Page 68 - Speedhorse November, 2018

“Vaccination is never 100% effective in protecting horses from influenza, so it’s important to practice biosecurity.”
Clinical Assistant Professor Dr. Katie Wilson, says equine inf luenza is caused by the orthomyxovirus equine inf luenza A type 2. “This is the same family as all
the other influenza viruses, including
the ones that infect humans. Within that family there are many different subtypes,” she says.
Clinical Assistant Professor Dr. Katie Wilson
Influenza In Horses
Infectious Respiratory Disease Prevention & Treatment
by Heather Smith Thomas
Influenza is caused by a virus and is one of the most common infectious diseases of the respira- tory tract in horses. This respiratory disease is
endemic in the equine population of the United States and throughout much of the world. Young horses, ages one to five, are generally the most sus- ceptible. Older horses tend to have some immunity if they have encountered the virus in the past, so they may not become as sick when they are exposed again to this disease.
vaccination or natural exposure may become short- term carriers. If they are recently infected they do not develop clinical signs of the disease themselves, but may shed the virus for a while and transmit it to other horses. The virus is spread via nasal secre- tions with horse to horse contact or coughing when another horse inhales the droplets containing the virus,” says Wilson.
SYMPTOMS
This virus causes upper respiratory disease and the horse will have a high fever. “It may be as high as 106 degrees or higher,” she says. “Most horses with influenza will be lethargic and off feed. They usually develop a clear nasal discharge to begin with, rather than thick snot, and will be coughing.
“The virus destroys the lining of the trachea and its cilia, which is the tiny hair-like appendages that continually move mucus and debris up out of the airway. This leaves a horse vulnerable to secondary bacterial infection on top of the influenza infection. This complication is common, and the horse may develop a thicker white/yellow nasal discharge and may be sicker and for a longer period. The respiratory infection may progress into pneumonia, which can sometimes be life-threatening if it is not recognized and treated early.”
Heather Smith Thomas
“The influenza viruses that horses get are called H3N8 subtypes. H and N refer to the type of glyco- protein on the surface of the virus. There are different classifications of the hemagglutinin and the neur- aminidase found on the surface of influenza viruses. There are 16 different hemagglutinins and nine different neuraminidases. Any combination of these hemagglutinins and neuraminidase makes each indi- vidual virus different,” she explains. In other words,
a human can’t get influenza from a horse. There are different classes, and within each class there can be very different strains.
“Horses get H3N8, but within that sub-type
there are also more categories based on the lineage of the virus. Most of the equine influenza virus in the U.S. is of a North American class. This is also found throughout most of Europe, which also has another lineage of influenza virus that we don’t see in the U.S. that can infect horses. Knowing the class of virus is important so that we can create the appropriate vac- cines for the horses.
“The virus is spread by horses that are actively infected. There is no carrier state. Horses don’t carry it for years like they do with herpes viruses. However, horses that have some immunity to influenza due to
Horses will usually recover faster from influenza if they don’t become dehydrated and undernourished.
66 SPEEDHORSE, November 2018
EQUINE HEALTH