Treating outbreaks of flu in vaccinated horses

The devastating flu outbreak that rampaged through the Australian horse population in 2007 was an important wake-up call reminding us that equine influenza virus is an ever-present threat. In Europe and America, the economic losses and welfare impact of flu are minimised by vaccination but still occasional outbreaks continue to occur. 

Equine Veterinary Journal recently published a series of articles focussing on equine influenza offering views from virologists, drug regulators, sporting authorities and the Animal Health Industry. In addition, flu researchers from the Irish Equine Centre, Kildare have shared important insights from their work on flu control in two recent research articles published in EVJ.

How do the flu vaccines work?

Vaccination stimulates the horse’s immune system to produce antibodies. These equine proteins recognise antigens, specific proteins on the surface of the virus. The antibody and antigen fit together, rather like a lock and key to prevent the virus entering the cells in the horse’s respiratory tract. Not only do these antibodies persist in the horse but the vaccine also stimulates an immunological memory. When the vaccinated horse meets the flu virus again, the immune system recognises the antigens as similar to the vaccine and quickly mounts a protective response to attack the invading virus.

Virus strains

Flu virus strains are named after two antigens on their surface, haemagglutinin (HA) and neuramidinase. HA is responsible for entry into equine cells lining the respiratory tract while the neuraminidase is involved in virus replication. The equine flu viruses all belong to the H3N8 family. Humans are affected by flu viruses of other strains but equine influenza poses no threat to people. Groups of viruses of closely-related strains are called Clades and currently there are two Clades of equine influenza viruses circulating globally.

Constant evasive action by the flu virus

Equine flu vaccines are all targeted at the HA protein. But the flu virus is smart, through a process called antigenic drift, the virus is continuously mutating and changing the genes that code for the HA antigenic site. As these changes occur, the vaccine becomes ever less effective. 

What’s the point in vaccination?

When an unvaccinated horse is infected with flu, first the virus invades the nasal passages and at this early stage, the horse begins to shed large amounts of virus and is highly contagious to other horses. Then, the horse develops clinical signs such as coughing, nasal discharge and fever. 




Author: Celia Marr