Editor's Note: This is from Understanding Equine Preventive Medicine by author and veterinarian Bradford G. Bentz, VMD. The book is available from www.ExclusivelyEquine.com.

Equine influenza is one of the most frequently encountered infectious diseases of horses. It affects the upper respiratory tract, producing illness associated with fever and coughing. There are numerous strains, and the occurrence of outbreaks may be associated with waning immunity of the equine population and emergence of a new strain. The risk of infection increases with horses that live in high-density and high-stress situations such as racetracks, training facilities, boarding stables, breeding farms, and show grounds. Equine influenza is introduced into a group of horses by a symptomatic or asymptomatic horse that is shedding the virus. The virus’ highly contagious nature is facilitated through the spread of the pathogen by coughing or exposure to such objects as contaminated feed buckets or feeding equipment, grooming implements, or tack. The equine immune response is capable, however, of rapidly eliminating the virus. Infection can, therefore, be controlled or prevented from entering a horse population by strict quarantine of new horses for 14 days and by appropriate vaccination. Horses that are infected with an influenza virus shed the organism in the nasal secretions for up to 10 days.

Influenza is highly transmissible and is, therefore, important to consider in the vaccination program of any horse that regularly encounters new horses, is undergoing stressful situations in shipping and/or competitions, or is likely to encounter high-risk situations for disease transmission.

Today there are various manufacturers of equine influenza vaccines. Furthermore, both intramuscular (killed vaccines) and intranasal (modified live) vaccines are now available. The use of killed vaccines in the horse is associated with increases in circulating antibody in the vaccinated animal, but repeated vaccination with certain licensed vaccines of this type has failed to provide protection during outbreaks or reduce the severity of the disease.  This may be related to the fact that these killed-virus vaccines cannot produce a local immune response at the level of infection (the respiratory system mucosa). Furthermore, the circulating antibody produced by the killed vaccines does not appear to be at high or persistent levels. However, these levels appear to be higher than those produced by the modified live vaccine (intranasal), and the killed vaccines are recommended for pregnant mares to facilitate the production of circulating antibody that is subsequently concentrated in the colostrum.

The levels of circulating antibody in the blood stream--even when there is a lot of it--do not necessarily mean the animal will be protected from the disease, suggesting that local responses in the upper respiratory tract may play an important role in the protection produced by this vaccine.  Therefore, it seems that a major factor in the production of immunity by the intranasal vaccine is the production of the "local" or mucosal immune response--actually to prevent infection by not allowing the virus to enter the body.  The circulating immunity produced cannot function unless the virus is already in the body, and then it is often not effective in preventing clinical signs.  One currently licensed, modified live vaccine (FluAvert I. N., Heska Corp, Fort Collins, Colo.) has received significant support according to the results of published data.  Field experience indicates that this vaccine is effective at preventing influenza.

Vaccination of foals with the killed vaccines in the presence of maternal antibody (derived from the colostrum) appears to induce a type of "immune-tolerance" during which foals and yearlings fail to respond to multiple doses administered over several months. For this reason it is important to initiate vaccination of foals after maternal antibody inhibition may be produced. Programs using the killed-virus vaccines (intramuscular) should therefore begin at nine months of age if the foals are isolated from new horses or horses that develop illness. This initial series should include three doses of vaccine administered at four- to six-week intervals. Foals that are born to unvaccinated mares may receive a primary vaccination series of three doses beginning at six months of age and at intervals of four to six weeks. The modified live-virus vaccine (intranasal) may be incorporated into the vaccination program instead of the killed (intramuscular) vaccine.

The modified live vaccine is recommended for administration to foals at 11 months of age. However, because it is unclear whether the maternal antibody interferes with the immune response to this vaccine, if vaccination of the foal with the modified live vaccine is performed before 11 months of age, it should be followed by a second dose when the foal is 11 months of age or older. Booster vaccination is recommended at six-month intervals and is not required before this as foals 11 months of age or older and previously unvaccinated adults appear to be protected after a single administration of the vaccine. Intranasal vaccination is particularly important when horses regularly commingle, in areas of frequent introduction of new animals, and for horses engaged in performance and showing. For programs that use the killed-virus vaccine (intramuscular), it is necessary to repeat vaccination in these horses at three- to four-month intervals in order to provide adequate circulating antibody levels in situations of high risk of exposure.

Because influenza outbreaks typically last three to four weeks and horses previously vaccinated or exposed to influenza are capable of mounting a rapid immune response, revaccination of healthy horses in the face of an outbreak is indeed beneficial in controlling the spread of disease. However, other control measures are also important, such as isolation of affected animals and appropriate management practices as previously described. The modified live vaccine is reported to be safe and highly effective in an outbreak, and may provide more rapid and complete immunity than the conventional killed vaccines.

About the Author

Bradford G. Bentz, VMD, MS, Dipl. ACVIM, ABVP (equine)

Brad Bentz, VMD, Dipl. ACVIM, ABVP, ACVECC, owns Bluegrass Equine Performance and Internal Medicine in Lexington, Ky., where he specializes in advanced internal medicine and critical care focused on helping equine patients recuperate at home. He’s authored numerous books, articles, and papers about horse health and currently serves as commission veterinarian for the Kentucky State Racing Commission.

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