What's New in Equine Vaccines?

The following information was given at a Seminar on Equine Nutrition and Health sponsored in part by the Virginia Horse Industry. It was held on October 17, 2008 at The Middleburg Agricultural Research and Extension (MARE) Center, Middleburg, Virginia. The following is being posted on HorseHints with the permission of the lecturer. This Seminar was beyond outstanding. My husband, Bill and I, feel so fortunate to have been able to attend.

"Infectious diseases present serious threats to the health of horses and the overall economy of the horse industry. Horse owners will spend significant money on vaccines and other remedies directed at preventing disease. Although vaccination may protect against several diseases, vaccines provide only limited protection against others, and, in the case of some diseases, no vaccines are available at all. In general, a wide variety of vaccine programs are required to address health risks in different equine populations. We must also remember that vaccination alone, in the absence of good management practices directed at infection control, is not sufficient for the prevention of disease. With so many available (and upcoming) vaccine options to maximize protection for horses, it is critical for horse owners to understand basic principles of both disease transmission and effective management practices. This article will review current vaccine technologies and provide some basic guidelines for effective vaccination programs and management practices.

First, clarification on some confusing terminology. Equine disease can be divided into two basic categories; infectious and contagious. Infectious disease is a clinically evident disease resulting from the presence of a pathogen (virus, bacteria, protozoa, etc.) that is able to cause contaminants. Contagious diseases (also called communicable diseases) include those infectious agents that are transmissible from one horse to another. Contagious diseases can be spread by body fluids/secretions, contaminated objects (dirty needles) and airborne inhalation to name a few. Common contagious diseases in horses include: Equine Infectious Anemia (AIA), Strangles, Influenza, Equine Herpes 1 (EHV 1) and Equine Herpes 4 (EHV 4), Salmonellosis, Rhodococcus equi and Equine Viral Arteritis (EVA).

Age: Many diseases present various levels of risk in different age groups and age can also have a significant influence on the immune response. In young/juvenile horses, the limited or low response to vaccines may present a serous problem. The general recommendation is to delay active vaccination programs until 6 months of age - provided the mare was appropriately vaccinated 2 to 6 weeks before foaling. It may also be difficult to generate an effective immune response even after maternal antibody (passive transfer) has wanted. This is likely due to a general lack of maturity of the immune response. More frequent vaccination may help address this challenge.

Use: Exposure of horses to infectious/contagious diseases and stresses that can increase susceptibility to infection can vary widely. For example, consider the risk of exposure to horses that are competing and showing compared with the risk of the backyard pasture ornament. Nevertheless, both groups are at risk from tetanus and vector-borne (insect bite) such as viral encephalitidies (EEE, WEE). Additionally, a mild viral respiratory infection can have a huge effect on a show horse but may be of little consequence in a pasture-maintained and infrequently used horse.

Disease Prevalence: This is an estimate of how common a disease is (total number of cases of the disease) within a population over a defined period of time. Disease prevalence is sometimes underestimated for diseases such as equine influenza or herpes virus due to subclinical infections. Knowledge of the regional distribution of organisms such a botulinum or rabies is critical in vaccination decisions. Other important examples would include Rhodococcus equi infection on broodmare farms or Strangles infection in a training/boarding facility.

Season: The seasonal nature of many diseases must be recognized in order to time vaccine administration appropriately. The presence of mosquitoes for diseases such as EEE or WEE is an obvious example. Diseases such as influenza are most commonly seen during the summer months while herpes virus is more common in the winter and spring. Seasonality of many diseases is dependent on the breeding season of horses.

Current vaccination strategies can be broadly divided into "live," "dead" and DNA vaccines. DNA vaccine technology is still being actively developed and currently has very limited commercial use.

Live Vaccines: Live vaccines employ an organism that can continue to replicate in the horse but has reduced pathogenicity (ability to cause disease). There are a number of advantages to utilizing live vaccines including a broad range of immune responses, longer lasting immunity and fewer doses. Disadvantages include increased risk of disease in immunocompromised or pregnant horses and contamination of the vaccine with other pathogens.

Modified Live Vaccines: Are generally produced in cell cultures where their pathogenicity is altered (mutated) yet they will still illicit a strong immune response. Examples of modified live vaccines used in horses include intranasal vaccines against Streptococcus equi (Strangles) and equine influenza virus.

Vectored Vaccines: With a 'vectored' vaccine, a specified piece of DNA from a pathogen (bacteria or virus) against which you want immunity is carried into the horse's cells by a carrier or vector that does not cause disease itself. An example of a suitable 'vector' is canarypox virus, which can infect cells but is unable to replicate within the cell. Currently a canarypox-vectored vaccine for West Nile Virus (Recomnitek/Merial) is on the market along with an influenza vaccine based on the same technology.

Dead Vaccines: Dead or 'killed' vaccines remain attractive due to their relative ease of preparation, lack of pathogenicity and inability to replicate and spread between hosts. Disadvantages include the requirement for multiple doses and multiple boosters along with questionable efficacy. Examples include Pneumobort (Ft. Dodge) for equine herpes virus.

Subunit Vaccines: The explosion of knowledge in the field of recombinant DNA has lead to the identification of specific antigens or proteins that are important for immunity to specific diseases. Subunit vaccines are those including only a part of the pathogen being vaccinated against. These types of vaccines are being actively researched for several pathogens including Equine infectious anemia (EIA) and equine protozoal myeloencephalitis (EPM).

1. Tetanus vaccination is appropriate for all horses
2. Active immunization of foals should not begin until the age of 6 months
3. Decision to vaccinate must be based on risk with the aim of minimizing use of vaccines
4. For transmissible diseases (flu, herpes), vaccinate the herd, not the individual
5. Maintain protective immunity throughout risk periods
6. Base vaccine selection on efficacy, not price
7. Benefits must exceed costs and risks

1. Implement pre-and post-arrival policies for new horses
2. Isolate new arrivals until they are tested for disease (or incubation period)
3. Isolate horses returning to premises after attending event sale, etc.
4. Isolate any suspect, ill, or shedder horse until it is no longer a threat to other horses
5. Avoid overpopulation and stress
6. Avoid contact between weanlings/yearlings and older (pregnant) horses
7. Isolating and practicing preventative hygiene is the case of an outbreak

ADULTS

• Tetanus - Frequency Annually -- Comments: Booster if wound
• EEE/WEE -- Frequency Annual to semi-annual -- Comments: 2x in mosquito heavy areas
• Influenza -- Frequency 2x tp 4x/year -- Comments: Internasal 2x/year
• Rhinopneumonitis -- Frequency 2x to 4x/year -- Comments: Killed Intranasal
• Rhinopneumonitis -- Frequency 5, 7, 9 m gestation -- Comments: Abortion prevention
• Rabies -- Frequency Annual
• WNV -- Frequency Annual to semi-annual -- Comments: 2x in mosquito heavy areas
• PHF -- Frequency Semi-annually --Comments: Efficacy?
• EPM -- Frequency Annually -- Comments: Efficacy?
• Strangles -- Frequency Semi-annually -- Comments: Intranasal
• Botulism (mare) -- Frequency 1 m pre-partum -- Comments: Initial 3 dose series
• EVA (stallion) -- Frequency Annually -- Comments: 1 m before breeding
• EVA (mare)-- Frequency Annually -- Comments: Isolate after breeding
• Rotavirus (mare) -- Frequency 8, 9, 10 m gestation -- Comments: Ineffective if given to neonate--not enough time to develop immunity

FOALS

• Tetanus, EEE, WEE, WNV, Rhino -- Boosters 2 to 3 -- Start 3-4 m unvaccinated; 6 m vaccinated
• Influenza -- Boosters 2 to 3 -- Start 6 m unvaccinated; 9 m vaccinated
• Rabies -- Boosters 1 to 2 -- Start 3-4 m unvaccinated; 6 m vaccinated
• Strangles -- Boosters 3-4 -- Start 4-6 m for injectable; 6-9 m for intranasal (I.N.)

--------------------2 (I.N.)--------------------------------------

• PHF -- Boosters 2 -- Start 5-6 m
• Botulism -- Boosters 3 -- Start 2, 4, 8 wks unvaccinated; 2 to 3 m vaccinated
• EVA -- Boosters 1 -- Start 6-12 m to intact colts

Biography: Mark V. Crisman, DVM, MS, D-ACVIM , Virginia - Maryland Regional College of Veterinary Medicine, Blacksburg, Va. Dr. Crisman received his doctorate in veterinary medicine in 1984, from the University of Warsaw, Poland, and his Master of Science in Veterinary Medicine in 1987 from Washington State University. From 1987 until the present, Dr.Crisman has served on the faculty of the Virginia-Maryland Regional College of Veterinary Medicine where his is currently Professor in Clinical Services/Medicine in the Department of Large Animal Clinical Sciences. He became a diplomate of American College of Veterinary Medicine in 1990 and is currently section chief of large animal medicine and surgery. He is also certified in acupuncture by the International Veterinary Acupuncture Society. Dr. Crisman served as primary veterinary consultant for Molecular Tool Inc. at the Johns Hopkins Bayview campus and is currently Director of the Molecular Diagnostics lab at the V-MRCVM. He has authored/co-authored over 70 refereed publications and book chapters. He was teacher of the year (2004) and serves on a variety of committees in the college. His research interests include immunopharmacology with an emphasis in inflammation and his current work is focused on oxidative stress and ischemia-reperfusion injury in horses with colic."