Abstract
Equine influenza viruses (EIV)—H3N8 continue to circulate in equine population throughout the world. They evolve by the process of antigenic drift that leads to substantial change in the antigenicity of the virus, thereby necessitating substitution of virus strain in the vaccines. This requires frequent testing of the new vaccines in the in vivo system; however, lack of an appropriate laboratory animal challenge model for testing protective efficacy of equine influenza vaccine candidates hinders the screening of new vaccines and other therapeutic approaches. In the present investigation, BALB/c mouse were explored for suitability for conducting pathogenecity studies for EIV. The BALB/c mice were inoculated intranasally @ 2×106.24 EID50 with EIV (H3N8) belonging to Clade 2 of Florida sublineage and monitored for setting up of infection and associated parameters. All mice inoculated with EIV exhibited clinical signs viz. loss in body weights, lethargy, dyspnea, etc, between 3 and 5 days which commensurate with lesions observed in the respiratory tract including rhinitis, tracheitis, bronchitis, bronchiolitis, alveolitis and diffuse interstitial pneumonia. Transmission electron microscopy, immunohistochemistry, virus quantification through titration and qRT-PCR demonstrated active viral infection in the upper and lower respiratory tract. Serology revealed rise in serum lactate dehydrogenase levels along with sero-conversion. The pattern of disease progression, pathological lesions and virus recovery from nasal washings and lungs in the present investigations in mice were comparable to natural and experimental EIV infection in equines. The findings establish BALB/c mice as small animal model for studying EIV (H3N8) infection and will have immense potential for dissecting viral pathogenesis, vaccine efficacy studies, preliminary screening of vaccine candidates and antiviral therapeutics against EIV.
Highlights
Equine influenza (EI) is an OIE listed highly contagious acute respiratory disease of equines viz. horses, mules, donkeys and zebras caused by Influenza A virus
Equine influenza viruses (EIV) like any other influenza viruses constantly evolve by the process of antigenic drift as a result of point mutations in haemagglutinin (HA) and neuraminidase (NA) genes leading to substantial change in the antigenicity of the virus [5]
Ferrets have been widely used as a model, lack of their availability worldwide with regard to inbred lines, specific pathogen free status, size, cost, requirement of specific housing, labour, intensive handling, natural ability to acquire other influenza virus infection [6] and lack of ferret specific biologicals make them difficult model for vaccine efficacy testing and routine EIV research
Summary
Equine influenza (EI) is an OIE listed highly contagious acute respiratory disease of equines viz. horses, mules, donkeys and zebras caused by Influenza A virus. EIVs like any other influenza viruses constantly evolve by the process of antigenic drift as a result of point mutations in haemagglutinin (HA) and neuraminidase (NA) genes leading to substantial change in the antigenicity of the virus [5]. This causes failure of the vaccines as antibodies elicited against earlier strains do not protect against newly evolved variants. An inbred strain- DBA/2 has been explored for influenza virus studies (H5N1, H3N2, H1N1, mice adopted PR/8, etc.) and found to be more susceptible without adaptation of virus as indicated by weight loss, severe form of disease and mortality [19, 20]. Increased pathology in DBA/2 mice has been correlated with enhanced inflammatory response [22] which warrants further studies to reveal the reason for differential host response in DBA/2 mice at genetic and molecular level to understand host-pathogen causal relationship
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