Equine influenza: What veterinarians must learn from the past

As researchers examine data from previous outbreaks, more insight is gained into reducing susceptibility to further epidemics.
Aug 01, 2012

"Those who cannot learn from history are doomed to repeat it." Philosopher George Santayana's famous quote reminds us to learn from the mistakes, failures, victories and successes of those who have come before us.

Fewer people, however, know that Santayana also said, "History is a pack of lies about events that never happened told by people who weren't there." That's the thing about history—some things are remembered, some quotes get repeated and immortalized, while some things are forgotten, and some words are unused and lost. What we eventually learn depends on the differences therein.

Any discussion of equine influenza must eventually become a conversation about its history since past outbreaks and attempts at management have helped teach us much of what we know about this disease.

The culprit

Equine influenza, an orthomyxovirus, is considered one of the most common infectious diseases of the respiratory tract in horses, and it is endemic throughout much of the world, with the notable exceptions of New Zealand and Ireland. It has been recognized in horses for centuries, and the history of the horse (and humankind) contains many notable epizootics, from the crippling outbreak in the United States and Canada in 1872 that paralyzed a horse-powered economy and affected nearly 90 percent of all horses in the country, to a more recent outbreak this past May in France that impacted the Fédération Équestre Internationale (FEI) 2012 Nations Cup show-jumping event.

Ideal conditions for disease spread

Equine influenza is highly contagious and spreads rapidly through groups of horses in aerosolized particles dispersed by coughing and sneezing. The highest risk for infection and most confirmed outbreaks center on situations in which large numbers of potentially stressed animals are in close contact. Horse shows and events combine the significant stressors of transport and competition and are, therefore, the situations most likely to result in infections.

Previously, it had been reported that the maximal distance for aerosolization in the horse (maximum coughing or sneezing distance) was 28 feet, so all horses within this distance from an infected horse potentially were at risk. But newer research from the 2007 influenza outbreak in Australia suggests weather factors may increase the range of potential infection.1

The researchers used information from the nearly 70,000 affected horses on more than 9,000 premises in this outbreak to determine whether specific conditions of humidity, temperature, rainfall and wind velocity could significantly influence the spread of equine influenza.1 They concluded that infection risk was decreased when the relative humidity was above 60 percent, on days when the maximum air temperature was 68 to 77 F (20 to 25 C) and when winds were less than 18.6 mph.

The researchers stated, "The interplay of meteorological factors during an actual influenza outbreak is less clear, and research into the contribution of weather to epidemic spread is scarce.... The relationships described [in our work] are of direct importance for managing disease risk during influenza outbreaks in horses, and more generally, advance our understanding of the transmission of influenza A virus under field conditions."1

Signs of disease

Horses infected with influenza virus have an incubation period of one to five days and may show mild to severe signs that include:

  • Serous to mucopurulent nasal discharge
  • A dry, hacking cough
  • An increased temperature (102 to 105 F [38.8 to 40.5 C])
  • Lethargy
  • Depression
  • Muscle soreness
  • Enlarged regional lymph nodes
  • Reluctance to eat or drink

These signs may last five to 10 days, and most affected horses recover in two to three weeks. Mortality ranges from 1 to 2 percent; since so many horses can potentially be affected in an outbreak, this number can be significant. Even though veterinary medicine was in its infancy and little was known about disease transmission or flu treatment at the time, it is worth noting that in the 1872 epizootic, the city of Philadelphia alone reported the loss of 2,500 horses, and a newspaper report from Farmingdale, N.Y., reported that 10 percent of all the heavy horses used in that city had died.