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VETERINARY VIEWS
EQUINE INFLUENZA by Nancy S. Loving, DVM
An outbreak of equine influenza virus can have a tremendous economic impact on the equine industry. With that in mind, there is great motivation to study this highly contagious disease and to develop effective vaccines for protection. In recent history, there have been significant outbreaks throughout the world that have rendered horses non-functional to do their work, or in worse cases, horses have suffered lasting negative medical effects or have died.
In more recent times, equine influenza
virus (EIV) isn’t as deadly as it was in the past. Immunization of pregnant mares allows foals
to receive some immunity from an immunized mare’s colostrum (first milk). Even still, a young horse stricken by the flu may develop bacterial pneumonia. In addition, EIV is highly infec- tious and causes high rates of sickness within
an equine population. This especially impacts the racing industry and organized competitive events. Even when clinical signs abate relatively quickly, full recovery can take at least 2-3 weeks before the horse is able to return to work. It is recommended that a horse recovering from EIV is rested for a week for every day of fever.
A wealth of research has been devoted to revealing the fine details about equine influ- enza virus such as what it is, how it mutates, vaccines for individual protection, strategies for herd protection, and cross-species transmission.
WHAT IS EQUINE INFLUENZA VIRUS (EIV)
The culprit for this persistent viral infec-
tion is the influenza A virus of H3N8 subtype. Subtyping is based on hemagglutinin (HA) protein and neuraminidase (NA) properties that allow the flu virus to target mammalian tissues, enter and infect the cells, reduce normal immune function, as well as create variations in relative susceptibility between species. While horses tend to retain susceptibility to H3N8, recent discovery indicates that cross-species infection can occur in dogs also susceptible to H3N8.
VACCINE EFFICACY
EIV has the ability to mutate because, as an RNA virus, its genetic diversity allows adaptation to a new environment or to escape a host’s immune system. Such muta- tion is referred to as antigenic drift, which modifies the virus enough to minimize
the efficacy of targeted vaccines. Primary neutralizing antibodies against EIV target hemagglutinin proteins; it is these antigens that mutate in response to immune pressure, resulting in “drift.”
Many current intramuscular vaccines against influenza are prepared from inactivated (killed) virus, which don’t typically induce a broad immune response. Any change in the influenza virus, as with antigenic drift, results in the horse receiving less than optimal protec- tion from vaccines. In addition, immunity from killed vaccines tends to be short-lasting and is mediated within a narrow immune response of mostly systemic immunoglobulins (IgG). This contrasts with natural infection that stimulates antibody as well as cell-medi- ated immune (CMI) responses directed against multiple proteins, not just the hemagglutinin. While primary protection against influenza infection is antibody related, CMI is helpful for virus clearance.
Another notable finding from EIV research into vaccines is that the timing of immuniza- tion has a large effect on protection. Any horse that is at high risk – based on age, training, travel, co-mingling – should be immunized every six months.
Studies by Merck Animal Health and vet- erinarians at the University of California Davis have examined surveillance on equine upper respiratory infections. EIV tends to be on the rise, surpassing the incidence of equine herpes 4 (EHV-4), which historically has been the most common upper respiratory tract infection. Even horses with a history of EIV vaccination are developing disease.
Another significant change has been seen – the age of infected horses. While his- torically most EIV cases occurred in young horses less than five years old, now more horses in the 6-10 year and 11-15-year old range are being infected.
Vaccination during an outbreak, even using inactivated vaccines, is helpful in limiting infection. Yet, there have been failures with every brand of inactivated EIV vaccine, likely due to a “mismatch” between killed vaccine strains and active circulating influenza virus in the field.
In contrast, a modified live virus (MLV) vaccine, given via intranasal administration, may confer more complete protection. The intrana- sal route presents the immune system with the entire virus, including all the internal proteins that don’t undergo the same degree of antigenic drift as the surface hemagglutinin proteins. The MLV intranasal vaccine is safe and stimulates clinical protection three weeks later.
Immunization of pregnant mares allows foals to receive some immunity from an immunized mare’s colostrum (first milk).
114 SPEEDHORSE April 2020