Vaccines for Horses: How They Work

Vaccines for Horses: How They Work

Vaccines work with a horse’s natural defense to develop immunity. A veterinarian explains vaccination types, vaccine failure and how often to vaccinate.

roan horse mare herd in pasture (Credit: Baru Forell)

text size

The American Quarter Horse Journal logo

By Dr. Thomas R. Lenz

Modern, commercially produced, government-approved vaccines are generally safe and effective.

Occasionally, I have a client ask why a horse that was vaccinated still developed the disease.

When germs (bacterial or viral) invade the horse’s body, they attack and multiply. This invasion causes the horse to become sick. Vaccines work with the horse’s natural defenses to develop immunity against specific diseases. They stimulate immunity by imitating an infection that doesn’t cause illness but does cause the horse’s immune system to react.

Blood contains two types of blood cells: 

  1. Red blood cells carry oxygen to tissues and organs.

  2. White blood cells fight infection.

The disease-fighting white blood cells are primarily macrophages, B-lymphocytes and T-lymphocytes:

  • Macrophages swallow and digest germs, as well as dead or dying cells. In the process, they leave behind parts of germs called antigens.

  • B-lymphocytes – in response to the foreign antigens – produce antibodies that attack the disease-causing germs and kill them.

  • T-lymphocytes are stimulated by the antigens to attack and destroy cells in the body that have been attacked by the invading viruses or bacteria.

The first time a horse’s body encounters a germ, it can take several days to make and use all the germ-fighting tools needed to overcome the infection. However, after the infection, the immune system creates and keeps a few unique T-lymphocytes that stimulate a much quicker response if that same germ attempts to infect the horse again.

Jump ahead to read about:

How Vaccines Work for Horses

Vaccines help create immunity to specific diseases by imitating an infection that doesn’t cause the disease but does stimulate the horse’s body to produce T-lymphocytes and antibodies against bacteria or viruses that produce that infection. 

T-lymphocytes remember that specific infectious agent and recruit B- and T-lymphocytes to fight that agent if it infects the horse in the future.

The immune system takes several days or weeks after vaccination against an infectious agent to produce enough white blood cells to fight off the infection, so it’s possible that a horse infected with a disease just prior to or immediately after vaccination can still develop symptoms. 

We see this situation often when horses are vaccinated only a few days before leaving for an event where they encounter horses that are shedding germs.

Types of Horse Vaccines

Live, attenuated vaccines contain a version of the living virus or bacteria that has been weakened so that it does not cause disease in horses with healthy immune systems. Because they are the closest thing to natural infection, they provide an excellent response by the immune system and usually require only one injection per year. The downside is they usually have to be reconstituted and it is critical that they be handled properly.

Most vaccines used in horses are inactivated vaccines where the germ has been killed during the process of making the vaccine.

Toxoid vaccines prevent disease by bacteria like Clostridium tetani, which causes tetanus, that produce toxins (poisons) in the body. During production of the vaccine, toxins are weakened so they cannot cause disease. When the horse’s immune system receives a toxoid vaccine, it develops systems to fight off natural bacteria-produced toxins.

Subunit vaccines include only parts of the virus or bacteria instead of the entire germ. Because they contain only the essential antigens and not the whole organism, side effects are less common.

Intranasal vs. Intramuscular Vaccines

With the exception of intranasal influenza and strangles vaccines, most vaccines should be administered deep in the muscle. Administering them subcutaneously (just under the skin) can decrease their effectiveness and the horse’s response. 

Vaccine Failure in Horses

Commercially available vaccines are regulated by the federal government and must meet rigid standards for stability, effectiveness and safety, so if handled and administered properly, they are seldom the reason for vaccine failure. 

Proper Refrigeration of Vaccines Is Essential

Proper storage of vaccines is one of the most critical aspects in ensuring the vaccine will provide the desired disease protection. Maintaining vaccines at the appropriate temperature is an important aspect of proper immunization. The label recommendations for storage of vaccines are to store them in the dark at 35-45 degrees Farenheit, avoid freezing and shake well to assure uniform suspension of the vaccine proper to administration.

Failure to follow label directions can result in reduced vaccine effectiveness, vaccine failure and an increased rate of local reaction after vaccine administration. Damage to the vaccine is most often due to exposure to heat, light or freezing. Sometimes, it’s due to a faulty refrigerator that is not cool enough or freezes the vaccine, or to leaving the vaccine at room temperature for an extended period of time. 

Vaccinating Foals

The most important cause of vaccine failure in foals is blockage of the vaccine by maternal antibodies. Foals are born with a limited ability to mount an immune response to disease during the first few weeks and months of life. To protect them from disease, they must depend on antibodies in the mare’s colostrum (first milk) that they ingest during the first few hours of life. Those antibodies will gradually diminish over time as the foal’s immune system matures. Vaccines administered while maternal antibodies are still high are blocked by the antibodies, so it is important to talk with your veterinarian about the proper time to start your foal’s vaccination program to prevent maternal antibody block and ensure a good immune response. 

How Often Should You Vaccinate Your Horse?

When vaccinating a foal or adult horse for the first time, it is critical that inactivated vaccines be given twice three or four weeks apart to build a good immune response and memory cell development. Failure to do so will dramatically diminish the horse’s immune response and is a common cause of vaccine failure. 

For most vaccines, the horse’s immunity to disease will gradually decrease over time, so boosters are usually recommended at least annually. In the case of influenza and rhino (equine herpesvirus 1 and 4), vaccination every six months is usually recommended to maintain a good immune response.

Your horse’s vaccination schedule should be tailored to your specific requirements and location, so ask your local American Association of Equine Practitioners veterinarian to help you develop a vaccination program to suit your horse’s individual needs. 

Additional Factors That Decrease Vaccine Effectiveness 

The effectiveness of live, attenuated vaccines can be decreased if: 

  • Used concurrently with antibiotics or chemical sterilizations.

  • Syringes are reused. 

  • Excessive alcohol is left on the skin. 

Heavily parasitized or malnourished horses might not be able to mount a good immune response when vaccinated. 

Stress, including extreme cold or heat, can also decrease a horse’s response to vaccination. 

Geriatric horses, especially those suffering from Cushing’s disease, might have a decreased ability to respond to vaccination.

Vaccination alone, in the absence of good nutrition and management practices directed at infection control, are not sufficient to prevent infectious disease.

Vaccines today are safe and effective, but realistically, no vaccine can prevent clinical disease in 100 percent of vaccinated animals. However, even in cases where the horse’s immunity, gained through vaccination, doesn’t completely protect the animal, the vaccine reduces the severity of the clinical signs and duration of illness.

About the Source: Dr. Thomas R. Lenz

Thomas R. Lenz, DVM, M.S., diplomate of the American College of Theriogenologists, is a trustee of the American Horse Council, past chairman of the AQHA Research Committee and past president of the American Association of Equine Practitioners. He is also a member of the American Quarter Horse Hall of Fame.