Page 698 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Respir atory system: 3.3 Medical conditions of the upper respir atory tr act 673

VetBooks.ir 3.3 Medical conditions of the upper respiratory tract

VIRAL DISEASES

EQUINE INFLUENZA appearance in Miami in 1963, H3N8 viruses were
disseminated worldwide by the global movement
Definition/overview of horses, dividing into two lineages in the 1980s,
Equine influenza is a highly contagious respira- known as European and American lineages. The
tory disease with high morbidity and low mortality gradual, but slow in comparison to human influ-
caused by orthomyxoviridae type A influenza viruses. enza viruses, evolution of H3N8 viruses has seen
Influenza is usually a self-limiting upper respiratory the dominance of American lineage viruses with
tract (URT) disease, but it may be complicated by near-disappearance of the European lineage viruses.
secondary bacterial respiratory infections. The virus The American lineage evolved into three further
occasionally causes lower respiratory tract (LRT) lineages in the 1990s (Florida, Kentucky and South
disease, myocarditis and post-viral fatigue syn- American), with dominance of the Florida lineage
dromes. Severe or fatal disease is very unusual except that further evolved into two clades (Florida Clade
in debilitated horses or neonatal foals. It affects 1 and Florida Clade 2) in the 2000s. Clade 2 viruses
naïve horses of any age, but disease is typically seen now dominate, with the large majority of influenza
in young horses when first introduced into training outbreaks across Europe in the 2010s caused by
yards. Large, economically devastating outbreaks them. Influenza viruses undergo gradual ‘antigenic
can occur in susceptible groups of horses (e.g. the drift’ whereby amino acid substitutions in antigenic
2007 Australian outbreak) or when new viruses with sites of the haemagglutinin molecule may create suf-
sufficient antigenic differences from vaccine strains ficient antigenic differences for vaccine- (or disease-)
enter groups of vaccinated horses. Infection causes induced antibodies to fail to neutralise the new virus,
pyrexia, depression, nasal discharge and a persistent resulting in vaccine breakdown. The OIE makes
harsh, dry cough. Horses usually recover with rest periodic recommendations for which strains should
and symptomatic treatment. Equine influenza is an be included in equine influenza vaccines. However,
‘Office International des Epizooties’ (OIE)-listed dis- in contrast to human influenza vaccines, regular fine-
ease and vaccination against equine influenza virus is tuning of equine influenza vaccine strains to match
compulsory under the rules of most racing authori- current circulating strains appears less important
ties and the Fédération Equestre Internationale because vaccine strains that are genetically ‘out of
(FEI). These vaccination programmes have proved date’ continue to provide effective protection, espe-
highly effective and have significantly reduced the cially when used as whole killed virus antigen with
number and extent of influenza outbreaks. suitable adjuvants.
The virus is transmitted from infected horses by
Aetiology/pathophysiology aerosol (across distances up to 50 m) or by respira-
Equine influenza viruses are type A orthomyxo- tory droplets, either directly from horse to horse or
viruses. They possess two major surface proteins, indirectly via fomites, including from hands. Virus
a neuraminidase (N) and a haemagglutinin (H). survival in the environment is short lived and envi-
Equine influenza viruses possess either H7N7 ronmental reservoirs of infection are not important
( formerly known as A/equine 1 viruses) or H3N8 in transmission. After entry into the horse, the virus
(formerly known as A/equine 2). H7N7 viruses have infects respiratory epithelial cells, causing destruc-
not circulated in Europe for many years and may be tion of ciliated columnar cells, loss of epithelial integ-
extinct. Equine influenza is now caused predomi- rity, exposure of cough receptors, increased mucus
nantly by H3N8 viruses worldwide. Since their first production and decreased mucociliary clearance.