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.