CHAPTER 43. Preventive Medicine
Rebecca S. McConnico
VACCINATION
I. Active immunization
A. Definition: Vaccination
B. Goal
1. Prolonged strong immunity
2. Free of adverse side effects
3. Ideal vaccine
4. Stable
5. Inexpensive
6. Effective
7. Adaptable to mass vaccination
C. Vaccination practices are guided by:
1. Tradition
2. Likelihood of exposure to a particular infectious agent
3. Limited research data
4. Scientific studies
D. Safety studies: Required by the Food and Drug Administration for market approval
1. Efficacy studies are necessary but not required
2. Controlled, blinded efficacy studies
a. Expensive
b. Time-consuming
II. Considerations for designing an effective vaccination program
A. Cost and risk of disease vs. cost and risk of vaccination
B. Zoonotic potential (e.g., rabies)
C. Endemic diseases vs. rare or disease-outbreak situations; natural vaccine vs. man-made vaccine
D. Vaccines available for horses in the United States
1. Tetanus
2. Equine herpes virus type 1 (EHV-1)
3. Equine herpes virus type 4 (EHV-4)
4. Eastern equine encephalomyelitis (EEE)
5. Western equine encephalomyelitis (WEE)
6. Venezuelan equine encephalomyelitis (VEE)
7. West Nile virus encephalomyelitis (WNV)
8. Influenza
9. Strangles
10. Rabies
11. Potomac horse fever
12. Rotavirus
13. Botulism
14. Anthrax
E. Fatal diseases in horses with a safe and effective vaccine such as tetanus
F. Often fatal neurologic disease
1. Arboviral encephalitis (Eastern, Western, Venezuelan, West Nile) with high mortality (greater than 50% for EEE)
2. Rabies (efficacy of vaccine for horses is unknown)
G. Highly contagious diseases of horses with safe and effective vaccine availability
1. Influenza
2. Herpes types 1 and 4: Respiratory disease and abortion. Remember that the herpes vaccine does not protect against the neurologic form of herpes (herpes myelitis)
3. Strangles vaccine ( Streptococcus equi): Several adverse side effects have been associated with strangles vaccines
H. Types of vaccines
1. Attenuated: Flu, herpes, equine viral arteritis (EVA), all encephalitis (EEE, WEE, VEE, WNV, rabies), strangles, tetanus, equine protozoal myelitis
2. Modified-live (e.g., intranasal; stimulate secretory immunoglobulin A)
a. Herpes
b. Influenza
c. Strangles: Aerosol, intranasal
3. DNA
a. Humoral and cell-mediated immunity
b. Recombinant: WNV
III. Vaccinating foals
A. A relative lack of immune responsiveness in young foals to currently available vaccines. The frequent use of vaccines in the face of persistent maternal immunity may actually induce a state of tolerance, which can prevent a satisfactory response to vaccines past 1 year of age
B. Tetanus
1. Initial vaccination series: 2 to 6 months of age
2. 6- to 8-week boosters are necessary if vaccination begins at 3 to 4 months of age
3. Yearly boosters are necessary after the initial series
4. Horses with a deep puncture wound or necrotic wound should be boostered (or initially vaccinated) regardless of vaccine status. It is not uncommon for these horses to receive tetanus antitoxin in addition to a toxoid
6. Important points:
a. Antitoxin may cause serum sickness in adults
b. Needs to be boosted yearly
c. Horses are the most susceptible species to tetanus the disease
7. Possible adverse side effect of using tetanus vaccine
a. Theiler disease
b. Acute hepatic necrosis
c. Serum sickness
d. Occur in the summer and fall
e. Occurs within 4 to 10 weeks of vaccine
f. Can be fatal
C. Equine influenza
1. A contentious issue in foal vaccination is the timing of the initial series of influenza vaccinations
2. Foals may be relatively immunologically unresponsive to many of the currently available vaccines
3. Nonprotective levels of antibodies may interfere with immune response
4. Equine “flu”
a. Secondary upper respiratory tract and pulmonary bacterial infections are common because the virus compromises defense mechanisms
b. Immunity resulting from natural disease lasts about 1 year
c. Vaccinal immunity lasts about 3 to 4 months
5. Equine “flu” vaccine
a. Needs to be type 2 strain with a date from the late 1980s or, ideally, the 1990s
b. Current circulating strain of flu
c. Vaccinate every 3 to 4 months in “traveling” horses
d. Modified live virus (MLV) vaccine at least 2 to 4 weeks before potential exposure (and not less than 10 days)
D. Rhinopneumonitis = EHV-1 and EHV-4
1. EHV-1
a. Abortion
b. Respiratory disease
c. Myelitis
2. EHV-4 Respiratory
3. Vaccine: EHV-1 and EHV-4
a. Pregnant mares
(1) For example, Pneumabort K, Ft. Dodge, Animal Health
(2) Administer at 5, 7, and 9 months of gestation (some veterinarians administer at 3 months)
(3) Only approved vaccine for herpes for pregnant mares
b. Foals: MLV, begin at 3 months of age
(1) Two additional vaccinations at 4- to 8-week intervals
(2) Depending on exposure potential
(3) Administer every 3 to 6 months
c. Adult horses not in contact with broodmares probably do not need to be vaccinated
d. Nonpregnant horses that are in contact with broodmares should be vaccinated for EHV-1 and EHV-4 every 3 to 6 months (at least twice a year)
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