Experimental transmission of avian-like swine H1N1 influenza virus between immunologically naïve and vaccinated pigs

Abstract

Background: Infection of pigs with swine influenza has been studied experimentally and in the field; however, little information is available on the natural transmission of this virus in pigs. Two studies in an experimental transmission model are presented here, one in immunologically naïve and one in a combination of vaccinated and naïve pigs.

Objectives: To investigate the transmission of a recent 'avian-like' swine H1N1 influenza virus in naive piglets, to assess the antibody response to a commercially available vaccine and to determine the efficiency of transmission in pigs after vaccination.

Methods: Transmission chains were initiated by intranasal challenge of two immunologically naïve pigs. Animals were monitored daily for clinical signs and virus shedding. Pairs of pigs were sequentially co-housed, and once virus was detected in recipients, prior donors were removed. In the vaccination study, piglets were vaccinated and circulating antibody levels were monitored by haemagglutination inhibition assay. To study transmission in vaccinates, a pair of infected immunologically naïve animals was co-housed with vaccinated recipient pigs and further pairs of vaccinates were added sequentially as above. The chain was completed by the addition of naive pigs.

Results and conclusions: Transmission of the H1N1 virus was achieved through a chain of six pairs of naïve piglets and through four pairs of vaccinated animals. Transmission occurred with minimal clinical signs and, in vaccinates, at antibody levels higher than previously reported to protect against infection.

Figure 1

Virus shed by individual pigs during transmission in naïve animals. Horizontal bars represent the period each pig was in the transmission chain, and each colour represents a different pig as indicated in the key. Y‐axis indicates viral RNA copy number, determined by qPCR of the NS1 gene, and X‐axis indicates day of study.

Figure 2

Box and Whisker plot showing the decline of haemagglutination inhibition (HI) antibody levels following vaccination of pigs with Gripovac^®. HI assays were carried out against the challenge strain A/sw/England/453/06.

Figure 3

Virus shed by individual pigs during transmission in vaccinated and naive animals. Horizontal bars represent the period each pig was in the transmission chain, and each colour represents a different pig as indicated in the key. Y‐axis indicates viral RNA copy number, determined by qPCR of the NS1 gene, and X‐axis indicates day of study. The first two and last four pigs were unvaccinated.