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Vaccination Programs
C H A P T E R
47
Vaccination Programs
ELIZABETH DAVIS
T
he primary goal of vaccination is protection from infectious disease that presents a significant threat to host health. The decision tree analysis used to establish a vaccine protocol is aimed at protecting the horse from disease that, if contracted, will result in a serious or fatal outcome as well as protecting against contagious infectious diseases that are severely debilitating and can lead to high morbidity rates.
Unvaccinated or Unknown Vaccine Status in an Adult Horse
VACCINE PROTOCOLS
Killed bivalent or trivalent vaccines are currently available for Eastern equine encephalomyelitis, Western equine en cephalomyelitis, and Venezuelan equine encephalomyelitis. The initial vaccine series should include a series of three vaccinations given at 3- to 6-week intervals. Depending on the geographic location, age, and health status of the individual horse, booster vaccines should be administered two to four times annually.
Factors to specifically consider in designing a vaccination program include whether the horse has been previously vaccinated, the age and health status of the horse, and, with mares, whether the horse is pregnant. Additional details regarding equine vaccine recommendations can be found at http://www.aaep.org/vaccination_guidelines.htm.
Initial Vaccine Series During the First Year of Life (In Foals From a Vaccinated Dam) The goal of the initial vaccine series is to induce optimal immunity so that at the time of challenge with infectious disease, the host is well protected with an immune response of pronounced antigenic specificity and memory. This is best achieved by initiating a vaccination series with three doses. The initial dose should be administered at 4 to 6 months of age, with an interval of 3 to 6 weeks passing until the second dose. The third dose should be administered at 10 to 12 months of age and should be followed by the interval recommended on the product label before the next vaccination. This interval is typically 6 to 12 months, depending on the disease.
Initial Vaccine Series During the First Year of Life (Vaccine Status of Dam Unknown or Unvaccinated) Specific label instructions should be followed, but in the instance in which the dam was unvaccinated or had an unknown vaccination status, vaccination protocols will generally be initiated when the foal is 3 to 4 months of age. The initial series will still include three vaccinations, a 3- to 6-week interval between the first and second vaccination, and a third vaccination approximately 8 weeks after the second vaccination. When vaccinating for seasonal diseases such as the viral encephalitides, the protocol may need to be abbreviated so that the three-dose series is completed before peak mosquito season.
Previously Vaccinated Adult Horse Consistent with label instructions, a booster vaccine is required at a specific interval. Depending on the disease, this may be a 6- to 12-month interval.
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Consistent with label instructions, an initial series of two vaccinations with a 3- to 6-week interval between doses should provide adequate protection against future challenge.
CORE VACCINES
Equine Encephalomyelitis (Sleeping Sickness)
West Nile Virus Protection against West Nile virus (WNV) encephalomyelitis, similar to other vector-mediated encephalitides, requires strict attention to control of insects as well as an effective vaccination program. Four vaccines are currently licensed by the U.S. Department of Agriculture for use in horses to aid in protection against WNV: two inactivated vaccines, a nonreplicating canarypox-vectored vaccine, and an inactivated flavivirus chimeric vaccine, are available. Administration of inactivated WNV vaccines, consistent with label instructions, involves administration of an intramuscular dose 3 to 6 weeks apart, followed by a 12-month revaccination interval. Because recombinant canarypoxvectored vaccine protective antigens are expressed in a canarypox vector that does not replicate in the horse, the vaccine contains an adjuvant. Label recommendations include intramuscular injection with a primary series including a 4- to 6-week interval, followed by a 12-month period before revaccination. Inactivated flavivirus chimeric vaccine with protective antigens expressed in a yellow fever virus vector also contains an adjuvant. Label instructions include a primary series of two vaccinations to be administered with a 3- to 4-week interval and followed by a 12-month period before revaccination. An adult horse that has previously been vaccinated for WNV should receive an annual vaccine in the spring, before the onset of the mosquito vector season. An adult horse that is unvaccinated for WNV, or is of unknown vaccine status, should receive a primary series of two vaccinations administered with a 3- to 6-week interval, followed by a 12-month interval before the next vaccination. Although there are limited data on the safety and efficacy of WNV vaccination of pregnant broodmares, it is common for veterinarians to
vaccinate these animals because of the inherent risk associated with contraction of the disease. A booster vaccination is recommended 4 to 6 weeks before expected foaling to provide WNV-specific immunoglobulin in the colostrum. Previous reports have demonstrated that foals vaccinated in the face of a significant maternal antibody titer can mount an effective endogenous humoral immune response to WNV. Current recommendations call for the initiation of a threedose series beginning at 4 to 6 months of age. The initial vaccination should be followed by a booster vaccination in 4 to 6 weeks, with a third vaccination at approximately 10 to 12 months of age, before the onset of vector insect season the following spring. Foals that are delivered by unvaccinated dams (or those with an unknown vaccine status) should receive WNV vaccines at 3 to 4 months of age, with the goal to complete the three-vaccine series before the peak insect season. The duration between the first and second dose should be approximately 30 days, with approximately 60 days passing between the second and third dose. If vaccination is taking place during the mosquito vector season, the entire protocol should be completed closer to an 8-week interval.
Tetanus Given the ubiquitous nature of the bacterium Clostridium tetani in the environment and in the equine gastrointestinal tract, optimal tetanus immunization protocols should be maintained for all horses. Toxoids are nontoxic derivatives of toxins that are used as antigens. All horses should be vaccinated with an annual booster. A booster vaccination should also be administered at the time of a penetrating wound or invasive procedure, particularly if the previous vaccine was administered longer than 6 months previously. Unvaccinated horses or those with an unknown vaccine status should receive a two-dose series, with a 4- to 6-week interval between vaccines. When vaccinating horses during the first year of life, a three-dose vaccine series should begin at approximately 4 to 6 months of age, with the initial two vaccines administered approximately 4 to 6 weeks apart and followed by a third booster vaccination at 10 to 12 months of age. If an injury occurs and vaccine status is not known, the vaccine series should be initiated with a priming vaccine. In addition, and at a distant muscular site, tetanus antitoxin should also be administered. A second booster vaccination should be administered approximately 4 to 6 weeks later to complete the vaccination series. Because of the risk for Theiler’s disease developing after the administration of tetanus antitoxin, the risks should be carefully considered. Pregnant broodmares should be properly vaccinated against tetanus. If they have not been previously vaccinated, they should receive a two-dose series with a 4- to 6-week interval. All mares should receive a booster tetanus vaccine approximately 4 to 6 weeks before expected foaling. This will ensure protection for the mare from tetanus secondary to a foaling injury, retained placenta, or endometritis. In addition, this will ensure adequate colostral antibody protection for the neonatal foal. Foals delivered to unvaccinated mares should receive their initial tetanus vaccine at 1 to 4 months of age. The initial series should include three vaccinations to ensure optimal immune response. The initial two vaccines are administered with a 4- to 6-week interval, followed by the third vaccination 4 to 6 weeks after the second. Annual booster vaccination is recommended for all horses. If the foal is born to an unvaccinated mare and is at high risk for the development of tetanus, tetanus antitoxin should be administered as previously described.
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Rabies Three inactivated vaccines are currently licensed for the protection of horses from rabies. Rabies virus is an excellent antigen that induces a pronounced serologic response following administration of a single vaccine. All horses maintained in areas where rabies is endemic should be vaccinated. Veterinarians should carefully read the label of rabies vaccines before administration. All adult horses should receive an annual booster as a component of a routine preventive health program. Pregnant broodmares may receive a booster vaccine 4 to 6 weeks before expected foaling date to provide adequate colostral immunoglobulin concentrations. Alternatively, because rabies is such a pronounced immunogen and serologic responses are maintained for extended periods of time, it may be preferable to vaccinate mares before breeding and not administer a booster before foaling. This practice will aid in reduction of the number of vaccines that late-term pregnant broodmares receive before foaling. Label instructions do not include approval for use in pregnant mares, but in endemic or high-risk situations, it is appropriate to vaccinate at-risk individuals. When rabies vaccine is administered to a horse in its first year of life and the vaccine status of the dam is current, the initial two-dose series should begin at approximately 6 months of age, with a booster vaccine administered in 4 to 6 weeks. An annual booster vaccination is recommended in all horses. Foals born to mares that were not vaccinated or those with an unknown vaccine status should be initially vaccinated at 3 to 4 months of age, followed by a booster vaccine in 4 to 6 weeks.
RISK-BASED VACCINES Respiratory Disease Equine Influenza
Equine influenza caused by the orthomyxovirus influenza A type 2 (A/equine 2) is one of the most common infections of the equine respiratory tract. Equine influenza rarely circulates among equine populations; rather, it is introduced into a group by an infected individual. Because this virus is rapidly removed by the immune system in horses, appropriate quarantine protocols (14-day quarantine upon arrival) and effective vaccine strategies can substantially limit the potential for disease outbreak. Therefore all horses on breeding and training facilities should be regularly vaccinated for equine influenza. Risk factors associated with disease include (1) young horses, 1 to 5 years of age; (2) ineffective serum concentrations of influenza-specific antibodies; (3) high-risk environment, such as frequent contact with large numbers of horses. Influenza is rapidly spread among groups of horses through the transmission of virus-containing fluids, such as respiratory secretions from infected individuals (e.g., spread through coughing). Horses with incomplete immune protection can become subclinically infected and subsequently shed virus. Immunity following vaccination with an inactivated vaccine may be short lived. Therefore vaccination with an approved vaccine with known efficacy should be selected for use as an aid to prevent disease (see also Chapter 39). Three influenza vaccines are commercially available at present: • Inactivated virus for intramuscular administration. These vaccines contain many of the A2 strains of influenza virus that are currently in circulation. Vaccination is recommended to include a three-dose priming series with the initial doses administered at a 4- to 6-week interval; the third dose should be administered 8 to 12 weeks
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after the second. These vaccines are well suited for a prefoaling series that will induce high levels of colostral immunoglobulin. • Modified live cold-adapted equine influenza A/2 vaccine for intranasal administration. This vaccine provides rapid protection to naïve individuals. Although the product is labeled for protection of horses 11 months of age or older, vaccination of horses 6 months of age and older is recognized to safely provide protection from disease. Label claims report 6 months of protection; however, it is recognized to confer protection for 12 months. Circulating immunoglobulin levels are not markedly elevated following vaccination; local mucosal immune protection is recognized to confer immunity to individuals following vaccination. • Canarypox-vectored vaccine for intramuscular administration. This product is safe to administer as early as 4 months of age. It induces a strong humoral response following vaccination, suggesting that this is also an appropriate product to administer during late-term pregnancy to enhance colostral antibody levels. Adult horses that have previously been immunized and are at risk for exposure can be effectively protected by vaccination twice annually. If risk for disease is relatively low, annual booster vaccination should be sufficient to provide protection. Previously unvaccinated adult horses can effectively be immunized with an individual dose of the modified live virus (MLV) intranasal vaccine, followed by repeat vaccination at 6-month intervals. Alternatively, an initial twodose series of the canarypox-vectored vaccine or a three-dose series of an inactivated vaccine should provide protection. When a three-dose series of the inactivated vaccine is administered, a period of 3 to 4 weeks between the initial two vaccinations, followed by a period of 3 to 6 months before the third vaccination, will provide optimal protection. Subsequent revaccination should be performed at 6- to 12-month intervals, depending on the use and risk for exposure of the horse. Broodmares previously vaccinated should receive a booster vaccine approximately 4 to 6 weeks before expected foaling with either an inactivated product or the canarypoxvectored vaccine. If a broodmare is previously unvaccinated, a three-dose series of inactivated product should be administered with an interval of 4 to 6 weeks between the first two vaccines, followed by a third dose 4 to 6 weeks before expected foaling. Alternatively, a two-dose series of the canarypox-vectored vaccine can be administered so that the second dose of vaccines is administered approximately 4 to 6 weeks before the expected foaling date. Horses in the first year of life and less than 11 months of age should begin receiving vaccination at approximately 6 months of age. If the MLV intranasal vaccine is selected, the initial dose should be administered at 6 to 7 months of age, with a booster vaccine administered at 11 to 12 months of age. If the canarypox-vectored vaccine is administered, it should be initiated at approximately 6 to 7 months of age with a booster vaccine administered in 4 to 6 weeks. If the inactivated vaccine is selected, it should be administered in a three-dose series with the initial dose at approximately 6 months of age, the second dose approximately 4 to 6 weeks later, and a third dose 3 to 6 months after that. In foals born to a mare that is suspected to be unvaccinated, some maternal antibody transfer is still likely; because of the effect of maternal antibody interference, the vaccine series should be initiated at approximately 6 months of age.
Equine Herpesvirus Types 1 and 4 Both equine herpesvirus type 1 (EHV-1) and type 4 (EHV-4) cause upper respiratory tract disease (equine rhinopneumonitis), but EHV-1 also causes abortion and neurologic disease. All horses on breeding and training farms should be regularly vaccinated against these viruses. Several inactivated products and one MLV vaccine are commercially available. Vaccination is recommended for use in pregnant mares to aid in the prevention of abortigenic EHV-1 infection and prevention of EHV-1– and EHV-4–induced respiratory disease in foals, weanlings, yearlings, and young performance and show horses that are included in the high-risk group for viral exposure. High antigen-load vaccines are labeled for protection from abortion in pregnant mares. None of the currently available vaccines has a label claim for prevention against neurologic disease. An initial series of three vaccinations is recommended, with a 4- to 6-week interval between doses. When the initial series is administered in foals, vaccination should be initiated at 4 to 6 months of age, with a 4- to 6-week interval between the first two doses and the third dose given at 10 to 12 months of age. Booster vaccines are recommended to maintain adequate immunity at approximately 6-month intervals. Pregnant broodmares should be vaccinated at 5, 7, and 9 months of gestation with an inactivated high antigen-load product to protect against abortion. Many clinicians also vaccinate at 3 months of gestation. To provide adequate colostral antibody protection, booster vaccination with an inactivated EHV-1 and EHV-4 vaccine should be administered 4 to 6 weeks before expected foaling. Other horses maintained on breeding farms, such as barren mares, stallions, and teaser stallions, should be vaccinated at the beginning of the breeding season and given boosters at 6-month intervals. Adult horses should be vaccinated every 6 to 12 months. Young horses, which are at higher risk for infection because of their age, likelihood of travel, and comingling, will be optimally protected with booster vaccinations given every 6 months.
Streptococcus equi equi Vaccination for protection against strangles is recommended in high-risk horses, such as those being introduced onto a farm with endemic disease. Two Streptococcus equi subsp equi vaccines are currently commercially available: an inactivated subunit vaccine, administered intramuscularly, and an MLV vaccine, administered intranasally. Specific information regarding disease pathogenesis and outbreak control has recently been reported. See the American College of Veterinary Internal Medicine (ACVIM) Consensus Statement (Sweeney et al., 2005) in Suggested Readings. Although vaccination appears to reduce disease severity, it is not without risk for adverse events. For this reason, when determining the indications for vaccination against S equi equi, consideration should be made to vaccinate only horses at high risk for exposure to this bacterium. A potential adverse event associated with use of the MLV vaccine is localized abscess formation. If this vaccine is administered concurrently with other vaccines, care should implemented to complete administration of the other vaccines first, and then prepare and handle the MLV product separately. Such precautions will aid in avoiding inadvertent intramuscular injection of live bacterial organisms. An additional potential complication associated with use of any S equi equi vaccine is the development of purpura hemorrhagica, a generalized immune-mediated vasculitis that can occur following recovery from S equi equi infection or following vaccination,
particularly in horses vaccinated during a course of disease outbreak. For these reasons, it is important for the equine clinician to carefully consider vaccine risks and consult the ACVIM 2005 Consensus Statement to ensure that the risk for adverse events is minimized. Horses not previously vaccinated against S equi equi that are receiving the MLV vaccine should receive an initial intranasal series of two doses given 3 to 4 weeks apart. Booster vaccination is recommended at 6- to 12-month intervals based on risk assessment and manufacturer instructions. Adult horses receiving the killed vaccine should be vaccinated with an initial intramuscular series of three vaccinations at 4- to 6-week intervals, with booster vaccination at 6- to 12-month intervals based on risk assessment and manufacturer instructions. Foals in high-risk situations should be vaccinated with the killed vaccine beginning at 4 to 6 months of age, with an initial series of three doses administered at 4- to 6-week intervals. Booster vaccinations should be administered at 6- to 12-month intervals. The intranasal MLV has been used in foals and may be administered with an initial two-dose series, starting at 6 to 9 months of age, with a 3-week interval between doses. The risk for adverse events associated with vaccine administration is increased when the MLV vaccine is administered to young foals.
Nonencephalitis Neurologic Disease Botulism
The potent toxins produced by the spore-forming bacteria Clostridium botulinum are associated with toxicoinfectious botulism, forage poisoning, wound botulism, and equine grass sickness. Most cases of shaker foal syndrome are caused by C botulinum type B. This organism is a significant threat in Kentucky and the mid-Atlantic states in foals 2 weeks to 8 months of age. A toxoid vaccine for C botulinum type B only is licensed for use in the United States. Current evidence suggests that equine grass sickness is a result of C botulinum type C; therefore the type B vaccine would be ineffective in protecting against this form of disease. The primary indication for the type B vaccine is to prevent shaker foal syndrome by inducing high colostral antibody concentrations in the vaccinated mare. At the time of this writing, there are insufficient data to support the suggestion that neonatal foals respond adequately to vaccination and derive protection from the disease. After passively derived maternal antibodies decline, foal vaccination is recommended in endemic regions. Vaccine recommendations for mares that have previously been vaccinated involve a booster vaccine approximately 4 to 6 weeks before expected foaling date. If the mare is receiving the series for the first time, the three-dose vaccine series should be targeted so that the third vaccine is administered approximately 4 to 6 weeks before the expected foaling date. Subsequent to the initial series, an annual booster should be administered, approximately 4 to 6 weeks before expected foaling. Foal vaccinations for botulism should begin early in life, particularly for foals born to unvaccinated mares. There is no evidence of maternal antibody interference with botulism vaccine administration to foals, so vaccination may begin in the face of circulating maternal antibodies. Foals born to unvaccinated mares should begin a three-vaccine series at 1 to 3 months of age, with a 4-week interval between vaccinations. Foals in high-risk situations have effectively been vaccinated as early as 2 weeks of life. Although not proved to provide complete protection, foals born to unvaccinated mares may benefit from plasma transfusion harvested from a vaccinated horse or administration of C botulinum type B
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antitoxin. Foals born to previously vaccinated mares should begin a three-vaccine protocol at 2 to 3 months of age, with a 4-week interval between vaccinations. Other horses that are maintained in a high-risk setting for development of type B C botulinum–associated disease should receive a series of three vaccinations at 4-week intervals, with an annual booster administered. Duration of immunity following recovery from botulism (in adult horses or foals) is highly variable, and vaccination is recommended following complete recovery from disease, starting with an initial threedose series, followed by an annual booster.
Abortive Disease
Equine Viral Arteritis Equine viral arteritis is caused by equine arteritis virus. The primary health hazards to horses are abortion in pregnant mares, illness and death in young foals, and the chronic carrier state in stallions. It is possible to develop effective programs for the prevention and control of equine viral arteritis based on the known characteristics of the causal agent and the epidemiology of the disease. Infection can be controlled by minimizing or eliminating direct or indirect contact with various secretions, excretions, or tissues of infected horses. An important component of successfully controlling disease is implementation of an appropriate vaccine strategy. The MLV vaccine effectively induces protection in vaccinated individuals. It is impossible to distinguish seroconversion secondary to vaccination from that resulting from natural exposure. It is therefore strongly recommended that all first-time vaccinates have serum collected for testing by a laboratory approved by the U.S. Department of Agriculture and confirmed negative for antibodies against equine arteritis virus, before the vaccine is administered. Because immunity is not induced immediately, the vaccine should be administered to stallions no less than 4 weeks before breeding. Similarly, mares should be vaccinated no less than 4 weeks before breeding. After being vaccinated, a mare should be isolated from any other horses that may transmit the virus because the mare will not be immediately protected after immunization. Pregnant mares and foals younger than 6 weeks should not be vaccinated.
Equine Herpesvirus Type 1 See recommendations for the management of EHV-associated rhinopneumonitis. Broodmares should be vaccinated during months 3, 5, 7, and 9 of gestation, with an inactivated product labeled for the protection against abortive EHV-1 (high–antigen-load vaccine).
Other
Anthrax Anthrax is a serious and commonly fatal septicemic condition resulting from infection with the bacterium Bacillus anthracis. Vaccination is recommended in horses where the disease is endemic and only in at-risk individuals because adverse reactions have been reported in young horses and miniature breeds. The reactions, which include injection site pain and swelling, generally resolve in a number of days. Adult horses that have been previously vaccinated should receive annual booster administration. Horses that have not been previously vaccinated should begin an initial two-dose series, administered subcutaneously, with a 4- to 6-week interval. Annual booster vaccines should be con tinued while horses are maintained in endemic regions. Routine vaccination for broodmares and young horses is not recommended.
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Potomac Horse Fever Equine monocytic ehrlichiosis caused by Neorickettsia risticii (formerly Ehrlichia risticii) is a seasonal disease of late spring to early fall. Clinical signs may include fever, diarrhea, laminitis, colic, and reduced gastrointestinal motility. In endemic regions, one component of reducing disease severity is vaccination. Two commercial killed vaccines are available for use in horses considered at high risk for the development of disease. Pregnant mares that develop disease are at risk for abortion, but neither of the commercially available vaccines claims to protect against this. Adult horses that have previously been vaccinated should be vaccinated before the peak season associated with disease, typically in the spring months. Booster vaccine administration is recommended at 3- to 4-month intervals in high-risk situations because the duration of immunity induced by vaccination appears to be short lived. Horses that have not been previously vaccinated should receive an initial series of two vaccines at 3- to 4-week intervals, with the second dose completed 2 to 3 weeks before the anticipated peak incidence of disease. Broodmares should receive a booster vaccine approximately 4 to 6 weeks before expected foaling. If the mare has not been previously vaccinated for Potomac horse fever, she should receive a twodose series, with the second dose administered approximately 4 to 6 weeks before expected foaling. In foals, the initial dose should be given at 5 to 6 months of age, with a second dose to follow in 3 to 4 weeks; a third dose should then be administered at 10 to 12 months of age. Boosters are recommended at 3- to 6-month intervals, depending on the risk for disease. Boosters should be given in May to June in endemic regions.
Rotavirus Rotavirus is a nonenveloped RNA virus that is an important infectious cause of foal diarrhea. Although many factors, such as environmental conditions and exposure to infected individuals on high-density farms, play important roles in development of diarrhea in foals, mare vaccination against rotavirus results in high antibody levels in colostrum and subsequently increased foal serum antibody concentrations. Evidence to support mare vaccination is based on field trials that have demonstrated attenuation of rotaviral diarrhea cases. One commercially available inactivated vaccine contains group A rotavirus and is indicated for use in mares to enhance colostral immunoglobulin concentration against equine group A rotavirus. Mares should receive a three-dose series of intramuscular vaccinations at 8, 9, and 10 months of gestation, regardless of previous vaccine status. This vaccine is used frequently in regions where large populations of mares and foals are housed; in addition, mares that will be shipped to high-density areas should be vaccinated in preparation for exposure to rotaviral challenge. It is important to ensure that foals receive an adequate quantity of colostrum from mares that have been vaccinated against rotavirus. There is no evidence to suggest that newborn foals require vaccination
against rotavirus. Serum colostral antibody titer declines at approximately 60 days of age; although there is some risk for development of rotaviral diarrhea at this age, the risk is considered low, and if disease develops, it is generally self-limited.
Snake Bite Vaccine Venomous snakebites occur in certain areas of North America. Based on risk for exposure, there may be an indication for vaccination of horses in high-risk areas with the Crotalus atrox (western diamondback rattlesnake) toxoid vaccine. At present, one toxoid vaccine with a conditional license is available for use in horses that are at risk for rattlesnake envenomation. The label suggests that there may be crossprotection against venom of the western rattlesnake (including the prairie, Great Basin, northern and southern, and Pacific varieties), sidewinder, timber rattlesnake, Massasauga, and copperhead. Partial protection may be provided against eastern diamondback venom, but not against water moccasin (cottonmouth), Mojave rattlesnake, or coral snake venom. The toxoid vaccine is labeled for use in horses older than 6 months of age. The primary series includes three doses to be administered at monthly intervals, with subsequent booster vaccination at 6-month intervals. Use of this vaccine in pregnant broodmares has not been specifically investigated, and if needed, direct consultation with the manufacturer is recommended.
Suggested Readings Horohov DW, Lunn DP, Townsend HGG, et al. Equine vaccination. J Vet Intern Med 2000;4:221-222. Krebs JW, Mandel EJ, Swerdlow DL, Rupprecht CE. Rabies surveillance in the United States during 2004. J Am Vet Med Assoc 2005;227:1912-1925. Kydd JH, Townsend HG, Hannant D. The equine immune response to equine herpesvirus-1: the virus and its vaccines. Vet Immunol Immunopathol 2006;111(1-2):15-30. Madigan JE, Pusterla N. Ehrlichial diseases. Vet Clin North Am Equine Pract 2000;16 (3):487-499. Powell DG, Dwyer RM, Traub-Dargatz JL, et al. Field study of the safety, immunogenicity, and efficacy of an inactivated equine rotavirus vaccine. J Am Vet Med Assoc 1997;211:193-198. Sweeney CR, Timoney JF, Newton JR, et al. Streptococcus equi infections in horses: guidelines for treatment, control and prevention of strangles, 2005; ACVIM Consensus Statement. Retrieved September 3, 2012, from http://www.acvim.org/ websites/acvim/index.php?p=22. Timoney PJ, McCollum WH. Equine viral arteritis. Vet Clin North Am Equine Pract 1993;9:295-309. Townsend HGG, Lunn DP, Bogdan J, et al. Comparative efficacy of commercial vaccines in naive horses: serologic responses and protection after influenza challenge. In: Proceedings of the 49th Annual American Association of Equine Practitioners Convention, New Orleans, 2003. Whitlock RH, Buckley C. Botulism. Vet Clin North Am Equine Pract 1997;13:107.
47
Vaccination Programs
ELIZABETH DAVIS
T
he primary goal of vaccination is protection from infectious disease that presents a significant threat to host health. The decision tree analysis used to establish a vaccine protocol is aimed at protecting the horse from disease that, if contracted, will result in a serious or fatal outcome as well as protecting against contagious infectious diseases that are severely debilitating and can lead to high morbidity rates.
Unvaccinated or Unknown Vaccine Status in an Adult Horse
VACCINE PROTOCOLS
Killed bivalent or trivalent vaccines are currently available for Eastern equine encephalomyelitis, Western equine en cephalomyelitis, and Venezuelan equine encephalomyelitis. The initial vaccine series should include a series of three vaccinations given at 3- to 6-week intervals. Depending on the geographic location, age, and health status of the individual horse, booster vaccines should be administered two to four times annually.
Factors to specifically consider in designing a vaccination program include whether the horse has been previously vaccinated, the age and health status of the horse, and, with mares, whether the horse is pregnant. Additional details regarding equine vaccine recommendations can be found at http://www.aaep.org/vaccination_guidelines.htm.
Initial Vaccine Series During the First Year of Life (In Foals From a Vaccinated Dam) The goal of the initial vaccine series is to induce optimal immunity so that at the time of challenge with infectious disease, the host is well protected with an immune response of pronounced antigenic specificity and memory. This is best achieved by initiating a vaccination series with three doses. The initial dose should be administered at 4 to 6 months of age, with an interval of 3 to 6 weeks passing until the second dose. The third dose should be administered at 10 to 12 months of age and should be followed by the interval recommended on the product label before the next vaccination. This interval is typically 6 to 12 months, depending on the disease.
Initial Vaccine Series During the First Year of Life (Vaccine Status of Dam Unknown or Unvaccinated) Specific label instructions should be followed, but in the instance in which the dam was unvaccinated or had an unknown vaccination status, vaccination protocols will generally be initiated when the foal is 3 to 4 months of age. The initial series will still include three vaccinations, a 3- to 6-week interval between the first and second vaccination, and a third vaccination approximately 8 weeks after the second vaccination. When vaccinating for seasonal diseases such as the viral encephalitides, the protocol may need to be abbreviated so that the three-dose series is completed before peak mosquito season.
Previously Vaccinated Adult Horse Consistent with label instructions, a booster vaccine is required at a specific interval. Depending on the disease, this may be a 6- to 12-month interval.
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Consistent with label instructions, an initial series of two vaccinations with a 3- to 6-week interval between doses should provide adequate protection against future challenge.
CORE VACCINES
Equine Encephalomyelitis (Sleeping Sickness)
West Nile Virus Protection against West Nile virus (WNV) encephalomyelitis, similar to other vector-mediated encephalitides, requires strict attention to control of insects as well as an effective vaccination program. Four vaccines are currently licensed by the U.S. Department of Agriculture for use in horses to aid in protection against WNV: two inactivated vaccines, a nonreplicating canarypox-vectored vaccine, and an inactivated flavivirus chimeric vaccine, are available. Administration of inactivated WNV vaccines, consistent with label instructions, involves administration of an intramuscular dose 3 to 6 weeks apart, followed by a 12-month revaccination interval. Because recombinant canarypoxvectored vaccine protective antigens are expressed in a canarypox vector that does not replicate in the horse, the vaccine contains an adjuvant. Label recommendations include intramuscular injection with a primary series including a 4- to 6-week interval, followed by a 12-month period before revaccination. Inactivated flavivirus chimeric vaccine with protective antigens expressed in a yellow fever virus vector also contains an adjuvant. Label instructions include a primary series of two vaccinations to be administered with a 3- to 4-week interval and followed by a 12-month period before revaccination. An adult horse that has previously been vaccinated for WNV should receive an annual vaccine in the spring, before the onset of the mosquito vector season. An adult horse that is unvaccinated for WNV, or is of unknown vaccine status, should receive a primary series of two vaccinations administered with a 3- to 6-week interval, followed by a 12-month interval before the next vaccination. Although there are limited data on the safety and efficacy of WNV vaccination of pregnant broodmares, it is common for veterinarians to
vaccinate these animals because of the inherent risk associated with contraction of the disease. A booster vaccination is recommended 4 to 6 weeks before expected foaling to provide WNV-specific immunoglobulin in the colostrum. Previous reports have demonstrated that foals vaccinated in the face of a significant maternal antibody titer can mount an effective endogenous humoral immune response to WNV. Current recommendations call for the initiation of a threedose series beginning at 4 to 6 months of age. The initial vaccination should be followed by a booster vaccination in 4 to 6 weeks, with a third vaccination at approximately 10 to 12 months of age, before the onset of vector insect season the following spring. Foals that are delivered by unvaccinated dams (or those with an unknown vaccine status) should receive WNV vaccines at 3 to 4 months of age, with the goal to complete the three-vaccine series before the peak insect season. The duration between the first and second dose should be approximately 30 days, with approximately 60 days passing between the second and third dose. If vaccination is taking place during the mosquito vector season, the entire protocol should be completed closer to an 8-week interval.
Tetanus Given the ubiquitous nature of the bacterium Clostridium tetani in the environment and in the equine gastrointestinal tract, optimal tetanus immunization protocols should be maintained for all horses. Toxoids are nontoxic derivatives of toxins that are used as antigens. All horses should be vaccinated with an annual booster. A booster vaccination should also be administered at the time of a penetrating wound or invasive procedure, particularly if the previous vaccine was administered longer than 6 months previously. Unvaccinated horses or those with an unknown vaccine status should receive a two-dose series, with a 4- to 6-week interval between vaccines. When vaccinating horses during the first year of life, a three-dose vaccine series should begin at approximately 4 to 6 months of age, with the initial two vaccines administered approximately 4 to 6 weeks apart and followed by a third booster vaccination at 10 to 12 months of age. If an injury occurs and vaccine status is not known, the vaccine series should be initiated with a priming vaccine. In addition, and at a distant muscular site, tetanus antitoxin should also be administered. A second booster vaccination should be administered approximately 4 to 6 weeks later to complete the vaccination series. Because of the risk for Theiler’s disease developing after the administration of tetanus antitoxin, the risks should be carefully considered. Pregnant broodmares should be properly vaccinated against tetanus. If they have not been previously vaccinated, they should receive a two-dose series with a 4- to 6-week interval. All mares should receive a booster tetanus vaccine approximately 4 to 6 weeks before expected foaling. This will ensure protection for the mare from tetanus secondary to a foaling injury, retained placenta, or endometritis. In addition, this will ensure adequate colostral antibody protection for the neonatal foal. Foals delivered to unvaccinated mares should receive their initial tetanus vaccine at 1 to 4 months of age. The initial series should include three vaccinations to ensure optimal immune response. The initial two vaccines are administered with a 4- to 6-week interval, followed by the third vaccination 4 to 6 weeks after the second. Annual booster vaccination is recommended for all horses. If the foal is born to an unvaccinated mare and is at high risk for the development of tetanus, tetanus antitoxin should be administered as previously described.
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Rabies Three inactivated vaccines are currently licensed for the protection of horses from rabies. Rabies virus is an excellent antigen that induces a pronounced serologic response following administration of a single vaccine. All horses maintained in areas where rabies is endemic should be vaccinated. Veterinarians should carefully read the label of rabies vaccines before administration. All adult horses should receive an annual booster as a component of a routine preventive health program. Pregnant broodmares may receive a booster vaccine 4 to 6 weeks before expected foaling date to provide adequate colostral immunoglobulin concentrations. Alternatively, because rabies is such a pronounced immunogen and serologic responses are maintained for extended periods of time, it may be preferable to vaccinate mares before breeding and not administer a booster before foaling. This practice will aid in reduction of the number of vaccines that late-term pregnant broodmares receive before foaling. Label instructions do not include approval for use in pregnant mares, but in endemic or high-risk situations, it is appropriate to vaccinate at-risk individuals. When rabies vaccine is administered to a horse in its first year of life and the vaccine status of the dam is current, the initial two-dose series should begin at approximately 6 months of age, with a booster vaccine administered in 4 to 6 weeks. An annual booster vaccination is recommended in all horses. Foals born to mares that were not vaccinated or those with an unknown vaccine status should be initially vaccinated at 3 to 4 months of age, followed by a booster vaccine in 4 to 6 weeks.
RISK-BASED VACCINES Respiratory Disease Equine Influenza
Equine influenza caused by the orthomyxovirus influenza A type 2 (A/equine 2) is one of the most common infections of the equine respiratory tract. Equine influenza rarely circulates among equine populations; rather, it is introduced into a group by an infected individual. Because this virus is rapidly removed by the immune system in horses, appropriate quarantine protocols (14-day quarantine upon arrival) and effective vaccine strategies can substantially limit the potential for disease outbreak. Therefore all horses on breeding and training facilities should be regularly vaccinated for equine influenza. Risk factors associated with disease include (1) young horses, 1 to 5 years of age; (2) ineffective serum concentrations of influenza-specific antibodies; (3) high-risk environment, such as frequent contact with large numbers of horses. Influenza is rapidly spread among groups of horses through the transmission of virus-containing fluids, such as respiratory secretions from infected individuals (e.g., spread through coughing). Horses with incomplete immune protection can become subclinically infected and subsequently shed virus. Immunity following vaccination with an inactivated vaccine may be short lived. Therefore vaccination with an approved vaccine with known efficacy should be selected for use as an aid to prevent disease (see also Chapter 39). Three influenza vaccines are commercially available at present: • Inactivated virus for intramuscular administration. These vaccines contain many of the A2 strains of influenza virus that are currently in circulation. Vaccination is recommended to include a three-dose priming series with the initial doses administered at a 4- to 6-week interval; the third dose should be administered 8 to 12 weeks
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after the second. These vaccines are well suited for a prefoaling series that will induce high levels of colostral immunoglobulin. • Modified live cold-adapted equine influenza A/2 vaccine for intranasal administration. This vaccine provides rapid protection to naïve individuals. Although the product is labeled for protection of horses 11 months of age or older, vaccination of horses 6 months of age and older is recognized to safely provide protection from disease. Label claims report 6 months of protection; however, it is recognized to confer protection for 12 months. Circulating immunoglobulin levels are not markedly elevated following vaccination; local mucosal immune protection is recognized to confer immunity to individuals following vaccination. • Canarypox-vectored vaccine for intramuscular administration. This product is safe to administer as early as 4 months of age. It induces a strong humoral response following vaccination, suggesting that this is also an appropriate product to administer during late-term pregnancy to enhance colostral antibody levels. Adult horses that have previously been immunized and are at risk for exposure can be effectively protected by vaccination twice annually. If risk for disease is relatively low, annual booster vaccination should be sufficient to provide protection. Previously unvaccinated adult horses can effectively be immunized with an individual dose of the modified live virus (MLV) intranasal vaccine, followed by repeat vaccination at 6-month intervals. Alternatively, an initial twodose series of the canarypox-vectored vaccine or a three-dose series of an inactivated vaccine should provide protection. When a three-dose series of the inactivated vaccine is administered, a period of 3 to 4 weeks between the initial two vaccinations, followed by a period of 3 to 6 months before the third vaccination, will provide optimal protection. Subsequent revaccination should be performed at 6- to 12-month intervals, depending on the use and risk for exposure of the horse. Broodmares previously vaccinated should receive a booster vaccine approximately 4 to 6 weeks before expected foaling with either an inactivated product or the canarypoxvectored vaccine. If a broodmare is previously unvaccinated, a three-dose series of inactivated product should be administered with an interval of 4 to 6 weeks between the first two vaccines, followed by a third dose 4 to 6 weeks before expected foaling. Alternatively, a two-dose series of the canarypox-vectored vaccine can be administered so that the second dose of vaccines is administered approximately 4 to 6 weeks before the expected foaling date. Horses in the first year of life and less than 11 months of age should begin receiving vaccination at approximately 6 months of age. If the MLV intranasal vaccine is selected, the initial dose should be administered at 6 to 7 months of age, with a booster vaccine administered at 11 to 12 months of age. If the canarypox-vectored vaccine is administered, it should be initiated at approximately 6 to 7 months of age with a booster vaccine administered in 4 to 6 weeks. If the inactivated vaccine is selected, it should be administered in a three-dose series with the initial dose at approximately 6 months of age, the second dose approximately 4 to 6 weeks later, and a third dose 3 to 6 months after that. In foals born to a mare that is suspected to be unvaccinated, some maternal antibody transfer is still likely; because of the effect of maternal antibody interference, the vaccine series should be initiated at approximately 6 months of age.
Equine Herpesvirus Types 1 and 4 Both equine herpesvirus type 1 (EHV-1) and type 4 (EHV-4) cause upper respiratory tract disease (equine rhinopneumonitis), but EHV-1 also causes abortion and neurologic disease. All horses on breeding and training farms should be regularly vaccinated against these viruses. Several inactivated products and one MLV vaccine are commercially available. Vaccination is recommended for use in pregnant mares to aid in the prevention of abortigenic EHV-1 infection and prevention of EHV-1– and EHV-4–induced respiratory disease in foals, weanlings, yearlings, and young performance and show horses that are included in the high-risk group for viral exposure. High antigen-load vaccines are labeled for protection from abortion in pregnant mares. None of the currently available vaccines has a label claim for prevention against neurologic disease. An initial series of three vaccinations is recommended, with a 4- to 6-week interval between doses. When the initial series is administered in foals, vaccination should be initiated at 4 to 6 months of age, with a 4- to 6-week interval between the first two doses and the third dose given at 10 to 12 months of age. Booster vaccines are recommended to maintain adequate immunity at approximately 6-month intervals. Pregnant broodmares should be vaccinated at 5, 7, and 9 months of gestation with an inactivated high antigen-load product to protect against abortion. Many clinicians also vaccinate at 3 months of gestation. To provide adequate colostral antibody protection, booster vaccination with an inactivated EHV-1 and EHV-4 vaccine should be administered 4 to 6 weeks before expected foaling. Other horses maintained on breeding farms, such as barren mares, stallions, and teaser stallions, should be vaccinated at the beginning of the breeding season and given boosters at 6-month intervals. Adult horses should be vaccinated every 6 to 12 months. Young horses, which are at higher risk for infection because of their age, likelihood of travel, and comingling, will be optimally protected with booster vaccinations given every 6 months.
Streptococcus equi equi Vaccination for protection against strangles is recommended in high-risk horses, such as those being introduced onto a farm with endemic disease. Two Streptococcus equi subsp equi vaccines are currently commercially available: an inactivated subunit vaccine, administered intramuscularly, and an MLV vaccine, administered intranasally. Specific information regarding disease pathogenesis and outbreak control has recently been reported. See the American College of Veterinary Internal Medicine (ACVIM) Consensus Statement (Sweeney et al., 2005) in Suggested Readings. Although vaccination appears to reduce disease severity, it is not without risk for adverse events. For this reason, when determining the indications for vaccination against S equi equi, consideration should be made to vaccinate only horses at high risk for exposure to this bacterium. A potential adverse event associated with use of the MLV vaccine is localized abscess formation. If this vaccine is administered concurrently with other vaccines, care should implemented to complete administration of the other vaccines first, and then prepare and handle the MLV product separately. Such precautions will aid in avoiding inadvertent intramuscular injection of live bacterial organisms. An additional potential complication associated with use of any S equi equi vaccine is the development of purpura hemorrhagica, a generalized immune-mediated vasculitis that can occur following recovery from S equi equi infection or following vaccination,
particularly in horses vaccinated during a course of disease outbreak. For these reasons, it is important for the equine clinician to carefully consider vaccine risks and consult the ACVIM 2005 Consensus Statement to ensure that the risk for adverse events is minimized. Horses not previously vaccinated against S equi equi that are receiving the MLV vaccine should receive an initial intranasal series of two doses given 3 to 4 weeks apart. Booster vaccination is recommended at 6- to 12-month intervals based on risk assessment and manufacturer instructions. Adult horses receiving the killed vaccine should be vaccinated with an initial intramuscular series of three vaccinations at 4- to 6-week intervals, with booster vaccination at 6- to 12-month intervals based on risk assessment and manufacturer instructions. Foals in high-risk situations should be vaccinated with the killed vaccine beginning at 4 to 6 months of age, with an initial series of three doses administered at 4- to 6-week intervals. Booster vaccinations should be administered at 6- to 12-month intervals. The intranasal MLV has been used in foals and may be administered with an initial two-dose series, starting at 6 to 9 months of age, with a 3-week interval between doses. The risk for adverse events associated with vaccine administration is increased when the MLV vaccine is administered to young foals.
Nonencephalitis Neurologic Disease Botulism
The potent toxins produced by the spore-forming bacteria Clostridium botulinum are associated with toxicoinfectious botulism, forage poisoning, wound botulism, and equine grass sickness. Most cases of shaker foal syndrome are caused by C botulinum type B. This organism is a significant threat in Kentucky and the mid-Atlantic states in foals 2 weeks to 8 months of age. A toxoid vaccine for C botulinum type B only is licensed for use in the United States. Current evidence suggests that equine grass sickness is a result of C botulinum type C; therefore the type B vaccine would be ineffective in protecting against this form of disease. The primary indication for the type B vaccine is to prevent shaker foal syndrome by inducing high colostral antibody concentrations in the vaccinated mare. At the time of this writing, there are insufficient data to support the suggestion that neonatal foals respond adequately to vaccination and derive protection from the disease. After passively derived maternal antibodies decline, foal vaccination is recommended in endemic regions. Vaccine recommendations for mares that have previously been vaccinated involve a booster vaccine approximately 4 to 6 weeks before expected foaling date. If the mare is receiving the series for the first time, the three-dose vaccine series should be targeted so that the third vaccine is administered approximately 4 to 6 weeks before the expected foaling date. Subsequent to the initial series, an annual booster should be administered, approximately 4 to 6 weeks before expected foaling. Foal vaccinations for botulism should begin early in life, particularly for foals born to unvaccinated mares. There is no evidence of maternal antibody interference with botulism vaccine administration to foals, so vaccination may begin in the face of circulating maternal antibodies. Foals born to unvaccinated mares should begin a three-vaccine series at 1 to 3 months of age, with a 4-week interval between vaccinations. Foals in high-risk situations have effectively been vaccinated as early as 2 weeks of life. Although not proved to provide complete protection, foals born to unvaccinated mares may benefit from plasma transfusion harvested from a vaccinated horse or administration of C botulinum type B
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antitoxin. Foals born to previously vaccinated mares should begin a three-vaccine protocol at 2 to 3 months of age, with a 4-week interval between vaccinations. Other horses that are maintained in a high-risk setting for development of type B C botulinum–associated disease should receive a series of three vaccinations at 4-week intervals, with an annual booster administered. Duration of immunity following recovery from botulism (in adult horses or foals) is highly variable, and vaccination is recommended following complete recovery from disease, starting with an initial threedose series, followed by an annual booster.
Abortive Disease
Equine Viral Arteritis Equine viral arteritis is caused by equine arteritis virus. The primary health hazards to horses are abortion in pregnant mares, illness and death in young foals, and the chronic carrier state in stallions. It is possible to develop effective programs for the prevention and control of equine viral arteritis based on the known characteristics of the causal agent and the epidemiology of the disease. Infection can be controlled by minimizing or eliminating direct or indirect contact with various secretions, excretions, or tissues of infected horses. An important component of successfully controlling disease is implementation of an appropriate vaccine strategy. The MLV vaccine effectively induces protection in vaccinated individuals. It is impossible to distinguish seroconversion secondary to vaccination from that resulting from natural exposure. It is therefore strongly recommended that all first-time vaccinates have serum collected for testing by a laboratory approved by the U.S. Department of Agriculture and confirmed negative for antibodies against equine arteritis virus, before the vaccine is administered. Because immunity is not induced immediately, the vaccine should be administered to stallions no less than 4 weeks before breeding. Similarly, mares should be vaccinated no less than 4 weeks before breeding. After being vaccinated, a mare should be isolated from any other horses that may transmit the virus because the mare will not be immediately protected after immunization. Pregnant mares and foals younger than 6 weeks should not be vaccinated.
Equine Herpesvirus Type 1 See recommendations for the management of EHV-associated rhinopneumonitis. Broodmares should be vaccinated during months 3, 5, 7, and 9 of gestation, with an inactivated product labeled for the protection against abortive EHV-1 (high–antigen-load vaccine).
Other
Anthrax Anthrax is a serious and commonly fatal septicemic condition resulting from infection with the bacterium Bacillus anthracis. Vaccination is recommended in horses where the disease is endemic and only in at-risk individuals because adverse reactions have been reported in young horses and miniature breeds. The reactions, which include injection site pain and swelling, generally resolve in a number of days. Adult horses that have been previously vaccinated should receive annual booster administration. Horses that have not been previously vaccinated should begin an initial two-dose series, administered subcutaneously, with a 4- to 6-week interval. Annual booster vaccines should be con tinued while horses are maintained in endemic regions. Routine vaccination for broodmares and young horses is not recommended.
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Potomac Horse Fever Equine monocytic ehrlichiosis caused by Neorickettsia risticii (formerly Ehrlichia risticii) is a seasonal disease of late spring to early fall. Clinical signs may include fever, diarrhea, laminitis, colic, and reduced gastrointestinal motility. In endemic regions, one component of reducing disease severity is vaccination. Two commercial killed vaccines are available for use in horses considered at high risk for the development of disease. Pregnant mares that develop disease are at risk for abortion, but neither of the commercially available vaccines claims to protect against this. Adult horses that have previously been vaccinated should be vaccinated before the peak season associated with disease, typically in the spring months. Booster vaccine administration is recommended at 3- to 4-month intervals in high-risk situations because the duration of immunity induced by vaccination appears to be short lived. Horses that have not been previously vaccinated should receive an initial series of two vaccines at 3- to 4-week intervals, with the second dose completed 2 to 3 weeks before the anticipated peak incidence of disease. Broodmares should receive a booster vaccine approximately 4 to 6 weeks before expected foaling. If the mare has not been previously vaccinated for Potomac horse fever, she should receive a twodose series, with the second dose administered approximately 4 to 6 weeks before expected foaling. In foals, the initial dose should be given at 5 to 6 months of age, with a second dose to follow in 3 to 4 weeks; a third dose should then be administered at 10 to 12 months of age. Boosters are recommended at 3- to 6-month intervals, depending on the risk for disease. Boosters should be given in May to June in endemic regions.
Rotavirus Rotavirus is a nonenveloped RNA virus that is an important infectious cause of foal diarrhea. Although many factors, such as environmental conditions and exposure to infected individuals on high-density farms, play important roles in development of diarrhea in foals, mare vaccination against rotavirus results in high antibody levels in colostrum and subsequently increased foal serum antibody concentrations. Evidence to support mare vaccination is based on field trials that have demonstrated attenuation of rotaviral diarrhea cases. One commercially available inactivated vaccine contains group A rotavirus and is indicated for use in mares to enhance colostral immunoglobulin concentration against equine group A rotavirus. Mares should receive a three-dose series of intramuscular vaccinations at 8, 9, and 10 months of gestation, regardless of previous vaccine status. This vaccine is used frequently in regions where large populations of mares and foals are housed; in addition, mares that will be shipped to high-density areas should be vaccinated in preparation for exposure to rotaviral challenge. It is important to ensure that foals receive an adequate quantity of colostrum from mares that have been vaccinated against rotavirus. There is no evidence to suggest that newborn foals require vaccination
against rotavirus. Serum colostral antibody titer declines at approximately 60 days of age; although there is some risk for development of rotaviral diarrhea at this age, the risk is considered low, and if disease develops, it is generally self-limited.
Snake Bite Vaccine Venomous snakebites occur in certain areas of North America. Based on risk for exposure, there may be an indication for vaccination of horses in high-risk areas with the Crotalus atrox (western diamondback rattlesnake) toxoid vaccine. At present, one toxoid vaccine with a conditional license is available for use in horses that are at risk for rattlesnake envenomation. The label suggests that there may be crossprotection against venom of the western rattlesnake (including the prairie, Great Basin, northern and southern, and Pacific varieties), sidewinder, timber rattlesnake, Massasauga, and copperhead. Partial protection may be provided against eastern diamondback venom, but not against water moccasin (cottonmouth), Mojave rattlesnake, or coral snake venom. The toxoid vaccine is labeled for use in horses older than 6 months of age. The primary series includes three doses to be administered at monthly intervals, with subsequent booster vaccination at 6-month intervals. Use of this vaccine in pregnant broodmares has not been specifically investigated, and if needed, direct consultation with the manufacturer is recommended.
Suggested Readings Horohov DW, Lunn DP, Townsend HGG, et al. Equine vaccination. J Vet Intern Med 2000;4:221-222. Krebs JW, Mandel EJ, Swerdlow DL, Rupprecht CE. Rabies surveillance in the United States during 2004. J Am Vet Med Assoc 2005;227:1912-1925. Kydd JH, Townsend HG, Hannant D. The equine immune response to equine herpesvirus-1: the virus and its vaccines. Vet Immunol Immunopathol 2006;111(1-2):15-30. Madigan JE, Pusterla N. Ehrlichial diseases. Vet Clin North Am Equine Pract 2000;16 (3):487-499. Powell DG, Dwyer RM, Traub-Dargatz JL, et al. Field study of the safety, immunogenicity, and efficacy of an inactivated equine rotavirus vaccine. J Am Vet Med Assoc 1997;211:193-198. Sweeney CR, Timoney JF, Newton JR, et al. Streptococcus equi infections in horses: guidelines for treatment, control and prevention of strangles, 2005; ACVIM Consensus Statement. Retrieved September 3, 2012, from http://www.acvim.org/ websites/acvim/index.php?p=22. Timoney PJ, McCollum WH. Equine viral arteritis. Vet Clin North Am Equine Pract 1993;9:295-309. Townsend HGG, Lunn DP, Bogdan J, et al. Comparative efficacy of commercial vaccines in naive horses: serologic responses and protection after influenza challenge. In: Proceedings of the 49th Annual American Association of Equine Practitioners Convention, New Orleans, 2003. Whitlock RH, Buckley C. Botulism. Vet Clin North Am Equine Pract 1997;13:107.