Influenza virus infection, as in people, is much more serious than other respiratory viruses. High fevers are the rule, along with severe coughing and heavy nasal discharge. Other body tissues may be invaded by the virus (muscles, joints, heart) and secondary bacterial infections in the lung on top of the viral pneumonia caused by influenza are common.
Horses are typically ill for longer than with other respiratory viruses and usually have a poor appetite. Time to complete recovery is often as long as two to four weeks, and horses should be stall rested for at least two weeks.
A study performed by the Japan Racing Association, published in the Journal of Veterinary Medical Science, looked at the effect of the human antiflu drug, oseltamivir phosphate (brand name Tamiflu, from Roche) in horses. Horses were divided into three groups of three, a control, preventative and treatment group.
The drug was given twice a day for five days starting either one day before an experimental infection, or at the onset of fever after experimental infection. The study found that horses treated after the appearance of fever shed the virus in their nasal secretions for only 2.3 days compared to six days for the untreated horses. Fever lasted an average of two days in treated horses, compared to eight days in untreated horses.
When the drug was used to try to prevent infection, the horses still became ill but shed the virus for five days rather than six, with fever lasting an average of 4.7 days compared to eight in untreated horses. This study also found significantly lower counts of the bacterium Streptococcus zooepidemicus, the most common secondary bacterial lung invader, in horses treated with this drug either before or after exposure to the virus.
The number of horses involved in this study was small, and a large study to confirm the findings is needed, but results suggest that while Tamiflu can’t prevent influenza infection, starting the treatment at the first sign of illness can have a significant impact on the amount of the virus that is shed and the duration of fever. This is consistent with how the drug works.
Tamiflu is an inhibitor of viral neuraminidase enzymes. After the flu virus invades the cell and multiplies, it uses these enzymes to ”explode” the infected cell so that the next generation of virus is released into the body. With the neuraminidase enzymes inhibited, the viruses are imprisoned inside the infected cells, which the horse’s immune system will then destroy.
Economics may influence how common this treatment becomes. At the dosage used in this study, an 1,100-lb. (500 kg) horse would require 2,000 mg/day. Even at the lowest generic prices we could find, this would cost about $150/day. However, the savings related to less down time in training and lower risk of secondary bacterial pneumonia may make it worth it in some cases.