Working with your own veterinarian, you can customize a vaccination and deworming program for your horse based on risk exposures.

One of the most confusing aspects of horse ownership can be sorting out annual vaccinations and deworming plans. There are so many different shots and deworming products available on the market today, how do you know what to use and when? The following simple explanations and charts should help shed some light on this confusing issue.

Equine Vaccinations
In March of 2008, the American Association of Equine Practitioners (AAEP) revised its recommendations for vaccinating your horse. Vaccines have been divided into two types: (1) core vaccines, which are recommended for all horses based on risk of death to your horse if exposed to these viruses, and (2) vaccines that should be given based on risk of exposure to certain viruses. Several prominent factors that play a part in exposure risk include: how much your horse travels or is exposed to horses who have traveled, the likelihood that you may breed any of your mares, and the developing immune systems of young foals. (See the AAEP recommendation chart on the next page.)

Core vaccinations. "Core" vaccinations are the ones you don’t want to skip. Risk of exposure to the viruses that make up the core group may not be very high, but the consequences of the infection could be death or permanent damage. In the case of rabies, there is also a very real risk the horse could transmit the disease to people working with him before anyone realizes the horse has rabies.

Non-core vaccinations. In evaluating whether to give non-core vaccinations, you should weigh risk of exposure when deciding. The rotavirus vaccine, for example, is only indicated for pregnant mares so that they can pass antibodies to their foals in the colostrum. Rotavirus only causes disease in young foals. Similarly, the botulism vaccine does not protect against the toxin strain typically found in adult horse cases of botulism, but does apply to the form that can develop in foals. Anthrax is only a threat in very specific areas of the country. For the other diseases, discuss your horse’s risk of exposure with your own veterinarian.

Effectiveness. Vaccines are most effective if the antibodies are present in high levels (called titers) when the horse is exposed to the disease. This means you can get best protection by correct timing of vaccines for seasonal diseases. The encephalitis viruses are spread by mosquitos, so it doesn’t make much sense to vaccinate for these diseases in the fall. A few weeks before start of mosquito season is best. If your horse is only around a lot of other horses in the summer, time your respiratory disease vaccinations to be given a few weeks before your most active riding season.

Pregnant mares & vaccines. Pregnant mares are a special consideration because the antibodies passed to foals in the mares’ colostrum (first secretion from the mammary gland) are critical for supplying them with antibody protection until their own immune systems can get up to speed. Mares are typically boostered 3 to 4 weeks before their expected foaling date to achieve maximal levels in the colostrum.

Foals. Timing of vaccination for foals depends to some extent on the vaccine, so details should always be worked out with your veterinarian. In general, foals from dams that were boostered close to foaling may not require their first vaccines until around 6 months of age, whereas foals from unvaccinated dams will require vaccinations a little earlier. However, you want to be careful to schedule these first vaccines to be completed at least 2 weeks before the stress of weaning.

Outbreak. One area of much confusion is whether horses should be vaccinated if there is a disease outbreak under way. There are many considerations in these situations. Vaccines do not provide immediate protection. If the horse has had the vaccine or the natural disease before, it will take from 1 to 2 weeks for any response to the vaccine to occur. If the horse has not had a vaccine or exposure before, most vaccines require a two-shot series and you won’t have full protection for at least 4 weeks. Also very important is whether the horse has already been exposed. If the outbreak has already hit and is running through the barn, there’s very little chance the vaccine will be able to work fast enough to protect these exposed horses, and it might even make things worse by adding to the burden on the immune system. If the disease has been located in your vicinity but not actually at your barn, a combination of vaccination and isolating the horses may be the way to go. Infectious disease dynamics can be complicated. Always work out a plan with your veterinarian.

Effective Deworming
The process of deworming your horse involves more than pulling any old dewormer off the feed store shelf. You need to consider your horse’s age, the drug being used, the health of your horse, and risk of exposure to parasites.

Most at risk. The very young (under 1 year), the very old, and debilitated horses have lowered resistance to parasites. Because of this, even the most effective dewormers often will not reduce egg counts to zero as they will in healthy adults. These horses are constantly reinfesting themselves and can build up large parasite burdens very easily. In these high-risk groups, if you’re not going to do fecal exams to see whether the horse needs deworming, you would be wise to stick very strictly to the deworming intervals in the Dewormer Drugs Chart on the next page.

On the flip side, healthy horses who are not very young or old often have a very vigorous immunity to parasites. The immune system cells lining their intestinal tract can manage to keep parasite infestations very low or even to none at all, especially if they live in uncrowded pasture conditions where they never have to eat close to fecal droppings.

Choosing dewormers. When buying a dewormer drug, train yourself to look at the active ingredients information on the label, rather than the brand name. There are several different brand names for many drugs. If you are trying to rotate dewormers, you need to make sure a different brand name isn’t actually the same drug you used last time. Knowing the actual active drug in the dewormer is also essential for knowing what the correct deworming interval is.

Parasite resistance to dewormer drugs is widespread. For small stronglyes, resistance has been found to all drugs except ivermectin and moxidectin. If you choose to use one of the drugs with known resistance problems in your program, you should plan to have your veterinarian perform a fecal exam after using that drug at least once a year. If you don’t do that, you may be inadvertently increasing the parasite burden in your horses and putting them at risk of building up high parasite levels during some stages of your deworming schedule.

Exposure. Exposure is a big part of risk. Low levels of parasite infestation do not necessarily have any obvious health consequences, but they do keep parasites alive and well in the environment. Because no dewormer gets each and every life stage of parasites, when you deworm your horse you will always be missing some immature worms. These are the ones that eventually mature and begin laying eggs during the deworming interval. The bottom line here is that no horse will ever be 100% parasite free, even if it’s the only horse on the property, because he will be a source of parasites himself.

Exposure can come from other horses too, of course. The more horses your horse is turned out with, and the more crowded the living conditions are, the higher the risk is of being exposed to parasites from another horse. Rotating horses through small paddocks or round pens on crowded facilities is also a high-risk situation. If you take your horse away from home for shows or trail rides, always be picky about how you let him graze (if at all) in high traffic areas, and never feed directly on the ground. A few mouthfuls in the wrong place can result in a heavy parasite infestation.

If you are like most people and have a horse who is heavily infested with parasites, or always has a negative reaction to dewormers, the thought of deworming him probably fills you with fear. This is truly a situation in which you are "between a rock and a hard place." However, the answer is definitely not to avoid deworming. Letting the parasites continue to drain the horse’s energy, nutrients, and health and damage his intestinal tract only further weakens him—including his immune system.

Deworming the heavily infested horse. First, involve your veterinarian. There are a variety of strategies for making the process easier on the horse. Anti-inflammatory drugs—such as flunixin meglumine (Banamine®)—can help control systemic reactions like fever or laminitis. Some veterinarians also use antihistamines. For the actual deworming, using a schedule that first deworms with a product that primarily targets only adults, like pyrantel or regular-dose fenbendazole, is a common strategy. Other veterinarians prefer to use ivermectin or moxidectin but at a lower-than-usual dose. After these less-aggressive approaches have reduced the parasite burden, your veterinarian will recommend a regular deworming appropriate to your situation.