Because it can negatively impact a racehorse’s career, equine influenza is best prevented.
F
ound worldwide, equine influenza is frequently seen in horses transported multiple times a year—as racing
season prepares to kick off for many horses, it pays to be vigilant for this disease. Influenza is the most common cause of upper respiratory infections in horses. It has a sudden onset and a short incubation period of one to three days, and disease outbreaks usually occur in horses age 1 to 3. This time generally corresponds with inter- mingling of horses at show grounds, racetracks and sales. Influenza affects older horses as well but clinical signs are usually milder and might not even be apparent. Although infections can occur at any time of the year, they seem to be more frequent in the winter and spring, when lower humidity and temperatures tend to enhance viral survival. The initial clinical sign that is most evident is the sud- den onset of high fever and lethargy.Rectal temperatures can climb as high as 106 degrees. Cough and nasal dis- charge accompany the fever but might not appear for sev- eral days following the initial fever spike. The cough may persist for up to three weeks. Endoscopic examination of the upper airway reveals pharyngitis and tracheitis. Also, the development of myalgia is diagnosed with the reluc- tance of many horses to move in the course of the disease. Infection in a susceptible horse begins with inhalation of the virus. The virus attaches to and proliferates in the epithelial cells of the entire respiratory tract. Studies have shown that tracheal mucociliary clearance rates are reduced in horses infected with equine influenza for up to 32 days, despite clinical improvement. This damage apparently impairs the transport of mucus in affected animals from the lower airway and predisposes them to secondary bacterial infection. A horse infected with the virus might be unfit for competition from 50 to 100 days, which could seriously impact a racehorse’s earning opportunities.
When a horse develops an acute fever and has a nasal discharge and cough, consider the possibility of equine influenza, equine herpes virus 1 and 4, equine viral arteri- tis and Streptococcus equi (Strangles) infection. A commer- cially available test detects the flu virus via a nasal swab, and results are available in as little as 15 minutes. The influenza virus belongs to the orthomyxovirus
group, and it is easily destroyed by common disinfectants like chlorine bleaches. Prevention of equine influenza is through vaccination. It has been suggested that upwards of 70 percent of the
equine population should be vaccinated to prevent epi- demics. If the vaccinated animal does become infected while its immunity is waning, the partial protection might not prevent infection but it will lessen the severity of the clinical signs. On the other hand, some studies indicate vaccination does not decrease the intensity or duration of the disease or enhance subsequent immune responses to future infections. The vaccines currently available in the United States contain bivalent or trivalent chemically inactivated influenza viruses. Manufacturers recommend an initial vaccination followed by a booster in three to four weeks; the animal is then revaccinated annually. Unfor- tunately, young horses need more frequent bolstering to maintain adequate protec- tion, so it becomes beneficial to revaccinate every four to six months.
Antibodies against equine influenza can be found in the colostrum of foaling mares. Vaccination trials have been conducted on foals born to mares vaccinated approxi- mately 30 days prior to foal-
By Tom Hutchins D.V.M., DABVP
ing; they concluded maternal antibodies to equine influ- enza interfered with the foal’s response to the vaccine up to 8 months of age. Foals from mares that had not been vaccinated against equine influenza were able to mount an immune response as young as 2 to 3 months. This proves young foals can respond to inactivated vaccines, and maternal antibodies do interfere with the foal’s response to influenza vaccine. This is not to say that vaccination of the pre-partum
mare isn’t recommended, but individual situations should be considered. Along with the appropriate time to vac- cinate the foal based on pre-partum preventative vaccines given to the mare. Consult with your local veterinarian for recommendations and guidance on vaccination protocols for your particular situation.
Tom Hutchins is a special contributor for Paint Horse Racing. To comment on this column, email feedback@apha.com.