Comparison of Pathogenicity and Transmissibility of Influenza B and D Viruses in Pigs

Abstract

Influenza viruses are important pathogens causing respiratory disease in humans and animals. In contrast to influenza A virus (IAV) that can infect a wide range of animal species, other influenza viruses, including influenza B virus (IBV), influenza C virus (ICV), and influenza D virus (IDV) have a limited host range. Swine can be infected with all four different genera of influenza viruses. IAV infection of pigs causes the well-known swine influenza that poses significant threats to human and animal health. However, influenza virus infection of pigs with IBV, ICV, and IDV are not well-characterized. Herein, we compared pathogenicity of IBV and IDV using intratracheal and intranasal infection of pigs, which are IAV seropositive, and commingled naïve pigs with the infected animals to determine their transmissibility. Both viruses caused fever and some lung lesions, replicated in the lungs of infected pigs, but only IDV transmitted to the contact animals. Although IBV and IDV displayed differing levels of replication in the respiratory tract of infected pigs, no significant differences in pathogenicity of both viruses were observed. These results indicate that both IBV and IDV can replicate, and are pathogenic in pigs.

Keywords: influenza B virus; influenza D virus; pathogenicity and transmissibility; pigs.

Figure 1

Body temperature, macroscopic, and microscopic lungs lesions of contact or infected pigs with influenza B virus (IBV), or influenza D virus (IDV). (A) The body temperature of infected or contact pigs is presented as the average body temperature of each group of pigs at indicated days. The number, such as 6/10, means that 6 out of 10 IDV-infected pigs showed fever at a defined dpi. (B) Macroscopic lung lesions of infected or contact pigs are presented as the average percentage ± SEM of gross lesions of three or four pigs in each group at the indicated days. (C,D) Microscopic lung (C) and nasal turbinate (D) lesions of infected pigs are presented as the Mean ± SEM of lesions of three or four pigs in each group at the indicated days. The asterisks (*) represent a statistically significant difference between groups (*: p < 0.05; **: p < 0.01).

Figure 2

Histologic nasal turbinate and lung sections from pigs infected with either IBV or IDV at 8 days post-infection. Top Row: Nasal turbinate sections from control and infected pigs. Turbinate from control (A) have normal ciliated respiratory epithelium (arrow) and minimal inflammatory cells in lamina propria (asterisk). The IBV (B) or IDV (C) infected nasal turbinates have prominent segment of squamous epithelium/metaplasia (arrow) with infiltrating neutrophils through lamina propria and epithelium and increased lymphocytes in lamina propria (asterisks). Bottom Row: Lung sections from control (D) and infected pigs (E,F) taken after collection of lavage. Lungs infected with IBV (E) have prominent diffuse interstitial pneumonia with increase in cellularity of alveolar septae (double arrows), along interlobular septae (single arrows), and with prominent (though mild) peribronchiolar lymphocytes (arrowheads). Lungs infected with IDV (F) have milder diffuse interstitial pneumonia (increased thickening and cellularity of alveolar septae including along interlobular septae) and minimal peribronchiolar inflammation. Lungs of control pigs have minimal increased cellularity noted components. Bars for nasal turbinate and lung section are 100 and 200 µm, respectively.

Figure 3

Virus titers in BALF and nasal swab samples collected from contact or infected pigs with IBV or IDV. (A) Mean of virus titers in BALF from contact or infected pigs with either IBV or IDV at indicated days. (B) Mean of virus titers in nasal swabs from contact or infected pigs infected with either IBV or IDV at indicated days. None of the contact pigs had virus in lung or nasal turbinates. Determination of virus titers was performed by calculating the 50% tissue culture infective dose (TCID₅₀)/mL in Madin–Darby canine kidney (MDCK) cells using indirect immunofluorescence assay (IFA). The number of pigs with positive virus isolation out of the total number of tested pigs is presented above of each bar. The error bars represent SEM. The asterisks (*) represent a statistically significant difference between groups (**: p < 0.01, and ***: p < 0.001).

Figure 4

Hemagglutination inhibition (HI) titer of serum samples from contact or pigs infected with IBV or IDV at 8 dpi (6 dpc). HI assay was performed for serum samples collected from three control pigs, three contact pigs, and four infected pigs at 8 dpi based on a standard protocol. HI titer ≥ 20 is judged as positive.