RECOMMENDATIONS FOR VACCINATION

Overview

     I will start with a few general comments regarding vaccination. I am not an advocate of the "5-way" (or more) vaccination (generally includes tetanus, influenza, rhinopneumonitis, eastern equine encephalitis and western equine encephalitis). My reasons for this are several. First, administration of this type of vaccine assumes that the company that makes a "5-way" makes equally effective/ or the best vaccine for each of these diseases. Second, the use of this vaccine produces vaccination and immunity to these 5 diseases all at the same time. The problem with this is that the length of immunity induced is not the same for all these 5 pathogens and the time and duration of exposure of your horse to these pathogens is highly variable. This leads to the false assumption that your horse is "protected" against each pathogen for the same amount of time and reliably protected when the horse is exposed to the pathogen, which varies with the time of year, movement of the horse and exposure to other environments and other horses. Finally, my personal opinion is that the immune response is unlikely to be as robust and reliable when there are 5 or more vaccines in the same shot, given in the same location in the horse at the exact same time.

     The vaccinations recommended for your horse vary with the use of your horse, where in the counrty the animal lives, where and how much travel the horse experiences, the concerns and preferences of the owner and caretakers and the allotted budget for yearly vaccination. In my opinion, there are at least 2 vaccines that ALL horses should receive regardless of any of these variable factors.

Mandatory Vaccines for ALL horses

          Tetanus Toxoid- mandatory vaccine to protect horses from tetanus to which horses are extremely sensitive. Administered as 2 shot initial series, then at least yearly thereafter (preferably every 6 months). Tetanus is a disease caused by a toxin produced by a type of bacteria, it is not caused by a bacterium or a virus itself.

        Rabies- mandatory vaccine to protect you, your horse and all other animals from exposure to rabies. This is an inexpensive vaccine that can easily prevent very unpleasant circumstances associated with any exposure of your horses to rabies from any source. I recommend an initial 3 shot series each 2 to 4 weeks apart, then yearly vaccination thereafter.

Vaccines Recommended for Horses That Show, Travel and/or are Exposed to New Horses AT ANY TIME AND FOR ANY DURATION

       Influenza- there are 2 major types of vaccine for this disease and they are modified live preparations of the virus and killed virus preparations. Generally speaking, I believe there is more evidence for superior efficacy of the modified live virus preparations. Of these modified live vaccine preparations, there are generally 2 modes of administration available. These include the typical intramuscular (IM) vaccine and the intranasal (IN) vaccine. For the last ~15 years, I have exclusively utilized the intranasal vaccines to control this disease in my client's horses. I believe this is a superior way to vaccinate for this disease as it more closely simulates natural infection and stimulates immunity and protection at the level of the nasal and pharyngeal mucosa, rather than focusing on immunity within the body that is operative AFTER the virus has already entered the body of the animal. There are a few companies that produce modified live intranasal vaccine for influenza. The strains of virus change over time with regard to their type and antigenicity. Consult with your veterinarian for information regarding the use of the product(s) that offer the best and most current protection. This vaccine (intranasal) requires 2 initial administrations between 2 and 6 weeks apart, then revaccination at 6 month intervals for continuous protection. If you and your veterinarian decide to use any intramuscular vaccine for influenza (modified live OR killed), you will need to administer the vaccine at 3 to 4 month intervals after an initial 2 or 3 dose series administered in 2 to 4 week intervals.

       Rhinopneumonitis (Herpes Virus I and IV)- this vaccination is considered to be very important in the control and management of disease outbreaks associated with (primarily) equine herpes virus I and IV. The clinical diseases caused by equine herpes virus generally includes viral respiratory disease, abortion in mares and neurological disease. With these viruses, you should know that the vast majority of horses, as early as the age of 3 or 4, will have been exposed to both of these viruses and most of those horses will have a latent infection with one or both of these viruses. What this means is that these horses will actually have the virus or viruses living within them in a state of latency, during which time there are no clinical signs of the infection and no active viral shedding. Therefore, these horses (the majority of the entire horse population), will all have the potential to serve as a reservoir and source of shedding and infection in response to any situation or stress that leads to acivation of these viruses from their latent state. What this means is that the most common clinical signs of infection with these viruses, respiratory disease, most often occurs in younger horses that have not been exposed to the viruses or that have not yet established latency and immunotolerance to them. Generally, as horses age, they become less and less apt to display clinical signs of virus infection, reacivation and shedding. Thus they serve to contaminate the environment and expose other horses without any overt signs of doing so. Older horses in particular, may develop clinical signs of respiratory disease, abortion or neurological disease when exposed to other "new" strains of the virus, when stressed, or sick from other problems.

       So, what does this mean for vaccination? It means that you cannot expect to keep your horses free from becoming infected and from establishing viral latency of these viruses. Furthermore, as your horse ages from 3 or 4 years of age, he/she will be less and less likely to show clinical disease due to hepes virus infection and shedding of the most common strains of virus. Furthermore, the currently available vaccines DO NOT prevent infection and cannot prevent neurological disease. So why vaccinate then? The major benefits of vaccination are 1) presmably reduced severity of clinical signs if and when infection does occur and 2) (perhaps most importantly) a reduction in the amount of virus and duration of shedding of virus when infection and viral latency is activated and shedding results. Thus, vaccinating your horse regularly is important to minimize illness should your horse become exposed to another herpes virus and is imperative to help minimize the shedding, spreading and exposure of all horses to these viruses that can be shed without any indication of its occurrence! Most vaccinations for herpes virus are intramuscular vaccines. They are also produced as killed or modified live formulations. Generally speaking, there is evidence to indicate that the modified live vaccines are superior in performance of reducing clinical signs of infection and reducing amount and duration of viral shedding. Intramuscular vaccines are usually recommended for 3 to 4 month intervals after an initial series of at least 2, preferably 3 vaccines, each 2 to 4 weeks apart. Calvenza is a brand of vaccine labeled for use both intramuscularly and intranasally after 2 initial intramuscular doses, then annually by either intramuscular or intranasal administration.       

Vaccinations Recommendable for Certain Time(s) of the Year and/or Certain Geographic Locations

       Viral Encephalitis (Eastern Equine Encephalitis, Western Equine Encephalitis, West Nile Viral Encephalitis, Venezuelan Equine Encephalitis)- why is viral encephalitis vaccination important during certain times of the year? The answer to this is based on the mode of trasnmission of the virus from the reservoir host animal to the horse. The transmission of these viruses occurs through biting insects, specifically, mosquitos. Therefore the duration of immunity required to protect your horse(s) against these diseases will depend on the length of the mosquito season. It is generally accepted that horse to horse transmission is not a significant modality of disease transmission. In the northernmost states, this duration is likely to be much shorter than in the southernmost states. In fact, in many warmer climates, protection is likely to be necessary year round. In states with shorter mosquito seasons, it may be adequate to vaccinate once yearly if the vaccination is timed well, before the mosquito season and if the mosquito season is likely to be less than 6 months long. This is because the duration of immunity of all 4 of these vaccines is generally regarded to be 6 months, so if the mosquito season is longer, you will need to vaccinate twice. Because the year is 12 months long and you will have to vaccinate twice in any area with a mosquito season exceeding 6 months, I recommend vaccinating every 6 months for continual protection against the necessary viruses in your geographic area. I think this is strongly advisable for all horse owners in the US since the mosquito season length may vary from year to year.

Why do I keep introducing the caveat of "geographical area"? Because in my opinion, it is unnecessary to vaccinate for venezuelan equine encephalitis in areas other than the southern most states, especially those that border Mexico. Otherwise I think it is very important, if not imperative, to vaccinate for Eastern Equine Encephalitis, Western Equine Encephalitis and West Nile Virus. 

       Potomac Horse Fever- this disease is caused by a bacterium that penetrates certain body cells- including white cells and intestinal cells. The clinical conditions caused by the organism may include one or all of the following; fevers of unknown origin, colic, loose stool to severe diarrhea and late term abortion. The vaccinations available to help prevent these clinical signs are thought to be helpful in preventing the manifestation of the disease assocaited with the organism, but clinical experience indicates that the vaccine is by no means completely protective. The duration of vaccine protection is relatively short and there appears to be numerous possible variants of this organism that may not be affected by vaccination-induced protection. Disease is believed to be transmitted by accidental ingestion of adult aquatic insects. Outbreaks of Potomac Horse Fever have been associated with mass emergence of burrowing mayflies. It is generally believed that disease transmission does not occur from horse to horse, and therefore cases are usually sporadic and isolated in number. For my practice, I generally leave vaccination for this diease up to the owner or caretaker. Certain areas of the country are more commonly affected by Potomac Horse Fever, and Kentucky is a state in which the disease is seen. The occurence of cases or Potomac Horse Fever is typically seen between the months of June to September and is greatest near large rivers and their tributaries (first described near or in the Potomac River Valley). The disease is reported in at least 40 states. Due to the properties of the vaccine, incomplete protection and short duration, this is not a vaccine that I routinely administer unless it is requested by the client or unless I feel the horse is likely to experience some reason to suspect an increased risk.  

       Botulism- Botulism is caused by the same genus of bacteria that causes tetanus. As is true for tetanus toxin, horses are highly sensitive to very low levels of botulism toxin. Tetanus toxin causes a spastic, rigid type of paralysis, whereas botulism toxin causes a flaccid, weak paralysis. Botulism is caused by toxins of numerous types. Unfortunately, the available vaccine for botulism is directed against toxin type B only and vaccination is only effective against type B toxin. It is fortunate that most cases of botulism do appear to be caused by type B toxin, so vaccination may be valuable in areas where botulism is recognized. Certain environmental sources may serve as more likely areas to pick up botulism toxin. These sources include contaminated feed and hay where decaying vegetal material and animal carcasses can harbor large concentrations of toxin, and can be ingested by and affect multiple horses. The botulism vaccine is highly effective, and in my opinion, usually a good idea, especially in areas of the country where cases of botulism are documented. In Kentucky and most nearby states, I do recommend vaccination for botulism. I recognize the added expense this incurs to the vaccination expenses, so I allow the owner and caretakers to make the final decision as to whether the vaccination is included. I do tell all my clients that if they ever go through the experience of having a horse that has botulism, they will vaccinate for the disease religiously. Treatment of the disease is possible, but can be very expensive, emotionally devastating and may or may not save the horse.

Vaccinations Recommendable Under Certain Circumstances

       Strangles Vaccine- As a general rule, I DO NOT recommend vaccination of most horses for strangles. In my experience, this vaccine is erroneously advocated by numerous equine veterinarians based of a poor understanding of the vaccine(s) available, the nature of the organism causing strangles (Streptococcus equi var equi) and the epidemiology of the disease it causes. What are the salient points you need to understand that explain why I usually don't advocate vaccination for strangles? 1) the disease known as "strangles", is generally a disease of younger horses, and once a horse has had the disease, it is generally conferred long lasting immunity against it. 2) it is a disease that exhibits relatively low mortality and again, once a horse has had it, it generally has long lasting immunity. 3) as a horse ages, the likelihood of strangles goes down with improving immune system function. Strangles is most likely to be seen on farms with high densities of horses of young ages, such as breeding farms, training centers with large numbers of young horses entering training or other farms with large numbers of yearlings, 2 year olds and young horses in high density. 3) the available vaccines are not always protective and 4) significant reactions to vaccination with both the intramuscular and intranasal vaccines are reported. Given these points and the population of healthy adult horses that I see in my performance practice, I do not feel the cost/benefits ratio warrants administration of the vaccine to to my clients' horses. The advice may be different in a situation/farm involving high densities of younger, immunologically naive horses.