Detection of influenza A virus in aerosols of vaccinated and non-vaccinated pigs in a warm environment

Today we are sharing a publication from the Torremorell lab regarding the impact of vaccination (both homologous and heterologous) on the detection of swine influenza virus in aerosols. The full publication is available in open access online on the PlosOne website.

Influenza A virus can be transmitted by direct and indirect contact and aerosols. Indeed, the virus has been detected and isolated from aerosols generated from pigs with and without immunity. Since then, there has been increased evidence of the role of aerosols in influenza transmission among swine.

Vaccination is used in swine populations as a strategy to mitigate clinical effects and the economic impact of influenza infections. It has also been proven to reduce shedding in pigs. Additionally, a study on the transmission of influenza in ferrets showed that high temperature may decrease the risk of airborne transmission. Therefore, we wondered if combining vaccination and high temperature would affect the detection of influenza virus in the air.

The objective of this study was to assess the effect of  vaccination on the generation of influenza A virus bioaerosols under warm conditions in pigs with varying degrees of cross-protective immunity.

Material and Methods

36 pigs of three weeks of age, seronegative for influenza were separated into four groups:

  1. vaccinated with an influenza strain identical to the one used for the challenge (homologous)
  2. vaccinated with a commercial vaccine containing multiple strains of influenza, all different from the challenge strain (heterologous, multivalent)
  3. vaccinated with a commercial vaccine containing one influenza strain different from the challenge strain (heterologous, monovalent)
  4. unvaccinated, which received an injection of saline instead

Pigs were challenged intranasally and intratracheally with a strain of H1N1 influenza virus, two weeks after the last vaccination.
Serum collected the day prior to the vaccination and at the end of the study 14 days post inoculation were tested via hemagglutination inhibition (HI) and ELISA.. Nasal swabs and oral fluids were collected and tested via PCR. Air samples were collected three times a day and tested via PCR and virus isolation. Temperature and humidity were recorded every five minutes.

Results

Hemagglutination inhibition and ELISA

Prior to infection, pigs in group 1 (Vaccinated, homologous) had significantly higher HI titers compared to the other three groups. In the group 3 (vaccinated, heterologous monvalent) 4 pigs had HI titers against the challenge strain, while pigs in groups 2 and 4 were negative against the challenge strain. All groups were HI positive against the challenge strain at necropsy, however HI titers were statistically different between group 4 and groups 1 and 3.

Proportion of pigs infected

The proportion of pigs infected was significantly higher in group 4 than in the vaccinated ones. Also, the percentage of infected pigs in group 1 was significantly lower than in group 2, but there was no difference with group 3.

Torremorell vacc pigs aerosol influenza proportion negative pigs
Proportion of negative pigs over time

Nasal swabs and oral fluids

Pigs in group 4 had higher amounts of nasal virus shedding most of the sampling days compared to vaccinated groups. Additionally, group 2 had higher levels of IAV compared with groups 1 and 4. Oral fluid results were in agreement with nasal swab.

 

Torremorell vacc pigs aerosol influenza nasal shedding
Nasal shedding over time

Air samples

All air samples in the vaccinated groups tested negative by RRT-PCR. Air samples collected at days 1, 2 and 3 from NON-VAC pigs tested positive by RRT-PCR but negative by virus isolation

Abstract

The 2009 influenza pandemic, the variant H3N2v viruses in agricultural fairs and the zoonotic poultry H5N9 infections in China have highlighted the constant threat that influenza A viruses (IAV) present to people and animals. In this study we evaluated the effect of IAV vaccination on aerosol shedding in pigs housed in warm environmental conditions. Thirty-six, three-week old weaned pigs were obtained from an IAV negative herd and were randomly allocated to one of 4 groups: 1) a homologous vaccine group, 2) a heterologous multivalent vaccine group, 3) a heterologous monovalent group and, 4) a non-vaccinated group. After vaccination pigs were challenged with the triple reassortant A/Sw/IA/00239/04 H1N1 virus. Environmental temperature and relative humidity were recorded throughout the study. Nasal swabs, oral fluids and air samples were collected daily. All samples were tested by RRT-PCR and virus isolation was attempted on positive samples. Average temperature and relative humidity throughout the study were 27°C (80°F) and 53%, respectively. A significantly higher proportion of infected pigs was detected in the non-vaccinated than in the vaccinated group. Lower levels of nasal virus shedding were found in vaccinated groups compared to non-vaccinated group and IAV was not detected in air samples of any of the vaccinated groups. In contrast, positive air samples were detected in the non-vaccinated group at 1, 2 and 3 days post infection although the overall levels were considered low most likely due to the elevated environmental temperature. In conclusion, both the decrease in shedding and the increase in environmental temperature may have contributed to the inability to detect airborne IAV in vaccinated pigs.

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