Equine Influenza

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Influenza in Horses Influenza is a highly contagious infection of the upper respiratory tract of horses

Overview Streptococcus equi infection (strangles), Equine influenza is a highly contagious equine viral arteritis, bacterial pneumonia, upper respiratory tract infection caused by chronic obstructive pulmonary disease, strains of the influenza virus type A. The pharyngitis (inflammation of the throat), common type of influenza A virus that cur- and a multitude of other less common rently circulates in horse populations is conditions.1 the subtype A2 (H3N8). An earlier subtype, Influenza is most commonly diagnosed A1 (H7N7) is now believed to be extinct in in unvaccinated horses and in young hors- horses and is no longer recommended for es between one and five years of age, espe- equine influenza vaccines.1 cially those that frequent areas with large Like other respiratory diseases of horses, groups of transient horses (such as race- influenza is a source of major economic tracks and show grounds).1,3 loss to the equine industry due to lost train- Older horses can also become infected ing days and veterinary costs.2 Further- with influenza, but they generally experi- more, influenza A infections often occur ence a milder disease than their younger as outbreaks that rapidly spread through counterparts. susceptible horse populations (e.g., at high- traffic facilities such as racetracks). Diagnosis Horses become infected by inhaling the A diagnosis of influenza A used to be pre- influenza A virus that is shed by infected, sumed based on history, clinical presenta- c kenz i e coughing horses or via equipment such as tion, and by ruling out other causes of fever, am ma feed buckets, tack, and grooming aids that p cough, and nasal discharge (i.e., strangles, were contaminated by an infected horse.1 One of the classic clinical signs associated with bacterial pneumonia). Now experts rec- equine influenza is a serous (clear, runny) nasal The virus then replicates in the lining (epi- ommend using the available technology discharge. thelial cells) of the upper respiratory tract. to identify the exact cause of the infection. These infected cells die, and there is in- infection can develop, resulting in pneumo- Accurately diagnosing the cause of respira- creased production of a watery discharge nia. The nasal discharge in these horses usu- tory disease is an important step in imple- as well as swelling and inflammation of the ally changes from serous to mucoid (green, menting proper management practices and respiratory tract. yellow, and thick), signifying a secondary controlling disease spread.4 An accurate di- bacterial pneumonia on the heels of the in- agnosis also is important for the industry to Clinical Signs of Influenza fluenza, which can be fatal if untreated. determine efficacy of the existing vaccines. The virus has a very short incubation It is important to remember that the se- Virus isolation, serology (determination period of only one to three days, and the verity of clinical signs is highly variable de- of acute and convalescent antibody titers), clinical signs of influenza are obvious three pending on the immune status of the horse. and stall side immunoassay kits that detect to five days after initial exposure to the vi- Sometimes horses can be infected and con- the influenza A virus can be used to diag- rus. Classic clinical signs associated with tagious without showing any clinical signs. nose affected horses. At the 2010 Ameri- equine influenza include a sudden onset of infection. These horses, in particular, can Association of Equine Practitioners’ of a high fever (up to 106°F), coughing, a risk circulating the virus throughout the (AAEP) Annual Convention, researchers serous (clear, runny) nasal discharge, and horse population. described the use of a polymerase chain sometimes mild swelling of the subman- Since the clinical signs of influenza reaction (PCR) test to diagnose equine in- dibular (under the jaw) lymph nodes. are nonspecific, distinguishing influenza fluenza (as well as equine herpesviruses-1 Rarely, veterinarians might note edema from other causes of upper respiratory and -4 and Streptococcus equi subsp equi). (swelling) of the distal limbs and trunk in tract infections can be challenging based According to the study authors, the PCR horses with influenza, which is referred to on presentation alone. Other diseases that test quickly (i.e., within 24 hours) and ac- as epizootic cellulitis.1,3 influenza could be mistaken for include curately diagnosed respiratory infections In some horses a secondary bacterial equine herpesvirus-1 and -4, rhinovirus, in 761 horses.4

This Fact Sheet may be reprinted and distributed in this exact form for educational purposes only in print or electronically. It may not be used for commercial purposes in print or electronically or republished on a Web site, forum, or blog. For more horse health information on this and other topics visit TheHorse.com. Published by The Horse: Your Guide To Equine Health Care, © Copyright 2011 Blood-Horse Publications. Contact [email protected]. Fact Sheet

Treatment completely recover and return to exercise in circulation. To accomplish this, surveil- Equine influenza infections are generally within three to six weeks of infection.1,3 In lance and laboratory diagnosis of equine self-limiting, and no specific treatment ex- more severe cases horses might require up influenza cases is essential. ists. Owners should isolate affected horses to 100 days of rest. Management also is important in dis- from healthy horses and institute general Horses that develop secondary bacterial sease prevention. Quarantine new horses supportive care. This includes encouraging infections require longer recovery periods for 14 days prior to mixing them with resi- the horse to eat and drink and administer- and have a more conservative prognosis dent horses to minimize the chance of in- ing non-steroidal anti-inflammatory drugs for return to previous athletic function due troducing the influenza virus to your herd. as prescribed by a veterinarian to control to damage to the lung tissues. Don’t share equipment or supplies between high fevers. Resting affected horses (typi- horses, especially if one spikes a fever, has cally one week for every day of fever) is im- Prevention nasal discharge, or is coughing. h perative. Do not resume training until the Vaccination plays an important role in horse’s coughing has subsided completely. prevention of influenza A infections. The References If the fever persists for three or more AAEP recommends vaccinating all at-risk 1. Ainsworth DM, Cheetham J. Disorders of the Respiratory System. In: Reed SM, Bayly WM, Sellon DC. Equine internal days and the nasal discharge becomes mu- horses.5 Vaccination schedules vary de- medicine, third edition. Saunders, USA. PP. 311-313. copurulent (containing pus and mucus), pending on current vaccine status, age, 2. West C. Equine Influenza Vaccine Protocols: Boosters then the horse should be reexamined for broodmare status, and potential exposure. are Best. www.TheHorse.com/15837 3. Merck Veterinary Manual. www.merckvetmanual.com/ development of a bacterial pneumonia. For example, the AAEP recommends mvm/index.jsp?cfile=htm/bc/121303.htm&word=equine Horses with a suspected pneumonia are administering a series of three boosters %2cinfluenza%2cvirus treated aggressively with antibiotics. Anti- to adult, unvaccinated horses followed 4. Pusterla N, Kass PH, Mapes S, et al. Voluntary surveil- lance program for important equine infectious respira- biotic selection is generally based on the by semi-annual vaccination. Discuss tory pathogens in the United States. AAEP Proceedings culture and sensitivity results performed vaccination protocols with your veterinar- 2010;56:423. on a tracheal aspiration (a fluid sample ian to develop a custom risk-based vaccine 5. AAEP Vaccination Guidelines. www.aaep.org/ vaccination_guidelines.htm collected from the trachea using a long, schedule most suitable for your horse. thin tube and saline). Like all influenza viruses, equine influ- Further reading and free health e- enza viruses mutate (change) over time, newsletter: www.TheHorse.com/influenza. Prognosis and the vaccines must periodically be up- Authored by Stacey Oke, DVM, MSc; For uncomplicated cases, horses should dated to keep up with the changing viruses reviewed by Tom Chambers, PhD

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