Q:I have a selection of seniors I care for: an ancient pony and two horses and a pony in their early to mid-20s. I seem to recall reading on The Horse.com that at a certain, shall we say, "advanced" age, horses may not need to be vaccinated for equine influenza. As a nurse I wonder if this may be similar to the recent swine flu situation where certain age groups did not need to be vaccinated because it was assumed exposure had occurred. I do know my dogs are not routinely vaccinated anymore, but have titers (concentrations of specific antibodies in a blood serum sample, a test that's generally more expensive) drawn to determine what they need, then they're vaccinated accordingly. My veterinarian has told me my 13-year-old Whippet no longer needs to be vaccinated, except for rabies, of course. Are you able to clarify how this applies to my horses?

Lori, via e-mail

A: Older horses are generally less susceptible to equine influenza infection, but horses that are continuously exposed to influenza can be infected with the virus. These horses are generally show horses that are traveling routinely. Older horses in a closed herd are far less likely to contract the virus. I think it is important to remember that equine influenza vaccines do not provide the same level of protection as do tetanus or the Eastern/Western equine encephalitis vaccines, and that influenza is characterized by high morbidity (causing illness) and low mortality (causing death), especially compared to tetanus and equine encephalitis.

Serologic response (immune response to vaccination) in the equid is variable between individuals. Titer levels can be suggestive of protection, but even horses with low titers have been shown to be protected