Illustrations by Kip Carter

Every year you sit down with your veterinarian and carefully plan your horse’s vaccination schedule. You make sure he gets vaccinated against all of the right diseases, at the right time of year. You pay special attention to the increased exposure risks he faces during competition season, and schedule appointments at just the right time to make sure he’s maximally protected.

You even plan your training rides around vaccination day in case he gets sore. Good for you?but don’t stop there. You may not know it, but not every vaccine is created equal. If you’re concerned enough to pay this close attention to your horse’s vaccination plan, you probably want to make sure he’s getting the safest, most effective vaccine available for each disease.

Gone are the days when vaccination simply involved injecting your horse with a ?dead? version of the virus or bacteria you were trying to protect against to stimulate antibodies against that disease. Although these killed vaccines still work and continue to make up a portion of your vaccination plan, scientific advances have led to some significant improvements in vaccine efficacy.

I’ll tell you what these changes are, and what they might mean for your horse. I’ll start by taking a look back at the very first vaccines we had available and how they worked. Then I’ll explain in more detail how your horse’s immune system operates beyond ?antibodies 101.? Finally, I’ll fill you in on exciting new developments in vaccine technology, including what they are, how they work, and how they can better protect your horse.

It Started With Smallpox
The story of smallpox is a shining example of medical accomplishment. This deadly viral disease was believed to have first appeared in human populations as early as 10,000 BC. Because of vaccination, the last naturally occurring case was reported in 1977?and in 1979 the disease was declared eradicated.

As early as the year 1000, the ancient Chinese started scratching matter from smallpox sores on people?s arms in a makeshift effort to protect themselves from contracting this disease. This early, haphazard form of vaccination persisted for centuries; then, in 1796, Edward Jenner used the cowpox virus to develop a smallpox vaccine that soon saw widespread use.

In 1885 a vaccination against the rabies virus was developed, followed closely by vaccines against many other diseases, including diphtheria, tetanus, cholera, plague, typhoid, and tuberculosis.

These early vaccines worked on simple principles. Injection of an inactivated form of a disease-producing organism would cause the recipient to respond by producing antibodies against that virus or bacteria. These antibodies would then be stored to remain ?at the ready? to wage war against invading organisms?and halt them in their tracks.

Vaccines against the equine herpes virus, introduced in the 1960s, were among the first to be used in horses. In fact, one of these early vaccines, Rhinomune, is still in wide use today and considered one of the most effective available against this complex virus.

By the 1970s there were vaccinations to protect your horse against tetanus, sleeping sickness, influenza, and strangles. Over the decades new vaccines have been developed to protect against emerging diseases, such as Potomac horse fever and West Nile virus. And as new vaccines are created, new technologies surface to make them even more effective. To understand how these new technologies can benefit your horse, first understand how his immune system works.

Antibodies and Beyond
You?ve probably heard a lot about antibodies. These Y-shaped proteins are produced by cells called B-lymphocytes in response to a foreign invader like a virus or bacteria (these bad guys are called antigens). The antibodies bind themselves to the antigen and prevent it from causing disease. One type of B-lymphocyte, the plasma cell, is a first responder, releasing antibodies the first time the antigen is detected.

A second type of B-lymphocyte, the memory cell, produces antibodies and saves them for later use. These stored weapons are at the ready to protect your horse when he’s exposed to the same antigen sometime in the future. This very basic system of protection is called humoral immunity, and is what most of us bring to mind when we think about vaccination. By administering a small dose of a killed or inactivated antigen, we stimulate the production of antibodies to help prevent disease.

An important factor to consider for disease prevention, however, is that antibodies don’t just circulate in your horse’s blood stream. There’s an entire army of antibodies that congregate in the mucous membranes (such as your horse’s nasal passages and upper airway) to attack invading organisms at their point of entry into your horse’s body. These mucosal antibodies prevent infection before it even begins.

A second, commonly overlooked arm of your horse’s immune system is the defense mechanism known as cell-mediated immunity. In this system, a ?clean-up crew? recognizes cells that have already been infected by a virus or bacteria. This group of cells (including natural killer cells and cytotoxic T-cells) patrols your horse’s body, detects infected or abnormal cells, then releases substances known as cytokines to destroy them.

Cell-mediated immunity is typically less specific than humoral?it attacks any abnormal cell rather than targeting a specific antigen. In fact, this system even plays a role in fighting cancer by wiping out the abnormal cells that invade the body.

Although we all tend to limit our thoughts to circulating antibodies when we talk about vaccines, all of these different features of your horse’s immune system are crucial for protecting him from invading organisms.

Vaccine Technology
Ads for vaccinations are loaded with buzzwords these days: recombinant vaccines, canary pox-vectored vaccines, adjuvants. What does it all mean? And what difference does it make?

Let?s take a look at the different technology options surrounding vaccination so you can understand why one vaccine may be better than another for protecting your horse.

Inactive (killed) vaccines.
What they are: An inactivated vaccine is created by ?killing? the antigen with heat, chemicals, or radiation to prepare it for injection. A subset of a killed vaccine is a toxoid, which is a vaccine that targets a disease caused by a toxin released by an organism rather than by the organism itself (such as tetanus). To create a toxoid vaccine, the toxin (rather than the organism) is inactivated prior to injection.

Pros: Killed vaccines are very safe and unlikely to cause side effects. Because they were one of the earliest types of vaccines to be developed, they are relatively easy to produce, which often makes them less expensive.

Cons: Inactivated vaccines may be less effective than other options, and generally produce only a humoral immune response (meaning no cell-mediated immunity).

Modified live (live attenuated) vaccines.
What they are: Modified live vaccines are produced by altering the antigen, usually by passing it through other cells (such as chick cells) for many generations until it is able to replicate in those other cells, but not in the horse. Your horse’s immune system responds, but the antigen can’t replicate and make him sick.

Pros: Modified live vaccines are typically more effective than killed vaccines. They often stimulate a response in cell-mediated immunity in addition to humoral immunity.

Cons: Modified live vaccines are more likely to produce side effects because of the stronger immune response they elicit. They also have the potential to revert to the form of virus or bacteria that can cause disease?especially in a horse with a weakened immune system. Modified live vaccines are generally not suggested for horses with compromised immune systems or for pregnant mares.

Subunit vaccines.
What they are: Subunit vaccines are created from a single protein or group of proteins from the disease-producing organism rather than from the entire organism. The protein(s) used are those most likely to cause an immune response, yet won?t cause disease. This type of vaccine includes live-vectored recombinant vaccines that use a virus or bacteria that doesn’t cause disease in the horse (such as canary pox), to introduce the proteins into your horse when he is vaccinated.

Pros: Subunit vaccines stimulate both a humoral and a cell-mediated response. They are very safe, with no chance for a reversion to virulence, meaning they can be used in horses with poor immune systems plus are generally safe during pregnancy.

Cons: Subunit vaccines may not mount quite as strong an immune response as other vaccines do. Also, because they involve more complicated technology, they may be more expensive and difficult to produce.

DNA vaccines.
What they are: DNA vaccines use a variety of different methods to deliver a part of the disease-producing organism?s genetic material into your horse. The genetic material is taken up by your horse’s cells, and tells those cells to reproduce. In a sense your horse’s own cells become a vaccine-producing factory.

Pros: DNA vaccines are extremely safe and very effective. As the technology develops they should also be fairly simple to produce. Research indicates they may be more effective at overcoming maternal antibodies, meaning they could greatly simplify foal vaccination protocols. They may also improve our ability to vaccinate against diseases caused by organisms other than viruses, such as bacteria and fungi.

Cons: For the most part DNA vaccines are experimental and not yet widely available.

Intranasal vaccines.
What they are: Intranasal vaccines of different types are delivered via a spray into your horse’s nose.

Pros: These vaccines stimulate a strong mucosal immune response and are thus often the most effective vaccines for preventing respiratory viruses and bacteria spread through the air. They typically have fewer side effects than their injectable counterparts.

Cons: Intranasal vaccines can be difficult to administer, particularly if your horse is head shy. In some cases, repeated administration of an intranasal vaccine can turn a good horse into a naughty one.

Adjuvants.
What they are: Adjuvants are substances added to vaccines to improve the immune response in a variety of different ways, such as promoting better absorption or distribution of the antigen within your horse’s body, or by stimulating a cell-mediated immune response.

Pros: Adjuvants improve vaccine effectiveness. They are often used in killed vaccines to help stimulate cell-mediated immunity. They may help reduce costs of vaccines by minimizing the amount of antigen needed to stimulate an immune response.

Cons: Addition of an adjuvant can increase the chance for side effects, such as a localized vaccine reaction at the injection site.

What It All Means
The West Nile virus is a good example of how different technologies play roles in your horse’s vaccination plan. Three different types of equine West Nile virus vaccinations are currently on the market. When this virus first struck North America, vaccine development was fast-tracked, and a killed vaccination was soon conditionally approved. This vaccine was widely administered and proved quite effective for controlling the spread of West Nile virus across the country.

As time wore on, additional vaccines were developed using more complex technology, including a canary pox-vectored recombinant vaccine (with an adjuvant) and a recombinant vaccine that uses the yellow-fever virus to introduce proteins of the virus (without an adjuvant).

Studies show these new vaccines to be more effective than the original killed vaccine. So which one should you use? If you live in a hotbed of West Nile virus and your horse is at high risk, one of the newer recombinant vaccines may be your best choice. But if you’re in an area where West Nile infections are uncommon, the original killed vaccine is likely to be plenty ?good enough.?

What about intranasal vaccines? An attenuated-live intranasal influenza vaccine is currently available, and generally accepted as the most effective influenza vaccine out there. But if your horse is head-shy it might be best to compromise on an injectable vaccine instead?and if you do, it’s probably wise to choose the one that’s most effective, with the least side effects.

Another example where vaccine technology has played a critical role is in the recent introduction of a vaccination against the deadly Hendra virus that first hit Australia in 1994, infecting and killing both horses and humans. Again, vaccine development was fast-tracked, and just last year a subunit vaccine generated from a single protein in the virus was made available. Because it only involves a single protein from the virus, there’s no risk it could cause disease, and addition of an adjuvant helps to make it more effective.

It’s really true that all vaccines aren?t created equal. By understanding how they work and how they?re made, you’ll be better equipped to make decisions about not only what diseases you’ll include in your horse’s vaccination, but also which vaccines to choose.