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Vaccines an Overview
Vaccines and Vaccination Overview for Horses
Introduction
Currently, there are equine vaccines available for:
- Tetanus: Injection of modified toxin (toxoid)
- Eastern, Western, Venezuelan Viral Encephalomyelitis: Injection of killed virus
- West Nile Viral Encephalomyelitis: Killed virus (Fort Dodge) / Modified live canarypox vectored (Merial)
- Influenza, many different strains: Injection / Intranasal
- Herpesvirus-1: Abortion, respiratory, and neurological disease
- Herpesvirus-4: Respiratory disease and abortion
- Strangles: Injection of killed bacterin / Intranasal vaccine (Fort Dodge)
- Rabies: Injection of killed virus
- Potomac Horse Fever: Killed rickettsia
- Equine Protozoal Myelitis (EPM): Killed protozoan
- Botulism: Injection of modified toxin (toxoid)
- Equine Viral Arteritis
- Anthrax
- Endotoxemia
- Rotavirus
- The incidence of these diseases differs in different parts of the world, countries, and even smaller geographical areas.
- The efficacy and safety of these vaccines vary remarkably.
- Even your goals affect which vaccines you should use.
How Vaccines Work: Killed vs Modified Live (ML) Vaccines
Put in its simplest terms, vaccines work by mimicking the presence of an infectious or toxic disease. The immune system senses the vaccine and then mounts a defense against the vaccine. The part of the vaccine the immune system senses is called an antigen and has to do with its shape and the position of molecular electrical charges. The next time the immune system senses this antigen, it mounts a quicker and stronger defense. Though infection might occur, this quicker and stronger defense may lessen or even prevent clinical signs from occurring.
Not all vaccines work the same. Many vaccines are killed disease organisms or are modified toxins called toxoids. Though killed or modified, they still retain some of the same surface shapes of the live organism or toxin, priming the immune system for the real thing. Other vaccines are live organisms but are modified so they are less pathogenic, meaning they do not cause serious disease. Viruses modified for vaccines are called modified live virus (MLV) vaccines. Bacteria would be called modified live bacterium (MLB) vaccines. Recently, we have seen more complex scenarios where harmless viruses are modified to elaborate antigens for diseases we want to protect our horses from. The virus is introduced into the horse, infects cells, and elaborates the antigens, producing immunity. These are called modified live virus vectored vaccines, and currently, there is a West Nile Virus vaccine that works this way.
To Vaccinate or Not
Your first step in deciding what to vaccinate for starts with compiling a list of infectious diseases that occur in your area and areas where you travel. Certainly, such a list starts with a talk with your local veterinarian, but there are other resources. Your local extension service may have an equine specialist on staff, and your state veterinarian along with the veterinary diagnostic lab are good sources for this information. However, the likelihood of contracting a disease is not the only factor in deciding what to vaccinate for. You should also answer these questions:
- How serious is this disease?
- How well does the vaccine work?
- What is the frequency and seriousness of the side effects of the vaccine?
- Does protecting the horse also protect other horses or the horse owner from contracting the disease?
- Does vaccination result in the persistence of the disease in the population?
- Does the vaccine create a diagnostic enigma or cause export restrictions?
- Does the vaccine create export or transport restrictions?
- Make a diagnosis more difficult if it creates a titer that is picked up on serological tests.
- Cause the use of resources (money mainly but also time) where they could be better used.
- Create an artificial sense of security, preventing more effective preventive measures.
- Lead to severe reactions, which, while rare, can occur with any vaccine but are more tragic if the vaccine is unnecessary.
Primary vs Booster Immunization
When you read about vaccines, you will often run across the terms "primary immunization" and "booster immunization". The immune system remembers past exposures to molecules it considers foreign and can therefore respond much quicker and stronger upon subsequent exposures. This second, stronger response is called the anamnestic response. Accordingly, the horse's immune system will react poorly to the first dose of most vaccines, and to prime the immune system, two vaccinations are given about 3 to 6 weeks following the first time a horse receives a vaccine. This first injection is called the "primary" vaccine, and the following ones are called "boosters". You will also see the first two injections called the primary series. After this, the vaccine is given regularly as often as necessary to maintain immunity.
Some vaccines do not require this primary series; for example, most rabies vaccines don't. You should always read the instructions that come with a vaccine to find out the suggested regimen for primary vaccination and boosters. Occasionally, you will find veterinary recommendations to booster more often than suggested by the manufacturer.
Maternal Antibody Inactivation of Vaccines in Foals
A frequent problem with foal vaccination is that maternal immunity, antibodies derived from the mare's colostrum, will inactivate vaccines given to the foal. This only occurs in mares who have been vaccinated for the disease that the foal is being vaccinated for. That is, it is specific to the vaccines the mare received. To properly vaccinate these foals, you have to wait until the mare's antibodies have declined to a low level. This gives rise to two problems:
1) The question of maternal inactivation in foals has not been researched for some vaccines, and we often do not know when they are gone.
2) Even where researched, the time before you can effectively vaccinate is variable.
Because of these problems, the following recommendations for primary vaccination of foals from vaccinated mares will be a compromise between the goals of protecting foals as early in life as possible while trying to minimize the interfering effect of persistent maternal antibodies. You see more and more where a third vaccination is recommended in the primary series to overcome this problem of unknown importance. Where known, you will find information on this subject in the specific disease articles linked below, and the information is incorporated into our schedule recommendations also linked below.
Adverse Vaccination Reactions
Vaccines work by stimulating the immune system, but the immune system can overreact and cause symptoms of disease. Fortunately, mild reactions are easily treated, and serious reactions to most properly given vaccines are uncommon.
Stiffness
Mild, local reactions to vaccines are common, with an incidence somewhere between 5% (for tetanus) and 40% (for strangles), for most routine vaccines. These reactions take the form of soreness, stiff gait, perhaps noticeable swelling, depression, and/or fever six to 72 hours post-injection, and can last from one to seven days. Poor technique and recent exposure to the disease you are vaccinating for can greatly increase the reaction rate and severity. Abscesses are a more serious local reaction that generally requires surgical drainage and antibiotics. Fortunately, these mild reactions are easily managed with phenylbutazone, with improvement seen within a couple of hours of administration. If a horse is known to react to a particular vaccination, it can be pretreated with bute to avoid this reaction.
Allergic Reactions
There are also generalized allergic reactions that occur much more rarely. These include:
For more information on symptoms, treatment, and prevention, see Vaccines and Adverse Reactions.
- Hives
- Anaphylaxis: rapid onset of breathing problems, possibly colic.
- Purpura: characterized by swelling in the hind or all four legs.
Frequently Given Vaccines Available
Here we briefly describe the disease and vaccines available for horses. The purpose is to help you decide whether you should use the vaccine or not. In-depth information on these diseases and vaccine schedules is available in the specific disease articles on the disease and the Vaccine Schedule article. Note: recommendations for vaccination will vary from brand to brand and even change from time to time for a specific vaccine; the instructions with the product should always be reviewed and followed.
Rabies: Injection of killed virus
Any mammal, with the possible exception of rodents, can contract rabies. The most common vectors are foxes, skunks, bats, and raccoons; rabies has become an epidemic in many parts of the Americas. However, humans rarely come into contact with rabies because we have created a buffer of well-vaccinated pets between us and the wild mammal population. Though rabies in horses is rare, by vaccinating your horse, you help protect yourself. Vaccinate horses if any small animal or wildlife cases occur in the surrounding areas.
Tetanus: Injection of modified tetanus toxin (toxoid)
Before effective vaccination, tetanus was a common, life-threatening disease with no specific treatment. The vaccine has a low adverse reaction rate and, after the primary series, is given yearly. The tetanus bacterium is a normal inhabitant of the gastrointestinal tract of many animals and has a worldwide distribution. Once on the ground, it forms spores that live for years. Tetanus gains entry through a dirt-contaminated penetrating wound. Experience suggests that the vaccine may even protect longer, but until this is proven, yearly boosters are best.
Vaccination (toxoid) vs Antitoxin
The toxoid vaccine should not be confused with the antitoxin. Both products are used to prevent tetanus, but the toxoid is a vaccine that stimulates the horse's immune system, while the antitoxin provides immediate protection by supplying preformed toxin antibodies. A change in the old recommendation of using tetanus antitoxins routinely in newborns and when wounds occur is warranted. The antitoxin can cause liver failure in a small percentage of horses several weeks to months following administration. Antitoxin should only be used in horses with wounds and no history of tetanus vaccination.
For More Information on Tetanus
Eastern, Western, Venezuelan Viral Encephalomyelitis: Injection of killed virus
Eastern and Western Equine Encephalomyelitis (EWEE) are related viruses that attack the nervous system of horses. The source and reservoir of the disease are infected birds. Symptoms include fever, incoordination, depression, coma, and death. Mosquitoes transmit the disease from birds to horses and humans. There is no cure once contracted, and mortality rates are high. Survivors often have permanent brain damage, remaining stuporous and/or uncoordinated. These diseases are endemic throughout North, Central, and South America. Thousands of horses die of viral encephalitis in the U.S. every year, but these numbers are small compared to the time before a vaccine was available. The incidence of these diseases varies significantly from year to year, chiefly influenced by rainfall and mosquito population. The vaccine has a low adverse reaction rate and, after a primary series, provides good protection for six months to a year.
Venezuelan
Venezuelan equine encephalomyelitis is not usually a problem in the US and Canada, so vaccination is not routinely recommended there. Central and South America do have a problem, so horses there should be vaccinated. VEE is a more serious illness with a higher death rate than EEE or WEE. Another important difference is that horses with VEE are contagious to other horses through blood transmission. A problem with vaccinating horses for VEE is the resulting serum titer makes it difficult to decide if a sick horse is suffering from this disease or some other problem. Vaccinated horses could also be restricted regarding where they may be shipped.
Japanese Encephalomyelitis
Recently, there has been a resurgence of Japanese Encephalomyelitis, requiring vigilance in the Asian countries where this disease is prevalent.
For More Information on Viral Encephalomyelitis
West Nile Viral Encephalomyelitis: Killed virus (Fort Dodge) / Modified canarypox vectored (Merial)
West Nile Virus has almost a worldwide distribution wherever mosquitoes occur. While not as severe in most cases, it is spread and has symptoms similar to the viral encephalitides described above. Two commercially available licensed vaccines are available in the US: West Nile Innovator by Fort Dodge Animal Health and Recombitek by Merial. Though very different in how they work, both vaccines have met USDA requirements for safety testing. Both have been shown to reduce the risk of infection with West Nile Virus by about 90% for 12 months following primary vaccination. However, in areas where the disease is endemic and mosquitoes are present year-round, twice-yearly boosters have been recommended. Though not labeled for this, experiments with Recombitek do show that the onset of immunity is much quicker, less than 26 days following the first shot of the primary series, so this vaccine would be preferred if you are in the middle of an outbreak but have not already given your horse its primary vaccination series.
Influenza, many different strains: Injection (killed) / Intranasal (MLV)
Equine influenza is caused by a virus that is easily spread from horse to horse and occurs anywhere horses congregate. Fever, depression, inappetence, cough, and serous nasal discharge are common. Horse owners with middle-aged horses who do not come into contact with other horses will not have much trouble with infectious respiratory diseases. Influenza can take a horse out of competition for several months, so horses that go to shows and frequently trail ride should be vaccinated regularly.
The injectable vaccines are of limited to unknown efficacy, with protection being fairly short-lived. Late in 1999, Heska released a modified live influenza vaccine that is given intranasally. Preliminary testing with relevant influenza strains indicates that following a single vaccination, horses had significantly decreased symptomatology and decreased shedding times of two relevant influenza strains. The vaccine is effective for at least six months following one administration of the vaccine. Maternal immunity inactivates the flu vaccine up to around 6-9 months of age, so vaccination for this should begin after the ninth month.
For More Information on Influenza
Herpesvirus 1 / 4: Killed and MLV injection
Equine herpes virus 1 / 4 (EHV-1/4) is responsible for several diseases in horses. The disease is usually transmitted through aerosolized secretions, the respiratory system, or contact with aborted fetuses, and is often first seen as a cold: fever, runny nose, and cough. EHV-1 can cause abortion in mid to late-term pregnant mares and, in young or stressed horses, neurological disease of the spinal cord. The neurological form is chiefly incoordination. Horses carry these viruses for a long time following infection, serving as the reservoir and source of infection. Immunity from killed vaccines is short-lived and incomplete. EHV-1 is available as an MLV injection and appears to provide longer and better immunity.
For More Information on EHV-1/4
Strangles: Injection of killed bacterin / Intranasal MLB vaccine (Fort Dodge)
Strangles is a bacterial disease, caused by Streptococcus, that affects the upper respiratory system. Though rarely fatal, it can produce a prolonged period of illness and is quite contagious. It's important to note that early in the course of the disease, it can be effectively treated.
The injectable Strangles vaccine is not routinely recommended because protection is only fair, of short duration, and the incidence of vaccination-related complications is much higher than with other vaccines. The intranasal MLB vaccine "Pinnacle" represents an improvement over the injectable form. The old problems of fever, stiffness, and abscessation at injection sites do not occur or are much less common with the new "Pinnacle" vaccine. In studies, the intranasal vaccinates consistently had fewer and less severe diseases than the non-vaccinated and injectable vaccinated groups. So far, the only common reaction to the intranasal vaccine reported is a mild clear discharge from the vaccinated nostril in less than 1% of the vaccinates. Though there is no labeling for it, there were no reactions in the pregnant mare studies. Colostral immunity and vaccine blockage are still open questions. The incidence of abscess and purpura has fallen to less than 1 in 10,000, and both were related to technique:
- Care must be exercised that none of the intranasal vaccine contaminates needles used for injection. The best practice is to complete all injections before handling the Strangles intranasal vaccination.
- Vaccination during an outbreak must be done carefully to avoid vaccinating any horse with already high antibody titers to Strangles.
Potomac Horse Fever: Killed rickettsia
PHF is a disease characterized by fever, diarrhea, and founder during the hot months of the year. Neorickettsia risticii was determined to be the cause, which persists in the environment in some species of snails and is transmitted to horses by caddis flies. Currently, there is a lot of confusion over where the disease is endemic. Many areas that were thought to be suffering from PHF were misdiagnosed. Before vaccinating, check with your State Veterinarian's Office or Laboratory to see what the known incidence is in your area. There are still questions about whether this vaccine is useful, as the incidence and severity of the disease in vaccinated populations is about the same as in unvaccinated populations.
For More Information on PHF
Uncommon Vaccines Given
These vaccines are not often used in horses for different reasons. Limited geographical range of the disease, limited or unknown efficacy, high reaction rates, and/or state approval for use are some of the reasons. In deciding whether to use these vaccines, it's important to thoroughly understand the disease and the consequences of vaccination. Links to articles on these diseases and vaccines are provided:
Vaccine Schedules
To summarize the above information, vaccine schedules depend on the age, use, and reproductive status of your horse. Just as important are your location, whether they have outside contact with other horses, and most importantly: your goals. We have an article that groups horses by some of these factors, primarily age, use, and reproductive status, but they should be looked at as general recommendations that need to be reviewed by your primary health provider: