A Review on Equine Influenza from a Human Influenza Perspective

Abstract

Influenza A viruses (IAVs) have a main natural reservoir in wild birds. IAVs are highly contagious, continually evolve, and have a wide host range that includes various mammalian species including horses, pigs, and humans. Furthering our understanding of host-pathogen interactions and cross-species transmissions is therefore essential. This review focuses on what is known regarding equine influenza virus (EIV) virology, pathogenesis, immune responses, clinical aspects, epidemiology (including factors contributing to local, national, and international transmission), surveillance, and preventive measures such as vaccines. We compare EIV and human influenza viruses and discuss parallels that can be drawn between them. We highlight differences in evolutionary rates between EIV and human IAVs, their impact on antigenic drift, and vaccine strain updates. We also describe the approaches used for the control of equine influenza (EI), which originated from those used in the human field, including surveillance networks and virological analysis methods. Finally, as vaccination in both species remains the cornerstone of disease mitigation, vaccine technologies and vaccination strategies against influenza in horses and humans are compared and discussed.

Keywords: epidemiology; equine; horses; human; influenza; vaccination.

Figure 1

Common pathways for the introduction of an equine influenza virus infected horse to a population.

Figure 2

Cross-species transmission of equine influenza A virus H3N8. Thick solid red arrow represents direct transmission event that has since become established and now circulates within the species. Thin red arrow represents isolation from the species as likely spill-over events. Dashed red arrow represents isolation following experimental exposure only. Dashed black arrow represents serological evidence of exposure only, from field or experimental sources.

Figure 3

Equine influenza virus surveillance pathway for laboratory confirmed infection. 1. Infectious horse noted to be sick by equine keeper and veterinary surgeon contacted 2. Veterinary surgeon suspects infectious process, takes samples during infectious phase, and requests equine influenza agent detection testing such as polymerase chain reaction (PCR) 3. Laboratory correctly identifies equine influenza, and result is reported through a surveillance network 4. Surveillance network shares epidemiological data and analyzes viral isolate to determine influenza strain.