rapid transmission to indirect-contact groups, further inducing within-pen transmission. Based on the transmission rate in MDA-positive and MDA-negative piglets and duration of shedding period, the reproduction number estimates were 5.7 [1.2 - 12.8] and 17.1 [8.9 - 29.4] respectively.

The presence of MDAs in piglets reduced but did not prevent early-life swIAV spread. As the virus spreads more slowly, the period with shedding animals is then expected to be longer at the population scale, which could increase swine flu within-herd persistence. Limitation of practices such as cross-fostering and mingling at weaning which enhance virus spread through direct contacts appears thus pivotal to control swIAV within-herd persistence. Likewise, airborne transmission was identified as a key component, highlighting the need to consider infectious aerosols in swIAV between room spread process.

Disclosure of Interest: None Declared.