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.

Bioaerosol sampling for airborne virus surveillance in swine facilities

Bioaerosol sampling refers to the methods by which one is able to collect the particles of biological origin (microbial, animal, or plant) in the air. This is useful information in swine production because many economically important pathogens can be transmitted by air from one farm to the next. 73 scientific reports were included in this review published in the journal Frontiers in Veterinary Science. The information regarding the presence of viruses in the air around swine settings is limited but their findings has been compiled in the figure below. Overall, bioaerosol sampling could be a promising way to conduct non-invasive viral surveillance among swine farms.

Viruses detected in radisuses around farms
Influenza A, PRRSV, PEDV detection downwind from farms with infected source populations

Abstract

Modern swine production facilities typically house dense populations of pigs and may harbor a variety of potentially zoonotic viruses that can pass from one pig generation to another and periodically infect human caretakers. Bioaerosol sampling is a common technique that has been used to conduct microbial risk assessments in swine production, and other similar settings, for a number of years. However, much of this work seems to have been focused on the detection of non-viral microbial agents (i.e., bacteria, fungi, endotoxins, etc.), and efforts to detect viral aerosols in pig farms seem sparse. Data generated by such studies would be particularly useful for assessments of virus transmission and ecology. Here, we summarize the results of a literature review conducted to identify published articles related to bioaerosol generation and detection within swine production facilities, with a focus on airborne viruses. We identified 73 scientific reports, published between 1991 and 2017, which were included in this review. Of these, 19 (26.7%) used sampling methodology for the detection of viruses. Our findings show that bioaerosol sampling methodologies in swine production settings have predominately focused on the detection of bacteria and fungi, with no apparent standardization between different approaches. Information, specifically regarding virus aerosol burden in swine production settings, appears to be limited. However, the number of viral aerosol studies has markedly increased in the past 5 years. With the advent of new sampling technologies and improved diagnostics, viral bioaerosol sampling could be a promising way to conduct non-invasive viral surveillance among swine farms.

Link to the full article

Science page: Evaluation of positive pressure filtration to reduce aerosol transmission of PRRSV during an experimental challenge of farm access points

This is our Friday rubric: every week a new Science Page from the Swine Health Monitoring Project. The previous editions of the science page are available on our website.

Key points from this week edition:

  • Dilute vaccine aerosolization combined with novel environmental sampling techniques allowed for testing of PRRSV aerosol entry into Positive Pressure Filtration (PPF) farm access points.
  • Under the experimental conditions of this study, positive pressure air speeds >1.85m/s resulted in no aerosol transmission.
  • Ensuring adequate positive pressure air speed through steps taken to increase access point pressure can further reduce the risk of aerosol PRRSV transmission on PPF farms.

The full report on positive pressure filtration and PRRSV transmission via aerosols is available.