VACCINES:

THE FIRST DEFENSE

“Vaccination is still the most practical and economical means of EIV protection,” says Craig. The American Association of Equine Practitioners lists equine influenza as a risk-based vaccine, meaning that veterinarians may recommend it based on your horse’s risk of exposure to the disease.

Generally, horses in the high-risk category frequently travel to shows or other venues where they encounter large numbers of other horses. “Our surveillance program reinforces the knowledge that travel is a significant risk factor for respiratory disease in the horse,” says Craig, “highlighting the fact that those horses who spend time trailering need an optimal vaccination program to stay healthy.”

But even horses who leave home only occasionally have some risk of exposure. When evaluating your horse’s situation your veterinarian will consider several questions, including, “What is the level of training the horse is in? Training can cause stress on the horse and compromise his immune system. How frequently does the horse travel? And where does the horse travel?” says Meg Green, DVM, manager of equine and large animal veterinary services with Merial, Inc.

Currently, three general types of influenza vaccine are available:

• Inactivated (killed virus), given as an intramuscular injection. If a horse has never been vaccinated, he needs a two- or three-dose series of this formula to confer full immunity, the first two at an interval of three to four weeks, followed by an interval of three to six months before the third. After that, the horse will need boosters at six-month or one-year intervals, depending on his degree of risk.

“In the killed vaccine, the whole virus must resemble the live form of the virus enough to stimulate the immune response, but the vaccine cannot cause infection, and the vaccinated horse cannot shed the virus,” says Green. “The drawback is that this type of vaccine stimulates only humoral immunity, which allows for circulating antibodies within the blood, and makes for a slower immune response.”

On the other hand, says Craig, “These vaccines provide good protection for horses with moderate exposure to the disease and are found in many combination vaccine products.”

• Modified live virus (MLV), administered intranasally. These vaccines contain a live virus that has been altered so that it can replicate enough to stimulate an immune response but cannot cause disease. A single dose confers full immunity for at least six months.

“These induce both humoral and cell-mediated immune responses,” says Green. “Cell-mediated immunity allows for a cell that is infected with a virus to be destroyed before the virus can rupture that cell. As a result, it reduces the amount of virus that can infect other cells, and this limits the amount of infection and reduces shedding in the environment.” However, because these vaccines contain whole viruses that have been modified, she adds, “there is a chance that they will revert to virulence, and they may produce illness in immunocompromised animals.” For this reason, some countries do not permit MLV vaccines.

Administering this vaccine intranasally also means that it induces immunity at the same site where the virus enters the body---in the pharynx and nasal passages. “This type of vaccine provides the best immune response