Equine influenza in its various forms can negatively impact the health of a horse and the pocketbook of its ownerAs sale time approaches and hopes of buying that Derby winner are on the rise, one is always leery of bringing his prized purchase home only to have it start coughing, spike a fever, develop a nasal discharge, and begin breathing heavily. These are the first signs of influenza, a potentially devastating disease in yearlings and other young horses. The stresses of shipping, training, and the commingling of these youngsters is generally all it takes to precipitate this highly contagious disease.
This virus primarily affects the respiratory tract. The incubation period (1-to-5 days) is short, and this disease can have a very sudden onset with a dramatic rise in body temperature to as high as 106 degrees. At the outset there is usually a dry, non-productive cough which can last up to ten days. (Influenza has been referred to as barn cough by many older horsemen.) Most uncomplicated cases will last up to two weeks with horses showing varying signs of depression, anorexia, mild bronchitis, a generalized muscle soreness, and a reluctance to move. Recovery periods vary from horse to horse.

Complications
The more complicated cases of influenza are associated with secondary bacterial infections, which can result in a mucopurulent type nasal discharge, abscessation of the regional lymph nodes, and bronchopneumonia. These are further complicated by infection of the cilia lining of the lung, thus altering the horses lung function and its ability to fight and clear infections. It is critical for these horses to be given complete rest (three months minimum) to allow for proper healing of the lung and its protective lining.
Severe cases also may result in the infection spreading throughout the body, damaging various muscles, including the heart. If these myocardial infections do indeed occur, a rest period of up to six months is essential for complete recovery. In rare cases, this infection can also spread to the brain, causing encephalitis. A premature return to training before these animals have had time for complete healing of the respiratory tract and other organs has been associated with future problemsincluding recurring respiratory infections, chronic obstructive pulmonary disease, and overall poor performance.
In young foals, influenza is usually quite severe. Infections mimic that of acute viral pneumonia and can lead to death within 24-to-48 hours. Many of these young animalsif they do survive the original viral infection are at much greater risk of developing a secondary bacterial infection.

Understanding influenza
Influenza is an orthomyxovirus with three distinct types known as A, B, and C. Horses, humans, and pigs are the only mammalian species naturally susceptible to infection by this virus. (Horses are infected only with type A.) Studies have shown that other species possess a specific gene that confers protection against this influenza virus.
To gain more understanding of influenza, one must first know how these virus strains were named
i.e., A/Equi-1/Prague/1/56 or A/Equi-2/Miami/1/63. A represents the seriological type of influenza virus. Equi designates the species from which the virus was isolated. The number (1 or 2) denotes the subtype of virus. Prague or Miami indicates where the original virus was located. 1 is the laboratory isolation number, and the last number indicates the year in which the virus was isolated.
Thus, one should be able to define the following isolated influenza viruses:
A/Equi-1/Prague/1/56; A/Equi-2/Miami/1/63; A/Equi-2/Lexington/1/63; A/Equi-2/Kentucky/1/81; and A/Equi-2/Alaska/1/91.
The first equine influenza virus ever isolated occurred in Prague, Czechoslovakia, in 1956. A virus resembling this strain was not isolated again until the spring of 1963. This isolation occurred in Lexington, Kentucky, where young Standardbreds were training at The Red Mile.
Nineteen sixty-three was a banner year for equine influenza. Not only was this the first year that the A/Equi-1 virus was isolated in the United States, but it also heralded the appearance of a new subtype virus called A/Equi-2. This new subtype was isolated in horses stabled at Hialeah racetrack in Miami, Florida. The Hialeah virus was soon transferred to other tracks and ultimately caused disease in horses of all ages throughout the world.
Knowledge of this outbreak aided researchers at the University of Kentucky, and they were able to quickly isolate the virus when it struck at the 1963 Keeneland sales. Upon isolation, it was incorporated with the original Prague 56 strain as a prototype strain to be used as a vaccine against this devastating disease.
Further work with these viral strains has revealed that a variant strain of the A2 virus also exists within the horse population. Thus, we now know of three different equine influenza-type viruses. The original A-1 Prague/56 virus that continues to circulate, causing very little actual disease, appears to be important in stimulating protective immunity. We also have the prototypic A-2 Miami/63 or A-2 Lexington/63 strains and the variant A-2 Kentucky/81 strain, which are currently causing most of the disease problems in the horse.
The relatively short incubation period of this disease (1-to-5 days) and the associated cough allows for large quantities of virus to be released in aerosol fashion to other horses in close proximity. Horses can also remain infectious for up to six days after the last clinical signs of disease. Thus, influenza is easily spread from stable to stable, and with the increase in international air travel, it can spread from country to country as well.

Mutating viruses
Selection pressures of the influenza virus within the horse population appears to be less than that of our human population. Within humans, the influenza virus tends to change yearly and vaccination of the majority of our susceptible population tends to limit the persistence of these strains.
The immune selection pressures of the horse appear to be not as efficient as that of man. Horses are more scattered, and a smaller percentage of the susceptible population are routinely vaccinated than that of the human population, so equine influenza viral strains are allowed to coexist. These strains can then interact, and genetic recombinations of these strains can occur, resulting in viral shifts and drifts of this disease. Thus, when outbreaks of equine influenza occur, veterinarians, researchers, and epidemiologists are prepared to isolate these viral strains and identify these newer pathogens as they become evident.
Diagnosis can be made by observing clinical signs and doing the necessary blood worknasopharyngeal swabs of the upper respiratory tract are chilled (not frozen) and sent to the appropriate lab for isolation and identification. It is important to differentiate between equine influenza and equine rhinopneumonitis. Serology is not only useful in obtaining titers for vaccination, but also titers obtained at different intervals can be diagnostic of acute disease.

Risk factors
The control of equine influenza is based upon a regular vaccination program and the proper management of clinically ill horses. Vaccination programs will vary according to the risk of disease. Horses of low exposure probability and stress conditions, such as individually owned horses in the country, are less likely to develop influenza. However, horses at racetracks, training facilities, and boarding stables are at high risk. Also, horses shipped from state to state and those involved in international air travel are at an even higher risk of disease. Under these conditions, the duration of immunity can be very short, and booster vaccinations are recommended every
2-to-3 months.
Horses, when vaccinated for influenza, develop antibodies for protection. However, these are rapidly metabolized, and within 100 days, vaccinated animals may have antibody levels that are no longer protective. This makes frequent booster immunization critical for those animals that are at a higher risk of developing influenza. The ability of the influenza virus to mutate further complicates this disease entity and may make even vaccinated horses susceptible to disease. Efforts are being made to update the current equine influenza vaccines at frequent intervals, thus providing more complete protection.

Summary
In summary, equine influenza can be a devastating disease in young horses. The prolonged recovery period (up to six months) in complicated cases can incur substantial financial losses for the owner. However, this recovery period is essential if the affected animal is expected to regain his health and reach his maximum potential.
To control equine influenza, a regular vaccination schedule must be combined with good stable management and the isolation of clinically ill horses from other animals. A vaccination program involves two immunizations at 3-to-4-week intervals, followed by an annual booster. Horses at higher risk should be boostered at more frequent intervals of every 2-to-3 months. To be effective, vaccines should contain the most current strains of the influenza viruses that are causing disease among the horse population.