In the United States, horse owners can vaccinate against more than a dozen equine diseases, including Eastern, Venezuelan, and Western equine encephalomyelitis (EEE, VEE, and WEE), West Nile encephalomyelitis, influenza (flu), two strains of equine herpes (EHV1 and EHV4), rabies, anthrax, botulism, strangles, tetanus, Potomac horse fever, rotavirus diarrhea, and equine protozoal myelitis. Overall, the best course is to review your horse's vaccination program with your veterinarian each year and determine what he needs based on his individual risks.

YOU VACCINATE YOUR horse to protect him against disease. But are you overvaccinating him?

Many horse owners have begun to ask that question-and to wonder whether they're doing damage while trying to do good. What's behind this worry, and how can you ensure that your horse s vaccination program is right for him? I'll try to answer those questions here.

Helpful or Harmful?

There's no question that vaccination can prevent, or lessen the severity of, many diseases. Vaccines work by priming the immune system against specific viruses, bacteria, and other disease-causing organisms. Some vaccines are made from killed or inactivated organisms (or products derived from them); others contain living but weakened versions of the organisms, potent enough to trigger an immune response without causing disease. The horse's body recognizes vaccine agents (killed or live) as foreign antigens and targets them.

This targeting response is complex. As part of it, white blood cells capable of recognizing the specific antigen multiply rapidly. Some, called "B cells," produce antibodies that circulate in blood and neutralize that antigen. Others, the "T cells," destroy infected body cells and help orchestrate the response. If the real disease causing agent shows up later, the horse can mount a faster and more powerful defense because white blood cells and antibodies targeted to that agent are ready and waiting for it.

Vaccination has saved the lives of countless horses over the years. But today some owners are asking if their horses are getting too many shots-and whether their horses need vaccinations at all. Their concerns are fueled in part by:

* New vaccination guidelines for small animals. As a result of these guidelines, many dogs and cats that previously got every shot every year now get some of those shots less often. It's logical to wonder if horses should be vaccinated less often, too.

* Safety concerns. Some people have been frightened by reports linking vac cinations to tumors in cats. Worries about the safety of human vaccines have also spilled over into veterinary medicine. For instance, in the late 1990s, a study suggested a link between vaccines containing mercury preservatives and autism in children. Larger studies failed to confirm the link, but childhood vaccines were changed to remove the preservatives.

* Alternative medicine. Some homeopaths and other members of the alternative-medicine community think that vaccines are inherently harmful. They believe that vaccinations produce a condition called vaccinosis, in which animals become unthrifty and exhibit a sort of chronic malaise because their systems are overwhelmed.

These concerns don't have much scientific grounding. So far, mainstream medical research hasn't found evidence that vaccinosis is a problem. There have been no reports of vaccination-related tumors in horses, or of harm caused by vaccines containing mercury-although it may make sense, where we have equal choices, to think about using a vaccine that doesn't have mercury. And pets and horses (and people!) need different vaccination schedules because they have different sensitivities and different risks of exposure to various disease-causing agents. Horses are far more likely to be exposed to tetanus, for example, and far more sensitive to the toxins produced by tetanus bacteria.

These facts don't mean that over-vaccination is not a danger, however. There are good reasons to avoid unwarranted vaccinations.

Vaccination Has Risks

Any vaccine-in fact, any injection-can have adverse effects. In rare cases, vaccines have caused severe allergic reactions, including anaphylaxis and death. Intramuscular (IM) injections can trigger a rare but potentially fatal infection with tissue-destroying Clostridium bacteria.

Mild reactions, such as fever and local muscle soreness from injections, are more common than these catastrophes-and easier to deal with. Light exercise may help minimize soreness after IM vaccination. Spacing vaccinations a few weeks apart, administering antihistamines or non-steroidal anti-inflammatory drugs, and changing to an alternate vaccine may help control specific reactions.

Certain vaccines carry more risk than others. Both injected and intranasal strangles vaccines can create problems, including local abscesses. In horses whose blood has high levels of antibodies against strangles, vaccination occasionally provokes a serious condition called purpura hemorrhagica. (This condition seems to appear when immune complexes-clusters of antibodies and antigens-damage blood and lymph vessels, producing anything from mild limb swelling to severe or fatal disease.)

In the United States, horse owners can vaccinate against more than a dozen equine diseases, including Eastern, Venezuelan, and Western equine encephalomyelitis (EEE, VEE, and WEE), West Nile encephalomyelitis, influenza (flu), two strains of equine herpes (EHV1 and EHV4), rabies, anthrax, botulism, strangles, tetanus, Potomac horse fever, rotavirus diarrhea, and equine protozoal myelitis. But as a veterinarian, I feel 1 should have a reason to recommend a vaccine and not give it just because it's available. In view of the risks-and costs-vaccination programs should be tailored to the individual horse and situation.

Which Vaccinations?

Some veterinarians designate certain vaccines as "core," and others as optional, based on risks of infection. Tetanus, EEE, WEE, and West Nile vaccines would be considered "core" for practically all horses in the US. Tetanus bacteria are everywhere, and mosquitoes spread EEE, WEE, and West Nile. In many regions, rabies would also be considered core.

If your horse is in a closed herd-that is, he doesn't go anywhere, and new horses don't come onto the property-these shots may be all he needs. In most cases, though, there are other risk factors to consider.

* Lifestyle: A horse that goes to shows or other venues where he'll encounter horses from other barns needs protection against diseases that spread from horse to horse, including flu, EHV, and perhaps strangles. Even stay-at-homes need protection if other horses come to the farm: Not only may these newcomers bring infection; they may carry these diseases, shedding viruses and infecting others, without showing symptoms.

* Regional risks: A horse should be vaccinated against diseases he's likely to encounter where he lives-or in any region he may be shipped to. For example, VEE hasn't been reported in the US since the 1970s; you don't need to vaccinate against it unless you're taking your horse south of the border. Botulism is a concern in the mid-Atlantic states and parts of the West, but not in most other parts of the country. Strangles is more widespread-it's huge in the upper Midwest-but it's not a problem everywhere. Because strangles vaccines can create problems, my feeling is that horses shouldn't automatically be vaccinated against this disease if they're not likely to be exposed, but this can be a tough call.

* Age: It would be nice to think that, after a lifetime of vaccination, an aged horse would have built up some level of immunity and so would not need many vaccinations. However, many older horses develop Cushing's disease, which suppresses the immune system. They need their shots.

* Farm history: A farm history of a specific disease, such as Potomac horse fever or rotavirus, may prompt a decision to vaccinate on just that farm.

When and How Often?

In most cases, vaccine-produced immunity fades over time, so the horse needs boosters. A good program times revaccination so that immunity will be strong when risk of exposure is high. Because your horse's body takes several weeks to respond to vaccines, you need to plan ahead.

* Boosters for some diseases, such as tetanus and rabies, can be given annually at any time of the year.

* Boosters for mosquito-borne diseases are usually given annually in spring, allowing the horse time to build immunity before mosquito populations peak in summer. In areas where mosquitoes are active year-round and the risk is especially high, horses may need to be vaccinated two (or, against West Nile, three) times a year.

* For flu and EHV, diseases that spread horse-to-horse, appropriate frequency depends largely on contact with other horses. Stayat-homes may do fine with an annual or semi annual booster, but performance horses typically get boosters three or four times a year.

* Revaccinating mares four to six weeks before foaling helps ensure high levels of antibodies in their colostrum (first milk), helping to protect their foals in the first months of life. Foals born to unvaccinated mares may need to start some vaccines earlier than they otherwise would.

Overall, the best course is to review your horse's vaccination program with your veterinarian each year and determine what he needs based on his individual risks. Although we don't have all the facts, many veterinarians and owners agree that the advantages of a regular vaccination schedule, offering the strongest protection available against significant diseases, outweigh the potential adverse effects of frequent vaccinations.

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Unanswered Questions

It's hard to know for sure whether horses really need boosters as frequently as authorities recommend-although most veterinarians would say they do-because we really don't have good measures of immunity in horses.

In theory, tests measuring antibody levels (titers) in blood could help answer the question: A very high liter in a healthy animal could suggest that a booster isn't needed.

Some small-animal vets are using titers as a guide to timing boosters. But we don't know what antibody levels are needed for protection against most equine diseases, so titer numbers don't mean much for horses. The only firm recommendation is for strangles: Researchers at the University of Kentucky advise that, to avoid the risk of purpura hemorrhagica, horses with titers of 1:1600 or greater should not be vaccinated.

Besides, immunity involves much more than antibodies-and a high titer is no guarantee of disease resistance. Modified live vaccines typically produce lower titers than killed vaccines, but they can leave the horse better able to resist disease (and have longer-lasting effects) because they stimulate other aspects of the immune response. Unfortunately, they also tend to have more side effects.

A lot of other questions remain, including what the effects are of giving multiple vaccinations in one session, or of vaccinating horses that have been exposed to disease. Researchers are looking for answers, and they're working on better and safer vaccines.