The Harry M. Zweig Memorial Fund for Equine Research

Emergence and Establishment of Influenza Virus in Equines

Dr. Gary Whittaker

Equine influenza remains one of the most common respiratory diseases of horses. It is a highly contagious viral disease that affects large numbers of thoroughbred race horses each year. In common with other species, the potential exists for pandemic influenza outbreaks, notably from birds, which would be accompanied by significantly increased morbidity and mortality. Equine influenza virus occurs in two serotypes H7N7 and H3N8. The first documented equine influenza virus was isolated from an extensive outbreak of respiratory disease in Czechoslovakia in 1956. The virus isolated from this outbreak became the prototype H7N7 equine influenza virus (A/equine/Prague/56). This virus spread around the world and continued to circulate in horses. Subsequent studies indicated that this virus was directly introduced from birds. In 1963, a second equine influenza virus was isolated (A/equine/Miami/63, H3N8), which also likely represents a direct introduction of an avian influenza virus. Equine H3N8 was recently was the cause of an epizootic in dogs.

The influenza virus coat protein (hemagglutinin or HA) is a major factor in the ability of influenza viruses to emerge and cause disease, and the so-called HA "cleavage site" is a primary factor in the ability of viruses to spread within a host. Before HA can mediate virus infection, it must be primed. In this priming step, the precursor HA is cleaved into two subunits. In humans, HA is normally cleaved by specific proteases released from cells of the respiratory epithelium, whose localization restricts tissue tropism to the respiratory tract. However viruses that possess a "polybasic stretch" of amino acids at their cleavage site (typically highly pathogenic avian influenza, HPAI) have the possibility of circumventing normal tissue tropism restraints, resulting in systemic lethal infections.

Equine influenza viruses are significant in that they can have features that are associated with HPAI, i.e. they possess a polybasic cleavage site. However, equine influenza viruses do not ordinarily undergo systemic spread in horses. What is unique about equine influenza viruses is that the HA can possess an additional, and highly conserved, 9–11 amino acid insert adjacent to the cleavage site. By molecular modeling, we have shown specific interactions of this insert, which likely result in attenuation of virus entry and spread within the host. Based on this, we hypothesize that the unique insert is critical to the adaptation of emergent influenza virus to the equine host.

We propose to carry out a structure-function analysis of two representative equine H7 viruses, A/equine/Prague/56 and A/equine/Cornell/74, as well as the prototype A/equine/Miami/63, H3N8 virus, with the goal of understanding the adaptation of these viruses to the equine host. Our specific aims will be to clone the equine HA genes and introduce these into recombinant influenza systems for mutagenesis studies, in combination with molecular modeling with the Cornell Theory Center. We will also generate an antigenic map of equine HA using a panel of monoclonal antibodies. These studies will be invaluable in our understanding of equine influenza virus and how novel emergent viruses adapt to the equine host. A study of this adaptation will be a critical part of our response to future influenza outbreaks in horses.