Equine Influenza

The pathogen responsible for equine influenza is a type A influenza virus. Two sub-strains can affect horses : H7N7 and H3N8.

Transmission between different species is rare, but transmission between birds/ dogs and horses have sometimes been observed. Transmission from the horse to Man has never been observed.
Flu viruses are modified over time and therefore do not respond to the horse’s immunity. These regular changes necessitate modifications in the composition of the vaccines.

Equine influenza is an extremely contagious disease : in a few hours it can affect a whole yard. Transmission is mainly through direct contact (between horses), the virus is not resistant in the outside environment, but contamination can also be indirect through contact with equipment or people.
Contamination is due to inhaling airborne particles emitted by sick horses when they cough. A sick horse excretes a very high virus count. These airborne particles can travel great distances.

Incubation

The incubation period [1] is between 2 to 5 days. The clinical signs observed are the following :                                                              General symptoms : Temperature (rectal temperature often >40°C) associated with depression and loss of appetite ;                  Respiratory symptoms :

• Heavy dry coughing, either occasional or in bouts ;
• Serous nasal discharge to begin with which can become mucopurulent, especially in the case of a secondary bacterial infection.

In a yard, clinical symptoms will vary from one horse to another and be more or less exacerbated depending on their vaccination status with regard to the disease.

Healing

Horses generally heal in 1 to 2 weeks, but the regeneration of the repiratory tract will take at least 3 weeks, time during which the horses should be kept at rest. Without rest, the respiratory tract is vulnerable to opportunistic bacteria[2] leading to secondary bacterial infections [3].

[1]Incubation period =time between contamination and the onset of the first clinical symptoms of an illness.
[2]Opportunistic bacteria = bacteria which is normally present in the horse’s organism without affecting him, but which can cause an illness due to a lack of defenses or a weakened immunity.
[3]PCR= acronym of « Polymerase Chain Reaction », this test enables detection of the DNA of the pathogen agent thanks to an enzyme amplifying technique.

The clinical symptoms and the quick spread through a yard will orient the vet’s diagnosis. The diagnosis is then established by a search for the virus (PCR*test) carried out from a naso-pharyngeal swab.

There is to date no specific treatment against equine influenza, which is a viral disease. When there is a secondary bacterial infection, antibiotics are administered. Symptomatic treatment can be prescribed to alleviate the clinical signs, in particular to bring the horse’s temperature down (non steroidal anti-inflammatories). Horses should then be allowed at least three weeks rest once the symptoms disappear, so as to allow the respiratory system to recover.

Vaccination

It is strongly recommended to vaccinate all equids, especially since the effectiveness and harmlessness of the available vaccines has been demonstrated . Vacination significantly reduces the clinical symptoms (especially the risk of secondary bacterial infection) and reduces the viral excretions responsible for spreading the illness. In a yard, or at an equestrian event it is important that a majority of horses, including those who do not move in and out of the yard, be innoculated. In fact an 80 % vaccination rate in a yard is considered to prevent an epidemic in a herd.
It is strongly advised to vaccinate foals at the age of 6 months, as this population is the most sensitive to the virus. Before 6 months old, the foal has acquired the mare’s anti-bodies (if she is vaccinated), and this, plus the fact that an immature immune system could potentially reduce the efficiency of the vaccine. Older horses also have an altered immune system and do not respond as well to vaccination, it is therefore recommended to increase the frequency of booster jabs.

Protocol

Typically, the vacination protocol advised by the instructions of the different vaccines available on the market is as follows :
•    3 first time injections : the first two between 3 to 6 weeks apart, the third being 5 to 6 months after the second ;
•    Annual booster injections
This protocol is subject to variations with regard to the rules of competitions and of the racing autorities.
Immunity appears around 2 weeks after the second first-time injection, and lasts between 6 to 12 months.

Booster jabs every six months are advised :
•    For horses frequently taking part in equestrian events with a high concentration of animals, and when the horses are stabled on site...
•    For older horses (>20 years old) whose immune response to the vaccine will lessen.

When is vaccination compulsary ?

For horses taking part in equestrian events;

For stud horses :
•    Stallions used in artificial insemination
•    Stallions and mares of certain breeds, where natural reproduction is used.
Other sanitary preventative measures should be respected to avoid disseminating the disease.

In France, equine influenza is not a regulated illness. Surveillance is carried out by the RESPE thanks to voluntary declarations submitted by horse keepers concerned, via sentinel veterinarians. More information regarding the sub-network « respiratory syndrome »of the RESPE
At the end of 2018and the beginning of 2019, 23 equine influenza clusters were declared to the RESPE up to the 13th of March 2019. Very diverse structures were concerned in sports, racing and leisure sectors. During the same period, equine influenza was widely in circulation in Europe.

Track record of confirmed equine influenza clusters declared to the RESPE between 2014 and 2019 :

number of places where horses are kept with at least one case of flu detected.