Page 700 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Respir atory system: 3.3 Medical conditions of the upper respir atory tr act 675

VetBooks.ir A viruses, and which cross react with equine influ- qPCR testing can be used to confirm the end of
virus shedding if required.
enza A viruses, are used at some competitions for
Equine influenza is controlled principally by
rapid initial diagnosis. Serology carried out using a
haemagglutination inhibition assay is the standard vaccination. A variety of vaccines are available
diagnostic test, but single radial haemolysis (SRH) including inactivated (whole killed virus) and sub-
is more useful for monitoring response to vacci- unit canary pox recombinant vaccines. These are
nation and in epidemiological studies. SRH titres all delivered by intramuscular injection. In North
provide a measure of protective immunity such that America, an intranasal modified live virus vaccine
horses with titres >150 mm generally show clinical is also available. Current OIE recommendations
2
and virological protection against challenge with are that vaccines should contain strains represen-
homologous virus and horses with titres >85 mm tative of American Florida Clade 1 and Clade 2
2
and <150 mm showing clinical protection. sublineages of virus, although vaccines containing
2
genetically ‘out of date’ strains can provide effec-
Management tive protection against currently circulating strains.
Influenza is usually a self-limiting disease provided Vaccination against equine influenza is required
clinical cases are properly managed. Affected horses by many organisations and has been mandatory in
should be rested for 3 weeks and, if stabled, kept racing and equestrian sports since the 1980s. The
in a dust-free environment with good air hygiene. primary vaccination course consists of two injec-
Continued training and poor air hygiene delay tions given 21–92 days apart, with a third dose given
recovery, predispose to secondary infection and may 150–215 days later. Annual boosters are given there-
cause chronic post-viral fatigue syndromes. Broad- after. Horses may not race until 8 days after any vac-
spectrum antimicrobials are often administered, but cination. Although British Horseracing Authority
unnecessary antimicrobial use should be avoided, (UK) rules specify that booster doses be given every
unless bacterial super-infection has been diagnosed. 12 months, there is evidence that to provide opti-
Non-steroidal anti-inflammatory drugs (NSAIDs) mum immunity in young horses in training, an addi-
may be used to control pyrexia if required. Other tional 6-month, rather than 12-month booster after
treatments may be employed (e.g. clenbuterol [to the third vaccination of the primary course may
improve ciliary clearance] and mucolytics) but are be required before adopting the routine of annual
not usually required. Antiviral influenza A drugs are boosters. FEI regulations require 6-monthly boost-
effective in horses in reducing severity and duration ers for all competition horses. An immunity gap
of disease, but selection of resistant virus strains is a may also occur in young Thoroughbreds between
concern and their use should therefore be restricted V2 and V3 of the primary course, due to decrease of
to humans. SRH antibody titres below those required to induce
Strict infection control including suspension of clinical protection. In the face of an epizootic,
movement on and off infected premises should be immunity can be maximised by giving the third
employed to prevent spread of infection to adja- vaccination of the primary course 2–3 months after
cent yards. Within the affected yard, barrier pre- the second vaccination and subsequent boosters at
cautions should be established to try to prevent 6-monthly intervals, taking into account that this is
spread. The virus is easily inactivated by many off-label use of vaccines. Mares may be vaccinated
disinfectants, including 1% bleach, 70% ethanol, 8–4 weeks before foaling to provide optimum lev-
iodine-based disinfectants, quaternary ammo- els of colostral antibody. Foals born to vaccinated
nium disinfectants, peroxygen disinfectants and mares should not be vaccinated for equine influenza
phenolics. Infected horses typically shed the virus before 4 months of age.
for 6–7 days, and it is advisable to maintain iso-
lation until there are no more clinical signs and Prognosis
body temperature is normal for at least 5 days. The prognosis for full recovery is good.