Influenza Challenges

Influenza continues to represent a serious infectious disease threat to humans and animals alike.

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Despite the fact that influenza is a well-known disease (Hippocrates  first described a disease resembling influenza in 412 B.C.), the virus continues to cause significant morbidity and mortality in humans and animals alike.

Influenza virus is the most common cause of equine viral respiratory tract disease, making a substantial economic impact on the horse industry annually. Outbreaks occur most frequently when susceptible animals are housed in close contact with one another, as is typical at racetracks, sales barns, and horse shows. Virus spread occurs through direct contact with infected horses, through contact with virus-contaminated droplets created by coughing and sneezing, and through contact with fomites (e.g., virus-contaminated water buckets, clothing, grooming equipment, tack, etc.). Clinical signs of infection in horses include fever, appetite loss, lethargy, nasal discharge (watery at first but typically becoming mucopurulent, meaning it contains pus and mucus), and coughing. In uncomplicated cases clinical signs resolve in approximately seven to 14 days, although coughing might persist longer. Complications can be severe and might include secondary bacterial pneumonia, myositis (muscle inflammation), myocarditis (heart muscle inflammation), and limb edema (fluid swelling).

Vaccinating against influenza is one of the most important ways to protect horses from the disease. Researchers have demonstrated that many of commercially available equine influenza vaccines are highly efficacious, protecting horses from experimental equine influenza challenge. So why has influenza remained such a serious threat to our horses’ health? 

First, influenza viruses are notorious for undergoing a process called antigenic drift—they accumulate small changes over time. Antigenic drift results in new virus strains that the immune system might not recognize. This is one of the main reasons why people and animals can get the flu more than once. It is also the main reason manufacturers must update influenza vaccines periodically with new virus strains. Although equine influenza viruses’ rate of antigenic drift is relatively slow compared to human influenza viruses, it is significant in terms of immunization efficacy. A number of the commercially available equine vaccines have recently undergone an update to provide better protection against currently circulating strains

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Written by:

Gabriele A. Landolt, Dr. med. vet., PhD, Dipl. ACVIM (LAIM), is a board certified Large Animal Internal Medicine specialist. She received her veterinary degree from the University of Zuerich, Switzerland and completed her specialty training at the University of Wisconsin, Madison. She currently holds the position of professor of equine internal medicine at Colorado State University.

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