Colds and flu in Horses

Flu testing at the height of the Australian outbreak
Flu testing at the height of the Australian outbreak in 2007

The most common cause of depression and lack of appetite without signs of colic is equine flu. These horses will have fever (most colics don’t) and may also have runny noses or cough.

These viruses are wherever horses congregate and young horses are especially susceptible. Though rarely fatal, these diseases count for tremendous loss of time and money in the horse business, writes Dr Robert N. Oglesby DVM.


Usually URT (upper respiratory tract) viral infections begin with depression, poor appetite, and a clear nasal discharge. It may look like a mild colic, but there will be no signs of abdominal pain and fever will be present. If you take the horse’s temperature at this early stage it may be as high as 105.5 degrees. Both heart rate and respiratory rate may be elevated in response to the fever.

Though the fever may be high and the horse mildly depressed, he will be easily aroused and will probably still eat treats. The fever is usually short lived though may return in several days for a short period. Several days later the nasal discharge may turn cloudy and a cough begins. Not all horses will have all theses symptoms and the severity will vary from individual to individual. The younger the horse and the less vaccinated for URT infections he is, the more severe the signs.

Influenza and The Herpes Viruses

Most of the URT infections are caused by one of two groups of viruses: the influenza group and the herpes group. The influenza virus tends to cause more cough while the herpes virus causes more of a snotty nose, but there is a lot of overlap in symptoms and the two can be hard to differentiate clinically.

Advanced techniques for rapid identification of influenza infection have become available. These kits were designed for humans but Morley et al tested them on horses with excellent results. Directigen Flu A, Becton-Dickinson Micro Systems, Cockeysville MD. and Directigen Kit, Centaur Inc., Overland Park KS, were designed to detect influenza antigens in nasal secretions of humans. It happens that the antigens are present in equine influenza strains also. These tests are easy, completed in 15 minutes and accurately rule in or out influenza as the cause of the symptoms. The test is very accurate on nasal secretions when used during the first 2 days of remarkable clinical signs. The test is somewhat less accurate on horses that have been ill longer than a few days of exhibiting mild signs. Nasopharyngeal swabs are better for this harder to detect group.

Strangles vs Viral URT infections

It is important to differentiate viral URT diseases from strangles which is a bacterial infection of the URT. Viruses are not treatable and self limiting with good nursing care. Strangles can be treated with penicillin to shorten its course and lessen the severity.

In general, horses with strangles will have:

  1. Come from an area with a recent history of the disease. Strangles outbreaks do not go unnoticed and your horse will not be the first.
  2. The fever is a little lower with strangles, in the 102 to 104 range, but more persistent. Bute has less of an effect on bringing down this fever.
  3. The lymph nodes under the jaw begin to enlarge about 4 days after the horse becomes ill and eventually bust open. Lymph nodes also enlarge with the viral URT but they do not continue to get bigger and bigger for several days in a row.


Once the virus gets hold of the horse there are no specific cures for the virus. Two important treatments are fever reducers and rest. For the fever butazolidine (bute) is very effective. It will also relieve the aches that go along with the flu making the horse feel better so he will eat and drink.

Even more important than bute is adequate rest. These viruses attack the respiratory lining reducing a horse’s resistance to infection by bacteria. The influenza virus also attacks the muscles and the heart. Rest allows the horse to heal with minimal complications. Usually 21 days of stall and paddock rest is adequate but if symptoms persist, particularly the cough, longer may be required.

Recently immune potentiating drugs have come on the market with some experimental work that they shorten the course of the disease. Recent concerns about their safety have been brought up. I will post new information here as it comes in.


There are a number of vaccines available for immunization against viral URT disease. They give moderate protection and are short lived. To get the full benefits requires an initial series of two shots about 30 days apart followed by boosters every 3 to 4 months.

In barns with a lot of young horses that travel, boosters every 2 months has been used effectively. For the horse that does not show or come into contact with a lot of other horses an initial series of 2 shots and yearly boosters will probably not prevent disease but will lessen the severity if he happens to come in contact with a virus. Young horses going to the trainers are particularly susceptible and boostering the vaccine 21 days before transport is well worth the cost.

Recent work suggests that the immune stimulants, containing a bacterial extract (Propionibacterium acnes) may shorten the course of viral respiratory diseases and when administered 2 and 5 days prior to shipping may lessen the incidence of respiratory disease in transported horses. Also from a prevention standpoint, transporting a horse facing backwards creates less stress, the most likely reason for the high incidence of respiratory disease following shipping.

Specific Recommendations when vaccinating:

  • Foals: Vaccinate at 6, 7, 12, 18 months of age.
  • Pleasure Horses: Annually and prior to exposure to groups of unknown horses.
  • Show and Performance Horses: Every 2 to 3 months.
  • Broodmares: Annually one month prior to foaling date and one month prior to shipping for breeding.


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This article reprinted with permission from, an internet information resource for the equestrian and horse industry since 1994. On the WWW at we have tens of thousands of documents on the web about horse care, diseases, and training.

First published on Horsetalk in 2005

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