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.

Influenza epidemiology in breed-to-wean farms and infection dynamics in nursery pigs

Fabian Chamba portrait photoEarlier this year, Dr. Fabian Chamba Pardo successfully defended his PhD under the supervision of Drs. Montse Torremorell and Marie Culhane. The focus of his thesis is influenza epidemiology with an emphasis on sow farms and nurseries. We share with you today a summary of his work.

Motivation

Influenza is an economically important disease in pigs and a public health threat. Breed-to-wean (BTW) farms play a central role in influenza epidemiology and control because piglets maintain and disseminate influenza A virus (IAV) to other farms. Despite the importance of piglets in influenza epidemiology, there is limited information on IAV infection parameters in piglets, risk factors that impact IAV prevalence in piglets at weaning, and how strategies that are implemented in BTW farms affect IAV infections in weaned pigs.

Objectives

In this thesis, my goal was to address some of the questions that are central to the transmission and control of influenza in BTW farms, especially infection in piglets ready to wean. The questions addressed are also critical to guide control strategies to mitigate IAV infections in the post weaning period. More specifically, I aimed to: 1) estimate herd-level prevalence and seasonality of influenza in BTW farms, 2) evaluate farm factors associated with IAV infection in piglets at weaning, 3) assess transmission patterns and parameters of influenza in nursery pigs based on IAV prevalence at weaning, and 4) evaluate the impact of maternally-derived antibodies (MDA) at weaning on IAV infection parameters in nursery pigs.

Research Chapter 1

Influenza herd-level prevalence and seasonality in breed-to-wean pig farms in the Midwestern United States

Article published in Frontiers in Veterinary Science: https://www.frontiersin.org/articles/10.3389/fvets.2017.00167/full

Results showed that IAV herd-level prevalence in piglets at weaning from Midwestern BTW farms is seasonal with higher infection rates in winter (December) and spring (May) than those in summer and fall. Additionally, influenza seasonality was partially explained by the seasonal variations of outdoor air absolute humidity and temperature. Finally, there was significant genetic diversity of influenza strains circulating in those farms and that, co-circulation of more than one genetically distinct clade over time was very common in the studied farms. This is critical knowledge that may help to identify high risk periods where influenza control measures can be placed. It may also help to create research opportunities on absolute humidity and influenza transmission in pigs and finally, it supports other studies that have shown that genetic diversity and circulation is wide and common and that new vaccines and vaccination strategies should take that into consideration.

Chamba herd level influenza prevalence in the Midwest
Influenza A virus herd-level prevalence in Midwestern US breed-to-wean pig farms.

Research Chapter 2

Breed-to-wean farm factors associated with influenza A virus infection in piglets at weaning

In this chapter, there were 24 farm factors evaluated for their association with influenza at weaning and among those, only IAV sow vaccination and the IAV-negative status of replacement breeding females (gilts) at entry to the herd were significantly associated with less IAV infected piglets at weaning. This is critical information that veterinarians and producers may use to manage IAV levels at weaning. In addition, there was also a lack of significant association with factors such as air filtration and farm density which may be indicative that endemic influenza infections are more important than airborne lateral transmissions between farms. Finally, disease control strategies such as herd closure, early weaning, batch farrowing, gilt isolation and gilt influenza vaccination were not fully evaluated in this study. Hence, more work is needed to further understand how to use these strategies to decrease influenza infections in pigs.

sow vac protocol - Copy
Influenza A virus (IAV) positive mean predicted probabilities over time for breed-to-wean farms with different sow vaccination protocols.

 Research Chapter 3

Influenza A virus transmission patterns and parameters in growing pigs

Results indicate that groups of piglets with different prevalence at weaning had different transmission patterns and parameters after weaning and these patterns were characterized by 1, 2 or no peaks of infection after weaning. Piglets with low prevalence at weaning had less influenza infections in the nursery. This information may help producers and veterinarians to make informed decisions when it comes to use control strategies such as sow vaccination aimed to reduce influenza infections in the nursery.

Figure 1

Research Chapter 4

Effect of maternally-derived antibodies on influenza A virus infection in growing pigs

In my last chapter, I reported that if pigs had high levels of strain-specific maternally-derived antibodies at weaning, IAV infection occurred later and it was of shorter duration after weaning. Piglets with hemagglutination inhibition (HI) titers of 1:40 or higher were less likely to test IAV positive at weaning and during the nursery. These results indicate that strain-specific maternally-derived antibodies generated with sow vaccination pre-farrow significantly reduce influenza infections at weaning and in the nursery.

Figure 1 (1)

Conclusions

Knowledge of influenza seasonality and what factors are significantly associated with influenza in breed-to-wean farms can help producers and veterinarians to better use and allocate influenza control strategies such as sow vaccination. In addition, lower prevalence of influenza at weaning due to high strain-specific maternally-derived antibodies levels may help decrease influenza spread from wean-to-finish farms. Reducing the burden of influenza in growing pigs should decrease influenza-associated economic losses and the generation of novel strains, including strains with pandemic potential. More studies are needed to further elucidate control strategies to limit influenza infections and spread in pigs.

Beneficial but variable effect of vaccination to control Salmonella in pigs

In this meta-analysis conducted by the Center for Veterinary Health Surveillance (Madrid, Spain) in collaboration with Drs. Conrado, Perez, and Alvarez from the University of Minnesota, the efficacy of vaccination to control Salmonella infection in pigs was evaluated. The meta-analysis reviewed a total of 44 studies focusing on Salmonella typhimurium or Salmonella choleraesuis. Included protocols were using either inactivated (killed) or live-attenuated vaccines.

Results showed that both vaccine types had a similar efficacy and that the most successful control strategies among the ones reviewed were using killed vaccines to control Salmonella choleraesuis.

Alvarez swine salmonella vaccine 2017

Abstract: Consumption or handling of improperly processed or cooked pork is considered one of the top sources for foodborne salmonellosis, a common cause of intestinal disease worldwide. Asymptomatic carrier pigs may contaminate pork at slaughtering; therefore, pre-harvest reduction of Salmonella load can contribute to reduce public health risk. Multiple studies have evaluated the impact of vaccination on controlling Salmonella in swine farms, but results are highly variable due to the heterogeneity in vaccines and vaccination protocols. Here, we report the results of an inclusive systematic review and a meta-analysis of the peer-reviewed scientific literature to provide updated knowledge on the potential effectiveness of Salmonella vaccination. A total of 126 articles describing the use of Salmonella vaccines in swine were identified, of which 44 fulfilled the inclusion criteria. Most of the studies (36/44) used live vaccines, and S. Typhimurium and S. Choleraesuis were the predominant serotypes evaluated. Vaccine efficacy was most often measured through bacteriological isolation, and pooled estimates of vaccine efficacy were obtained as the difference in the percentage of positive animals when available. Attenuated and inactivated vaccines had similar efficacy [Risk Difference = − 26.8% (− 33.8, − 19.71) and − 29.5% (− 44.4, − 14.5), respectively]. No serotype effect was observed on the efficacy recorded for attenuated vaccines; however, a higher efficacy of inactivated vaccines against S. Choleraesuis was observed, though in a reduced sample.

Results from the meta-analysis here demonstrate the impact that vaccination may have on the control of Salmonella in swine farms and could help in the design of programs to minimize the risk of transmission of certain serotypes through the food chain.

Link to the full article

Reducing the likelihood of a piglet reservoir when dealing with influenza in swine herds

Drs. White, Torremorell and Craft from the University of Minnesota recently published an article in Preventative Veterinary Medicine regarding practices that can decrease the likelihood of creating an endemic piglet reservoir in the case of an infection by swine influenza. Indeed, a stochastic model was developed considering that the pigs were in one of the following categories: Susceptible, Exposed,  Infectious, Recovered, or Vaccinated. Loss of immunity over time and differences between naturally infected and vaccinated animals were taken into account. Several scenarios were evaluated regarding their impact on piglet prevalence: timing of gilt introductions, gilt separation, gilt vaccination upon arrival, early weaning, and sow vaccination strategies.

In this model, homologous mass vaccination and early weaning were the most efficacious interventions. By combining frequent homologous mass vaccination, early weaning, gilt separation, gilt vaccination and longer periods between gilt introductions reduced endemic prevalence overall by 51% relative to the null scenario and the endemic prevalence in piglets by 74%.

influenza-a-piglet-reservoir-torremorell

Abstract: Recent modelling and empirical work on influenza A virus (IAV) suggests that piglets play an important role as an endemic reservoir. The objective of this study is to test intervention strategies aimed at reducing the incidence of IAV in piglets and ideally, preventing piglets from becoming exposed in the first place. These interventions include biosecurity measures, vaccination, and management options that swine producers may employ individually or jointly to control IAV in their herds. We have developed a stochastic Susceptible-Exposed-Infectious-Recovered-Vaccinated (SEIRV) model that reflects the spatial organization of a standard breeding herd and accounts for the different production classes of pigs therein. Notably, this model allows for loss of immunity for vaccinated and recovered animals, and for vaccinated animals to have different latency and infectious periods from unvaccinated animals as suggested by the literature. The interventions tested include: (1) varied timing of gilt introductions to the breeding herd, (2) gilt separation (no indirect transmission to or from the gilt development unit), (3) gilt vaccination upon arrival to the farm, (4) early weaning, and (5) vaccination strategies of sows with different timing (mass and pre-farrow) and efficacy (homologous vs. heterologous). We conducted a Latin Hypercube Sampling and Partial Rank Correlation Coefficient (LHS-PRCC) analysis combined with random forest analysis to assess the relative importance of each epidemiological parameter in determining epidemic outcomes. In concert, mass vaccination, early weaning of piglets (removal 0–7 days after birth), gilt separation, gilt vaccination, and longer periods between introductions of gilts (6 months) were the most effective at reducing prevalence. Endemic prevalence overall was reduced by 51% relative to the null case; endemic prevalence in piglets was reduced by 74%; and IAV was eliminated completely from the herd in 23% of all simulations. Importantly, elimination of IAV was most likely to occur within the first few days of an epidemic. The latency period, infectious period, duration of immunity, and transmission rate for piglets with maternal immunity had the highest correlation with three separate measures of IAV prevalence; therefore, these are parameters that warrant increased attention for obtaining empirical estimates. Our findings support other studies suggesting that piglets play a key role in maintaining IAV in breeding herds. We recommend biosecurity measures in combination with targeted homologous vaccination or vaccines that provide wider cross-protective immunity to prevent incursions of virus to the farm and subsequent establishment of an infected piglet reservoir.

Link to the full article