Equine Influenza (Horse Flu)
Questions & Answers
- What is equine influenza (horse flu)?
- Can horse flu infect people?
- How can horse flu spread among other horses?
- Can equine influenza virus spread between horses and other mammals?
Equine influenza (horse flu) affects the nose, throat, and sometimes the lungs. It is caused by influenza viruses that commonly spread in horses and closely related animals, like donkeys and zebras. The viruses cause flu-like signs and symptoms in those animals, similar to those caused by seasonal flu viruses in people.
“Horse flu” viruses originally spread from birds to horses. Horse flu viruses, or “equine influenza viruses” (abbreviated EIV) have historically been Type A influenza viruses belonging to two subtypes: H7N7 and H3N8.
- The EIV H7N7 subtype was first reported in the 1950s and last reported in the 1970s. It is now considered to be extinct.
- The EIV H3N8 subtype was first reported in horses in the United States in the 1960s and still spreads in horses globally today.
- An H3N8 virus subtype has been reported in dogs, horses, and birds, but these viruses are different in each species.
In experimental settings, horse flu viruses have been shown to be able to infect people, and a few people in contact with infected horses have developed antibodies (a sign of infection) to horse flu viruses, but no people exposed to horse flu viruses have become ill, according to the World Organization for Animal Health. In general, horse flu viruses pose a low threat to people. While horse flu viruses are currently not well adapted to people, it is possible that one day one of these viruses could change in a way that would allow it to infect and spread easily among people. That is why CDC and its partners in animal health closely monitor the changes that occur in circulating flu viruses. This kind of surveillance is particularly important for tracking changes to animal flu viruses that could spread between animals and people.
Avian (bird) flu A(H3N8) viruses continue to spread in birds, but these viruses are different from those that spread among horses. While rare, human infections with avian (bird) flu A(H3N8) viruses have been reported globally in the past and recently. EIV H3N8 spread to dogs in the United States in the late 1990s, adapted to become a canine influenza virus while spreading in dogs. It was last reported in 2016, suggesting that this virus is now extinct in dogs.
Horse flu viruses are highly contagious among horses and are thought to spread mainly through droplets made when the animal coughs or sneezes. Virus can then enter the eyes, nose or mouth or be breathed in by other horses nearby. Horse flu viruses are currently not well adapted to infecting people.
Horses can shed EIV even before they show signs of illness. Horse flu viruses also can spread indirectly through objects that have the virus on them, like clothing, equipment, and brushes. If virus from on an object is inhaled, absorbed, or ingested by a susceptible horse, then the virus could spread. EIV can also be found on surfaces where infected animals are housed or transported.
Practicing good infection control and hygiene measures in herds, including cleaning hands and clothing, is important, as well as limiting access to people who have not been exposed during an outbreak. Horses will generally start to show signs of infection with horse flu within one to three days. Once introduced into an area with a susceptible population of animals, it can spread quickly and can cause large outbreaks among horses. Crowding and transportation are factors that make spread among animals more likely.
Horse flu viruses first spread to horses from birds. Avian influenza A(H3N8) viruses circulating in birds infected horses, and those viruses eventually adapted to horses and started regularly spreading in horses as equine influenza A(H3N8) viruses. These viruses were first isolated from horses in 1963. Infection can occur from exposure to saliva and mucus from infected animals.
In the late 1990s, EIV H3N8 infected and adapted to dogs and started regularly spreading in them as canine influenza A(H3N8) virus. As H3N8 viruses became adapted to each host, the H3N8 viruses in birds, horses and dogs all became different and distinct from one another. While different H3N8 viruses continue to spread in birds and horses, the canine influenza H3N8 virus is now extinct in dogs.
EIV H3N8 spread to dogs and after spreading and adapting to dogs became canine influenza virus H3N8, which is now extinct in dogs. In 2004, cases of an unknown illness in dogs (initially greyhounds) affecting the nose, throat, and sometimes the lungs were reported in the United States. An investigation showed that this illness was caused by equine influenza A(H3N8) viruses. Scientists believe this virus jumped species (from horses to dogs) and adapted to cause illness in and spread among dogs, especially those housed in kennels and shelters. No human infections with CIV H3N8 were reported. CIV H3N8 is now extinct in dogs.
Horses infected with EIV may experience fever (elevated body temperature between 39 and 41ﹾC) and a dry cough. Very sick horses may have wheezing when breathing. Other signs, such as loss of appetite, fatigue (tiredness), and runny nose, may be present. One or more of these signs may not be present in horses vaccinated with an EIV vaccine, which are available in the United States.
Vaccines for horses to protect against horse flu have been available since the 1970s. They include the most commonly spreading horse flu viruses with different vaccines including different vaccine viruses. Those vaccines are widely used to vaccinate domesticated horses. Recommendations for which viruses should be included in vaccines are regularly proposed by an expert panel from the World Organization for Animal Health.