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Vaccinations for Your Horses
All horses are at risk of development of tetanus, an often fatal disease caused by the anaerobic, spore-forming bacterium, Clostridium tetani. Tetanus toxoid is a core equine vaccine and is indicated in the immunization program for all horses. Clostridium tetani organisms are present in the intestinal tract and feces of horses, other animals and humans, and are abundant as well as ubiquitous in soil. Spores of Cl. tetani survive in the environment for many years, resulting in an ever-present risk of exposure of horses and people on equine facilities. Tetanus is not a contagious disease but is the result of Cl. Tetani infection of puncture wounds (particularly those involving the foot or muscle), open lacerations, surgical incisions, exposed tissues such as the umbilicus of foals and reproductive tract of the postpartum mare (especially in the event of trauma or retained placenta).
It is generally accepted that tetanus toxoid administered per manufacturer recommendations yearly is both safe and effective. Survival of horses with tetanus was strongly associated with previous vaccination. Tetanus is included with the yearly 5-in-1 vaccine.
Adult horses, previously vaccinated against tetanus:
Vaccinate annually.Horses that sustain a wound or undergo surgery 6 or more months after their previous tetanus booster should be revaccinated with tetanus toxoid immediately at the time of injury or surgery.
Note: The severity of the wound does not predict the risk for development of tetanus. Superficial wounds have resulted in clinical tetanus in horses.
Adult horses, previously unvaccinated against tetanus, or of unknown vaccine history:
Administer a primary 2-dose series of tetanus toxoid with a 4- to 6-week interval between doses. Vaccinate annually thereafter.
Tetanus antitoxin is indicated to provide passive immunity in situations where the horse is at risk of tetanus infection and has not been immunized according to labeled recommendations for tetanus. If the veterinarian determines that administration of tetanus antitoxin is indicated, then it should be administered. Pregnant mares previously vaccinated against tetanus: Vaccinate annually 4 to 6 weeks before foaling, both to protect the mare should foaling-induced trauma or retained placenta occur and to enhance concentrations of colostral immunoglobulins. Pregnant mares unvaccinated against tetanus or of unknown vaccine history: Administer a 2-dose primary series of tetanus toxoid with a 4- to 6-week interval between doses. Revaccinate 4 to 6 weeks before foaling.
EEE, VEE, WEE
In the United States, equine encephalitides for which vaccines are available include eastern equine encephalomyelitis (EEE), western equine encephalomyelitis (WEE), Venezuelan equine encephalomyelitis (VEE) and West Nile Virus encephalomyelitis. The distribution of EEE has historically been restricted to the eastern, southeastern and some southern states while outbreaks of WEE have been recorded in the western and mid-western states. Variants of WEE have caused sporadic cases in the northeast and southeast, most notably Florida. VEE occurs in South and Central America but has not been diagnosed in the United States for more than 20 years. The availability of licensed vaccine products combined with an inability to completely eliminate risk of exposure justifies immunization against EEE and WEE as core prophylaxis for all horses residing in or traveling to North America and any other geographic areas where EEE and/or WEE is endemic.
Transmission of EEE/WEE/VEE
Is by mosquitoes, and infrequently by other bloodsucking insects, to horses from wild birds or rodents, which serve as natural reservoirs for these viruses. Human beings are also susceptible to these diseases when the virus is transmitted to them by infected mosquitoes; however, horse-to-horse or horse-to-human transmission by mosquitoes is highly unlikely, because the amount of virus in the blood of horses affected by EEE or WEE is small. The risk of exposure and geographic distribution of EEE and WEE vary from year-to-year with changes in distribution of insect vectors and reservoirs important in the natural ecology of the virus. EEE activity in mosquito and birds, and resultant disease in humans and equids, continues to cause concern along the East Coast and demonstrates northward encroachment. VEE is a reportable foreign animal disease. Epidemics of VEE occur when the virus undergoes genetic change and develops greater virulence in avian and mammalian hosts. These viral variants are able to multiply to high levels in the horse and then the horse becomes a reservoir in these outbreaks.
EEE/WEE vaccines currently available are formalin inactivated adjuvant whole virus products. Early testing of bivalent (EEE/WEE) vaccines was performed by intracranial challenge with either EEE and WEE; the formalin inactivated preparations demonstrated 100% protection.Currently, the only available VEE vaccine is a killed product. This vaccine is included in the yearly 5-in-1 vaccine.
Vaccination Schedules EEE/WEE:
Adult horses previously vaccinated against EEE/WEE Annual revaccination must be completed prior to vector season in the spring. In animals of high risk or with limited immunity, more frequent vaccination or appropriately timed vaccination is recommended in order to induce protective immunity during periods of likely exposure. Pregnant mares, previously vaccinated against EEE/WEE vaccinate 4 to 6 weeks before foaling.
Pregnant mares, unvaccinated or having unknown vaccine histories immediately begin a 2-dose primary series with a 4-week interval between doses. Booster the vaccine at 4 to 6 weeks before foaling or prior to the onset of the next vector season.
WEST NILE VIRUS
This virus has been identified in all of the continental United States, most of Canada and Mexico. Several Central and South American countries have also identified WNV within their borders. The virus is transmitted from avian reservoir hosts by mosquitoes (and infrequently by other bloodsucking insects) to horses, humans and a number of other mammals. West Nile virus is transmitted by many different mosquito species and this varies geographically. The virus and mosquito host interactions result in regional change in virulence of the virus and no prediction can be made regarding future trends in local activity of the viruses. Horses and humans are considered to be dead-end hosts for WNV; the virus is not directly contagious from horse to horse or horse to human. Indirect transmission via mosquitoes from infected horses is highly unlikely as these horses do not circulate a significant amount of virus in their blood. This vaccine is not included in the yearly 5-in-1 vaccine it is a separate vaccination.
Initial series of two intramuscular injections administered 3 to 6 weeks apart followed by a 12-month revaccination interval.
Adult horses previously vaccinated
Vaccinate annually in the spring, prior to the onset of the insect vector season.
Adult horses previously unvaccinated or having unknown vaccinal history
Rabies is an infrequently encountered neurologic disease of equids. While the incidence of rabies in horses is low, the disease is invariably fatal and has considerable public health significance. It is recommended that rabies vaccine be a core vaccine for all equids. Exposure occurs through the bite of an infected (rabid) animal, typically a wildlife source such as raccoon, fox, skunk, or bat. Bites to horses occur most often on the muzzle, face, and lower limbs. The virus migrates via nerves to the brain where it initiates rapidly progressive, invariably fatal encephalitis.
The vaccines are given by intramuscular injection.
- Adult horses previously vaccinated against rabies:
- Annual revaccination.
- Adult horses previously unvaccinated against rabies or having unknown vaccine history:
- Administer a single primary dose. Revaccinate annually.
- Pregnant mares, previously vaccinated against rabies:
- Vaccinate 4 to 6 weeks before foaling.
- Administration of rabies vaccine prior to breeding of the mare reduces the number and type of vaccines given in the period prior to foaling.
- Pregnant mares, previously unvaccinated or of unknown vaccine history:
- Vaccinate 4 to 6 weeks before foaling.
- Foals of mares vaccinated against rabies:
- Administer a primary series. The first dose of vaccine should be administered no earlier than 6 months of age. The second dose should be given 4 to 6 weeks later. Revaccinate annually thereafter.
- Foals of mares NOT vaccinated against rabies:
- Administer according to label directions. The first dose of vaccine should be administered at 3 to 4 months of age. Revaccinate annually thereafter.
Horses exposed* to confirmed rabid animal
- Horse currently vaccinated against rabies: Immediate revaccination by a licensed veterinarian and observation (as directed by public health officials) for 45 days for development of clinical signs of rabies.
- Unvaccinated horse: Euthanatize immediately. If the owner is unwilling to have this done then horse should be closely monitored under veterinary supervision for 6 months. Public health officials may establish requirements and conditions for monitoring of exposed, unvaccinated animals.
- *Rabies exposure and transmission occur only when the virus is introduced into bite wounds, into open cuts in skin, or onto mucous membranes from saliva or other potentially infectious material such as neural tissue.
PHF (Potomac Horse Fever)
Equine monocytic ehrlichiosis is caused by Neorickettsia risticii (formerly Ehrlichia risticii).
- Clinical signs are variable but may include:
- Fever, mild to severe diarrhea, laminitis, mild colic, and decreased abdominal sounds.
- The currently available commercial vaccines are killed, adjuvanted products. Two of these are also available combined with a rabies vaccine
- This is a separate vaccine that can be given with your spring vaccine series.
- Due to the seasonal incidence of disease, vaccination should be timed to precede the anticipated peak challenge during the summer months or fall.
Adult horses, previously vaccinated:
- Manufacturers recommend revaccination at 6- to 12-month intervals.
- Adult horses, previously unvaccinated or with unknown vaccinal history:
- Administer a primary series of 2 doses, at a 3- to 4-week interval. Peak protection occurs 3 to 4 weeks after the second dose.
Streptococcus equisubspecies equi (S. equi var. equi) is the bacterium which causes the highly contagious disease strangles (also known as “distemper”). Strangles commonly affects young horses (weanlings and yearlings), but horses of any age can be infected. Vaccination against S. equi is recommended on premises where strangles is a persistent endemic problem or for horses that are expected to be at high risk of exposure. Following natural infection, a carrier state of variable duration may develop and intermittent shedding may occur. The organism is transmitted by direct contact with infected horses or sub-clinical shedders, or indirectly by contact with: water troughs, hoses, feed bunks, pastures, stalls, trailers, tack, grooming equipment, nose wipe cloths or sponges, attendants’ hands and clothing, or insects contaminated with nasal discharge or pus draining from lymph nodes of infected horses. Streptococcus equi has demonstrated environmental survivability particularly in water sources and when protected from exposure to direct sunlight and disinfectants, and can be a source of infection for new additions to the herd.
Infection by S. equi induces a profound inflammatory response.
- Clinical signs may include
- Fever (102-106o F)
- Dysphagia or anorexia
- Stridor; lymphadenopathy (+/- abscessation)
- Copious mucopurulent nasal discharge.
Vaccination should not be expected to prevent disease. However, appropriate pre-exposure vaccination with these products appears to attenuate the severity of clinical signs in affected horses, should disease occur, and has been shown to reduce the incidence of disease by as much as 50% during outbreaks.
Modified live vaccine
An intranasal product has been shown to stimulate a high level of immunity against experimental challenge. The inductive sites are the pharyngeal and lingual tonsils. Vaccinal organisms must reach these sites in sufficient numbers to trigger protective responses; therefore, accurate vaccine delivery is critical to vaccine efficacy. In a small percentage of cases, residual vaccinal organism virulence may result in formation of slowly developing mandibular or retropharyngeal abscesses.
- Adult horses previously vaccinated
- Vaccinate every 6 to 12 months based on risk assessment and manufacturers’ recommendations.
- Adult horses unvaccinated or having unknown vaccinal history
- Manufacturers’ recommendations are for primary vaccination with a series of 2 or 3 doses administered at intervals of 2 to 4 weeks, depending on the product used, followed by annual revaccination. Revaccinate at 6- month intervals, regardless of the injectable product used.
Modified live vaccine:
- Administer intranasal a 2-dose primary series with a 3-week interval between doses. Semiannual (6-month intervals) or annual revaccination is recommended.
Intestinal Parasites and Your Horse
Internal parasites are small organisms that live a portion of their life cycle in a host animal. They live in internal organs, body cavities, and tissues while gaining their nutritive source by feeding on the host animal. The horse is affected by many different species of parasites. The nature and extent of damage varies with the parasite.
Parasite infestation causes loss of nutrients or blood from the host, resulting in serious medical problems. Horses heavily burdened with parasites will have a loss of condition due to a depletion of nutrients and blood, decreased growth, and reduced reproductive and athletic performance.
Numerous internal parasites infect horses, but there are only a few that commonly cause significant health problems. To establish an effective parasite control program, it is important to first understand the life cycle of parasites. Successful prevention and control programs are effective because they interrupt the life cycles of parasites. In some areas of the country, the primary class of internal parasites that cause health problems for horses are nematodes, such as large and small strongyles, ascarids, and tapeworms. Other internal parasites perhaps of lesser significance, such as pinworms and botfly larvae, are often considered when designing a parasite control program.
Strongyles are grouped as either large or small. The three primary species of large strongyles that infect the horse are Strongylus vulgaris, Strongylus endentatus, and Strongylus equinus.
The adult form of all strongyles (large or small) live in the large intestine. Adult strongyles produce eggs that are passed out in the feces into the horse’s environment. These eggs then develop into infective larvae that exist on the pasture vegetation or in stalls. The horse is infected when it consumes grass, feed, or water contaminated with infective larvae. These larvae are very resistant to harsh environmental conditions because of a protective sheath. Strongyles can survive in a freezing environment, but a hot and dry environment will often kill them. The infective larvae survive up to 31 weeks at winter temperatures, compared to up to seven weeks at summer temperatures.
The larvae of large strongyles migrate through various parts of the body. Strongylus vulgaris, the bloodworm, will burrow into and migrate in the walls of the arteries that are the primary blood supplier to the small and large intestines. This migration can result in the formation of blood clots, which can disrupt the blood flow to the intestines and cause scar tissue formation in affected arteries. After approximately 120 days, the larvae move to the lumen of the large intestine, where maturation is completed. As adults, these parasites will lay several thousand eggs each day, completing the life cycle. The entire life cycle takes six to seven months.