IN-DEPTH: IMMUNOLOGY/VACCINATION/PARASITE CONTROL


mation regarding vaccination in horses less than a year of age. Adverse reactions have been reported in young and miniature horses, and caution should be used when use is considered in such instances.


Botulism


Botulism is a serious and frequently fatal infectious disease that develops secondary to liberation of toxin from Clostridium botulinum, most commonly Cl. botulinum type B and C. Four forms of disease occur in horses: wound botulism, shaker foal syn- drome, forage poisoning, and equine grass sickness. An inactivated toxoid Cl. botulinum type B vaccine is licensed for use in horses in the United States. The indication for vaccination is for the prevention of shaker foal syndrome in horses less than 1 year of age. Pregnant broodmares should be vaccinated in at-risk geographic regions such as central Kentucky and the mid-Atlantic region. There is no evidence for cross protection against other Cl. botulinum tox- ins; therefore, vaccination against type B will not protect against other forms of disease. The vaccine protocol involves an initial three-dose


series in pregnant broodmares with a final vaccine booster administered approximately 4 weeks before the expected foaling date. This protocol will pro- vide the mare the opportunity to maximize colostral antibody concentration at the time of parturition. Annual boosters are recommended at time of pre- foaling vaccination. Foals in high risk regions should begin a vaccine series at 2 to 3 months of age. Foals born to unvaccinated mares or foals moving to high risk geographic locations should begin the three-dose vaccine series at 2–4 weeks of age with boosters at 4 week intervals. Other horses located in high risk regions should receive an initial three- dose series with annual booster vaccination. Horses that have suffered from Cl. botulinum type B infection that have recovered from disease should begin a three-dose vaccine series following recovery.


Equine Influenza


Equine influenza is a common upper respiratory infection resulting from viral colonization of the upper respiratory tract. Equine influenza is classi- fied among the orthomyxoviruses, which are single stranded RNA viruses. Influenza viruses are clas- sified among three types depending on surface and internal protein antigens, A, B, and C, yet only type A influenza has been reported to infect horses. Major viral antigens include neuraminidase and hemagglutinin. Two type A subtypes that are known to cause disease in horses include H7N7 and H3N8. Among viruses H3N8 is considered to be of greater pathogenicity than H7N7. Influenza is most common in horses commingled


under stressful conditions such as race or show training. Infection occurs via inhalation of viral particles, which are abundant in an environment containing infected horses, resulting from severe paroxysmal coughing that occurs in affected indi-


398 2014  Vol. 60  AAEP PROCEEDINGS


viduals. Once inhaled, the virus infects the respi- ratory ciliated epithelium, leading to loss of the mucociliary escalator, which substantially dimin- ishes pathogen and particle clearance. Therefore, viral infection predisposes affected individuals to secondary bacterial colonization. Clinical manifestation of disease includes a short incubation period of 1 to 3 days, high fever, depres- sion, and paroxysmal coughing, which can be severe. Nasal discharge typically begins as a serous fluid, yet with disease progression and bacterial contami- nation mucopurulent discharge may be observed. There are currently three influenza vaccines available:


1) Inactivated (intramuscular administration): This vaccine contains many of the A2 stains of influenza that are currently in circula- tion. These vaccines are recommended to in- clude a three-dose priming series with the initial dose administered witha4to6 week interval; the third vaccine should be admin- istered 8 to 12 weeks after the second dose. These vaccines are well suited for pre- foaling series, which will provide high levels of colostral immunoglobulin.


2) Modified live cold-adapted equine influenza A/2 (intranasal administration): This vac- cine is administered intranasally and has been shown to effectively provide rapid pro- tection to naïve individuals.3,26,27 The prod- uct is labeled for protection of horses 11 months of age or greater. Vaccination in 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 individu- als following vaccination.


3) Canary pox-vectored vaccine (intramuscular ad- ministration): This product is administered as an intramuscular injection and is safe to admin- ister as early as 4 months of age. A strong humoral response is observed following vaccina- tion, suggesting that this is also an appropriate product to administer during late term preg- nancy to enhance colostral antibody levels.


Adult horses that have previously been immu-


nized and are at risk for exposure can be effectively vaccinated twice annually. If risk for disease in relatively low, annual booster vaccination should be sufficient to provide protection from disease. Pre- viously unvaccinated adult horses can effectively be vaccinated with an individual dose of the MLV intranasal vaccine and boostered at 6 month inter- vals. Alternatively, an initial two-dose series of the canary pox vectored vaccine or a three-dose series of


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